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HEMICAL COMPOSITION OF THE EARTH'S CRUST

FIGURE 2.2 Model of the crystal structure of halite (or table salt).

and electrons. A proton is a subatomic particle that contributes mass and a single positive electrical charge to an atom. A neu-tron is a subatomic particle that contributes mass to an atom but is electrically neutral. An electron is a single negative electric charge that contributes a tiny percentage of mass to an atom. Electrons are regarded as moving very rapidly within specific energy levels, which are depicted as shells (figure 2.3A).

Protons and neutrons form the nucleus of an atom. though the nucleus occupies an extremely tiny fraction of the lume of the entire atom, practically all the mass of the atom concentrated in the nucleus. The atomic mass number is the number of neutrons and protons in an atom. The atomic s number of the oxygen atom in figure 2.3B is 16 (8 protons s 8 neutrons). Heavier elements have more neutrons and pro-s than do lighter ones. For example, the heavy element gold an atomic mass number of 197, whereas helium has an atomic mass number of 4. The number of protons controls the "character" of an ele-nt more than does the number of other subatomic particles. The atomic number of an element is the number of protons in each atom. We can refine our earlier definition of an element by

adding that each atom of an element has the same number of protons. Gold has an atomic number of 79, or 79 protons per atom; oxygen always has 8 protons; hydrogen always has 1 pro-ton; chlorine has 17; and sodium has 11. (Other atomic numbers are listed in appendix C as well as in appendix D, the periodic table of elements.) The number of neutrons (and therefore the mass of an ele-ment) can vary within limits. Isotopes of an element are atoms containing different numbers of neutrons but the same number of protons. For example, the most common isotope of oxygen has 8 neutrons; far less abundant is the oxygen isotope with 10 neutrons. In geology, radioactive isotopes are important for determining the age of rocks, as described in the chapter on geo-logic time.

 


 

Ions

1nlorine and sodium are more typical elements because if an ectron shell is complete, the atom is electrically out of bal--ace. Note that sodium in figure 2.5 has a complete inner shell ith 2 electrons and a second shell, also filled, with 8 electrons. ,ne more electron would neutralize all 11 protons in the icleus, but an eleventh electron alone in a shell is extremely ;stable, so the sodium atom normally does without it. In each dium atom, then, .the 11 protons (11+) and 10 electrons 0-) add up to a single excess positive charge (+1). Such an -om is an ion, an electrically charged atom or group of atoms. The sodium ion can be abbreviated as Na+. Chlorine, with an atomic number of 17, has a complete --er shell with 2 electrons and a complete second shell of 8 e,:trons around this. A neutral chlorine atom would have only electrons in the third shell, but this shell requires 8 electrons an extra electron is captured and incorporated in it. The chlo-ne ion then contains 18 electrons and 17 protons, and so has a :Tie excess negative charge (Cl-). Positive and negative ions are attracted to each other. In a stalline structure the mutual attraction is one way atoms are Id in place or bonded to one another. Bonding is the attach-frit of an atom to one or more adjacent atoms (see box 2.2).



HEMICAL COMPOSITION OF THE EARTH'S CRUST

stimates of the chemical composition of Earth's crust are - -sed on many chemical analyses of the rocks exposed on = _rth's surface. (Models for the composition of the interior of Earth—the core and the mantle—are based on more indirect idence.)

Table 2.1 lists the generally accepted estimates of the


 

,aed silicate structure is rearranged into the single-chain sili-_ate structure of pyroxene. In this case, Mg±2 ions occupy the • itive ion positions between chains as shown in figure 2.11A. The amphibole group is characterized by two parallel chains double-chain silicate structure) in which every other tetrahedron a chain shares an oxygen atom with the adjacent chain (see _ ure 2.9). In even a small amphibole crystal, millions of paral-lel double chains are bonded together by positively charged ions. Chain silicates tend to be shaped like columns, needles, or :ven fibers. The long structure of the external form corresponds -3 the linear dimension of the chain structure. Fibrous aggre-:ates of minerals are called asbestos (see box 2.3).


 

The pyroxene group and the amphibole group, which are and double-chain silicates, respectively, each contain a _tuber of minerals. Augite is the most common pyroxene, and rnblende is the most common amphibole. The mica group is characterized by minerals with a sheet ,:ate structure. The two most common micas are biotite and ..covite. Biotite is a dark-colored, iron/magnesium-bearing -ca. Muscovite mica lacks iron and magnesium and is trans-_rent or white. The clay mineral group is another group of sheet silicates ,ee box 2.4). Clays are abundant on the Earth's surface and in :dimentary rocks but make up only a minor percentage of the -ust as a whole. Olivine is not among the most common minerals in the -ust. It is, however, the predominant mineral in the upper man-: therefore, it is vastly more abundant in Earth as a whole than - e minerals that form most of the crust.

Nonsilicate minerals include native elements, which are min-erals composed of only one element. Gold is a native element, as are diamond and graphite, both of which are composed solely of carbon. Other nonsilicates are classified according to the pre-dominant negatively charged ions in their crystal structures. For instance, halite is a chloride because the negatively charged ions in the crystal are C1-. If the mineral contains CO3-2 ions, it is a carbonate. Sulfides have S-2 ions, sulfates SO4-2, and oxides 0-2 (but without Si, S, or C bonded to the oxygen atoms). Nonsilicate minerals are also more abundant on the Earth's surface than in the crust as a whole. Calcite (calcium carbon-ate, or CaCO3) is the most common nonsilicate mineral and is usually found at or near the Earth's surface. Limestone and marble are rocks composed mainly of calcite. Ore minerals, or economic minerals, are minerals of com-mercial value; most are not silicates. Among the ore minerals are iron oxides (the minerals magnetite and hematite) mined for


 

Luster is either metallic or nonmetallic. A metallic luster gives a substance the appearance of being made of metal. Metal-lic luster may be very shiny, like a chrome car part, or less shiny, like the surface of a broken piece of iron. Nonmetallic luster is more common. The most important type is glassy (also called vitreous) luster, which gives a sub-stance a glazed appearance, like glass or porcelain. Most silicate minerals have this characteristic. The feldspars, quartz, the micas, and the pyroxenes and amphiboles all have a glassy luster. Less common is an earthy luster. This resembles the sur-face of unglazed pottery and is characteristic of the various clay minerals. Some uncommon lusters include resinous luster (appearance of resin), silky luster, and pearly luster.


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 sur-face of the softer mineral. For example, quartz can always scratch calcite or feldspar. Substances can be compared to Mohs' hard-ness scale (table 2.3), 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. Rather than carry samples of the ten standard minerals, a geologist doing fieldwork usually relies on common objects to test for hardness (table 2.3). A fingernail usually has a hardness of about 2 1/2. If you can scratch the smooth surface of a min-eral with your fingernail, the hardness of the mineral must be less than 2 1/2 (figure 2.14). A copper coin or a penny has a hardness between 3 and 4; however, the brown oxidized surface of most pennies is much softer, so check for a groove into the coin. A knife blade or a steel nail generally has a hardness slightly greater than 5, but it depends on the particular steel alloy used. A geologist uses a knife blade to distinguish between softer minerals, such as calcite, and similarly appear-ing harder minerals, such as quartz. Ordinary window glass, usually slightly harder than a knife blade (although some glass, such as that containing lead, is much softer), can be used in the same way as a knife blade for hardness tests. A file (one made of tempered steel for filing metal, not a fingernail file) can be used for a hardness of between 6 and 7. A porcelain streak plate also has a hardness of around 6 1/2.



Date: 2016-01-03; view: 808


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