AN INTRODUCTION TO MINERALS

Transcription

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2 AN INTRODUCTION TO MINERALS Vivien Gornitz New York City August 1998 Copyright 1998 The New York Mineralogical Club

3 The image below is from the Bulletin of the New York Mineralogical Club Volume 3, No. 1: The Minerals of New York City and Its Environs by James G. Manchester, January, G.F. Kunz Col. Plate No. 50 (x½) GARNET, VAR. ALMANDITE Broadway and 35th Street, Manhattan Island, New York City ii

4 PREFACE This booklet has grown from a set of notes prepared for the Study Group of the New York Mineralogical Club, held at the American Museum of Natural History in New York City between 1995 and It provides an introduction to basic concepts of mineralogy and crystallography that govern the physical properties and morphology of crystals, mineral clusters, and aggregates. This information will give mineral collectors and rockhounds a sounder basis for recognizing and identifying minerals. In addition, mineral enthusiasts will also develop a keener esthetic appreciation of the factors that contribute to the formation of a well-crystallized specimen. The material in this booklet is intended to bridge a gap between the colorfully-illustrated field guides and the more technically-oriented texts and reference books. Vivien Gornitz, Ph. D. New York City June, 1998 iii

5 ACKNOWLEDGMENTS The author expresses her sincerest appreciation to Mitchell Portnoy for his tireless efforts, professionalism, and originality in arranging the layout, graphic design, and printing this booklet. Thanks are also extended to John Betts and Ken Colosky for their helpful comments on mineral recognition and fluorescence, respectively. Illustration Credits Figures 1.1, 2.1, 2.2, 2.3, 2.5, 2.7, 2.9, 2.10, 2.13, 3.1, 3.2, 3.8, 3.9, 3.10a-c, 4.2, and 4.5 are reprinted by permission of John Wiley and Sons, Inc., from Th e Man u al o f Mine ralo g y, by C. Klein and C.S. Hurlbut, Jr. Copyright 1993 by John Wiley and Sons, Inc. Figures 4.3 and 4.4, B.M. Loeffler and R.G. Burns, Shedding Light on the Color of Gems and Minerals, Am e ric an Sc ie ntis t, V. 64, p (1976). Figures 3.4 and 3.5, The Tourmaline Group, by R.V. Dietrich, Van Nostrand Reinhold, (Now part of John Wiley and Sons.) Figures 2.14 and 6.2, Ro c ks and Mine rals, by A Mottana, R. Crespi, and G. Liboria; Edited by M. Prinz, G. Harlow, and J. Peters. Simon and Schuster, Inc. Copyright 1978, by Arnaldo Mondadori Editore S.P.A., Milan. Figure 5.5, E. Fritsch and G.R. Rossman, An Update on Color in Gems. Part 3. Ge m s and Ge m o lo g y, Summer All other illustrations or diagrams are originals by the author or from sources in the public domain. iv

6 TABLE OF CONTENTS Chapter 1. Physical Properties of Minerals Chapter 2. Basics of Crystallography Chapter 3. Crystal Growth Chapter 4. Origin of Color in Minerals and Gems Part I Chapter 5. Origin of Color in Minerals and Gems Part II Chapter 6. Classification of Minerals Chapter 7. Mineral Recognition Bibliography About the New York Mineralogical Club About the Author List of Figures Chapter 1 Figure 1.1. Common mineral habits Figure 1.2. Chatoyancy (left) and star effect (right) Figure 1.3. Comparison of the Mohs scale of hardness to absolute measurements of hardness Figure 1.4. Examples of cleavage Chapter 2 Figure 2.1. Examples of crystals and crystal forms Figure 2.2. Comparison of external appearance and internal arrangement of atoms in anhydrite Figure 2.3. Some crystal habits ideal and distorted Figure 2.4. Examples of symmetry Figure 2.5. Arrangement of pentagons and hexagons Figure 2.6. Symmetry elements of the three crystals shown in Figure 2.1 Figure 2.7. The six crystal systems Figure 2.8. Sphalerite crystal Figure 2.9. (a) The orthorhombic unit cell of sulfur. (b) The tetragonal unit cell of scheelite Figure Sulfur crystal two dipyramids Figure Pyrite crystal showing a cube modified by an octahedron and dodecahedron Figure (a) Orthorhombic crystal of topaz, showing prisms and dipyramid (b) Monoclinic crystal of orthoclase, showing prisms and pinacoids Figure 2.13a A 41 screw axis Figure 2.13b Glide plane Figure The 14 Bravais lattices v

7 Chapter 3 Figure 3.1. Schematic illustration of crystal growth Figure 3.2. Defects in crystal structures Figure 3.3. Trigons and solution pits in diamond Figure 3.4. Relationship between color zoning and chemical variations in tourmaline Figure 3.5. Slices of color-zoned tourmaline cut perpendicular to the c-axis. Figure 3.6. Successive sections through a chiastolite crystal Figure 3.7. Schematic illustration of three stages of growth in a hoppered cube crystal (halite) Figure 3.8. Growth features Figure 3.9. Examples of epitaxial growth. (a) Staurolite on kyanite; (b) Plagioclase on microcline. Figure 3.10a. Contact twins Figure 3.10b. Penetration twins Figure 3.10c. Polysynthetic twins Figure 3.10d. Cyclic twins Figure Relation between lattices in a twinned crystal Chapter 4 Figure 4.1. Figure 4.2. Figure 4.3. Figure 4.4. Figure 4.5. Figure 4.6. The visible spectrum as part of the electromagnetic spectrum Relative energies of the orbitals in neutral, many-electron isolated atoms Absorption spectra of two Fe+2-bearing minerals Absorption spectra of three Cr+3-bearing minerals Schematic illustration of fluorescence. Schematic illustration of the production of color centers Chapter 5 Figure 5.1. Figure 5.2. Figure 5.3. Figure 5.4. Figure 5.5. Three types of band gaps and their relation to colors of minerals Band gaps and the colors of diamond The refraction of light in a crystal. The example shown is for diamond The dispersion of white light by a prism The diffraction of light in precious opal Chapter 6 Figure 6.1. Figure 6.2. Figure 6.3a. Figure 6.3b. The silicate tetrahedron basic building block of the silicate minerals Schematic illustration of the structure of silicates Structure of pyroxene Structure of amphibole Chapter 7 Figure 7.1. Figure 7.2. Figure 7.3. Figure 7.4. Etched topaz crystal Magnetite or lodestone Schematic illustration of test for double refraction Homemade dichroscope vi

8 CHAPTER 1 PHYSICAL PROPERTIES OF MINERALS Introduction Minerals exist everywhere in the soil underfoot, the arid wastelands of deserts, the rocks of the loftiest mountain ranges, the ocean depths, and beyond the earth, to the farthest reaches of the solar system. Minerals have benefitted human society, from the most ancient flint and chert tools, to the metal and pottery artifacts and colorful pigments of the early civilizations, to the steel, glass, ceramic, and silicon products of our technological era. Mineralogy, or the scientific study of minerals, is one of the oldest of the physical sciences. It has contributed extensively to the development of geology, crystallography, inorganic chemistry, physics, and materials science. The observation of Nicolaus Steno, in 1669, that the angles between crystal faces of quartz remained constant, no matter how distorted or misshapen the crystal, became the foundation for the science of crystallography. In the 18th and 19th centuries, new discoveries in chemistry and mineralogy were closely intertwined. The famous Swedish chemist, Jon Jacob Berzelius ( ) devised the basis for the chemical classification of minerals, still widely used today (see Chapter 6). The modern phase of mineralogy began in 1913, when the Bragg father and son team used X-ray diffraction of crystals, for the first time, to infer the internal arrangement of sodium and chlorine atoms in halite. Within the last few decades, highly sophisticated new instruments have been devised that can probe extremely minute portions of a crystal to detect subtle variations in chemical composition, study tiny inclusions, and even see the actual atoms in a crystal. Aside from their value to human society and to science, an important motivation for collecting minerals is their sheer beauty and variety. The dramatic shapes and vivid colors of well-formed crystals have a sculptural quality that represents nature's finest artwork. A keener appreciation of the attributes of a desirable, collectible specimen depends on a knowledge of how crystals grow in nature, as well as a recognition of their shapes, symmetries, and physical characteristics. This booklet provides the basic information needed by the novice collector or mineral enthusiast to begin to recognize and identify the common minerals. The material presented here can be used in conjunction with well-illustrated field guides. Although not all-inclusive, this booklet highlights some of the more important topics that will give the collector a basic understanding of mineralogy. 1

9 Mineral Definition A mineral is a homogeneous solid that occurs in nature and that has a specific, but not always fixed, chemical composition, and repetitive atomic arrangement. It is usually formed by inorganic processes. Occurs in nature. Materials having the same chemical composition as minerals, but produced in the laboratory are, strictly speaking, not true minerals. Such gemstones or crystals should be described as lab-grown or synthetic. Homogeneous solid. This implies a uniform material or phase. However, homogeneity may depend on scale. What appears to be homogeneous to the naked eye may be inhomogeneous under higher magnification. For example, microcline often contains very fine narrow blebs or streaks of albite that are visible even to the naked eye. Specific but not always fixed chemical composition. Atoms are usually present in specific proportions. For example, quartz has one atom of silicon for each two atoms of oxygen (SiO 2). However, for many minerals, atoms or ions (atoms that have either lost or gained electrons and therefore carry an electrical charge) can be interchangeable within certain limits. The degree of chemical variability is constrained by two basic principles: 1. Preservation of electrical neutrality. The crystal has to remain electrically neutral, overall. This means that the sum of the positive charges has to equal the sum of the negative charges. For + + example, one potassium (K ) ion can replace one sodium (Na ) ion in the alkali feldspars (both + carry the same charge). On the other hand, for the plagioclase feldspars, one Na and one aluminum (Al ) replace one calcium (Ca ) and two Al ions. To maintain electrical neutrality, the number of silicon atoms must therefore be reduced from three in albite (the sodium endmember of the series) to only two in anorthite (the calcium end member of the plagioclase series) Albite: NaAlSi3O 8 1Na :1Al :3Si Anorthite: CaAl Si O 1Ca :2Al :2Si Atomic or ionic size. The size of the substituting atom or ion has to fall within certain limits, due to the constraints of solid geometry, i.e., the way in which spheres are packed in threedimensional space. A repetitive atomic arrangement. This is characteristic of all crystalline solids. Crystalline implies a repeating pattern of atoms, ions, or molecules arranged in a three-dimensional array or lattice. The term crystalline should not be confused with crystal, which is a solid bounded by flat (planar) surfaces (called faces) that are related to each other by symmetry. (Symmetry will be explained further in the next chapter). The external symmetry of a crystal is a reflection of its internal (atomic-level) symmetry. A crystalline solid does not necessarily have well-developed crystal faces, but does have an ordered arrangement of its atoms. For example, obsidian (volcanic glass) and opal do not have an internal crystalline arrangement. They are called mineraloids. 1 1 Glass (both volcanic and man-made) and opal are warmer to the touch than quartz, which is crystalline. The repetitive array of atoms in quartz allows heat to be conducted more efficiently, which makes it feel cooler. 2

10 Usually formed by inorganic processes. This is a tricky part of the definition, as interactions between minerals and living systems are fairly common in nature. For example, calcite or aragonite found in mollusk shells, corals, and pearls, as well as hydroxyapatite in bone are inorganic compounds that form as a result of biological processes. Hence, these are not considered to be true minerals. On the other hand, whewellite, or calcium oxalate, is a naturally-occurring organic compound and therefore qualifies as a mineral. The term organic as used in chemistry refers to compounds of carbon other than carbonate or carbon dioxide. Note that there are two different connotations of the word inorganic: 1. nonbiological and 2. non-carbon-containing (other than carbonates). Physical Properties of Minerals Physical properties of minerals arise from their chemical and structural makeup. Inasmuch as many physical properties can be rapidly evaluated by visual inspection of hand specimens or by fairly simple tests, they are very useful to the mineral collector for identification. Crystal Habits (see Figure 1.1). Crystals are solids bounded by smooth, flat faces, producing characteristic forms. Crystal forms consist of a set of faces that are related to each other by symmetry (for example, the 6 faces of a cube, the 8 faces of a octahedron, or the 6 faces of a hexagonal prism). Crystal forms and principles of symmetry will be described further in Chapter 2. Minerals tend to occur in specific habits. The habit of a crystal refers to its overall shape or morphology the ensemble of commonly-occurring forms. Some common habits of groups or aggregates of crystals are listed here. Acicular thin needles, e.g., rutile, tremolite-actinolite. Capillary, filiform hairlike or threadlike crystals, e.g., native silver. Fibrous thin fibrous crystals, e.g., asbestiform serpentine (chrysotile), crocidolite (amphibole asbestos). Dendritic branched like a tree, e.g., pyrolusite, native gold, silver. Reticulated intersecting groups of crystals, e.g., cerussite. Bladed elongated, flat crystals, e.g., kyanite. Drusy thin layer of small crystals, often coating another mineral, e.g., quartz on chrysocolla. Botryoidal grapelike, e.g., hematite, malachite. (Reniform kidney-shaped; mammillary, colloform spherical masses generic term). Scaly or Lamellar flat, thin plates, e.g., molybdenite, specular hematite Micaceous very thin flakes, like mica e.g., muscovite, biotite, chlorite, talc. Plumose feathery, e.g., some agates. Prismatic elongated crystal prisms, e.g., beryl, tourmaline, quartz. Granular composed of more-or-less equally-sized small grains (also used to describe certain igneous rocks like granite or diabase). Massive nondescript, formless compact masses. 3

11 Figure 1.1. Common mineral habits. Luster and Color Luster refers to the way in which light is reflected from a mineral surface, creating a particular kind of sheen. It depends on a) the index of refraction (the ratio of the speed of light in air to the speed of light in the mineral) and b) the state of aggregation or surface polish (e.g., a well-polished surface of hematite is steel-gray, but its powder or streak is reddish-brown). Types of Luster Metallic opaque, no light transmitted. Polished surfaces are very reflective, e.g., native metals (gold, silver, copper), hematite, magnetite, pyrite. Submetallic duller on a polished surface. May be translucent in thin slices, e.g., cuprite, cinnabar, wolframite. Non-Metallic: Adamantine diamond-like, brilliant, sparkling. Characteristic of minerals with a high index of refraction; e.g., diamond, zircon, cerussite, (sphalerite). Vitreous or Glassy most common minerals, e.g., quartz, garnet. Pearly like pearl or mother-of-pearl, weakly iridescent, e.g., talc, apophyllite cleavage faces. Resinous resin-like as with sphalerite, sulfur. Silky characteristic of fibrous minerals like asbestos, pectolite, malachite. Waxy chalcedony, nephrite. Earthy dull luster due to rough surface or microcrystalline aggregate. 4

12 Color can be either characteristic of the mineral (ideochromatic), because of the internal presence of a color-producing element, such as copper in azurite (blue) and malachite (green), manganese in rhodochrosite (pink-red), or due to occasional impurities, inclusions, or lattice defects (allochromatic) e.g. fluorite, quartz, topaz, beryl, etc. The color of a mineral that we see is light that has been reflected from the surface of the mineral. It is caused by the selective absorption of various wavelengths of light. The wavelengths of reflected light are complementary to the wavelengths that have been absorbed by the mineral. In anisotropic crystals (e.g., non-cubic or non-isometric crystal systems see Chapter 2), light is broken into two rays that travel through the crystal with slightly different velocities, creating double refraction, as in calcite. In such crystals, light can also be absorbed to varying extents in different directions hence the color changes depending on how the crystal is oriented. This phenomenon is called pleochroism. Pleochroism is best observed in plane polarized light using a Polaroid filter (see also Chapter 7). Extremely pleochroic minerals where color changes can be seen with the naked eye include kunzite, tanzanite, tourmaline, iolite (cordierite). Optical Phenomena These are phenomena in which the luster or color of the mineral surface seems to shift or move as the viewing angle changes. Play-of-Colors e.g., opal; caused by diffraction from tiny, regularly-spaced microscopic spheres, causing changing flashes of color as the stone is moved. Labradorescence e.g., labradorite; caused by diffraction from platy structures due to microscopic-scale intergrowth of two plagioclase phases that differ slightly in chemical composition. Chatoyancy a band of light forming at right angles to closely-packed parallel fibers (e.g. rutile) producing a cat's eye effect (e.g. chrysoberyl) or tiger's eye (fibrous crocidolite replaced by quartz). Other minerals occasionally displaying chatoyancy include tourmaline, sillimanite, aquamarine (Fig. 1.2). Asterism results from the intersection of parallel sets of fibers in more than one direction. Sixrayed stars (ruby, sapphire, rose quartz) form when fibers intersect in three directions at 120 apart (Fig 1.2). A cross results from the intersection of two sets of inclusions or fibers at 90 (e.g., almandine garnet, black diopside). Figure 1.2 (Left). Chatoyancy. Orientation of inclusions at right angles to the beam of light. (Right) Star effect produced by intersection of three sets of inclusions 120º apart. 5

13 Hardness Hardness is the resistance of the mineral to abrasion. It is tested by scratching one mineral with another or with a knife, not by breaking with a hammer! (See tenacity). The commonly-used Mohs scale of hardness (see Figure 1.3) is a relative scale. Note that diamond is over five hundred times as hard as talc. Figure 1.3. Comparison of the Mohs scale of hardness to absolute measurements of hardness. Tenacity Tenacity is the resistance to breaking or crushing. Brittle breaks easily (quartz). Flexible can be bent without breaking (talc, chlorite). Elastic returns to original shape after bending (mica). Malleable can be hammered into thin sheets (gold and silver). Ductile can be drawn into thin wires (gold and silver). Sectile can be cut into thin shavings with a knife (gold, silver). (The last three are characteristic properties of metals.) Cleavage, Parting, Fracture. Cleavage is the tendency of a mineral to break along planes that are parallel to directions of weakness within the crystal structure (Figure 1.4). It is a fundamental property of the mineral; i.e., all specimens of the mineral display cleavage. Basal Cleavage 1 direction: micas, graphite. Prismatic 2 directions: pyroxenes, amphiboles. Cubic 3 directions: halite (rock salt). Octahedral 4 directions: fluorite, diamond. Rhombohedral 3 directions: calcite, siderite. Dodecahedral 6 directions: sphalerite. 6

14 Figure 1.4. Examples of cleavage. (a) cubic; (b) octahedral; dodecahedral; (d) rhombohedral; (e) prismatic and pinacoidal; (f) pinacoidal (basal, platy). Parting is a less regular tendency for the mineral to break in certain planar directions. Not all specimens of the mineral exhibit it. It is often caused by processes such as twinning or high pressure. Fracture is breakage other than cleavage or parting. Common types are: Conchoidal shell-like, e.g.. quartz. Fibrous, splintery fibrous minerals, e.g., natrolite. Hackly sharp-edged, metallic, e.g., native copper, gold. Uneven Other than the above. Specific Gravity The specific gravity denotes the ratio of the weight of mineral to the weight of an equal volume of water. It depends on the nature of the atomic composition and tightness of chemical bonding. HIGH >10 platinum, gold, silver 5-10 galena, iron, cassiterite, cuprite, pyrite 4-5 sphalerite, corundum, chalcopyrite, rutile, barite 3-4 apatite, fluorite, hornblende, diamond 2-3 quartz, feldspar, micas, calcite, gypsum 1-2 borax, many evaporites LOW Miscellaneous Properties Piezoelectricity The pressure on a polar axis produces an electrical current. This property is present only in minerals of crystal classes lacking a center of symmetry (see next chapter). Examples: quartz used in quartz watches; tourmaline. Pyroelectricity heating produces an electrical current. Analogous to piezoelectricity. Magnetism attraction to a magnet. Examples: magnetite (strong) pyrrhotite ilmenite (weak) 7

15 8 NOTES

16 CHAPTER 2 BASICS OF CRYSTALLOGRAPHY Introduction Minerals often occur in regular, geometric solids known as crystals. The regularity and symmetry of crystals on the visible scale are a direct manifestation of a three-dimensional, ordered arrangement of the constituent atoms or molecules. Crystallography is the science that deals with the study of crystals and crystalline solids: what controls their external shapes and how they grow, and also how the atoms or molecules are arranged in space. While crystallography originated as a part of mineralogy, it has now expanded to include all forms of solid matter, including molecules of biological importance. Basic Definitions Cry s tal: A solid bounded by smooth, flat planar surfaces, called faces (Fig 2.1). Figure 2.1. Examples of crystals and crystal forms. (a) Quartz m=hexagonal prism r,z=rhombohedrons (b) Apophyllite p=tetragonal pyramid a=tetragonal prism c=pinacoid Pyrite a=cube e=pyritohedron Cry s tallin e : A solid material in which the constituent atoms, ions, or molecules are arranged in an ordered pattern which repeats indefinitely in three dimensions. The symmetry of the crystal visible to the naked eye (e.g., Fig. 2.2a, anhydrite) is the external reflection of its internal atomic symmetry (Fig. 2.2b). Figure 2.2. A comparison of the external appearance and internal arrangement of atoms in anhydrite. (a) Anhydrite crystal (b) Anhydrite atomic structure 9

17 Cry s tal Fo rm : A set of crystal faces related to each other by symmetry. Figure 2.1 shows typical forms of three common minerals; each form is labeled by a different letter. The basic crystal forms are illustrated in standard mineralogy textbooks and field guides; references to these are listed at the end of this booklet. Several common examples are illustrated in Figures 2.1 and 2.2. Novices often ask how to distinguish between a natural crystal face and a cleavage surface, especially for those crystals where the morphology could follow cleavage directions. This could occur, for example, with rhombohedrons of calcite, octahedrons of fluorite or diamond, and prisms of pyroxene. One clue is the luster. A freshly-cleaved surface is shinier, and observation with a hand lens will show breaks or steps parallel to the cleavage direction. In contrast, a natural crystal face may show growth features, such as etch pits, hillocks (or raised areas), and striations. Cry s tal Hab it: This refers to the usual shape and appearance of the crystal. The assemblage of forms that are typically present in a crystal constitutes its habit. Thus in Figure 2.1a, quartz crystals are typically hexagonal prisms modified by rhombohedrons. Pyrite (Fig. 2.1c) shows the cube modified by a pyritohedron. Other typical habits include octahedron for diamond, cube for fluorite, hexagonal prism for beryl, dodecahedron for garnet, etc. Habits of crystal clusters are described in Chapter 1. Note that, while crystals may become distorted in shape, due to circumstances of growth, the angles between equivalent faces remain constant. This is known as Steno's law of interfacial angles, after the discoverer (Fig. 2.3). Figure 2.3. Some crystal habits ideal and distorted: (a) Cube. (b) Octahedron. Dodecahedron. Symmetry Symmetry refers to the repetition of a point or design motif by some geometrical operation. These operations include: 1) rotation around an axis 2) reflection across a mirror plane, and 3) inversion through a center. Symmetry can be found in a wide variety of objects, including crystals (Fig. 2.4). Familiar examples are the bilateral (two-sided symmetry) of vertebrates, including human beings, five-fold symmetry of starfish, six-fold symmetry of snowflakes; also the two-dimensional repeating patterns of wallpaper, textiles or brick walls. In crystals, however, the requirement of filling three-dimensional space on an atomic level eliminates 5-fold, 7-fold, etc., types of symmetry (e.g., to use a two-dimensional analogy, try tiling a floor with regular pentagons, without leaving gaps; Fig. 2.5). 10

18 Figure 2.4. Examples of symmetry. (a) Rotational Symmetry. (Left) 3-fold rotation in tourmaline; (Center) 4-fold rotation in a Native American basket; (Right) 6-fold rotation in beryl. (b) Reflection m Inversion through a center. C is the center of symmetry. Figure 2.5. (a) Arrangement of pentagons with 5-fold symmetry axes, perpendicular to the page, leaving gaps between them. (b) Arrangement of hexagons with 6-fold axes, perpendicular to the page, forming a perfect fit, as in honeycombs. (a) (b) 11

19 Symmetry Operations 1) Rotation around an axis. Rotational symmetry means the number of times a crystal can be turned around an axis through a full circle (360 ), bringing it to an equivalent position (Fig. 2.4a). A1 1-fold symmetry once in a 360 rotation A2 2-fold symmetry twice in a 360 rotation A 3 3-fold symmetry three times in a 360 rotation A4 4-fold symmetry four times in a 360 rotation A6 6-fold symmetry six times in a 360 rotation 2) Reflection across a mirror plane (m). The mirror plane divides the crystal into two equivalent halves that are related to each other as mirror images (e.g., like left and right hands; Fig. 2.4b). 3) Inversion through a center (I). Equivalent points are related to each other by passing a line from the point on the surface through the center to the equivalent point on the opposite side (Fig. 2.4c). Figure 2.6. Symmetry elements associated with the three minerals of Figure 2.1. (a) Quartz Hexagonal 32 Crystal Systems (b)apophyllite Tetragonal 4/m2/m2/m Pyrite Cubic 2/m _ 3 These are reference systems defined by a set of three (or four) axes in three-dimensions (Fig. 2.7). Each crystal system has its unique combination of axial lengths and angles between them. In most cases, these reference axes are selected to coincide with rotational symmetry axes or to be perpendicular to mirror planes. They are also usually taken parallel to the intersection edges of the most common or prominent crystal faces. There are six crystal systems (although some texts divide the hexagonal system into two separate ones, namely the hexagonal and the trigonal or rhombohedral divisions). The six crystal systems are: 1. Isometric (or cubic). All three axes are equal in length (a 1 = a 2 = a 3) and mutually perpendicular to each other. 2. Tetragonal. Two axes are equal in length (a 1 = a 2 c). All three axes are mutually perpendicular. 3. Orthorhombic. All three axes are unequal in length (a b c). All three axes are mutually perpendicular. 4. Hexagonal. Three axes are equal in length and 120 apart (a 1 = a 2 = a 3). The fourth axis is not equal in length to the other three, but is perpendicular to the plane in which they lie. 5. Monoclinic. All three axes are unequal in length (a b c). The b-axis is selected to be perpendicular to the plane containing the a and c axes. 6. Triclinic. All three axes are unequal in length (a b c), and none of the angles between them is

20 Figure 2.7. The six crystal systems. Figure 2.8. Sphalerite crystal with etch pits showing tetrahedral symmetry. 13

21 Crystal Classes (Point Groups) Crystals can be further subdivided on the basis of a common set of symmetry elements into 32 c ry s tal c las s e s, distributed among the six crystal systems. Table 2.1 lists the 32 crystal classes according to their crystal system and symmetry. The symbols summarize the symmetry operations. For example, under Symmetry Elements, 2A 2 means 2 rotational axes of twofold symmetry, 3m means 3 mirror planes, I is an axis of inversion, and so on. The notation under Crystal Class is a more shorthand way of summarizing the symmetry elements. For example, in the isometric system, the symmetry elements of the hexocahedral class (with the most symmetry) are: 4/m32/m. The 4 refers to the four 4-fold rotational axes of the cube. Since there are 3 cube directions, there are 3 4-fold axes. The /m means that there is a mirror plane perpendicular to the 4-fold axis; hence a total of three mirror planes perpendicular to the cube axes, 3 refers to 3-fold axes along cube diagonals (of which there are 4), the bar over the 3 means a combination of the 3-fold rotation with a center of symmetry, 2 refers to a set of 6 2-fold axes between cube edges, and the last /m equals 6 mirror planes perpendicular to these. For other crystal systems, the highest symmetry axis is listed first, followed by other symmetry axes and mirror planes. A careful examination of surface features on crystal faces, such as growth striations, natural or corrosion pits, or variations of luster, reveals the true symmetry of the crystal, which may be lower than that suggested by the actual habit. For example, pyrite often shows striations on its cube faces oriented parallel to the three cube directions. The real symmetry of pyrite is therefore _ lower than that of a complete cube; it is that of the diploidal class, with symmetry elements 2/m 3 (Fig. 2.1c and Fig. 2.6c; Table 2.1). Similarly, an apparent octahedron of sphalerite shows four shiny and four striated or pitted faces (Figure 2.8). In reality, the octahedron consists of two interpenetrating _ tetrahedra. The true symmetry of sphalerite is therefore that of the hextetrahedral class, i.e., 4 3m, with 3 fourfold axes combined with a center, 4 threefold axes and 6 mirror planes (Table 2.1) Table 2.1. The 32 crystal classes and their symmetry operations. Crystal System Crystal Class Symmetry Elements Crystal System Crystal Class Symmetry Elements Triclinic Monoclinic Orthorhombic Tetragonal 1 1 _ 2 m 2/m 222 mm2 2/m2/m2/m 4 4 _ 4/m 422 _ 4mm 4 2m 4/m2/m2/m none I 1A 2 1m I, 1A 2, 1m 3A 2 1A 2, 2m I, 3A 2, 3m 1A 4 1A _ 4 I, 1A 4, m 1A 4A2 1A _ 4, 4m 1A 4, 2A 2, 2m I, 1A, 4A, 5m 4 2 Hexagonal Isometric 3 3 _ 32 3m _ 3 2/m 6 6 _ 6/m 622 6mm _ 6m2 6/m2/m2/m 23 2/m3 _ 432 _ 4 3m_ 4/m 32/m 1A_ 3 3 1A 3, 3A2 1A _ 3, 3m 1A 3, 3A 2, 3m 1A_ 6 6 I, 1A 6, 1m 1A 6, 6A2 1A _ 6, 6m 1A 6, 3A 2, 3m I, 1A 6, 6A 2, 7m 3A 2, 4A3 _ 3A 2, 3m, 4A 3 3A _ 4, 4A 3, 6A2 3A, 4 4A _ 3, 6m 3A _ 4, 4A 3, 6A 2, 9m (1A = 1A + I)

22 Unit Cell The unit cell is the smallest volume enclosing a group of atoms, ions, or molecules that when extended repeatedly in three dimensions produces the macroscopic (eye visible) crystal. It is the basic repeat unit (Fig. 2.9). The edges of the unit cell are taken to coincide with the crystallographic axes, which in most cases are also symmetry axes (see above). Figure 2.9. (a) The orthorhombic unit cell of sulfur. (b) The tetragonal unit cell of scheelite (CaWO 4). Axial Ratios (a) (b) In all of the crystal systems other than the isometric, the crystallographic axes differ in length. The axial ratios are the ratios of the lengths of the edges of the unit cells, which lie parallel to the crystallographic axes. For example, the orthorhombic mineral sulfur (Fig. 2.9a) has unit cell dimensions a = 10.47, b = 12.87, and c = (where 1 = one hundred millionth of a centimeter). Taking the length along the b axis as 1, the lengths of the a and c axes relative to b are in the ratios (10.47/12.87) = 0.813, and (24.49/12.87) = 1.903, respectively. The axial ratios are then written as 0.813:1:1.903, in the sequence a:b:c. The tetragonal mineral scheelite (Fig. 2.9b) has unit cell lengths a 1 = a 2 = 5.25 and c = The axial ratios are therefore 1:1:2.17. Miller Indices Miller Indices are the reciprocals of the relative distances at which the crystal faces intersect the crystallographic axes. For example, in Fig. 2.9a, the front face of the sulfur crystal intersects the a axis at a length of 1 unit distance, but is parallel to both the b and c axes. Thus the intercepts of this plane with the crystal axes are 1,,. respectively. The rhombic pyramid face (shaded) in Fig intersects the a, b, and c axes at unit lengths 1, 1, and 1 respectively, but the less steeply inclined rhombic pyramid intersects the crystallographic axes at 2 (a), 2 (b), and 2/3 (c). Dividing the latter by the common factor of 2 results in the proportions 1 (a), 1 (b), and 1/3 (c). Note that these represent relative lengths. Since it is cumbersome to deal with infinity and fractions, the fractions are first cleared and the reciprocals are obtained by inverting the numbers. These are the Miller Indices. The Miller Indices for the sulfur crystal (Fig. 2.9a) are then 100 (front face), 010 (right side face), and 001 (top face). The Miller Indices for the shaded pyramid face (Fig. 2.10) become 111, and for the less inclined pyramid are 113. The general symbol for Miller Indices for a crystal face is (hkl), where the h, k, and l represent the reciprocals of the intercepts along the a, b, and c axes respectively. The general symbol for a crystal form is {hkl}. Examples of Miller Indices for crystal forms on three common minerals are shown given in Figures 2.11 and

23 Figure 2.10 (left). Sulfur crystal two dipyramids. Figure 2.11 (right). Pyrite crystal showing a cube modified by an octahedron and dodecahedron. Figure (a) Orthorhombic crystal of topaz, showing prisms and dipyramid. (b) Monoclinic crystal of orthoclase, showing prisms and pinacoids. (a) (b) Crystal Lattices The crystal lattice is the three-dimensional periodic array resulting from the repetition of a basic unit (i.e. the unit cell) in three directions in space. Crystal lattices contain the same symmetry elements discussed in conjunction with the 32 crystal classes (also known as point groups), which refer to macroscopic crystals. However, there are two additional symmetry operations in 3-D lattices. These are: (1) Screw axis. This combines a rotation with translation along the rotational axis. Two, three, four, and six-fold screw axes can exist. For example, a 41 screw axis indicates a 90 rotation each 1/4 of the unit length along the axis (Fig. 2.13a). Both left and right hand screw axes are possible. (2) Glide plane. This combines reflection across a plane with translation. The design motif is translated forward one-half the repeat distance parallel to the mirror plane before being reflected (Fig. 2.13b). 16

24 Figure 2.13a (left). A 41 screw axis. Figure 2.13b (right). Glide plane. (a) Bravais Lattices (b) There are 14 distinct types of unit cell which possess the symmetry properties of the 32 crystal classes (or point groups) discussed earlier. When these unit cells are repeated indefinitely in space, they generate the points or nodes of a crystal lattice. The 14 lattices are called the Bravais Lattices (Fig. 2.14). When the 14 Bravais lattices are combined with the symmetry operations belonging to the 32 crystal classes, and the additional symmetry operations of three-dimensional lattices (i.e. the screw axis and glide plane), this results in 230 unique patterns known as the 230 space groups. These space groups are the three-dimensional analogs of the 32 point groups. Figure The 14 Bravais lattices. 17

25 Polymorphism The ability of a material of the same chemical composition to exist in more than one crystal structure, depending on different physical conditions of temperature and pressure is called polymorphism. The polymorphs are considered to be distinct minerals. Familiar examples include graphite and diamond (both C), calcite and aragonite (CaCO 3), and pyrite and marcasite (FeS 2). One polymorph is usually less stable than the other under normal surface terrestrial conditions. For example, diamond, aragonite, and marcasite are less stable at the earth's surface (i.e. metastable), and given enough time will invert to the more stable form. (However, the chemical bonds in diamond are so strong that for all practical purposes it will persist indefinitely). Diamond and Graphite A more strikingly divergent pair of polymorphs can scarcely be imagined! Both diamond and graphite are chemically identical made of the element carbon. But there the similarity ends. Diamond is the hardest known substance; graphite one of the softest. Diamond had a highly brilliant luster (i.e. adamantine); graphite has a dull, submetallic, greasy luster. Diamond is a perfect heat conductor; graphite is not. The reason for this disparity lies in the crystal structure of the two minerals. In diamond, each carbon atom is tightly bonded to four other carbon atoms at the corners of a tetrahedron (Fig. A). Diamond also displays perfect octahedral (111) cleavage. In graphite, each carbon atom is linked to three others forming hexagonal sheets, much like a honeycomb or bathroom tiles (Fig. B). The sheets are stacked one on top of the other, like pages in a book, but they are connected to each other by much weaker bonds. Thus, graphite has perfect basal (0001) cleavage. The ability of carbon sheets to glide past each other makes graphite an excellent lubricant. This example dramatically illustrates the close connection between the atomic structure of minerals and their macroscopic physical properties. Figure A. Crystal structure of diamond. Figure B. Crystal structure of graphite. Isostructuralism There are minerals that differ in chemical composition but have the same crystal structures. Examples include (1) halite (NaCl), sylvite (KCl), chlorargyrite (AgCl), and galena (PbS); (2) the orthorhombic carbonates aragonite (CaCO 3), strontianite (SrCO 3), cerussite (PbCO 3), and witherite (BaCO 3). Isostructural groups form an important element in the mineral classification system (see Chapter 6). 18

26 CHAPTER 3 CRYSTAL GROWTH Crystal Growth in Nature One way in which minerals form in nature is by the solidification of magmas, or molten silicate material, producing a variety of igneous rocks (for example, granite, diorite, gabbro, and their volcanic equivalents: rhyolite, andesite, basalt). Typical igneous rock-forming minerals include quartz, feldspars, pyroxenes, amphiboles, and olivine. Toward the latest stages of crystallization from a granitic magma, a water- and volatile-rich fluid phase separates out. Pegmatite minerals crystallize from this hot, fluidrich phase. Pegmatite minerals often grow unimpeded into open cavities or pockets, thus forming large, well-terminated crystals. Typical pegmatite minerals include quartz, microcline, and other feldspars, muscovite, beryl, tourmaline, topaz, kunzite, and less commonly uranium, thorium, and rare earth minerals. Minerals can also crystallize from hydrothermal solutions of hot, magmatic emanations or heated aqueous solutions of non-magmatic origin. These form ore vein or replacement-type deposits (typical minerals include pyrite, galena, chalcopyrite, barite, fluorite). Less commonly, minerals can grow directly from the vapor state, as in fumerole deposits near active volcanoes (e.g. sulfur). A more detailed description of how minerals form in nature will appear in a separate booklet (Volume II). Ideal Crystal Growth The first stage in the growth of a crystal is that of nucleation, in which a small nucleus (or seed) of atoms or ions forms in a saturated solution, or a freezing melt. Ideally, the crystal continues to grow by adding atoms or ions to the surface of the seed, layer by layer. The rate of growth depends on a number of external and internal factors. Examples of the former include temperature, pressure, the degree of supersaturation of the solution, etc. Internal factors controlling crystal growth relate to the density of lattice points in a plane. Although the most densely-populated lattice planes are the most stable, the less densely-populated planes usually grow the fastest because fewer particles need to be added per unit area. However, the rapidly-growing faces eventually grow themselves out of existence, leaving the slowergrowing, but more stable faces to generate the actual crystal (Fig. 3.1). Figure 3.1. Schematic illustration of crystal growth. (a) The original crystal nucleus (shaded) is a cross-section of an octahedron (o). The octahedral faces are growing more rapidly than the cube faces (a), and eventually disappear. (b) Four stages of growth of the crystal from the initial octahedron (1), to intermediate cube-octahedra (2,3), and final cube (4). 19

27 Growth Imperfections In the real world, crystal growth is far from perfect. These imperfections occur on all scales, ranging from atomic-scale structural defects, to macroscopic phenomena, such as zoning, skeletal growth, parallel growth, twinning, and incorporation of inclusions. Structural Defects Figure 3.2 illustrates various types of structural defects that can occur during crystal growth. Schottky defect in which an atom (or ion) is missing from its normal lattice position. Frenkel defect in which the atom/ion has been dislodged from its normal position and is wedged between adjacent atoms/ions. Interstitial impurity is one in which a foreign atom/ion is inserted into the crystal structure. Edge dislocation is one in which an extraneous row or plane of atoms is wedged into the crystal structure. Screw dislocation is one in which atomic planes wind ramp-like around a screw axis. Lineage structure a mosaic of sub-grains that differ very slightly in their orientation of lattice planes. Structural defects, such as Frenkel defects (Fig. 3.2b) can give rise to color centers in which electrons (negatively-charged particles) are trapped at the site of a missing ion (e.g. in fluorite, blue halite). Interstitial impurities may produce similar color effects. Screw and edge dislocations are also important for crystal growth, because new atoms or ions are deposited preferentially along the ledges that these provide (Figs. 3.2d and e). Edge dislocations are furthermore important, in that they allow the crystal to deform under stress, by slippage of atoms along these planar defects. Such solid-state deformation occurs in nature, as for instance, in the flow of a glacier downhill under the force of gravity, or tectonic plate motions in the earth's upper mantle. Screw and edge dislocations can be revealed by etching with acid, leaving characteristic pits or depressions. 20

28 The growth spirals are revealed by etching, because solution takes place preferentially at sites of emergent screw dislocations. Diamonds often exhibit triangular growth depressions known as trigons. The apices of the trigon point to the edges of the octahedral face. On the other hand, triangular pits caused by corrosion and solution have edges parallel to the octahedral face (Fig. 3.3). Sometimes, the interaction between two (or more) spiral growth steps produces a growth hillock or mound, visible on the surface of the crystal face. Figure 3.3. Trigons and solution pits in diamond. Other Growth Phenomena Zoning The chemical composition or temperature of the solution or melt from which crystals grow rarely remains constant throughout the growth process. Variations in chemical composition (or temperature) will result in zoning, forming bands of different color and chemical composition. Figure 3.4. Relationship between color zoning and chemical variations in tourmaline. Sections cut perpendicular to the c-axis. Zoning also illustrates the successive stages of crystal growth (Fig. 3.5). Zoning can also result from deposition of impurities on selected crystal faces, resulting in Maltese-cross (e.g., halite) or hourglass (e.g., gypsum) patterns (Fig. 3.6). Minerals that commonly display zoning include fluorite, amethyst, ametrine, tourmaline (Figs. 3.4, 3.5), corundum, feldspar. The presence of angular zones in gemstones is often a diagnostic indicator of natural (as opposed to lab-created) origin. 21

29 Figure 3.5. Slices of color-zoned tourmaline cut parallel and perpendicular to the c-axis, showing successive changes in crystal habit going from trigonal pyramid (inner core) to hexagonal prism (outer rim). Figure 3.6. Successive sections through a chiastolite (var. andalusite) crystal, showing sector zoning. Phantom Crystals Closely related to the phenomenon of zoning is the development of phantom crystals. Phantoms exhibit outlines of the crystal from earlier growth stages, enclosed in the later crystal. Phantoms develop as a result of interruptions in the continuity of deposition, probably caused by fluctuations in composition or temperature of the surrounding solution or melt. During a hiatus in growth, the crystal may acquire a light coating of dust, or solid or fluid inclusions (see below), before growth resumes. Thin layers of these impurities, such as chlorite in quartz, outline the pre-existing crystal. Although the habit and color of the phantom generally corresponds to the outer crystal, changes in both color (e.g. amethyst-quartz) and habit (calcite, tourmaline [Fig. 3.5]) are known. Skeletal Growth Under rapid growth conditions where the crystallizing solution is highly supersaturated, atoms or ions are added more rapidly to the edges and corners of growing crystals than to the face centers. This results in hollow, stepped depressions (hoppers), or branched, tree-like forms (dendritic, or skeletal habit). Figure 3.7 illustrates the preferential growth at cube corners of a hopper crystal, such as halite. Other minerals showing hopper growth include quartz, pyromorphite, vanadinite, native gold, silver; also manmade bismuth. Figure 3.7. Schematic illustration of three stages of growth in a hoppered cube crystal such as halite. 22

30 Skeletal (or dendritic) growth is also common in minerals. Examples includes pyrolusite (Mn oxide) on limestone and in agate (moss agate), native gold, silver, copper; also ice on windowpanes. Parallel Growth and Epitaxy Parallel growth occurs when an aggregate of crystals of the same mineral grow with their crystallographic axes and faces parallel. Such aggregates, although they appear to consist of several different crystals, are really a single crystal, because the internal (atomic) structure maintains the same orientation throughout the specimen. Common examples include quartz and barite (Fig. 3.8). Figure 3.8. Growth features. (a) Quartz scepter (b) Parallel growth in quartz Parallel growth in barite When two different mineral species grow one over the other in a non-random (or oriented) manner, this constitutes epitaxy, or epitaxial growth. Although the two crystals differ in their structures (and unit cell dimensions) because of non-identical chemical compositions, they also share certain atomic planes that have a reasonably good fit (or relatively small amount of mis-match) between the two individuals. The phenomenon of certain types of oriented overgrowths, intergrowths, and replacements is closely related to epitaxy. Examples include overgrowths of staurolite on kyanite (Fig. 3.9a), plagioclase feldspar on microcline (Fig. 3.9b), rutile on hematite. Figure 3.9. Examples of epitaxial growth. (a) Staurolite on kyanite; (b) Plagioclase on microcline. Twinning A twin is an intergrowth of two or more crystals of the same mineral, in which the different crystals are related to each other by symmetry operations such as: 1) mirror plane reflection (twin plane), 2) rotation around a common (twin) axis, and 3) inversion around a common point (twin center). However, the twin planes, axes, or centers are distinct from and do not coincide with the usual symmetry elements of the single crystal. The common surface along which the crystals are joined is called the composition plane (or twin plane, Fig. 3.10a); the common axis is the twin axis (Fig. 3.10b). 23

31 Types of Twins Contact twin. Shares a common plane (Examples: quartz Japan law; quartz Brazil law; gypsum swallow-tails; calcite, Fig. 3.10a). Penetration twin. Shares a common axis (Examples: fluorite, orthoclase feldspar Carlsbad twin; staurolite fairy cross and St. Andrew's cross, Fig. 3.10b). Polysynthetic twin. A multiple twin, sharing a common plane. Twinning is repeated successively many times across this plane (Examples: plagioclase feldspar; calcite Fig. 3.10c). Cyclic twin. A multiple twin in which the successive twin planes are not parallel. (Examples: rutile, chrysoberyl, aragonite, Fig. 3.10d). Figure 3.10a. Contact twins. (left) Quartz-Japan twin; (center) Gypsum Swallow-tail twin; (right) Quartz Brazil twin. Figure 3.10b. Penetration twins. (top left) Fluorite (Pyrite); (top right) Orthoclase Carlsbad twin; (bottom left) Quartz Dauphine twin; (bottom right) Staurolite Fairy Cross. 24

32 Figure 3.10c. Polysynthetic twins. (left) Albite; (right) Calcite. Figure 3.10d. Cyclic twins. (left) Rutile; (center) Chrysoberyl; (right) Aragonite. In the twinned crystal, the orientation of the crystal lattice has been shifted with respect to the original orientation, such that the two lattices are related to each other in a symmetrical manner. For example, imagine a lattice oriented on a N-S, E-W grid (like the streets in many mid-western cities), and then rotated by 60 to the left (west). The unrotated and rotated grids are related to each other by a mirror plane passing diagonally N30ºW (Fig. 3.11). The dashed line outlines the twinned crystals. Re-entrant angles (or notches) are diagnostic of twinned crystals, but are not always infallible guides. Re-entrant angles may also appear on crystals in parallel growth, or in dendritic growth (e.g. snowflakes). Twinning represents a departure from perfect crystal growth, in that the twinned portion of the crystal has assumed a different crystallographic orientation from that of the untwinned part. Thus, twins can result from some accident during growth, or a nucleation error, in which the original lattice orientation is disturbed, and growth proceeds in a different direction, which, however, maintains a symmetrical relation to the original one. (Other causes of twinning, unrelated to growth accidents, arise from temperature changes, or physical deformation). 25

33 Figure Relation between lattices in a twinned crystal. 26

34 CHAPTER 4 THE ORIGINS OF COLOR IN MINERALS AND GEMS PART I The Physical Basis of Color Color is one of the most obvious physical properties of minerals. As noted in Chapter 1, certain minerals are characterized by specific colors, for example, the green of malachite or the yellow of sulfur. In most cases however, color is not a diagnostic property, rather, it is due to trace amounts of impurities that are not always present or that can vary in composition. The human eye is sensitive to visible light which occupies a rather narrow region of the electro-magnetic spectrum between 400 and 700 nanometers (nm, or one billionth of a meter; see Fig. 4.1). Beyond the violet lies the ultraviolet, with wavelengths shorter than 400 nm. X-rays and gamma rays have even shorter wavelengths. The infrared lies beyond the red, with wavelengths above 700 nm. The microwave and radio wave regions have even longer wavelengths. The energy of visible light (and other forms of electromagnetic radiation) increases in direct proportion to its frequency and decreases in inverse proportion to the wavelength. Figure 4.1. The visible spectrum as part of the electromagnetic spectrum (top). One nanometer is one billionth of a meter. The color of a mineral is created by the interactions of light with the electrons of atoms in a crystal. This interaction results in the selective absorption of certain wavelengths of visible light. The human eye perceives the remaining wavelengths of transmitted or reflected light as color. If no absorption occurs, the mineral is white or colorless. If all visible wavelengths are absorbed, the mineral is black. 27

35 Electrons are negatively-charged particles that surround the nucleus of an atom. According to the uncertainty principle of modern physics, one cannot precisely measure both the position or velocity of an electron at the same time. Thus the motion of electrons cannot be well-described in terms of circular or elliptical orbits, like miniature planets. Instead, electrons are described as having a certain probability of occupying particular regions of space around the atomic nucleus. Regions with a high probability of containing electrons are called orbitals. Electrons are arranged in a series of shells [1 (K), 2 (L), 3 (M), 4 (N)...], extending outward from the nucleus. Each shell can be divided further into a number of subshells or orbitals (e.g. 1s; 2s, 2p; 3s, 3p, 3d;...etc.; Fig. 4.2). The innermost shells and subshells are generally filled with pairs of electrons that do not participate in chemical reactions, nor do they contribute to the creation of color. Only transitions involving unpaired electrons from the outermost shell or subshell (the valence electrons) will produce color. Figure 4.2. Relative energies of the orbitals in neutral, many-electron isolated atoms. A fundamental property of matter on the atomic scale is that electrons can exist only in certain discrete states, each with a precisely defined energy. The lowest possible energy level is the ground state; all higher energy levels are excited states. The energy levels can be visualized as rungs on a ladder, with highly irregular spacings (Fig. 4.2). Light, or other electromagnetic radiation, can be absorbed only if those wavelengths have exactly the right amount of energy to raise an electron from the ground state to an excited state. When the electron drops down to the ground state, it emits radiation corresponding to the energy difference between the two levels. Figure 4.2 shows the relative energies of electron shells and subshells in a neutral, isolated atom. The presence of other atoms or ions can alter the spacing and relative sequence of these energy levels, hence alter the absorbed or transmitted wavelengths of light. The physical mineralogist, Kurt Nassau, lists fifteen distinct causes of colors, of which at least a dozen apply to minerals and gems. These can be combined into several groups, among which are: Crystal Field Transitions a. Transition metal compounds: malachite (Cu), turquoise (Cu), rhodocrosite (Mn), olivine (Fe), almandine (Fe). b. Transition metal impurities: ruby (Cr), emerald (Cr), alexandrite (Cr), aquamarine (Fe, in part), jadeite (Fe, Cr). 28

36 Molecular Orbital Transitions a. Charge transfer: blue sapphire, vivianite, lapis lazuli. b. Organic compounds: coral, amber. Color Centers: fluorite, blue halite, smoky quartz, amethyst. Energy Bands a. Metals: copper, silver, gold, brass. b. Pure semi-conductors: galena, cinnabar, diamond. c. Doped semi-conductors: blue and yellow diamond. Optical Phenomena a. Dispersion: diamond, zircon, sphene, cubic zirconia (synthetic). b. Scattering: moonstone, stars. c. Interference: iridescent chalcopyrite, bornite, iris quartz. d. Diffraction: opal, iris agate, labradorite. In this chapter, crystal field, molecular orbital transitions, and color centers are described. Energy bands and optical phenomena will be discussed in the next chapter. Crystal Field Transitions Valence electrons in minerals are generally paired with electrons from adjacent atoms to form the bonds that hold the crystal together. The bonds are usually strong enough so that electronic transitions occur mostly in the ultraviolet and therefore remain invisible. (Such higher energy transitions are associated with shorter wavelength radiation; i.e., ultraviolet, rather than visible light). However, for certain chemical elements, electrons from partially-filled inner subshells can exist in excited states that fall within the visible spectrum. These electronic transitions can produce vivid colors in minerals. Crystal field transitions are electronic transitions between partially-filled 3 d orbitals (see Fig. 4.2) of transition metal elements (for example: Ti, V, Cr, Mn, Fe, Co, Ni, and Cu). Of these, iron (Fe) is the most abundant in the earth's crust and is therefore the major source of color in rock-forming minerals Traces of Fe generate greens, blues, grays, while Fe produces yellows, oranges, reds, and browns. Minerals whose colors have been attributed to crystal field transitions are listed in Table 4.1. In crystals, positively-charged transition metal ions are surrounded by negatively-charged oxygen ions. +2 In peridot, the gem variety of olivine, Fe can occupy two geometrically distinct sites in the crystal +2 lattice that differ slightly in their atomic surroundings. Each Fe is surrounded by six oxygen ions that +2 form two-slightly different types of distorted octahedrons. The Fe ion has six 3 d electrons, of which four are unpaired. (There are a total of five possible d orbitals, each of which can be occupied by 2 electrons). The crystal field transitions involving these unpaired electrons occur mostly in the infrared region, but they extend partly into the red (Fig. 4.3). The remaining wavelengths that are transmitted fall predominantly in the yellow and green part of the spectrum, producing the yellowish-green color of this gemstone. +2 In almandine garnet, Fe is surrounded by 8 oxygen ions, forming a distorted cube. This change in the +2 atomic environment surrounding the Fe ion shifts the energies of the possible electronic transitions further into the infrared. However, lesser absorption peaks also occur in the violet-blue, green, and yellow-orange. Only the red region transmits light, resulting in the characteristic burgundy color of this gemstone (Fig. 4.3). 29

37 Table 4.1. Minerals whose colors are due to crystal field transitions. Ion Mineral (Variety) Color V (VO ) apophyllite light green V Cr Mn Mn Fe Fe Co Cu zoisite (tanzanite) grossular (tsavorite) beryl (some emerald) chrome diopside andradite (demantoid) beryl (emerald) chrysoberyl (alexandrite) corundum (ruby) beryl tourmaline (rubellite) beryl (morganite) spessartine andradite chrysoberyl actinolite (nephrite) olivine (peridot) almandine spinel erythrite dioptase azurite malachite turquoise blue, purple bright green green green green green green/red-purple red red pinkish-red pink yellow-orange yellow-green yellow green yellow-green burgundy red blue pink green blue green green-blue +2 Ni chrysoprase apple green +2 Figure 4.3. Absorption spectra of two Fe -bearing minerals peridot, a gem variety of olivine, and almandine, a member of the garnet group. 30

38 +2 In both peridot and almandine, Fe forms an integral part of the crystal structure. The color of many minerals and gems, however, is caused by the presence of only trace amounts of color-producing impurities. Crystal field theory explains why the same impurity is responsible for the strikingly-different +3 colors of ruby, emerald, and alexandrite. In all three minerals, small amounts of chromium ions (Cr ) +3 replace aluminum ions (Al ), which are surrounded by six oxygen ions in a distorted octahedral configuration. In emerald, (a gem variety of beryl, a ring silicate mineral), the six oxygens surrounding +3 Al are also shared with silicon and beryllium tetrahedra. This gives the bonding in beryl a somewhat covalent character (that is, the electrons are shared to a greater extent between the atoms). On the other hand, in ruby, a gem variety of corundum, or aluminum oxide, the bonding is more strongly ionic (there is a greater separation of electrical charge). Consequently, the crystal field environment in ruby is stronger than in emerald. The relative differences in crystal field strength is expressed in the absorption spectra of these two gems (Fig. 4.4). In ruby, the main absorption peaks are centered in the violet and in the green-yellow, with transmission in the blue and red (Fig. 4.4), resulting in the violet-red to orangered color of this gemstone. On the other hand, the absorption peaks of emerald are shifted toward lower energy (that is, longer wavelengths) relative to the corresponding peaks in ruby. Therefore, in emerald, the absorption peaks lie in the violet-blue and especially yellow, orange, and red, resulting in strong transmission in the green (Fig. 4.4). Figure 4.4. Absorption spectra of three Cr+3-bearing minerals: ruby, a gem variety of corundum, emerald, a gem variety of beryl, and alexandrite, a gem variety of chrysoberyl. In alexandrite, a gem variety of chrysoberyl, the bonding character and crystal field environment of +3 the Cr ions are intermediate between those of ruby and emerald. Therefore, the energies of the electronic transitions producing the spectral absorption peaks also lie in between those of ruby and emerald (Fig. 4.4). The main absorptions in alexandrite occur in the violet-blue and yellow-orange, forming comparable transmissions in both green and red. Since the transmitted colors have approximately equal intensities, the actual color observed depends on the nature of the illumination. In incandescent light, alexandrite appears red, since this light source is richer in the red component. In +3 daylight or in fluorescent light, the mineral appears bluish-green. Vanadium (V ) in corundum has a similar absorption spectrum to that of alexandrite and is often used in imitation alexandrites. 31

39 Fluorescence Minerals that luminesce as they are exposed to ultraviolet light are fluorescent. The phenomenon of fluorescence is similar to that creating color, described above. The ultraviolet radiation excites electrons and raises them to higher energy levels. In returning to the ground state, they emit light of the same wavelength. If, however, they fall back to an energy level intermediate between the excited and ground states, they will emit light of lower energy (i.e., longer wavelength) which lies in the visible range (Fig. 4.5). Not all specimens of a mineral show fluorescence, not even those from the same locality. Impurities, such as transition metal ions, are effective activators. Fluorite, in spite of its name, only fluoresces occasionally. When it does, it fluoresces blue. Other minerals that often show fluorescence include scheelite, willemite, calcite, scapolite, diamond, hyalite, and autunite. Figure 4.5. Schematic illustration of fluorescence. Ultraviolet lamps are used to test for fluorescence. Minerals often react differently to different wavelengths of UV light. Therefore, two sources of UV radiation are commonly used: shortwave UV nanometers (a nanometer is one billionth of a meter) and longwave UV nm. Fluorescent reactions to UV light are also a standard procedure for testing gemstones. Molecular Orbital Transitions In some minerals, electrons can be shared between more than one atom. They are bound less tightly and less energy is needed to create an excited state. Such electrons are said to occupy molecular orbitals. Transitions involving molecular orbitals are responsible for a wide range of colors in minerals. The most common type of molecular orbital transition in minerals involves charge transfer, in which electron densities are transferred or shifted from one atom to another. One class of molecular orbital transition involves oxygen-to-metal charge transfer. In this class, the -2-3 metal ion is surrounded by oxygen ions in isolated, discrete units, e.g. CrO 4 in crocoite, and VO 4 in vanadinite. Charge transfer transitions involving these two minerals produce strong violet, blue, and green absorptions, allowing transmission of orange to red wavelengths. 32

40 More common in minerals is the metal-to-metal charge transfer, which involves transfer of electrons between transition metal ions that can occur in variable oxidation states. The charge transfers of Fe Fe and Fe Ti are among the most common transitions in minerals. The Fe Fe transition is responsible for the intense blue color and strong pleochroism of many minerals, including: glaucophane, magnetite (black), cordierite (gem variety iolite), vivianite, and lazulite. The Fe Ti charge transfer produces colors in minerals such as: andalusite (green-brown), benitoite (blue), sapphire (variety of corundum; colors: blue [also contributing, Fe Fe ]; purple [Cr also present]; green [also Cr and Ti ]); kyanite (blue); sillimanite (blue); vesuvianite (or idocrase, brown). Examples of minerals whose colors are caused by molecular orbital transitions are given in Table 4.2. Table 4.2. Minerals whose colors are due to molecular orbital transitions. A. Metal Metal Charge Transfer Ion Mineral Color Fe Fe vivianite beryl (aquamarine) cordierite (iolite) blue, green blue, yellow blue, violet Fe Ti B. Oxygen Metal Charge Transfer benitoite corundum (sapphire) kyanite andalusite blue blue blue green/brown Ion Mineral Color Cr +6 V +5 Fe +3 crocoite vanadinite beryl (heliodor) C. Other Molecular Orbital Transitions orange orange-red, brown yellow Ion Mineral Color +3 S lazurite (lapis lazuli) blue Color Centers Color centers arise from structural defects in the crystal lattice. For example, an excess electron can occupy the site of a mission ion or an interstitial impurity. Conversely, the absence of an electron creates a hole (Fig. 4.6). In fluorite, an electron fills the vacancy left by a negative ion, in order to maintain electrical neutrality. The electrons trapped in fluorite undergo transitions which absorb wavelengths of visible light, producing a purplish color. In quartz, some Al can substitute for Si. The resulting + + charge imbalance is compensated by the presence of H or Na ions. This makes it easier to remove an electron from an adjacent oxygen atom by means of exposure to gamma or x-rays, leaving an unpaired electron or hole. The remaining electron can occupy a set of excited states, causing light absorption, +3 leading to the brownish or smoky colors of smoky quartz. In amethyst, Fe is the impurity, rather +3 than Al, and a purplish color is formed. Examples of minerals whose colors are due to color centers are given in Table

41 Table 4.3. Minerals whose colors are due to color centers. Mineral Type of Color Center Color - halite Cl vacancy blue - fluorite F vacancy purple quartz (smoky) Al for Si brown quartz (amethyst) Fe for Si purple +2 + microcline Pb for 2K green-blue diamond N 3 - aggregate of N atoms yellow Figure 4.6. Schematic illustration of the production of color centers in (a) fluorite, and (b) smoky quartz. In (a), the color center results from the replacement of a fluoride ion by an electron. In (b), the replacement of Al + H (or Na ) for Si weakens the structure, so that radiation can remove an electron from the oxygen ion, leaving an unpaired electron or hole. Additional Reading K. Nassau, The Causes of Color. Scientific American, v. 243, p B.M. Loeffler and R.G.Burns, Shedding light on the color of gems and minerals. American Scientist, v. 64, p

42 CHAPTER 5 THE ORIGINS OF COLOR IN MINERALS AND GEMS PART II Introduction The previous chapter described three basic causes of color which involve electronic transitions in crystals: crystal field transitions, molecular orbital transitions, and color centers. All of these processes involve electronic transitions in which electrons are localized on a single atom or associated with several atoms. In this chapter, we cover two additional important sources of color in minerals: colors due to band gaps, and to optical phenomena. In the first case, colors arise from electronic transitions in which the electrons are delocalized over the entire crystal, as in metals. In the second case, colors arise from the interaction of light with internal structures in the crystal on scales larger than the atomic scale. Colors Caused by Band Gaps In metals, valence electrons are not attached to particular atoms and can move freely throughout the crystal. This accounts for the good electrical and heat conductivity properties of metals. The metal has very many closely-spaced electronic energy levels forming a single, continuous band that is completely filled with electrons. Since a metal has a continuum of excited states, it can absorb all wavelengths, and should therefore appear black. However, the metal's electrons can return to the ground state and re-emit this energy and therefore reflect light. producing a metallic luster. In certain metals, some wavelengths are absorbed (and reflected) more efficiently than others, producing a distinct color, such as the reddish color of copper, the bright yellow of gold, or the slightly darker yellow of untarnished brass. In semi-conductors (these include sulfides and sulfarsenides, also silicon), a gap in energy levels, or a band gap, separates the lower-energy band filled with valence electrons from the empty, higher-energy, excited states of the conduction band (Fig.5.1). Thus, there is a minimum energy that light must have in order to raise an electron from the top of the valence band to the bottom of the conduction band. The color of the semi-conductor depends on the magnitude of this energy gap. 35

43 Figure 5.1. Three types of band gaps and their relation to colors of minerals. A. The band gap is larger than the energy of visible light. All visible light is transmitted. Minerals are colorless. B. The band gap lies in the visible range. The higher-energy portion of the spectrum is absorbed (violetblue). Minerals are colored yellow to red. C. The band gap is in the infrared. All visible light is absorbed. The minerals are black. Three scenarios are possible, depending on the energy of the band gap. If the energy of the band gap lies in the ultraviolet (i.e. the band gap is large), visible light cannot provide enough energy to propel the electron from the lower to the higher energy band. Consequently all visible wavelengths are transmitted and the mineral is colorless (Fig. 5.1A). This is the case for pure insulators, such as many metal oxides and silicates, that lack impurities or elements that would otherwise produce colors by other mechanisms. If the energy of the band gap lies in the visible range, the higher energy radiation (violet to green) is absorbed, allowing transmission in the yellow to red (Fig. 5.1B). Examples include cinnabar HgS, cuprite Cu O, proustite Ag AsS (Table 5.1) Table 5.1. Minerals whose colors are due to band gap transitions. Mineral cinnabar covellite cuprite diamond galena greenockite molybdenite orpiment proustite realgar sphalerite Color red blue red colorless, blue, yellow black yellow black yellow red orange-red colorless 36

44 Finally, if the energy of the band gap lies in the infrared (i.e. the band gap is relatively small), visible light has sufficient energy to promote electronic transitions from the valence band to the conduction band. In this case, all visible wavelengths are absorbed, and the mineral appears black, or has a metallic luster due to re-emission of the light. Such is the case of semi-conductors (e.g. galena, pyrite). Diamond is an example of a mineral, which in the pure state, has a relatively wide band gap, hence is an insulator, and is normally colorless. However, traces of nitrogen and boron can substitute for carbon in the crystal lattice of diamond, producing bright yellow and blue colors, respectively. The presence of nitrogen provides extra electrons that occupy an additional or donor energy level in between the valence band and conduction band of diamond (Fig. 5.2A). Although the energy needed to promote electrons into the conduction band is still high (i.e., in the ultraviolet), the donor band is wide enough for some violet light to be absorbed, allowing yellow light to be transmitted (Fig. 5.2A). Figure 5.2. Band gaps and the colors of diamond. Boron, on the other hand, has fewer electrons than carbon. Therefore, it introduces an acceptor energy band within the diamond band gap. The energy needed to excite an electron from the diamond valence band to the acceptor level is relatively low (in the IR). Absorption takes place in the infrared to the green, resulting in transmission of blue color (Fig. 5.2B). This process is responsible for the beautiful blue color of the Hope diamond. Optical Phenomena Optical phenomena involve the interaction of light waves with structures larger than atomic-scale in the crystal. These phenomena include: 1. Dispersive refraction, such as the fire in gemstones. 2. Interference of light from thin films. 3. Scattering of light from particles or planar elements. 4. Diffraction of light. Examples of these are given in Table

45 Table 5.2. Optical phenomena. Phenomenon Cause Example Iridescence, orient Interference of visible light Iris quartz, ammolite, pearls Play-of-color, labradorescence Diffraction of visible light Opal, labradorite (spectrolite) Adularescence Chatoyancy Asterism Aventurescence Change-of-color Scattering of light by particles smaller than visible wavelengths Scattering of light by oriented, parallel needles or growth tubes Scattering of light by several sets of parallel needles Reflection of light by large, platy inclusions Changes in differential absorption with changing illumination Albite, adularia (moonstone) Chrysoberyl, tourmaline, sillimanite, beryl Corundum, quartz, diopside Aventurine quartz, feldspar (sunstone) Chrysoberyl (alexandrite), corundum, spinel Dispersive Refraction When a ray of light strikes a transparent crystal, a portion is reflected at the surface, but the remainder passes through the crystal. The ray is bent, or refracted toward the perpendicular from the surface (Fig ). A fundamental property of a mineral or gemstone is its refractive index. The refractive index is the ratio of the sine of the incident angle (as measured from the perpendicular) to the sine of the refracted angle (Fig. 5.3). Refraction is caused by the slowing down of light as it passes from a less dense medium (such as air) to a denser one (such as a crystal). When light enters a crystal, the degree to which it is slowed down depends on the wavelength. For example, red light, with the longest wavelength, has the greatest velocity, and is refracted the least. On the other hand, violet light, with the shortest wavelength, has the least velocity, and is refracted the most (Fig. 5.4). This separation of light into its component wavelengths is called dispersion and gives rise to the spectral colors seen in a glass prism. Dispersion is responsible for the fire of gemstones, such as diamond, andradite garnet, sphene, synthetic rutile, and cubic zirconia. Minerals that exhibit high dispersion often tend to have a high index of refraction, which gives them a brilliant luster as well. Figure 5.3. The refraction of light in a crystal. The example shown is for diamond. 2 Or indices; crystals other than those of the cubic system have more than one index of refraction. 38

46 Double Refraction Light waves passing through a crystal belonging to the cubic (isometric) system, or glass, move with the same speed in all directions. The rays slow down and are bent (refracted) slightly, as they go from air through the crystal. In contrast, light rays penetrating crystals of all other systems are split into two polarized rays that vibrate in planes perpendicular to each other and that travel with different speeds. This splitting of the light into two beams is called double refraction, or birefringence. Doubly refracting crystals therefore have two indices of refraction. Double refraction is easily seen in a transparent crystal or cleavage fragment of calcite ( Iceland spar ). When the calcite is placed over a printed page, the letters appear double. As the calcite is rotated, one of the letters moves around, whereas the other remains fixed. The ray producing the moving letter represents the extraordinary ray, while the stationary ray corresponds to the ordinary ray (Fig. A). Doubly refracting crystals fall into two major groups: uniaxial and biaxial crystals. Crystals of the tetragonal and hexagonal systems are uniaxial, i.e., there is one direction within the crystal which has only one index of refraction, corresponding to the ordinary ray. This direction, also known as the optic axis, coincides with the c-crystallographic axis. In any other direction, the light is broken into two rays. The ordinary ray vibrates in the basal plane, perpendicular to the c-axis (Fig. B) On the other hand, the extraordinary ray vibrates at right angles to the vibration direction of the ordinary ray, in a plane containing it and the c-axis (Fig. B). The index of refraction of the ordinary ray is constant in all directions, whereas that of the extraordinary ray varies with direction. The maximum difference in indices of refraction occurs when the extraordinary ray travels in the basal plane. This maximal difference in refractive indices is called the birefringence. The indices of refraction and the birefringence are important physical properties of crystal, minerals, and gems. Biaxial crystals, or those belong to the orthorhombic, monoclinic, or triclinic systems, display two directions with single refraction (two optic axes). These directions, however, do not necessarily coincide with crystallographic axes. To complicate matters further, biaxial crystals have three indices of refraction. A fuller explanation of the optical behavior of biaxial crystals is dealt with in standard textbooks on optical mineralogy (e.g., Nesse, An Introduction to Optical Mineralogy). Figure A. Double Refraction in calcite. Two images of the dot are produced. One remains fixed, as the calcite rhomb is rotated (the ordinary ray); the other rotates along with the calcite (the extraordinary ray). Figure B. A more detailed view of the two rays traveling through calcite. The ordinary ray (ù) vibrates in the basal plan (0001; dark shading); the vibration direction is shown by the arrows. The extraordinary ray (å) vibrates in a direction perpendicular to that of the ordinary ray. This vibration lies in a plan containing it and the c-axis (diagonal lightly shaded plan). 39

47 Figure 5.4. The dispersion of white light by a prism. Interference of Light When light passes through a crystal that is covered by a thin film of an impurity (or surface alter-ation), the rays reflected from the surface and the interior move at slightly different velocities and get out of phase. This results in interference, in which the waves which are completely out of phase cancel out while the one that are in phase are strengthened or reinforced. Since the wave phase also depends on wavelength, certain colors are enhanced while others are suppressed. This results in an iridescent appearance on the surface of the mineral. Examples include iris quartz, or the colorful tarnish (due to surface oxidation) of minerals such as bornite and chalcopyrite ( peacock ore ). The colors of pearls are also partly due to interference. Pearls consist of alternating layers of aragonite and conchiolin (a fibrous nitrogenous material) which differ in refractive indices. Reflected light from the surfaces of these layers interferes with incoming light to create the iridescent colors of pearls called orient. Scattering of Light Light can be scattered by extremely fine sets of parallel fibrous inclusions, or by planar elements in the crystal (for example due to polysynthetic twinning or to exsolution). In the first case, a gemstone cut from a mineral with one set of parallel fibers will show a band of light at right angles to the fibers (or inclusions). This property is known as chatoyancy (chat [Fr.] = cat) and is exhibited by cat's eye (chrysoberyl) and tiger's eye (quartz replacing fibrous crocidolite). If the mineral contains several intersecting sets of parallel fibers, it will form a star (asterism). Most familiar are the star ruby and star sapphire, with three sets of fibers (usually needle-like rutile) oriented perpendicular to the a- crystallographic axes at 120 to each other (see Fig. 1.2). Rose quartz is another example. Four-rayed stars are seen in some garnets and diopside. Some feldspars, notably adularia and albite, contain fine-scaled exsolution lamellae, in which two feldspars of slightly different chemical composition have formed as a result of slow cooling. The scattering of light across these planar elements results in a type of iridescence called schiller or adularescence, producing a pale, bluish-white shimmer in the gem variety, moonstone. Diffraction of Light The play of colors seen in precious opal is produced by the presence of a regular three-dimensional array of equal-sized microscopic spheres of hydrous silica, whose diameter is less than the wavelength of visible light. The uniformly-spaced spheres act as a diffraction grating which causes white light to be scattered (Fig. 5.5). The resulting multiple wavelets interfere with one another, breaking light into its spectral colors. This is the visible light analog of x-ray diffraction, in which the array consists of the atoms in the crystal lattice. Ironically, the silica spheres of opal are amorphous, that is, the atoms of silicon and oxygen are irregularly arranged. Ordinary opal lacks the regular pattern of spheres and scattering of white light produces a milky opalescence, instead. 40

48 Figure 5.5. The diffraction of light in precious opal. Iris agate (not to be confused with iris quartz) is another mineral in which the play of colors is due to diffraction. In this case, the diffraction is caused by the finely-banded structure. The bands in iris agate consist of regularly-alternating zones with contrasting crystal grain sizes and concentrations of impurities. At high magnifications, one can observe zones of coarser-grained quartz crystals, around mm in diameter, alternating with chalcedony fibers. Labradorite also owes its play of colors to diffraction. Here, the diffraction is caused by the presence of fine, regularly-spaced exsolution lamellae. 41

49 42 NOTES

50 CHAPTER 6 CLASSIFICATION OF MINERALS Basis of the Mineral Classification System The systematic classification of minerals is based on both the chemical composition and the crystal structure. There are around 10 major classes of minerals, which are divided into chemically-based families, that can be further subdivided into structurally-similar groups. Members of a group are often isostructural (i.e. share the same crystal structure). A group consists of individual mineral species. Mineral species can form part of a continuous series. That is, atoms or ions can replace each other extensively in the crystal structure. Species can be subdivided further into chemical varieties. Examples of the classification hierarchy are given below. 1. Mineral Class: Carbonate Family: Group: calcite (hexagonal-rhombohedral) Species: calcite CaCO 3, magnesite MgCO 3, siderite FeCO 3, rhodochrosite MnCO 3, smithsonite ZnCO3 Varieties: cobaltian calcite, manganoan calcite 2. Mineral Class: Silicate Family: Nesosilicate Group: garnet (isometric) Species: pyrope (magnesium aluminum silicate) almandine (iron aluminum silicate) spessartine (manganese aluminum silicate) (The above three form a s e rie s ) grossular (or grossularite) (calcium aluminum silicate) andradite (calcium iron silicate) uvarovite (calcium chromium silicate) (The above 3 form another s e rie s.) Variety: rhodolite (a mixture of pyrope and almandine) hessonite, tsavorite (varieties of grossular) demantoid (green gem variety of andradite) Major Mineral Classes Native Elements a. Metals (Isometric) Gold Au, silver Ag, copper Cu; platinum Pt; iron Fe and nickel-iron (Ni, Fe), from meteorites. b. Semi-metals (hexagonal-rhombohedral) Arsenic As, antimony Sb, bismuth Bi c. Non-metals Diamond C, graphite C, sulfur S 43

51 Sulfides, Arsenides, Sulfarsenides Galena PbS, sphalerite ZnS, covellite CuS, chalcocite Cu2S, pyrite FeS 2, chalcopyrite CuFeS 2, cinnabar HgS, arsenopyrite FeAsS, realgar AsS, stibnite Sb S Oxides and Hydroxides a. Hematite group (hexagonal-rhombohedral): hematite Fe2O 3, corundum Al2O 3, ilmenite FeTiO 3 b. Spinel group (isometric): spinel MgAl2O 4, magnetite Fe3O 4, franklinite ZnFe2O 4 c. Rutile group (tetragonal): rutile TiO 2, cassiterite SnO2 d. Miscellaneous: cuprite Cu2O, zincite ZnO, chrysoberyl BeAl2O4 e. Goethite group (orthorhombic): goethite á-feo(oh), diaspore á-alo(oh) (a constituent of bauxite, an ore of aluminum) f. Brucite Mg(OH) 2 g. Gibbsite Al(OH) another constituent of bauxite Halides Halite NaCl, sylvite KCl, fluorite CaF 2 3 Carbonates a. Calcite group: (hexagonal-rhombohedral) calcite CaCO 3, magnesite MgCO 3, siderite FeCO 3, rhodochrosite MnCO 3, smithsonite ZnCO3 b. Dolomite group: dolomite CaMg(CO 3) 2, ankerite CaFe(CO 3) 2 (both form a solid solution series) c. Aragonite group: aragonite CaCO 3, witherite BaCO 3, strontianite SrCO 3, cerussite PbCO3 d. Miscellaneous: malachite Cu CO (OH), azurite Cu (CO ) (OH) Nitrates, Borates Niter KNO, borax (sodium borate hydrate) Sulfates a. Barite group: barite BaSO 4, celestite SrSO 4, anglesite PbSO4 b. Other sulfates: gypsum CaSO 4.2H2O, anhydrite CaSO 4, antlerite Cu3SO 4(OH) 4, alunite (potassium aluminum hydroxl sulfate) Phosphates, Arsenates, Vanadates a. Apatite group: apatite Ca 5(PO 4) 3(F,Cl,OH), pyromorphite Pb 5(PO 4) 3Cl, vanadinite Pb 5(VO 4) 3Cl, mimetite Pb 5(AsO 4) 3Cl b. Others: vivianite Fe 3(PO 3) 2.8H2O, turquoise (copper aluminum phosphate hydrate), variscite (aluminum phosphate hydrate), wavellite (similar in composition to variscite) Tungstates, Molybdates Scheelite CaWO, wulfenite PbMoO, wolframite (Fe,Mn)WO

52 Silicates The silicates constitute the main building blocks of the earth's crust and mantle, and comprise around 25% of all known minerals. The basic structural unit of silicates is the silicate tetrahedron in which one silicon atom is surrounded by four oxygen atoms that form the corners of a tetrahedron (Fig. 6.1). The tetrahedra can exist as discrete, isolated units or can share oxygens and thereby link into more complex structures, somewhat analogous to the polymerization of carbon in organic chemistry. Silicates are divided further into families based on how the silicate tetrahedra are linked: a. Nesosilicates (Fig. 6.2a) isolated tetrahedra, tend to form equant crystals. e.g. olivine, garnet group (see above), topaz, staurolite. b. Sorosilicates (Fig. 6.2b) two tetrahedra sharing one oxygen. e.g. hemimorphite, epidote, vesuvianite. c. Cyclosilicates (Fig. 6.2c) rings in which two oxygens of each tetrahedron are shared. Rings of 3 e.g. benitoite (rare); Rings of 6 e.g. tourmaline, beryl d. Inosilicates (Fig. 6.2d) single or double chains. Crystals tend to be elongated, prismatic, acicular, or fibrous; well-developed cleavage in two directions (Fig. 6.3). Bonds holding atoms together within chains are stronger than the bonds holding adjacent chains together. e.g. single chains pyroxenes enstatite MgSiO 3, diopside CaMgSiO 3, augite; double chains amphiboles hornblende, tremolite-actinolite. e. Phyllosilicates (Fig. 6.2e) sheets in which three oxygen atoms of each tetrahedron are linked to other tetrahedra. Crystals tend to be flat, platy, have well-developed cleavage in one direction (basal cleavage). Here too, bonds holding silicate tetrahedra together within sheets are stronger than bonds holding adjacent sheets together. Examples: micas muscovite, biotite, phlogopite, lepidolite, fuchsite. Clays kaolinite, illite, smectite (montmorillonite group); chlorite group; serpentine group. f. Tectosilicates (Fig. 6.2f) framework silicates in which all four oxygen atoms are shared between adjacent tetrahedra. e.g. quartz, feldspars, and zeolites. Figure 6.1. The silicate tetrahedron basic building block of the silicate minerals. 45

53 Figure 6.2. Schematic illustration of the structure of silicates. Figure 6.3a. Structure of pyroxene viewed perpendicular to the c-axis.. Note the relationship between the cleavage directions and the spacing of atomic planes. The trapezoids are the pyroxene chains seen end on. Figure 6.3b. Structure of amphibole viewed perpendicular to the c-axis. The trapezoids are the double chains of amphibole seen end on. 46

54 CHAPTER 7 MINERAL RECOGNITION Overview The preceding chapters provide the background information needed for recognition and identification of minerals. A number of simple, physical tests can be made to help narrow down the possibilities among minerals. Some textbooks or field guides summarize descriptive data in a set of identification tables, which are organized according to easily-observed physical properties. By following a consistent, logical sequence of steps, one can soon arrive at a reasonably accurate identification for the more common minerals. Most identification schemes assume that crystals are either visible to the eye or can be seen under low-power magnification (e.g., a 10x hand lens or loupe). Massive material is more difficult to identify, but in many cases a combination of properties, such as color, streak, density, may suffice. Extremely fine-grained material or rare minerals may require advanced testing procedures for positive identification. Basic equipment for mineral testing includes a penknife, a 10x hand lens, a streak plate, a small vial of dilute hydrochloric acid (to test for the presence of carbonate minerals), and a small horseshoe magnet. Most of these tools can be obtained at mineral and gem shows, and through scientific equipment supply companies, such as Ward's Scientific. The more serious collector may also consider buying either a binocular or petrographic microscope. The binocular microscope provides a 3-D view of the surface of the specimen and is useful for observing very small crystals. The petrographic microscope has built-in polarizing filters which allow determination of the optical properties of transparent or translucent crystals. A more thorough discussion of the principles and applications of optical mineralogy can be found in texts, such as Nesse, An Introduction to Optical Mineralogy. Identification Strategy An example of a straightforward, easy-to-use identification strategy is that based on luster, hardness, cleavage, streak, and color presented in the determinative tables (Appendix) of Klein and Hurlbut, Jr., Manual o f Mine ralo g y (1993). Luster and color are the first characteristics of a minerals usually observed. However, color is often not a very diagnostic property, for reasons explained in Chapter 4, and thus is ranked lower in the testing scheme. Luster is used as the first-cut separation between metallic to sub-metallic minerals and non-metallic ones. Next, for metallic minerals, comes hardness on the Mohs scale (Fig. 1.2). Metallic minerals are subdivided into 3 major hardness classes: very soft (<2 ½), soft (2 ½ - 5 ½), and hard (>5 ½). The hardness of a mineral is measured by scratching one mineral or substance of known hardness against the unknown specimen. A series of tests is made with materials of increasing or decreasing hardness, to narrow down the hardness range of the unknown. The softest minerals (i.e., those less than or equal to 2 on the Mohs scale) can be easily scratched by a fingernail. This includes minerals such as talc (1), gypsum (2), molybdenite (1-1 ½), covellite (1-1 ½), and chlorite (2-2 ½). A copper coin can be used to test minerals softer than around 3 (e.g., calcite, chalcocite, native metals). A penknife separates medium-hard minerals (i.e., greater than 6) from softer ones (i.e., less than 5 ½ - 47

55 6). A piece of quartz (7) will scratch glass, feldspars, apatite, but not beryl (7 ½-8), phenacite (7 ½-8), or corundum (9). Diamond (10), the hardest known substance, will scratch all other minerals. Only another diamond can cut or scratch diamond. Note that the Mohs scale is a scale of relative hardness. In actuality, the hardness scale is logarithmic. The true hardness of diamond is over 500 times that of talc (see Fig. 1.3)! For non-metallic minerals, streak is the second most distinguishing characteristic. Non-metallic minerals are broken further into two groups: those with colored streaks vs those with colorless streaks. The streak is the powder produced by rubbing a mineral on a streak plate or an unglazed ceramic tile. The ceramic tile has a hardness of around 6. Therefore, the streak is most useful for minerals softer than around 6. Usually, the streak is a paler color of the mineral or is colorless. But for a number of metallic to submetallic minerals, the streak may differ from the color or luster, and may therefore be diagnostic. For example, the streak of hematite is a distinctive reddish-brown, whereas its crystals are a dark, steel-gray. Brassy pyrite and chalcopyrite, bronze bornite, pale silvery-pink nickeline (niccolite) all have a black streak, and dark brown goethite has a yellow-brown streak. For non-metallic minerals, the next property to look for is cleavage or breakage along planes (Fig. 1.4). Note whether or not cleavage is present (are there breaks along flat, parallel surfaces?), and if present, how easy is it to cleave, and in how many different directions. Some minerals may occasionally break along more or less flat surfaces (parting), (e.g., corundum, garnet), but this characteristic is not as regularly developed as true cleavage, not always present, and therefore not a fundamental property of the mineral. It is caused by phenomena such as twinning, exsolution (an intimate intergrowth of two phases of slightly different composition that develops during slow cooling of a crystal), or by geological deformation. Other physical properties to examine include the color, crystal form and symmetry, and specific gravity. As noted above, the color is not always a reliable guide, because of the presence of different kinds of impurities. Another complicating factor is the tarnish on many metallic minerals. To determine the true color and luster, scratch a small, unobtrusive surface with a penknife to reveal a fresh surface. Crystal form and symmetry (see Chapter 2 ) are also useful guides in mineral identification. Examine crystals for the presence of 2, 3, 4, and 6-fold axes and mirror planes. Even small drusy crystals may show well-developed forms visible with a hand lens. Recall that the distribution of etch pits, striations, or variations in luster on crystal faces provide clues to the overall crystal symmetry (Fig. 7.1) Growth striations are often associated with particular crystal forms. Examples include the horizontal striations of hexagonal prisms of quartz, or vertical striations on hexagonal or trigonal prisms of tourmaline. Figure 7.1. Etched topaz crystal showing different etch patterns on unlike crystal faces, which provides clues to the overall symmetry. 48

56 Specific gravity is the ratio of the weight of a given volume of mineral to the weight of an equal volume of water. It is measured by weighing the specimen accurately in both air and water, and dividing the weight in air by the air-water weight difference. One can get an approximate sense of the specific gravity of a sufficiently large and pure specimen simply by holding it in the hand. For example, native metals, galena, pyrite, and barite, will feel distinctly heavier than comparably-sized specimens of quartz, calcite, or feldspar. Minerals such as garnet, fluorite, topaz, and corundum lie somewhere in between these extremes. A number of minerals, notably those containing iron, are magnetic that is, they are attracted to a magnet. Some minerals showing distinct magnetic properties include native iron (rare as a terrestrial mineral, but occurring in iron meteorites), magnetite, ilmenite, pyrrhotite, and maghamite (a polymorph of hematite). Figure 7.2 Magnetite or lodestone, showing magnetic attraction for iron objects, such as nails. Optical properties are also helpful for transparent to translucent crystals. Double refraction is a characteristic property of non-cubic minerals. Some simple tests can be made to distinguish doublyrefracting crystals or cut gemstones from singly-refracting cubic crystals or glass. The first test involves placing the crystal or stone between two polarizing lenses or filters, which have been rotated until they appear dark (i.e., their polarization directions are at right angles to each other and no light passes through). This is the crossed polarizer position (Fig. 7.3). As the doubly-refracting crystal or gemstone is rotated between the crossed polarizing filters, the stone will alternately turn light and dark every 45. Cubic crystals or glass will remain dark throughout the rotation. Note however, that if a crystal is oriented with its optic axis direction parallel to the viewing direction, it will appear dark throughout the rotation. To check for this possibility, turn the stone over in several direction and repeat the test. 49