Minerals

Minerals

The term mineral is often used to denote any material that occurs naturally in the ground, including oil and natural gas . However, mineralogists and geologists restrict its use to naturally occurring solids having specific chemical compositions. For example, all solid forms of pure silica (SiO₂) are minerals, including natural glass and quartz , but coal is not a mineral because it has no definite and universal chemical composition.

Solids produced by living things—bones, shells, pearls, and the like—are a special case. Scientists usually consider these objects non-minerals even when they have definite a chemical composition, as do the calcium carbonate (CaCO₃) shells of marine animals. The distinction is more professional than physical; mineralogists study minerals, but biologists study shells and bones, so shells and bones must not be minerals. However, biological solids that have been completely rearranged at the atomic level are officially regarded as minerals. For example, graphite and diamond formed by metamorphosis of coal are minerals.

Because solidity is part of the definition of a mineral, substances may change from mineral to non-mineral or vice versa by melting or solidifying. Liquid water has a definite chemical composition (H₂O) but is not considered a mineral because it is not solid; ice , however, is a mineral. Magma or molten lava are not minerals because they have no definite, universal composition and are liquids; solidified, they become mixtures of specific minerals.

The atoms making up a mineral may be arranged either randomly, like mixed marbles in a bag, or in an orderly pattern, like squares on a chessboard. If a mineral's atoms show long-range organization, the mineral is termed crystalline. The objects commonly called crystals are crystalline minerals of relatively large size that happen to have developed smooth faces. Many crystals, however, are too small to see with the naked eye, and most have imperfectly developed faces or none at all. Most rocks consist of chunks of several crystalline minerals fused together. In some rocks, such as granite , these individual pieces are large enough to see, while in others, such as slate , they are too small.

If a mineral's atoms are randomly arranged it is termed an amorphous mineral or a mineraloid. The most common amorphous mineral is glass—the solid formed by cooling magma or molten lava so quickly that its atoms do not have time to organize into crystals. Molten lava quenched in air or water, or intrusive magma cooled rapidly by contact with rock form glasses. All glasses are metastable; that is, they tend to lapse into crystalline form, much as water molecules in cold vapor organize themselves into snowflakes. In the case of glasses, this spontaneous crystallization process is termed devitrification. The processes of devitrification causes glasses to be rare in proportion to their age. Most natural glasses date from the last 60 or 70 million years, a mere tenth of the time since the beginning of the Cambrian Period . The remainder have devitrified.

Because oxygen, silicon , and other elements may be present in any ratio in a glass, depending on the composition of the original melt, some mineralogists do not consider glasses minerals and restrict the term mineral to naturally occurring crystals. For the remainder of this article, the term mineral will be used in this restricted sense.

Earth's crust and mantle consist almost entirely of minerals, yet the number of known minerals is less than 3,000. Two factors limit the number of possible and actual minerals. First, a crystal's atoms must be arranged in some periodically repeating, three-dimensional pattern, but only a finite number of such patterns exists. Second, there are only a few score naturally occurring elements, many of which are rare and eight of which—oxygen, silicon, aluminum, iron , calcium, sodium, potassium, and magnesium, in order of decreasing commonness—comprise

98.5% of Earth's crust by weight. Oxygen alone makes up approximately 47% of the crust by weight (over 90% by volume), and silicon makes up approximately another 27%. The number of minerals that can form is therefore finite, and many of those that could theoretically form do so rarely.

The atoms of the two most common elements on earth, silicon and oxygen, readily arrange themselves into tetrahedra (four-sided pyramids) having a silicon atom at the center and an oxygen atom at each point. This unit is the silicate radical, (SiO₄)⁴⁻. Silicate radicals can link into sheets, chains, or three-dimensional frameworks by sharing oxygen atoms. If every oxygen atom participates in two tetrahedra, then the overall ratio of silicon to oxygen is 1:2, and the resulting chemical formula is that of silica, SiO₂. Minerals built mostly of silica are termed silicate minerals. The mineral quartz is pure crystalline silica; other silicate minerals result when atoms of elements other than silicon are introduced at regular intervals. For example, some of the tetrahedra in the silicate framework may be centered on aluminum atoms rather than silicon atoms. In this case, atoms of other elements (usually calcium, potassium, or barium) must be present to balance the ionic charges in the framework. The silicate minerals having this particular structure are the feldspars, which make up approximately 60% of the earth's crust by volume.

When atoms of elements other than silicon unite with oxygen to form the basic building block of a mineral, nonsilicate minerals result: carbonates from carbon (e.g., calcite [CaCO₃]), sulfates from sulfur (e.g., anhydrite [CaSO₄]), phosphates from phosphorus (e.g., apatite [Ca₅(PO₄)₃F]), and the oxide minerals, in which O²⁻ alternates with positively charged ions (e.g., spinel [MgAl₂O₄]). Other mineral groups do not involve oxygen at all, including the halides (e.g., salt [NaCl]), the sulfides (e.g., pyrite [FeS₂]), and the native elements (pure sulfur, carbon, gold, etc.).

Although for simplicity's sake chemical formulas have been identified with mineral species in the preceding paragraph, the identity and properties of a mineral depend not only on what kinds of atoms compose it but on the arrangement of these atoms in space . Diamond and graphite, for instance, both consist entirely of carbon atoms and so have the same chemical formula (C), but differ in structure. A mineral's structure, in turn, depends partly on its chemical formula and partly on its history, that is, on the changes in pressure, temperature , and chemical context through which it has passed in reaching its present state. A simple example of a mineral structure recording process is the production of glass by rapid cooling of molten silica. To hold a piece of glass is to know a small, specific piece of history; this silica must have cooled rapidly. The dependence of mineral formation on time and temperature is exactly analogous to cookery. Indeed, geologists routinely speak of how the formation of minerals in large bodies of cooling magma is influenced by the "baking" of the magma. Minerals are therefore studied not only for their directly useful properties but for what their very existence reveals about the history of the earth.

See also Crystals and crystallography; Minerology; Obsidian