Minerals

1. Minerals Dr. R. B. Schultz

2. The earth is made of rocks, which are in turn made of minerals. In this part of the course we'll learn how to identify common minerals and rocks. In order for something to be classified as a mineral, it must meet five (5) criterion: Minerals are: ·        1. Naturally occurring, ·        2. Inorganic, ·        3. Have known chemical compositions ·        4. Have definite physical properties. 5. Are solid ·        They are usually (although not always) crystalline.

3. Mineral Classification Minerals are classified based on chemical composition and crystal structure. Minerals are made of different ions bonded together. Ions are charged atoms ·Cations are positively charged whereas ·Anions are negatively charged 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 Crystal structure Crystal structure depends on sizes of and charges on ions Polymorphs -- same chemical composition, different crystal structures Mafic silicate minerals Most common minerals are silicates All silicate minerals contain silicon and oxygen Silicates that also contains iron or magnesium are called mafic silicate minerals Mafic silicate minerals are dark in color Examples: of mafic silicates: olivine, pyroxene, amphibole, and biotite mica Felsic silicates Felsic silicates are silicate minerals that don't contain magnesium or iron, are light in color Examples: feldspar, quartz, clay minerals, muscovite mica Silicate mineral structures 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 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 Tetrahedra may link together by sharing oxygens Isolated tetrahedral structure Cations serve as links between tetrahedra; no sharing of oxygens e.g. olivine, and garnet, which also happen to be mafic silicates Single chain silicates Adjacent tetrahedra form a chain by sharing 2 of their oxygens with neighboring tetrahedra e.g. pyroxenes, which also happen to be mafic silicates Double chain silicates Two chains can link up by sharing oxygens e.g. amphiboles, which are mafic silicates too Sheet silicates Sheets are formed when each tetrahedron shares 3 of its oxygens with its neighbors e.g. 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 e.g. feldspar, quartz, which are also non-mafic Common non-silicate minerals Calcite -- calcium carbonate -- Limestone is made of calcite. Dolomite -- calcium magnesium carbonate Gypsum -- calcium sulfate Galena -- lead sulfide Pyrite -- iron sulfide Halite -- sodium chloride (table salt) How to Identify Minerals: Physical Properties Geologists determine the identity of an unknown mineral by describing its physical properties. They then use a reference book to find out what mineral has those properties. We will learn to describe the physical properties. 1. Habit refers to the overall shape of the mineral use terms like: "equant" (3 dimensions of the mineral have about the same length, like a cube or sphere), “elongate" (one direction is long but the other 2 are short, like a pencil), or "platy" (one dimension is short, other 2 are long like a sheet of paper) isolated tetrahedra & framework silicate minerals tend to be equant in habit; chain silicates tend to be elongate, sheet silicates are platy 2. Cleavage Refers to very smooth, flat, shiny breakage surfaces These special breakage surfaces correspond to zones of weak bonding in the crystal structure To describe cleavage, 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) 3. Hardness Refers to "scratchability" of the minerals harder minerals will scratch softer minerals Rank minerals according to hardness using the Moh's scale Fingernails are about 2.5 on Moh's scale; glass or steel knife is about 5.5; use these common items to estimate hardness of a mineral. 4. Color Varies in many minerals, e.g. quartz Some minerals come in just one color; other are many colors/many varieties 5. Streak Refers to color of mark left by rubbing mineral against a streak plate (unglazed porcelain) streak does not vary even if color does. 6. Other Properties Some minerals are magnetic Some minerals effervesce ("fizz") in dilute acid Specific gravity (like density) Moh's Hardness Scale (Commit this to memory) 1.0TALC 2.0GYPSUM 2.5 FINGERNAIL 3.0CALCITE 3.5COPPER PENNY 4.0FLUORITE 5.0APATITE 5.5STEEL KNIFE BLADE/GLASS PLATE 6.0ORTHOCLASE FELDSPAR 7.0QUARTZ 8.0TOPAZ 9.0CORUNDUM (RUBY) 10 DIAMOND

4. Crystal structure Crystal structure depends on sizes of and charges on ions Most common mineral group is the silicates All silicate minerals contain silicon and oxygen 1. Mafic silicate minerals contain iron or magnesium and are dark in color. Examples: olivine, pyroxene, amphibole, and biotite mica 2. Felsic silicates don't contain magnesium or iron, and are light in color. Examples: feldspar, quartz, clay minerals, muscovite mica

5. Mafic silicate Felsic silicate

6. Common non-silicate minerals Fluorite – used as a toothpaste additive Calcite -- calcium carbonate -- Limestone is made of calcite. Dolomite -- calcium magnesium carbonate Gypsum -- calcium sulfate Galena -- lead sulfide Pyrite -- iron sulfide Halite -- sodium chloride (table salt)

8. How to Identify Minerals: Physical Properties Geologists determine the identity of an unknown mineral by describing its physical properties. They then use a reference book to find out what mineral has those properties. We will learn to describe the physical properties. 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.

9. Non-metallic mineral Metallic mineral

10. Cleavage Refers to very smooth, flat, shiny breakage 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) NO cleavage

11. Hardness Refers to "scratchability" or resistance to being scratched. Harder minerals will scratch softer minerals. Geologists rank minerals according to hardness using the Moh's scale Moh's Hardness Scale (Commit this to memory) 1.0TALC 2.0GYPSUM 2.5 FINGERNAIL 3.0CALCITE 3.5COPPER PENNY 4.0FLUORITE (Note the spelling!) 5.0 APATITE 5.5STEEL KNIFE BLADE/GLASS PLATE 6.0ORTHOCLASE FELDSPAR 7.0QUARTZ 8.0TOPAZ 9.0CORUNDUM (RUBY) 10.0 DIAMOND

12. Color Varies in many minerals, e.g. quartz VERY unreliable. Some minerals come in just one color; other are many colors/many varieties. Streak Refers to color of mark 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) Specific gravity (like density) galena has a high specific gravity.

13. Key Terminology Mineral Chemical composition Crystalline structure Ions Cation Anion Silicate Mafic Felsic Silica tetrahedron Single chain Double chain Sheet silicate Framework silicate Non-silicate Physical properties Habit Luster Streak Cleavage Hardness Moh’s Hardness Scale