Chapter 4. Minerals. 1. vvhat are nearly all rocks made of?

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1 . - -z~... Chapter 4 Minerals ::-)~1::: CHAPTER 4-SKILL SHEET 1: MINERAL PROPERTIES Nearly all rocks are composed of the elements, compounds, and mixtures that Earth scientists call minerals. A mineral is defined as a natural chemical solid ofinorganic origin that has well-defined properties and a specific range of composition. That is, each mineral has a unique and uniform chemical make up, which gives it uniform chemical and physical properties. We can test for these properties to identify minerals. There are thousands of minerals, but most of them are rare. The great majority ofthe rocks that we see are composed of only about a dozen of the most common minerals. Some rocks contain no minerals. For example, coal is made of carbon from the accumulation of the remains offossil plants. Because ofits organic origin, coal contains no minerals. Some limestone FIGURE 4-1. Malachite is a copper ore mineral. is derived from the hard parts of shellfish and coral. This kind oflimestone may therefore contain few or no minerals. On the other hand, ice is a mineral because it fits the definition above. Yet, no rocks contain ice as a mineral constituent. Some rocks contain only a single mineral. Quartzite is composed ofquartz, either pure or with minor impurities. Marble is predominantly calcite. However, most rocks contain a variety of minerals. Granite usually contains feldspar, quartz, mica, and amphibole. Other minerals, such as magnetite and pyroxene, may also be present. 1. vvhat are nearly all rocks made of? 2. How many different minerals are there? 3. Of the,thousands of minerals, how many are very common? Minerals are identified by their observable properties. Geologists have selected certain observations that are most useful in identifying different minerals. Color is one of the most readily observed characteristics. Some minerals are easy to identify by their color. SKILL SHEET 1: MINERAL PROPERTIES 41

2 Almandine (a variety of garnet) is always dark red. Pyrite is a brassy yellow. However, many light-colored minerals can be discolored by small amounts ofimpurities. For example, quartz may be colorless (clear), white, pink, green, brown, or even black. In addition, some different minerals are the same color; for example white quartz and calcite. Therefore, although color is easy to see, it can also be misleading. 4. Why is color of limited use in identifying minerals? 5. What causes many light-colored minerals to display a wide variety of colors? 6. Name one mineral that can be almost any color. FIGURE 4-2. Selenite gypsum has a glassy luster, while pyrite has a metallic luster. Luster is the way that light behaves at the surface of a mineral. Geoscientists usually characterize luster as metallic (shiny with no entry oflight) or nonmetallic. It is important to note that light does not penetrate the surface of a mineral that has a metallic luster. Minerals with this kind ofluster look like they are made ofa hard metal. Transparent or translucent surfaces cannot have a metallic luster. Nonmetallic lusters include glassy, pearly, waxy, and earthy (dull). It takes a little practice and a fresh mineral surface to correctly identify luster; yet, it can be one of the most useful properties in mineral identification. When you observe luster, ask yourself, "Does this look like it could be made from a hard metal7" 7. Whatisluster? 8. What is the luster of fresh aluminum foil? 9. What types of luster are shiny, but not metallic? Most mineral samples do not contain perfectly shaped crystals. However, when a mineral sample does contain crystals, crystal shape can be very useful in identifying minerals. Calcite and quartz are commonly transparent and colorless with a glassy luster. However, quartz forms hexagonal (six-sided) crystals, while calcite usually forms rhombohedral crystals that look like a cube pushed over toward one corner. 42 CHAPTER 4 MINERALS LABORATORY MANUAL

3 Sheets Cubic Tetrahedral Hexagonal Mica family Galena Calcite Quartz FIGURE 4 M 3. Each of these minerals shows a characteristic crystal shape. Crystal shapes are determined by the arrangements and bonding of atoms and molecules. For example, halite (rock salt) crystals are cubic or rectangular solids. This is because the molecules in halite are locked into fiat rows, layer upon layer. The molecules in calcite are in offset rows. Therefore, calcite crystals are not cubic with 90 corners, but rhombohedral. 10. What name is applied to the geometric and symmetrical shape of minerals? 11. What do crystals oftable salt look like? 12. What is the most common crystal shape ofcalcite 13. What determines the shape of the crystals of a particular mineral? 0.E of.e <Ii c: c: Cleavage is the way a mineral splits, generally along fiat planes. Cleavage depends on the arrangement and bonding of molecules. Minerals tend to split along the planes.of weak bonds between their atoms. In specifying the cleavage properties of a mineral, scientists count the number of nonparallel planes ofcleavage, and the angle between those cleavage planes. For example, the mica minerals split in one direction, forming thin 0 sheets. Halite (rock salt) crystals cleave in three directions at right angles (90 ) to each 0.g other. Many minerals cleave parallel to the crystal faces. However, some, like quartz, do 'til.~.2 not split parallel to any crystal face. 0-0 '" :l5 ii :::l (5 (5.c.s::. v Cleavage in Cleavage in Cleavage in 1f) 1f) " one direction two directions three directions 1f) 1f) :::ie :::ie <: <: 1' : 1: r:n Ol.~ 0! Mica e Feldspar Halite FIGURE 4-4. Number of cleavage directions is one way to specify cleavage. SKILL 1: MINERAL PROPERTIES 43

4 Fracture is an uneven breakage. Asbestos breaks into thin fibers. Quartz fractures along curved, seashell-like surfaces. This is a property known as conchoidal fracture. The fracture ofgarnet produces surfaces that are flat enough to look like cleavage planes, even though they are not true cleavage surfaces. 14. Why do minerals cleave only in specific directions? 15. What property is similar to cleavage, but the breakage is not controlled by the weak atomic bonds? 16. What are the angles at which cleavage surfaces meet in halite? The hardness of a mineral is determined by using it to scratch other solids. We test hardness by drawing the pointed edge of the unknown mineral across a clean surface of a known substance. A substance will scratch only materials that are either softer or have the same hardness. Ifthe known substances is not scratched by the mineral, the mineral is softer. Figure 4-5 shows Mohs' Scale of Hardness. Hardness is determined by the strength of atomic and molecular bonding in a mineral. The diagrams in the figure illustrate the hardness of several common materials MOHS' SCALE OF HARDNESS ~(11J 1. Talc 6. Feldspar 2. Gypsum 7. Quartz 3. Calcite 8. Topaz 4. Fluorite 9. Corundum 5. Apatite 10. Diamond FIGURE 4-5. Mohs' Scale of Hardness Fingernail Penny 3 Window Steel file 2.5 glass What is the hardest mineral on Mohs' scale? 18. How is hardness usually tested? 19. What two index minerals on Mohs' scale can be scratched by a fingernail? 20. Diamonds are quite brittle. That is, they are easily shattered. However, what kind of damage is unlikely to occur to a diamond because of its hardness? - 44 CHAPTER 4 MINERALS LABORATORY MANUAL

5 1M...+/' The streak test shows the color ofthe powder ofa mineral. vve usually test the streak by rubbing a cor ner ofthe mineral across a white, unglazed porcelain streak plate. Minerals that have a metallic luster often leave a streak that is a different color from the surface of the sample. For example, pyrite is brassy yellow in color. However, the streak ofpyrite is green to black. Specific gravity is the ratio of the density of a mineral to the density of water. As a ratio, specific gravity has no units. However, specific gravity is the same number as the density in grams per cubic centimeter. Many common minerals have a specific gravity (density) in the range of (a density of glcm 3 ). FIGURE 4-6. To test for streak, rub the sample across a porcelain streak plate. 21. What test allows us to observe the color of the powder of a mineral sample? 22. Specific gravity compares the density of a mineral to the density of what other substance? 23. For what group of minerals is the streak test especially useful? 24. Why is there no unit of measure included in specific gravity? There are a number other properties that are found in only a few minerals. These d d..s.e <ii <Ii special properties can make those minerals very easy to identify. For example, trans <:: 0:.S! parent crystals of calcite can break light into two images (double refraction) as 16.9 ~ :a :i3 shown in Figure 4-7. Uranium (uranite) is a mineral that shows radioactivity. (See :::l :::l (5 a Figure 4-8.).s:::.s::: (J 0 Other minerals also have (f) (J) 0 a (J Q some unusual properties. Cal (f) (J) ::2 :a cite bubbles with acid if a <t «r-. r strong enough acid solution is applied. Magnetite is the only.e.e 0>.;: Ol common mineral that is strongly.~ :>. c- - o 0 attracted by a magnet. (It is (J Q strongly magnetic.) Halite (rock salt) "has a salty taste. (In general, you should not taste substances in the science labora FIGURE 4-7. Transparent calcite FIGURE 4-8. Uranium ore is tory, unless instructed to do so.) shows double refraction. radioactive. - SKILL SHEET 1: MINERAL PROPERTIES 45

6 25. What property of magnetite is unusual, but easy to test? 26. What property of calcite can make one line appear as two lines? 27. What property makes halite easy to identify? 28. Words are to sentences as minerals are to 29. What is the complete definition of a mineral? 46 CHAPTER 4 MINERALS LABORATORY MANUAL