lab 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) \ 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 minerals 4 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

6 r 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 Special Properties of Minerals Certain minerals have special properties. Most minerals containing the carbonate anion (CO 3 2- ) such as calcite (CaCO 3 ) react with dilute hydrochloric acid (Figure 11). Some minerals are naturally radioactive

A.1. Test the hardness of the minerals in the kit by scratching them with a quartz crystal (Hardness 7), a nail (Hardness 4.5) or metal screw. Separate the minerals into those that were scratched and those that resisted scratching. You may or may not need all of the blanks below. 9 Minerals scratched by a quartz (H ≈ 7) If a mineral is scratched by quartz its H <7 Number ____2_____________ Mohs hardness __ (<7.0)____ Number _____3____________ Mohs hardness ___(<7.0)_______ Number___ 7______________ Mohs hardness __(<7.0)_________ Number ____8_____________ Mohs hardness __(<7.0)_________ Number_________________ Mohs hardness ___________ Number _________________ Mohs hardness ___________ Number_________________ Mohs hardness ___________ Number _________________ Mohs hardness ___________ Minerals scratched by a nail (H ≈ 4.5) Number ____1_____________ Mohs hardness __ (<4.5)____ Number __2_______________ Mohs hardness ___(<4.5)________ Number __6_______________ Mohs hardness ___(<4.5)________ Number ____7_____________ Mohs hardness ___(<4.5)________ Number _________________ 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_____3____________ Mohs hardness _>5.5______ Number _____5____________ Mohs hardness __>5.5_________ Number______8___________ Mohs hardness ___>5.5________

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______1___________ Mohs hardness __<2.5_________ Number ____2_____________ 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 ____8____________ ___N___________ _Mohs hardness _>5.5_____________ Number _____2___________ _____Y_______Mohs hardness __<4.5____________ Number ________________ _______________ Mohs hardness ______________ Number ________________ _______________ Mohs hardness ______________ 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____________ __lead ore, ceramic glaze____ c. sulfur So2______________ car batteries, water processing ___________ d. gold __Au____________ ___________Jewelry, medals____ e. fluorite __CaF2____________ _________aluminum, open-hearth steel _ f. calcite __CaCO3____________ _____abrasive,

Quartz: no cleavage directions, but fractures conchoidally and scratches glass Pyrite: no cleavage, brassy metallic luster Magnetite: hardness 5.5 to 6 and 1 cleavage direction, dark gray and magnetic Intermediate Minerals (2.5 ≤ Hardness ≤ 4.5) Fluorite: hardness of 4 and 4 cleavage directions – see Figure 4 Calcite: hardness of 3 and 3 cleavage directions not a 90 o - rhombohedral cleavage – see Figure 4 Halite: hardness of 2.5 and 3 cleavage directions at 90 o – cubic cleavage – see Figure 4 Galena: hardness of 2.5 to 2.7 and cubic cleavage, metallic luster and high specific gravity Soft Minerals (H ≤ 2.5) scratched by fingernail Muscovite: hardness of 2.0 to 2.5 and 1 direction of perfect cleavage, colorless to white color mica 12 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 Hardness : scratched by ___ quartz _______ but not by ____ nail ________ _______ 2 of 90_ _________ 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? _____ Y ____________ Cleavage not present _____ y ____________ Luster: metallic, vitreous (glassy), earthy _______ Y __________ Special properties: magnetism or salty ____ Dark Grey _____________ Color Unknown #1 is ____________ Graphite ________________________________________________

Unknown #2 Hardness : scratched by _ everything _________ but not by ____________ ______ 3 of 90___________ 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? _______ Y __________ Cleavage not present ______ N ___________Luster: metallic, vitreous (glassy), earthy ________ N _________Special properties: magnetism or salty ________ Clear _________ Color Unknown #2 is ________ Calcite ____________________________________________________ Unknown #3 Hardness : scratched by _ Nail _________ but not by _ Fingernail ___________ 13 ______ 1 of 90___________ 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? _______ N __________ Cleavage not present _______ Y __________ Luster: metallic, vitreous (glassy), earthy ________ N _________Special properties: magnetism or salty _______ Grey __________Color Unknown #3 is ______________ Quartz ______________________________________________ Unknown #4 Hardness : scratched by __ Nothing ________ but not by ____________ ___3 of 90______________ 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? ________ N _________ Cleavage not present _____ Y ____________ Luster: metallic, vitreous (glassy), earthy

Unknown #7 Hardness : scratched by __ Everything ________ but not by ____________ _____ 1 ____________ 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? ________ N _________ Cleavage not present ______ Y ___________ Luster: metallic, vitreous (glassy), earthy ______ N ___________Special properties: magnetism or salty ______ Black ___________ Color Unknown #7 is _________ Flourite ___________________________________________________ Unknown #8 Hardness : scratched by __ quartz ________ but not by ____ nail ________ _______ 2 not 90__________ 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? _____Y ____________ Cleavage not present 15 ______ _N __________Luster: metallic, vitreous (glassy), earthy ______N ___________Special properties: magnetism or salty _____ pink to brown ____________ Color Unknown #8 is ________ Pryite ____________________________________________________ Once you finish Laboratory Exercise 2, please submit it using Canvas. If you are on campus, you may place completed laboratory assignments in Dr. Puckette’s mailbox in NRC 105. If you have questions concerning the lab exercises, please contact Dr. Puckette.

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