Laboratory Exercise 2 Minerals

Laboratory Exercise 2: Minerals that Form Common Rocks Rocks are made up of minerals, which are naturally occurring, inorganic solid matter with an orderly crystalline structure (arrangement of atoms). Minerals also have a definite chemical composition. Some minerals are composed of one element. Examples include graphite (carbon – C), diamond (carbon – C), sulfur (S), gold (Au) and silver (Ag). In this laboratory exercise you will learn to identify minerals based on their physical properties. These properties, based on the order of importance are hardness, cleavage, crystal habit, luster, streak and color. Notice that color is the least important physical property. This is because the same mineral can have a number of colors due to minor amounts of impurities. For instance, quartz can be clear, pink, yellow, black, white and purple. These forms of quartz have the same chemical composition and are all quartz. Minerals can have special properties such as being magnetic, reacting to dilute hydrochloric acid, being fluorescent and glowing when exposed to ultraviolet light, and radioactive. Learning Objectives After you have completed this laboratory exercise, you should be able to: 1. List and describe the common physical properties of minerals. 2. Use physical properties to identify an unknown mineral. 3. Recognize a set of minerals that form common rock types. 4. List the minerals that are important to society. Definition of Mineral Geologists define mineral as a naturally occurring, inorganic solid that possesses an orderly crystalline structure and a set chemical formula. Minerals exhibit all of these characteristics: 1. Naturally occurring . Minerals form naturally by geologic processes. Man-made materials, though they might have the same chemical formula as a natural substance, are not minerals. For instance, a synthetic ruby is not a mineral, but a natural ruby is. 2. Solid . Only solid crystalline substances are minerals. Ice, the solid form of water, is considered a mineral. Liquid mercury is an exception to this rule. 3. Orderly crystalline structure . Minerals are crystalline substances with an orderly arrangement of atoms and molecules. Some naturally occurring solid materials such as volcanic glass are not considered minerals. Glass is quenched and becomes solid before a crystalline structure can form. Glass in considered amorphous (without form). 4. Inorganic . Minerals are inorganic, which means they are not substances formed primarily from living things. Living things do form minerals and many marine organisms secret a mineral shell. Our definition means that substances such as coal and solid petroleum are not minerals. Coal is made up of compounds similar to minerals, but in coal these building blocks are called macerals. 5. Chemical formula : Most minerals have a set chemical formula. For instance, quartz is SiO 2 , which means that quartz consists of silicon (Si) and oxygen (O) atoms bonded together in a ratio of one (1) 1

silicon atom for every two (2) oxygen atoms. Because many atoms are of similar size, in nature they freely substitute for each other. Because this occurs, the chemical formulae for certain minerals vary or occur within acceptable ranges. Figure 1. Quartz crystals from Arkansas, U.S.A. Quartz is naturally, occurring, solid, crystalline, inorganic (does not come from carbon in plants or animals), and has the chemical formula SiO 2 . (From Essentials of Geology , 11 th ed. 2012) Relationship between minerals and rocks If two or more minerals combine, they form a rock. Sandstone, a common sedimentary rock, is often made up of numerous small grains of quartz (each from a separate quartz crystal) that are cemented together. Granite, a common igneous rock, can be made up of the following minerals: (1) quartz, (2) mica, and (3) feldspar. Each component by itself is a mineral, but combining these three minerals forms the rock we call granite. Identifying Minerals Geologists identify minerals using their physical properties that are a manifestation of the mineral’s chemical composition and crystalline structure. The common properties include: hardness, cleavage or fracture, crystal habit, luster, streak and color. 1. Hardness. Hardness is the resistance or mineral to abrasion or scratching. It is the most important test for identifying minerals. Hardness is tested by rubbing two mineral specimens against each other, or rubbing the mineral specimen against glass, or by scratching the mineral with a knife blade or nail. Geologists use the Mohs scale of hardness, which is arranged from the softest (1) to the hardest (10) as shown in Figure 2. 2

Figure 3. Cleaving mica minerals. The thin sheet was produced by splitting a mica mineral parallel to the direction of perfect cleavage. The bond between the atoms is weakest in this direction. (From Essentials of Geology , 11 th ed. 2012) 3. Crystal habit . Minerals can form spectacular crystals if the mineral forms in a cavity or other unrestricted space. Quartz (Figure 1) always forms specific crystal shapes that are different from the shape formed by calcite (Figure 6). Galena, the mineral we mine to extract lead, usually crystallizes as cubes or what geologists call cubic form. Interestingly, galena also cleaves to form cubes and has cubic cleavage or cleavage in three directions, all at 90 o to each other (see Figures 4 and 7). 4. Luster . Luster is the appearance of the mineral in reflected light. Some mineral like galena (Figure 7) have brilliant metallic luster on a freshly cleaved surface. Others like quartz, appear glassy and have what is called a glassy or vitreous luster. 5. Streak . If a mineral is powdered, the color of the powdered form is called streak. We use streak anytime we write with a graphite pencil because we leave a thin trail of powdered graphite on the paper. The same is true when we write on the sidewalk with chalk. The powdered chalk pattern is streak. What is interesting about streak is that the color of a mineral can be different than the color of its streak. Examples include pyrite or “fool’s gold” that is brassy colored in reflected light, but gives a dark green-gray streak when powdered (Figure 8). 6. Color . Color is the least preferred physical property used to identify minerals. The reason that geologists do not use color to identify minerals is simple. Many minerals come in a variety of colors. Small amounts of impurities in a mineral will change its color. As an example, examine the colors of quartz in Figure 9. Each of the four examples shown in Figure 9 is quartz (SiO 2 ) with the same crystal structure and chemical formula. However, because of small amounts of other materials trapped in the crystals, they are white (milky quartz), clear (crystal quartz – Figure 10), purple (amethyst quartz), yellow (citrine quartz) or black (smoky quartz). While quartz can show a range colors, certain 4

minerals such as sulfur never change. All sulfur is yellow. A general rule regarding minerals and the rocks they form is: minerals containing iron (Fe) are darker colored. As an example, white to clear mica (muscovite) contains the element potassium (K). In contrast, when iron (Fe) substitutes into the mica structure, the mica becomes dark-colored (see dark colored mica in Figure 3) and is called biotite. Figure 4. Common cleavage directions exhibited by minerals. (From Essentials of Geology , 11 th ed. 2012) 5

Figure 7. Galena (PbS) a source of lead (Pb) exhibits metallic luster (right), cubic crystal form (left) and cubic cleavage (right) when broken. All angles are 90 o (From Essentials of Geology , 11 th ed. 2012). Figure 8. A comparison of the color of pyrite and its powder or streak, which is much darker. (From Essentials of Geology , 11 th ed. 2012) 7

Figure 9. Examples of quartz (SiO 2 ) with the same chemical formula and crystal structure, but different color. Clear quartz is called crystal , black quartz is smoky , yellow quartz, citrine , white quartz, milky and purple quartz is amethyst . (From Essentials of Geology , 11 th ed. 2012) Figure 10. Clear quartz (crystal) that is broken to show conchoidal fracture, not cleavage. (From Essentials of Geology , 11 th ed. 2012) 8

Minerals scratched by a quartz (H ≈ 7) If a mineral is scratched by quartz its H <7 Number A-3 Mohs hardness (<7.0) ____ Number A-4 Mohs hardness (<7.0) ____ Number A-5 Mohs hardness (<7.0) ____ Number B-4 Mohs hardness (<7.0) ____ Number B-5 Mohs hardness (<7.0) ____ Number B-6 Mohs hardness (<7.0) ____ Number B-7 Mohs hardness (<7.0) ____ Number B-8 Mohs hardness (<7.0) ____ Minerals scratched by a nail (H ≈ 4.5) Number A-3 Mohs hardness (<4.5) ____ Number A-4 Mohs hardness (<4.5) ____ Number A-5 Mohs hardness (<4.5) ____ Number B-6 Mohs hardness (<4.5) ____ Number B-7 Mohs hardness (<4.5) ____ A.2. Take the subset of minerals not scratched by a nail, but scratched by quartz, and test their ability to scratch glass. Suggested glass to use is a pickle jar or similar type glass container. Glass beverage bottles work well for this test. List the minerals that scratched glass. These minerals have a hardness between 5.5 and 7.0 Minerals that scratch glass (H ≈ 5.5), but were scratched by the quartz crystal. Number B-4 Mohs hardness _>5.5 but <7.0 Number B-5 Mohs hardness _>5.5 but <7.0 Number B-8 Mohs hardness _>5.5 but <7.0 A.3. Another test for soft minerals is the human fingernail (H ≈ 2.5). Test the minerals in your collection with your thumbnail to determine the ones that are softer than the thumbnail. Number A-3 Mohs hardness <2.5 Number B-7 Mohs hardness < 2.5 Number _________________ Mohs hardness ___________ 10

Exercise B: Cleavage and Color B.1. Cleavage. Examine the minerals in your collection and determine which ones have smooth faces that reflect light. For reference, look at Figures 4, 5 and 6. You may or may not need all of the blanks below. The information sheet in each bag should help with cleavage. Mineral samples with cleavage Cleavage at 90 o (Y/N) Number ______6 _________ _____N _________ Mohs hardness _____<4.5 _____ Number 7 N Mohs hardness H<_2.5 Number 8 Y Mohs hardness > 5.5 but < 7.0 Number 3 N Mohs hardness <_ 2.5 Number 4 Y Mohs hardness 2.5 <_ H <_4.5 Number ________________ _______________ Mohs hardness ______________ Exercise C: Uses of Minerals Below is a list of common minerals. Using your book or internet resources, determine (1) the chemical formula and (2) how these minerals are used by modern society. For instance, diamond has industrial uses in abrasives and saw blades to cut hard materials like rocks. By far, most gem-quality diamonds are used for jewelry. Mineral name Chemical formula Uses a. diamond C abrasive, saw blades, jewelry b. galena Pbs Primary ore of lead c. sulfur S Vulcanization & Sulfuric acid d. gold Au Dentistry & Jewelry e. fluorite CaF2 Fluorine – lenses for microscopes and cameras f. calcite CaCO3 Construction material, abrasive, and soil treatment g. corundum AI203 Abrasive, optical glass, & polishing metals h. feldspar KAISi3O8 Fluxing agents in ceramics and glass production i. magnetite Fe2+Fe3+2O4 Ore of iron & material of power plants j. graphite C Lead in pencils & an electrical conductor k. talc Mg3Si4O10(OH)2 Baby, foot, and first aid powders l. gypsum CaSO4 2H2O Wallboard, cement, and soil conditioning m. chalcopyrite CuFeS2 Ore of copper and healing properties 11

Biotite: hardness of 2.0 to 2.5 and mica cleavage, dark brown to black iron-rich mica Selenite gypsum: hardness of 2.0 and 1 cleavage direction, but not perfect like mica Graphite: hardness 1 to 2 and 1 cleavage direction, soft gray mineral with greasy feel Sulfur: hardness of 1.5 to 2 and cleavage not prominent, always yellow and sulfur smell Talc: hardness of 1.0, cleavage is not apparent due to small size, soapy feel Unknown #1 A-3 Hardness : scratched by quarts, nail, and fingernail 1 Direction forming flexible sheets Cleavage (cleavage planes in one, two or three directions). If more than one cleavage plane is present, are the angles between the cleavage surfaces square (90 o ) or another angle? Metallic Luster: metallic, vitreous (glassy), earthy Magnetism Special properties: magnetism or salty Silver & black color Unknown #1 is graphite Unknown #2 A-4 Hardness : scratched by quartz and nail but not by fingernail 3 directions @ 90, cubic Cleavage (cleavage planes in one, two or three directions). If more than one cleavage plane is present, are the angles between the cleavage surfaces square (90 o ) or another angle? Vitreous Luster: metallic, vitreous (glassy), earthy Salty Special properties: magnetism or salty Clear color Unknown #2 is Halite Unknown #3 A-5 Hardness : scratched by quartz and nail but not by fingernail 3 directions @ 90, cubic Cleavage (cleavage planes in one, two or three directions). If more than one cleavage plane is present, are the angles between the cleavage surfaces square (90 o ) or another angle? Metallic, but tarnishes dull Luster: metallic, vitreous (glassy), earthy Magnetism Special properties: magnetism or salty Bright sliver & metallic luster color Unknown #3 is Galena Unknown #4 B-4 Hardness : scratched by quartz but not by nail or fingernail 13

Cleavage not present Metallic Luster: metallic, vitreous (glassy), earthy Salty Special properties: magnetism or salty Brassy color Unknown #4 is Pyrite Unknown #5 B-5 Hardness : scratched by quartz but not by nail or fingernail Cleavage not present Metallic Luster: metallic, vitreous (glassy), earthy Magnetism Special properties: magnetism or salty Black/ dark gray color Unknown #5 is Magnetite Unknown #6 B-6 Hardness : scratched by quartz and nail but not by fingernail 3 directions, not 90 - rhombohedral Cleavage (cleavage planes in one, two or three directions). If more than one cleavage plane is present, are the angles between the cleavage surfaces square (90 o ) or another angle? Vitreous Luster: metallic, vitreous (glassy), earthy Magnetism Special properties: magnetism or salty Colorless with a white line color Unknown #6 is Calcite Unknown #7 B-7 Hardness : scratched by quartz, nail, and fingernail Single perfect Cleavage (cleavage planes in one, two or three directions). If more than one cleavage plane is present, are the angles between the cleavage surfaces square (90 o ) or another angle? Pearly Luster: metallic, vitreous (glassy), earthy Magnetism Special properties: magnetism or salty Brown/Black color Unknown #7 is Biotite Unknown #8 B-8 Hardness : scratched by quartz but not by nail or fingernail Two directions @ 90 Cleavage (cleavage planes in one, two or three directions). If more than one 14