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Minerals

Dr. R. B. Schultz


What is the difference between a mineral and a rock? The earth is made of rocks, which are in turn made of minerals. Minerals are the “building blocks� of rocks. In order for something to be classified as a mineral, mineral it must meet five (5) criterion: Minerals are: Naturally occurring, Inorganic, Have known chemical compositions, Have definite physical properties, Are solid at room temperatures for long periods of time They are usually (although not always) crystalline.


How are minerals classified? Minerals are classified based on: chemical composition (what elements they are composed of) crystal structure (how the elements are arranged)

Minerals are made of different ions bonded together. Ions are charged particles Cations are positively charged (example: Na+1) Anions are negatively charged (example Cl–1) Common ions in earth's crust: O - most common ion (anion) Si, Al, Fe, Ca, Na, K, Mg, (Cations) Minerals are made mainly of these ions


How are minerals structured microscopically? Crystal structure depends on sizes of and charges on ions Most common mineral group is the silicates The crust and mantle of the Earth make up about 80% of the volume of the planet and are predominantly made of silicates. All silicate minerals contain silicon (Si) and oxygen (O) Mafic silicate minerals contain iron and magnesium and are dark in color Examples: olivine, pyroxene, amphibole, and biotite mica

Felsic silicates don't contain magnesium or iron, and are light in color Examples: feldspar, quartz, clay minerals, muscovite mica


What are silicate minerals and what are they composed of? Basic building block: silica tetrahedron Silica tetrahedron is a silicon ion bonded to 4 oxygen ions Silicon is positively charged (+4) Oxygen is negatively charged (-2) Net charge on tetrahedron: -4 Because entire tetrahedron is negatively charged, it is attracted to cations Tetrahedra may link together by a cation (e.g. Mg, Fe, Na, Ca, K) serving as a bridge, or may link together by sharing oxygens


Silica Tetrahedron


What possible structures can be made with a silica tetrahedra? Simple Tetrahedra Cations serve as links between tetrahedra; no sharing of oxygens Examples: olivine, and garnet, (also mafic silicates) Single chain silicates Adjacent tetrahedra form a chain by sharing 2 of their oxygens with neighboring tetrahedra Examples: pyroxenes, which also happen to be mafic silicates Double chain silicates Two chains can link up by sharing oxygens Examples: amphiboles, which are mafic silicates too Sheet silicates Sheets are formed when each tetrahedron shares 3 of its oxygens with its neighbors Examples: micas, biotite (mafic) and muscovite (non-mafic), and clay minerals, which are non-mafic silicates Framework silicates Every oxygen in each tetrahedron is shared to form 3-D framework Examples: feldspar, quartz, (also non-mafic)


Olivine:

Isolated Tetrahedra

Pyroxene:

Single Chain

Amphibole: Double Chain Micas:

Sheets

Feldspars:

Simple Framework

Quartz:

Complex Framework


What are NON-Silicate Minerals? Minerals that do NOT contain Silicon and Oxygen as a tetrahderal structure. Fluorite: used as a toothpaste additive Calcite: calcium carbonate; limestone is made of calcite. Gypsum: calcium sulfate ; used for wall board (drywall) Galena: lead sulfide; mined for lead Pyrite -- iron sulfide; mined for iron Halite -- sodium chloride (table salt); for your french fries

Fluorite

Halite

Calcite


How does one identify a mineral? Geologists determine the identity of an unknown mineral by describing its physical properties. properties Each mineral has its own unique set of physical properties and characteristics. Physical Properties include: Luster refers to the light reflected off of the mineral and its overall quality. Minerals can be termed: glassy, opaque, transparent, shiny, or most commonly: metallic and non-metallic. One of the first determinations a geologist must make is whether the mineral in metallic or non-metallic. Habit refers to the overall shape of the mineral. Mineral scientists (mineralogists) use terms like: "equant" “elongate" or "platy"

Isolated tetrahedra & framework silicate minerals tend to be equant in habit; chain silicates tend to be elongate, sheet silicates are platy.


Non-metallic mineral

Metallic mineral


How does a mineral break and what are its characteristics? Cleavage is the term applied to a mineral’s preferred orientation of breakage when it splits along smooth flat surfaces. These special breakage surfaces correspond to zones of weak bonding in the crystal structure. To describe cleavage, one must determine the number of unique cleavage planes (directions) and their angle with respect to each other (e.g. salt breaks into cubes, with cleavage in 3 directions, all at 90 degrees) An irregular break (no smooth surfaces) is termed fracture. fracture

One plane of cleavage NO cleavage: fracture

Three planes of cleavage


So what’s the difference between “cleavage” and “fracture”?


What is the hardness of a mineral? Pretty much what it says: the ability to resist abrasion or scratching. Harder minerals or objects will scratch softer minerals. Softer minerals or objects get scratched. Geologists rank minerals numerically according to hardness using the Moh's Hardness Scale Note objects are 0.5’s and minerals are whole numbers. Moh's Hardness Scale (Commit this to memory) 1.0 TALC 2.0 GYPSUM 2.5 FINGERNAIL 3.0 CALCITE 3.5 COPPER PENNY 4.0 FLUORITE (Note the spelling!) 5.0 APATITE 5.5 STEEL KNIFE BLADE/GLASS PLATE 6.0 ORTHOCLASE FELDSPAR 7.0 QUARTZ 8.0 TOPAZ 9.0 CORUNDUM (RUBY) 10.0 DIAMOND


Is color important? Not really! Minerals vary greatly in color and thus color is unreliable as a physical property in many minerals, e.g. quartz Some minerals come in just one color; color may be useful in their identification Streak Refers to color of powder residue left by rubbing mineral against a streak plate (unglazed porcelain). Streak does not vary even if color does. Other Properties Some minerals are magnetic (i.e., magnetite) Some minerals effervesce ("fizz") in dilute acid (calcite) Galena has a high specific gravity (like density)


These are all varieties of the mineral quartz:

Milky quartz Citrine Amethyst Smoky quartz Although they are all different colors, their luster, hardness, fracture, streak, and other properties are identical.

Rose quartz


A good way to go about identifying the minerals is: First, determine the luster (either metallic or non-metallic). If metallic: determine the streak color and hardness; go to appropriate identification table. if non-metallic, determine fracture or cleavage and hardness; go to appropriate identification table Use the mineral identification tables to eliminate what the minerals CANNOT be. Use process of elimination. Read descriptions carefully. Do NOT try to memorize every physical property of each mineral. Rather, know one or two diagnostic properties that will tell you the mineral name every time. Example: galena always makes cubes, always is silver-grey and always is very heavy.


What will the mineral identification quiz consist of? 1.

Moh’s Hardness Scale: you will be given the numbers 1 through 10; you are to write the names of the minerals and objects next to their appropriate hardness according to those as defined by Moh’s Scale (see slide on Moh’s Hardness Scale).

2.

You will be given five (5) mineral samples from those we have identified in class. You will be asked: Its hardness on Moh’s Scale (you are allowed to use the tools/objects) Whether the mineral has cleavage or fracture Is it a silicate or non-silicate? What is the family name of the mineral? (Example: micas, feldspars, silicates, non-silicates) What might a practical use be for the mineral? (paperweight or jewelry are NOT practical uses)

3. You may NOT use your notes or flow charts on the quiz. However, you will be provided with the names of all minerals studied.


Key Terminology Mineral Crystalline structure Cation Silicate Felsic Single chain Sheet silicate Non-silicate Habit Streak Hardness

Chemical composition Ions Anion Mafic Silica tetrahedron Double chain Framework silicate Physical properties Luster Cleavage Moh’s Hardness Scale


Minerals