Identifying Minerals

Identifying Minerals

Beryl, feldspar, quartz Rock-forming Minerals

Rock-forming minerals Common minerals that make up most of the rocks of Earth s crust Only a few dozen members Composed mainly of the 8 elements that make up > 98% of the continental crust

Most abundant elements in crust

Elemental abundances in continental crust Figure 3.26

Minerals: Building blocks of By definition a mineral is Rock rocks Naturally occurring (formed by nature) Inorganic (never living sort of) Crystalline (Ordered internal molecular structure) Solid (definite shape and volume) Definite chemical composition (made with same elements every time sort of) An aggregate of several minerals (sort of ) Example: limestone (all calcite)

Crystalline (Ordered internal molecular structure) Crystals are atoms packed in a particular order.

Ordered structure (definite pattern)

Figure 3.3 (top)

Rock

To Identify a Mineral: Cleavage Fracture Hardness Density Luster Streak Color

Cleavage Tendency to break along planes (due to weak bonding) Produces flat, shiny surfaces It is the number of planes or directions, not the number of surfaces! Useful to know the angles between adjacent planes Handy tip: pinch the surfaces to find out how many cleavages.

One plane of cleavage Two planes of cleavage Three planes of cleavage Four planes of cleavage

Common cleavage directions Figure 3.22

Fluorite, halite, and calcite all exhibit perfect cleavage

Figure 3.21 (bottom)

Cleavage versus crystal form Planes of weakness throughout the mineral. Kept when broken. Only an external expression of atomic structure. Lost when broken. Note: Cleavage planes can be repeated, like a series of steps.

Fracture Absence of cleavage when a mineral is broken Random break with respect to the crystal structure. Example: conchoidal fracture Curved, shell-like breaks such as what is seen in broken glass.

Conchoidal fracture Figure 3.23

Hardness How easy a mineral can be scratched All minerals are compared to a standard scale called the Mohs scale of hardness 1 is the softest, 10 is the hardest. Rub a mineral (or object) of known hardness against one of unknown hardness or vice versa. If it scratches the other one, it is harder (or vice versa).

Figure 3.20A

Hardness Talc Gypsum Calcite Fluorite Apatite Orthoclase Quartz Topaz Corundum Diamond http://mineral.galleries.com/, www.minerals.net

Density (Specific Gravity) Density = mass/volume Specific Gravity Mass of a mineral / mass of an equal volume of water Average value = 2.7 Example 1: if a block of a mineral one centimeter on each side weighs 3 grams, and a block of the same size filled with water weighs 1 gram, the ratio would be 3:1, so the mineral would have a density of 3. Example 2: lead has a density of 11.3 g/cm 3 ; water has a density of 1 g/cm 3. Not the easiest thing to use to identify a mineral, but can be useful

Luster: the way a mineral reflects light Two basic categories Metallic (and submetallic) Nonmetallic Other common terms for nonmetallic minerals include: vitreous/glassy silky dull/earthy pearly

Galena (PbS) displays metallic luster (and submetallic) Figure 3.16

Streak Color of a mineral in its powdered form The color of the steak does not always match the color of the mineral Rarely is different among the same minerals, so it is rather reliable Only can be used on minerals that are softer than porcelain because tested on a porcelain plate (hardness 6.5)

Streak

Color Helpful, but unreliable for mineral identification Often many colors produced due to slight changes in mineral chemistry or impurities.

Quartz (SiO 2 ) exhibits a variety of colors Figure 3.32

Ability to transmit light Opaque: no light transmitted through it Translucent: when light but not an image is transmitted through it. Transparent: when light and an image is transmitted through it.

Other properties Texture Magnetism Reaction to hydrochloric acid Malleability Double refraction Taste Smell

Figure 3.24

Figure 3.25

Texture How a mineral feels to the touch Can be smooth, rough, ragged, greasy, soapy

Mineral Groups Groups are separated based on crystal structure Therefore, have similar properties Cleavage Hardness Silicate Groups include: Amphiboles Feldspars Micas Pyroxenes Quartz (sort of)

Amphiboles Two cleavages at about 60 and 120 Rich in iron and magnesium, so have a dark green to black color. Example: hornblende

Feldspars Two cleavages at about 90 Orthoclase (potassium feldspar) and plagioclase (sodium and calcium feldspar; albite and anorthite, respectively) are the two most common members

Micas Sheet structure gives them one perfect cleavage. Cleave so well that almost always in sheets.

Pyroxenes Two cleavages about 90. Example: augite

Quartz Not really a group, but very common, so worth learning about! Hardness of 7 No cleavage; fractures conchoidally Any color

Physical properties help identify Crystal shape or habit minerals Habit: common shape of a crystal External expression of a mineral s internal structure Won t always see crystal shapes due to competition for space and rapid loss of heat Minerals need time and space to grow!

Crystal shape Crystals grow outward from a central seed. Crystal form is maintained until the edges meet

Figure 3.19A

Figure 3.19B

Figure 3.14C

Figure 3.14A

Figure 3.14B

Some minerals have more than one habit Figure 3.18

Crystal habit Zeolite Quartz

Quartz

That s all

Talking Gypsies Calculate Flour Appetites Or Quarrel To Pass Corrupt Diamonds A way to remember it Talc Gypsum Calcite Fluorite Apatite Orthoclase Quartz Topaz Corundum Diamond