What is a mineral

I MA N Minerals are naturally occurring, solid, inorganic compounds or elements.

Real-World Reading Link Look around your classroom. The metal in your desk, the graphite in your pencil, and the glass in the windows are just three examples of how modern humans use products made from minerals.

Mineral Characteristics

Earth's crust is composed of about 3000 minerals. Minerals play important roles in forming rocks and in shaping Earth's surface. A select few have helped shape civilization. For example, great progress in prehistory was made when early humans began making tools from iron.

A mineral is a naturally occurring, inorganic solid, with a specific chemical composition and a definite crystalline structure. This crystalline structure is often exhibited by the crystal shape itself. Examples of mineral crystal shapes are shown in Figure 4.1.

Naturally occurring and inorganic Minerals are naturally occurring, meaning that they are formed by natural processes. Such processes will be discussed later in this section. Thus, synthetic diamonds and other substances developed in labs are not minerals. All minerals are inorganic. They are not alive and never were alive. Based on these criteria, salt is a mineral, but sugar, which is harvested from plants, is not. What about coal? According to the scientific definition of minerals, coal is not a mineral because millions of years ago, it formed from organic materials.

Www What Crystal Mineral Com

86 Chapter 4 • Minerals

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Definite crystalline structure The atoms in minerals are arranged in regular geometric patterns that are repeated. This regular pattern results in the formation of a crystal. A crystal is a solid in which the atoms are arranged in repeating patterns. Sometimes, a mineral will form in an open space and grow into one large crystal. The well-defined crystal shapes shown in Figure 4.1 are rare. More commonly, the internal atomic arrangement of a mineral is not apparent because the mineral formed in a restricted space. Figure 4.2 shows a sample of quartz that grew in a restricted space.

^p Reading Check Describe the atomic arrangement of a crystal.

Solids with specific compositions The fourth characteristic of minerals is that they are solids. Recall from Chapter 3 that solids have definite shapes and volumes, while liquids and gases do not. Therefore, no gas or liquid can be considered a mineral.

Each type of mineral has a chemical composition unique to that mineral. This composition might be specific, or it might vary within a set range of compositions. A few minerals, such as copper, silver, and sulfur, are composed of single elements. The vast majority, however, are made from compounds. The mineral quartz (SiO2), for example, is a combination of two atoms of oxygen and one atom of silicon. Although other minerals might contain silicon and oxygen, the arrangement and proportion of these elements in quartz are unique to quartz.

Restricted Minerals
Figure 4.2 This piece of quartz most likely formed in a restricted space, such as within a crack in a rock.

Vocabulary

Academic vocabulary

Restricted small space; to have limits The room was so small that it felt very restricted

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Albite

Oligoclase

Labradorite

Anorthite

Albite

NaAlSi3O8

Oligoclase

Figure 4.3 The range in composition and resulting appearance is specific enough to identify numerous feldspar varieties accurately.

Labradorite

Anorthite

Anorthite Sample

Variations in composition In some minerals, such as the ones shown in Figure 4.3, chemical composition can vary within a certain range depending on the temperature at which the mineral crystallizes. For example, plagioclase feldspar ranges from white albite (AHL bite) to gray anorthite (ah NOR thite). This color difference is due to a slight change in the mineral's chemical composition from sodium-rich to calcium-rich. At high temperatures, calcium is primarily incorporated, and at low temperatures sodium is primarily incorporated. At intermediate temperatures, a mixture of calcium and sodium is incorporated in the crystal structure producing a range of colors, as shown in Figure 4.3.

Rock-Forming Minerals

Although about 3000 minerals occur in Earth's crust, only about 30 of these are common. Eight to ten of these minerals are referred to as rock-forming minerals because they make up most of the rocks in Earth's crust. They are primarily composed of the eight most common elements in Earth's crust. This is illustrated in Table 4.1.

NaAlSi3O8

Table 4.1

Most Common Rock-Forming Minerals

Quartz

Feldspar

Mica

Pyroxene*

SiO2

NaAlSi3O8 - CaAl2Si2O8 & KAlSi3O8

K(Mg,Fe)3(AlSi3O1o)(OH)2 KAl2(AlSi3O1o)(OH)2

MgSiO3

CaMgSi2O6

NaAlSi2O6

Amphibole*

Olivine

Garnet*

Calcite

Ca2(Mg,Fe)5Si8O22(OH)2 Fe7Si8O22(OH)2

Fe3Al2Si3O12

Ca3Al2Si3O12

Representative mineral compositions

Minerals from magma Molten material that forms and accumulates below Earth's surface is called magma. Magma is less dense than the surrounding solid rock, so it can rise upward into cooler layers of Earth's interior. Here, the magma cools and crystallizes. The type and number of elements present in the magma determine which minerals will form. The rate at which the magma cools determines the size of the mineral crystals. If the magma cools slowly within Earth's heated interior, the atoms have time to arrange themselves into large crystals. If the magma reaches Earth's surface, comes in contact with air or water, and cools quickly, the atoms do not have time to arrange themselves into large crystals. Thus, small crystals form from rapidly cooling magma, and large crystals form from slowly cooling magma. The mineral crystals in the granite shown in Figure 4.4 are the result of cooling magma. You will learn more about crystal size in Chapter 5.

^ Reading Check Explain how contact with water affects crystal size.

Minerals from solutions Minerals are often dissolved in water. For example, the salts that are dissolved in ocean water make it salty. When a liquid becomes full of a dissolved substance and it can dissolve no more of that substance, the liquid is saturated. If the solution then becomes overfilled, it is called supersaturated and conditions are right for minerals to form. At this point, individual atoms bond together and mineral crystals precipitate, which means that they form into solids from the solution.

Minerals also crystallize when the solution in which they are dissolved evaporates. You might have experienced this if you have ever gone swimming in the ocean. As the water evaporated off your skin, the salts were left behind as mineral crystals. Minerals that form from the evaporation of liquid are called evaporites. The rock salt in Figure 4.4 was formed from evaporation. Figure 4.5 shows Mammoth Hot Springs, a large evaporite complex in Yellowstone National Park.

Salt Geology

Rock salt

Figure 4.4 The crystals in these two samples formed in different ways. Describe the differences you see in these rock samples.

Rock salt

Figure 4.4 The crystals in these two samples formed in different ways. Describe the differences you see in these rock samples.

Figure 4.5 This large complex of evaporite minerals is in Yellowstone National Park. The variation in color is the result of the variety of elements that are dissolved in the water.

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CAREERS in EARTH SCIENC1

Lapidary A lapidary is someone who cuts, polishes, and engraves precious stones. He or she studies minerals and their properties in order to know which minerals are the best for certain projects. To learn more about Earth science careers, visit glencoe.com.

Identifying Minerals

Geologists rely on several simple tests to identify minerals. These tests are based on a mineral's physical and chemical properties, which are crystal form, luster, hardness, cleavage, fracture, streak, color, texture, density, specific gravity, and special properties. As you will learn in the GeoLab at the end of this chapter, it is usually best to use a combination of tests instead of just one to identify minerals.

Crystal form Some minerals form such distinct crystal shapes that they are immediately recognizable. Halite—common table salt—always forms perfect cubes. Quartz crystals, with their double-pointed ends and six-sided crystals, are also readily recognized. However, as you learned earlier in this section, perfect crystals are not always formed, so identification based only on crystal form is rare.

Luster The way that a mineral reflects light from its surface is called luster. There are two types of luster—metallic luster and nonmetallic luster. Silver, gold, copper, and galena have shiny surfaces that reflect light, like the chrome trim on cars. Thus, they are said to have a metallic luster. Not all metallic minerals are metals. If their surfaces have shiny appearances like metals, they are considered to have a metallic luster. Sphalerite, for example, is a mineral with a metallic luster that is not a metal.

Minerals with nonmetallic lusters, such as calcite, gypsum, sulfur, and quartz, do not shine like metals. Nonmetallic lusters might be described as dull, pearly, waxy, silky, or earthy. Differences in luster, shown in Figure 4.6, are caused by differences in the chemical compositions of minerals. Describing the luster of nonmetallic minerals is a subjective process. For example, a mineral that appears waxy to one person might not appear waxy to another. Using luster to identify a mineral should usually be used in combination with other physical characteristics.

^p Reading Check Define the term luster.

Figure 4.6 The flaky and shiny nature of talc gives it a pearly luster. Another white mineral, kaolinite, contrasts sharply with its dull, earthy luster.

Figure 4.6 The flaky and shiny nature of talc gives it a pearly luster. Another white mineral, kaolinite, contrasts sharply with its dull, earthy luster.

Flaky Mineral Contains Silicon Oxygen
Kaolinite

90 Chapter 4 • Minerals

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Table 4.2

Mohs Scale Interactive Table To explore

, , more about Mohs scale of hard-of Hardness ness, visit glencoe.com.

Mineral

Hardness

Hardness of Common Objects

Diamond

10

Corundum

9

Topaz

8

Quartz

7

streak plate = 7

Feldspar

6

steel file = 6.5

Apatite

5

glass = 5.5

Fluorite

4

iron nail = 4.5

Calcite

3

piece of copper = 3.5

Gypsum

2

fingernail = 2.5

Talc

1

Hardness One of the most useful and reliable tests for identifying minerals is hardness. Hardness is a measure of how easily a mineral can be scratched. German geologist Friedrich Mohs developed a scale by which an unknown mineral's hardness can be compared to the known hardness of ten minerals. The minerals in the Mohs scale of mineral hardness were selected because they are easily recognized and, with the exception of diamond, readily found in nature.

^p Reading Check Explain what hardness measures.

Talc is one of the softest minerals and can be scratched by a fingernail; therefore, talc represents 1 on the Mohs scale of hardness. In contrast, diamond is so hard that it can be used as a sharpener and cutting tool, so diamond represents 10 on the Mohs scale of hardness. The scale, shown in Table 4.2, is used in the following way: a mineral that can be scratched by your fingernail has a hardness equal to or less than 2. A mineral that cannot be scratched by your fingernail and cannot scratch glass has a hardness value between 5.5 and 2.5. Finally, a mineral that scratches glass has a hardness greater than 5.5. Using other common objects, such as those listed in the table, can help you determine a more precise hardness and provide you with more information with which to identify an unknown mineral. Sometimes more than one mineral is present in a sample. If this is the case, it is a good idea to test more than one area of the sample. This way, you can be sure that you are testing the hardness of the mineral you are studying. Figure 4.7 shows two minerals that have different hardness values

Figure 4.7 The mineral on top can be scratched with a fingernail. The mineral on the bottom easily scratches glass.

Determine Which mineral has greater hardness?

Figure 4.7 The mineral on top can be scratched with a fingernail. The mineral on the bottom easily scratches glass.

Determine Which mineral has greater hardness?

Matt Meadows

Halite

Figure 4.8 Halite has perfect cleavage in three directions; it breaks apart into pieces that have 90° angles. The strong f bonds in quartz prevent cleavage from forming. Conchoidal fractures are characteristic of microcrystalline minerals such as flint. ls An to

Figure 4.8 Halite has perfect cleavage in three directions; it breaks apart into pieces that have 90° angles. The strong f bonds in quartz prevent cleavage from forming. Conchoidal fractures are characteristic of microcrystalline minerals such as flint. ls An to

Cleavage and fracture Atomic arrangement also determines §

how a mineral will break. Minerals break along planes where =

atomic bonding is weak. A mineral that splits relatively easily and (l) ; evenly along one or more flat planes is said to have cleavage. To ; identify a mineral according to its cleavage, geologists count the : number of cleaved planes and study the angle or angles between them. For example, mica has perfect cleavage in one direction. It breaks in sheets because of weak atomic bonds. Halite, shown in Figure 4.8, has cubic cleavage, which means that it breaks in three directions along planes of weak atomic attraction.

Recognize Cleavage and Fracture

How is cleavage used? Cleavage forms when a mineral breaks along a plane of weakly bonded atoms. If a mineral has no cleavage, it exhibits fracture. Recognizing the presence or absence of cleavage and determining the number of cleavage planes is a reliable method of identifying minerals.

Procedure ^ Blt^

Part 1

1. Read and complete the lab safety form.

2. Obtain five mineral samples from your teacher. Separate them into two sets—those with cleavage and those without cleavage.

3. Arrange the minerals that have cleavage in order from fewest to most cleavage planes. How many cleavage planes does each sample have? Identify these minerals if you can.

4. Examine the samples that have no cleavage. Describe their surfaces. Identify these minerals if you can. Part 2

5. Obtain two more samples from your teacher. Are these the same mineral? How can you tell?

6. Use a protractor to measure the cleavage plane angles of both minerals. Record your measurements.

Analysis

1. Record the number of cleavage planes or presence of fracture for all seven samples.

2. Compare the cleavage plane angles for Samples 6 and 7. What do they tell you about the mineral samples?

3. Predict the shape each mineral would exhibit if you were to hit each one with a hammer.

Quartz, shown in Figure 4.8, breaks unevenly along jagged edges because of its tightly bonded atoms. Minerals that break with rough or jagged edges are said to have fracture. Flint, jasper, and chalcedony (kal SEH duh nee) (microcrystalline forms of quartz) exhibit a unique fracture with arclike patterns resembling clamshells, also shown in Figure 4.8. This fracture is called conchoidal (kahn KOY duhl) fracture and is diagnostic in identifying the rocks and minerals that exhibit it.

Streak A mineral rubbed across an unglazed porcelain plate will sometimes leave a colored powdered streak on the surface of the plate. Streak is the color of a mineral when it is broken up and powdered. The streak of a nonmetallic mineral is usually white. Streak is most useful in identifying metallic minerals.

Sometimes, a metallic mineral's streak does not match its external color, as shown in Figure 4.9. For example, the mineral hematite occurs in two different forms, resulting in two distinctly different appearances. Hematite that forms from weathering and exposure to air and water is a rusty red color and has an earthy feel. Hematite that forms from crystallization of magma is silver and metallic in appearance. However, both forms make a reddish-brown streak when tested. The streak test can be used only on minerals that are softer than a porcelain plate. This is another reason why streak cannot be used to identify all minerals.

^P Reading Check Explain which type of mineral can be identified using streak.

Color One of the most noticeable characteristics of a mineral is its color. Color is sometimes caused by the presence of trace elements or compounds within a mineral. For example, quartz occurs in a variety of colors, as shown in Figure 4.10. These different colors are the result of different trace elements in the quartz samples. Red jasper, purple amethyst, and orange citrine contain different amounts and forms of iron. Rose quartz contains manganese or titanium. However, the appearance of milky quartz is caused by the numerous bubbles of gas and liquid trapped within the crystal. In general, color is one of the least reliable clues of a mineral's identity.

Figure 4.9 Despite the fact that these pieces of hematite appear remarkably different, their chemical compositions are the same. Thus, the streak that each makes is the same color.

<Foldables|

Incorporate information from this section into your Foldable.

Figure 4.10 These varieties of quartz all contain silicon and oxygen. Trace elements determine their colors.

Examples Double Refraction Minerals

Red jasper

Figure 4.10 These varieties of quartz all contain silicon and oxygen. Trace elements determine their colors.

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Table 4.3

Special Properties of Minerals

Property

Mineral

Example

Double refraction occurs when a ray of light passes through the mineral and is split into two rays.

Calcite—Variety Iceland Spar

Effervescence occurs when reaction with hydrochloric acid causes calcite to fizz.

Calcite

Magnetism occurs between minerals that contain iron; only magnetite and pyrrhotite are strongly magnetic.

Magnetite Pyrrhotite

Interactive Table To explore more about the special properties of minerals, visit glencoe.com.

Iridescence—a play of colors, caused by the bending of light rays.

Agate

Fluorescence occurs when some minerals are exposed to ultraviolet light, which causes them to glow in the dark.

Fluorite Calcite

Fluorite Cleavage Fracture

Special properties Several special properties of minerals can also be used for identification purposes. Some of these properties are magnetism, striations, double refraction, effervescence with hydrochloric acid, and fluorescence, shown in Figure 4.3. For example, Iceland spar is a form of calcite that exhibits double refraction. The arrangement of atoms in this type of calcite causes light to be bent in two directions when it passes through the mineral. The refraction of the single ray of light into two rays creates the appearance of two images.

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Responses

  • andwise
    What rock is made up of the fewest minerals?
    8 years ago
  • luke
    What is an example of a subjective mineral?
    8 years ago
  • alisia
    Why hematite occurs with different luster, colors and crystal forms?
    8 years ago
  • uwe
    Are the shapes shown in figure a the mineral's natural shape?
    8 years ago
  • nerea
    What all minerals are, meaning they are not alive and never were alive?
    3 years ago
  • roxanne
    Are the shapes shown on figure A the minerals natural shape?
    2 years ago

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