Minerals page 1. Minerals

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1 Minerals page 1 Minerals Introduction Most people are familiar with the commonly applied classification of all physical entities into the three categories of animal, vegetable and mineral. We now know that these subdivisions are not in the strictest sense discrete. Some organisms exist that combine animal locomotion with plant photosynthesis. Others, such as viruses, grow and reproduce under some conditions, and crystallize like minerals under other conditions. However, the general implications of this fundamental breakdown are valid: minerals are non-living and occur in many species as do plants and animals. The study of minerals is the area in geology most commonly translated into a satisfying hobby for the layman. Indeed, one need not be a full-fledged geologist to appreciate the aesthetic and intrinsic value of minerals, and to enjoy identifying and collecting them. The primary objective of this unit is to introduce minerals selected from a kingdom of several thousand species, and to define the various physical criteria used to distinguish one mineral from another. Definition In order to be considered a mineral, a substance must satisfy four criteria: 1. It must be inorganic 2. It must be naturally occurring 3. It must have a definite chemical make-up 4. It must be crystalline in nature Inorganic: Minerals are not, and never were, living. The only gray area is carbon. Carbon is product of organic processes, but remains accepted as a mineral by convention. Naturally Occurring: Minerals are not man-made, but happen naturally. Man-made substances may be chemically identical to minerals, but area not classified as minerals. Synthetic rubies, emeralds sapphires, and other gems may be flawless, but have no investment value. Definite Chemical Make-up: The basic identity of a mineral is its chemistry. Each species contains certain elements in specific proportions. For example, quartz has the formula Si0 2. This means that for every type of quartz, there is one silicon atom for every two oxygen atoms. Crystalline in nature: In minerals, the atoms are organized in orderly networks at specific distances and angles to one another. Glass is not a mineral because it lacks this ordered internal structure.

2 Minerals page 2 Physical Properties Each mineral species has a discrete chemistry, and can therefore be identified using certain chemical tests. This is not very practical for fieldwork, and mineralogists have established a series of physical properties, which will suffice to distinguish most minerals from each other. In mineral identification, not all of these properties apply to every specimen. Some minerals have one specific test that serves to identify them quickly. For example, some minerals have an unusual smell, while others break in a certain way. Often the tests are used together to form and overall identification of a mineral. Students should be able to perform simple tests to help determine the identification of each mineral. I. Hardness Hardness is a relative scale, evaluated on a level of 1 to 10, with 1 being very soft, to 10, which is the hardness of a diamond, the hardest of all minerals. Each mineral on this scale can scratch a mineral with and equal or lower number, but not one with a higher number. For example, calcite can scratch calcite, gypsum, talc, or your fingernail, but nothing with a harness greater than three. All minerals have a hardness somewhere in the 1 to 10 range, but may not be a whole number. Sphalerite can scratch calcite(3) but not fluorite(4), so it is a 3.5. Minerals with a hardness of less than 6 will not withstand prolonged wear as jewelry. Many gemstones are variants of quartz, and have a hardness of 7. When testing for hardness, make a small scratch, about ¼ long, to avoid unnecessary damage to the samples. Wipe your finger across the scratch to be sue it isn t merely a powder line made by the other sample. Use firm pressure, but you don t need to be excessive. When testing on a glass plate, always place the glass flat on the table to avoid injury. 1 TALC 2 GYPSUM 2 ¼ Average fingernail 3 CALCITE 3 ½ Copper penny 4 FLUORITE 5 APATITE 5 ½ Steel nail, knife blade 6 Glass plate 6 ORTHOCLASE 7 QUARTZ 8 TOPAZ 9 CORUNDUM 10 DIAMOND Moh s Hardness Scale for Minerals

3 Minerals page 3 II. LUSTER: Luster is the appearance of a mineral s surface in reflected light. Metallic Luster: These minerals have a metallic glint, often silver or gold, and reflect considerably. Metallic minerals are always opaque, no matter how thin they are sliced. Non-Metallic Luster: These fall into several subcategories, the most common are: Adamantine: Brilliant, like a polished diamond Vitreous: Glassy, like glazed porcelain or quartz Resinous: Like resin, Sphalerite is an example Pearly: Like pearls, Talc, Alabaster Gypsum Silky: Results from parallel fibers; asbestos, Satin Spar Gypsum Earthy: Dull, little reflection III. COLOR: This would seem to be a simple attribute to deal with, and yet it really does not work very well for mineral identification. Colors are subjective to the observer and are unreliable for many specimens. Quartz, fluorite, calcite, and other fundamentally colorless minerals may take on the full spectrum of hues depending in trace element contamination. Smaller crystals often look lighter in aggregate. In some specimens a thin surface coating of exotic material may be misleading. Color is most dependable for the metallic minerals. For the purpose of this class, light and dark are the only color categories we will use. Light colors are: white, clear, pink, light gray, salmon, and yellow. Dark colors are: dark gray, brown, black, red, and green. Metallic minerals are divided into silver and gold. IV. STREAK: This is the color of the mineral when powdered. Streak is generally constant, even when the color of the uncrushed mineral varies. Most non-metallic minerals have a non-diagnostic white streak, but metallic samples have a dependable streak. Specimens may be crushed in a mortar, but the usual practice is to drag them across the abrasive surface of an unglazed tile. If the specimen is harder than the streak plate (5 ½ to 6) then it will not leave a streak.

4 Minerals page 4 V. CLEAVAGE This is the tendency of a mineral to break along certain planes determined by internal atomic arrangement and bonding strength. Common types of cleavage are: 1. ONE DIRECTION (perfect cleavage): This looks like pages in a book and thin sheets can be pulled off easily. 2. TWO DIRECTIONS: This is often difficult to see, but usually two sides come to an angle greater than 90 o. 3. THREE DIRECTIONS: Three directional cleavages fall into two categories, either cubic, where all the angles are exactly 90 o, or non-cubic, where the angles can be greater or less than 90 o. Cubic looks like a box, or cube, while non-cubic looks like a rhomboid. 4. FOUR DIRECTIONS: These look like two pyramids stuck together, base to base. There are many other types of cleavage, but these are the only ones we will be discussing in class. VI. VII. FRACTURE This is the non-planar breakage of minerals. There is no noticeable cleavage planes. Fracture may be described as rough, splintery, or conchoidal (breaks like glass, into curves and/or circles). CRYSTAL FORM: Most minerals will take on a definite external geometric form when unobstructed growth is allowed. This form is an expression of their orderly internal atomic arrangement or crystallinity. Whole crystals and crystal faces are therefore and aid to identification. A complete study of crystallography is beyond our scope in this class.

5 Minerals page 5 VIII. MISCELLANEOUS: Other properties can be specific to one mineral, or are less common and only diagnostic for a few specimens. Natural Magnetism: magnetite Feel: talc fees soapy or greasy Specific Gravity: galena is heavy, talc is light Reacts with acid: calcite will fizz when it comes into contact with a 10% HCL solution Tenacity: reaction under stress; brittle (galena), flexible (talc), elastic (muscovite), malleable (native copper) Double refraction: calcite will give a double image when looked through Fluorescence: emits color under ultraviolet light Odor: sulfur smells like rotten eggs Taste: halite tastes like salt