Chapter 2

Atoms, Elements, and Minerals

Index

Rocks & MineralsRock – Naturally formed, consolidated material composed of grains of one or more minerals. (There are a few exceptions to this definition.)Fig. 2.1

Mineral – A naturally occurring, inorganic, crystalline solid that has a definite chemical composition.Fig. 2.2

AtomsElement – A substance that cannot be broken down to other substances by ordinary chemical methods. Each atom of an element possesses the same number of protons.Atom – Smallest possible particle of an element that retains the properties of that element.Proton – A subatomic particle that contributes mass and a single positive electrical charge to an atom.Neutron – A subatomic particle that contributes mass to an atom and is electrically neutral.Electron – A single, negative electric charge that contributes virtually no mass to an atom.Fig. 2.3

Inner AtomNucleus – Protons and neutrons form the nucleus of an atom. Although the nucleus occupies an extremely tiny fraction of the volume of the entire atom, practically all the mass of the

atom is concentrated in the nucleus.

Isotopes – Atoms (of the same element) that have different numbers of neutrons but the same number of protons. (in the nucleus)

Atomic . . . .

Atomic mass number – The total number of neutrons and protons in an atom.

Atomic number – The total number of protons in an atom.

Atomic weight – The sum of the weight of the subatomic particles in an average atom of an element, given in atomic mass units.

Chemical Activity– Many geological processes can be explained as

chemical reactions. Some rocks form as a result of chemical reactions between substances. Understanding a few basic concepts of chemistry will clarify why and under what conditions chemical reactions occur.Atoms that are not electrically neutral tend to react (or combine) with other atoms to neutralize the electrical imbalance. Each atom not only seeks electrical neutrality but wants each of its shells to be full of electrons. The innermost shell is full when it possesses 2 electrons. Outer shells each require 8 electrons for an atom to be non-reactive (this is true for the second and third shells; elements having additional shells are more complicated.)

• Fig. 2.4

IonsIons – An electrically charged atom or group of atoms.

Fig. 2.5

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Chemical Composition of the Earth’s Crust

Pg. 33, Table 2.1

The Important Minerals

It is useful to be able to associate the names of important minerals with the physical properties that identify them of course, what constitutes an “important” mineral depends on your perspective. To a miner or prospector, an important mineral is one that is commercially valuable (and is, by implication, relatively uncommon). A “rock hound” is interested in collecting any mineral that is pretty or unusual. A gemologist specializes in those varieties of minerals that are of gem quality (diamonds, emeralds, etc.). A mineralogist is a scientist who studies the chemistry and crystallographic structure of minerals. In this book, the minerals we regard as important are those that help us understand the nature of the earth. We are particularly interested in rock-forming minerals because they make up most of the rocks of the earth’s crust.

– Continued

Of the several thousand identifiable minerals on earth, most are rare and not important to geology (many occur at only a single site on the globe). Only a few hundred are classified as rock-forming minerals. Even most of these are relatively uncommon in comparison with the few minerals that make up the vast bulk of the earth’s crust. The five minerals groups listed in the upper third of table 2.2 account for well over 90% of the earth’s crust. These are the minerals whose names recur most often in this book.

The Physical Properties of Minerals

Color

Streak

Luster

Hardness

Simple Chemical Tests

External Crystal Form

Cleavage

Fracture

Specific Gravity

Other Properties

Color

The first thing most people notice about a mineral is its color. For some minerals, color is a useful property. Because color is so obvious, beginning students tend to rely too heavily on it as a key to mineral identification. Unfortunately, color is also apt to be the most ambiguous of physical properties. If you look at a number of quartz crystals, for instance, you may find specimens that are white, pink, black, yellow, or purple. Color is extremely variable in quartz and many other minerals because even minute chemical impurities can strongly influence it. Obviously, it is poor procedure to attempt to identify quartz strictly on the basis of color.

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StreakA pulverized mineral gives a color, called a streak, that usually is more reliable than the color of the specimen itself, scraping the edge of a minerals sample across an unglazed porcelain plate leaves a streak that may be diagnostic of the mineral. For instance, hematite always leaves a reddish brown streak though the sample may be brown or red or silver.Unfortunately, few of the silicate minerals – the most common minerals – leave an identifying streak because most are harder than the porcelain plate.

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LusterThe quality and intensity of light that is reflected from the surface of a mineral is termed luster. (A photograph cannot show this quality.)Luster is either metallic or nonmetallic.Metallic – Gives a substance the appearance of being made of metal.

NonMetallic

Glassy luster – like glass or porcelain Silky luster – appearance of silk

Earthy luster – like unglazed pottery Pearly luster – appearance of

pearlResinous luster – appearance of resin

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Hardness

The property of “scratchability,” or hardness, can be tested fairly reliably. For a true test of hardness, the harder mineral or substance must be able to make a groove or scratch on a smooth, fresh surface of the softer mineral. For example, quartz can always scratch calcite or feldspar. Substances can be compared to Mohs’ hardness scale, on which ten minerals are designated as standards of hardness. The softest mineral, talc (used for talcum powder because of its softness), is designated as 1. Diamond, the hardest natural substance on earth, is 10 on the scale.Fig. 2.13

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Mohs’ Hardness ScaleTable 2.3 on pg. 40

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Fig. 2.13Pg. 41

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External Crystal FormThe crystal form of a mineral is a set of faces that have a definite geometric relationship to one another. What most people call a “crystal” is a mineral with well-developed crystal faces. To geologists a crystal usually means any homogenous solid that is crystalline (with or without crystal faces).

Fig. 2.14 Fig. 2.15 Fig. 2.18

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Fig. 2.14Pg. 41

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Fig. 2.15Pg. 41

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Fig. 2.18Pg. 42

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CleavageThe internal order of a crystal may be expressed externally by crystal faces, or it may be indicated by the minerals’ tendency to split apart along certain preferred directions. Cleavage is the ability of a mineral to break, when struck, along preferred directions.A mineral tends to break along certain planes because the bonding between atoms is weaker there.Cleavage is one of the most useful diagnostic tools because it is identical for a given mineral from one sample to another. Cleavage is especially useful for identifying minerals when they are small grains in rocks.

Fig. 2.19 Fig. 2.20

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Fig. 2.19Pg. 44

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Fig. 2.20Pg. 44

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FractureFracture is the way a substance breaks where not controlled by cleavage. Minerals that have no cleavage commonly have an irregular fracture.

Some minerals break along curved fracture surfaces known as conchoidal fractures (Fig. 2.23). These look like the inside of a clam or conch shell.

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Fig. 2.23Pg. 45

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Specific Gravity

It is easy to tell that a brick is heavier than a loaf of bread just by hefting each of them. The brick has a higher density, weight per given volume, than the bread. Density commonly expressed as specific gravity, the ration of a mass of a substance to the mass of an equal volume of water.

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Other PropertiesProperties that are useful in only a few instances include taste and smell. Halite obviously tastes salty; few other minerals have any taste at all. An “earthy” smell is characteristic of some clay minerals when they are moistened.Plagioclase feldspar commonly exhibits striations – straight, parallel lines on the flat surfaces of one of the two cleavage directions (Fig. 2.24)The mineral magnetite (an iron oxide) owes its name to its characteristic physical property of being attracted to a magnet.A clear crystal of calcite exhibits an unusual property. If you place transparent calcite over an image on paper, you will see two images (Fig. 2.25)

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Fig. 2.24Pg. 46

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Fig. 2.25Pg. 46

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Simple Chemical TestsOne chemical reaction is routinely used for identifying minerals. The mineral calcite, as well as some other carbonate minerals (those containing CO3

-2), reacts with a weak acid to produce carbon dioxide gas. In this test, a drop of dilute hydrochloric acid applied to the sample of calcite bubbles vigorously, indicating that CO2 gas is being formed. Normally this is the only chemical test that geologists do during field research.

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The Rock CycleFig. 2.27 on pg. 48

Word Example of Rock Cycle

As described in chapter 1, magma is molten rock. Igneous rocks form when magma solidifies. If the magma is brought to the surface by a volcanic eruption, it may solidify into an extrusive igneous rock. Magma may also solidify very slowly beneath the surface. The resulting intrusive igneous rock may be exposed later after uplift and erosion remove the overlying rock (as shown in Fig. 1.13) the igneous rock, being out of equilibrium, may then undergo weathering and erosion, and the debris produced is transported and ultimately deposited (usually on a sea floor) as a sediment. If the unconsolidated sediment becomes lithified (cemented or otherwise consolidated into a rock), it becomes a sedimentary rock. As the rock is buried by additional layers of sediment and sedimentary rock, heat and pressure increase. Tectonic forces may also increase the temperature and pressure. If the temperature and pressure become high enough, usually at depths greater than several kilometers below the surface, the original sedimentary rock is no longer in equilibrium and recrystallizes. The new rock that forms is called a metamorphic rock. If the temperature gets very high, the rock melts and becomes magma again, completing the cycle. The cycle can be repeated.

A Plate Tectonic ExampleFig. 2.28 on pg. 49

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Fig. 2.1Pg. 28

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Fig. 2.2Pg. 28

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Fig. 2.3Pg. 30

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Fig. 2.4Pg. 31

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Fig. 2.5Pg. 31

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BondingArticle on pg. 32

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Table 2.2Pg. 38

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