To expand somewhat on the definition of rock, the term may be said to describe an aggregate of minerals or organic material, which may or may not appear in consolidated form. Consolidation, which we will explore further within the context of sedimentary rock, is a process whereby materials become compacted, or experience an increase in density. It is likely that the image that comes to mind when the word rock is mentioned is that of a consolidated one, but it is important to remember that the term also can apply to loose particles.
The role of organic material in forming rocks also belongs primarily within the context of sedimentary, as opposed to igneous or meta-morphic, rocks. There are, indeed, a handful of rocks that include organic material, an example being coal, but the vast majority are purely inorganic in origin. The inorganic materials that make up rocks are minerals, discussed in the next section. Rocks and minerals of economic value are called ores, which are examined in greater depth elsewhere, within the context of Economic Geology.
The definition of a mineral includes four components: it must appear in nature and therefore not be artificial, it must be inorganic in origin, it must have a definite chemical composition, and it must have a crystalline internal structure. The first of these stipulations clearly indicates that there is no such thing as a man-made mineral; as for the other three parts of the definition, they deserve a bit of clarification.
At one time, the term organic, even within the realm of chemistry, referred to all living or formerly living things, their parts, and substances that come from them. Today, however, chemists use the word to describe any compound that contains carbon and hydrogen, thus excluding carbonates (which are a type of mineral) and oxides such as carbon dioxide or carbon monoxide.
The third stipulation, that a mineral must be of nonvarying composition, limits minerals almost exclusively to elements and compounds—that is, either to substances that cannot be chemically broken down to yield simpler substances or to substances formed by the chemical bonding of elements. The chemical bonding of elements is a process quite different from mixing, and a compound is not to be confused with a mixture, whose composition is highly variable.
Another way of putting this is to say that all minerals must have a definite chemical formula, which is not possible with a mixture such as dirt or glass. The Minerals essay, which the reader is encouraged to consult for further information, makes reference to certain alloys, or mixtures of metals, that are classified as minerals. These alloys, however, are exceptional and fit certain specific characteristics of interest to mineralogists. The vast majority of the more than 3,700 known varieties of mineral constitute either a single element or a single compound.
The fact that a mineral must have a crystalline structure implies that it must be a solid, since all crystalline substances are solids. A solid, of course, is a type of matter whose particles, in contrast to those of a gas or liquid, maintain an orderly and definite arrangement and resist attempts at compression. Thus, petroleum cannot be a mineral, nor is "mineral spirits," a liquid paint thinner made from petroleum (and further disqualified by the fact that it is artificial in origin).
Crystalline solids are those in which the constituent parts are arranged in a simple, definite geometric pattern that is repeated in all directions. These solids are contrasted with amorphous solids, such as clay. Metals are crystalline in structure; indeed, several metallic elements that appear on Earth in pure form (for example, gold, copper, and silver) also are classified as minerals.
The type of crystal that appears in a mineral is one of several characteristics that make it possible for a mineralogist to identify an unidentified mineral. Although, as noted earlier, there are nearly 4,000 known varieties of mineral, there are just six crystal systems, or geometric shapes formed by crystals. Crystallographers, or mineralogists concerned with the study of crystal structures, are able to identify the crystal system by studying a good, well-formed specimen of a mineral, observing the faces of the crystal and the angles at which they meet.
Other characteristics by which minerals can be studied and identified visually are color, streak, and luster. The first of these features is not particularly reliable, because impurities in the mineral may greatly affect its hue. Therefore, mineralogists are much more likely to rely on streak, or the color of the powder produced when one mineral is scratched by a harder one. Luster, the appearance of a mineral when light reflects off its surface, is described by such terms as vitreous (glassy), dull, or metallic.
Minerals also can be identified according to what might be called tactile properties, or characteristics best discerned through the sense of touch. One of the most important among such properties is hardness, defined as the ability of one mineral to scratch another. Hardness is measured by the Mohs scale, introduced in 1812 by the German mineralogist Friedrich Mohs (1773-1839).
The scale rates minerals from 1 to 10, with 1 being equivalent to the hardness of talc, a mineral so soft that it is used for making talcum powder. A 2 on the Mohs scale is the hardness of gypsum, which is still so soft that it can be scratched by a human fingernail. Above a 5 on the scale, roughly equal to the hardness of a pocketknife or glass, are potassium feldspar (6), quartz (7), topaz (8), corundum (9), and diamond (10).
Other tactile parameters are cleavage, the planes across which the mineral breaks, and fracture, the tendency to break along something other than a flat surface. Minerals also can be evaluated by their density (ratio of mass to volume) or specific gravity (ratio between the mineral's density and that of water). Density and specific-gravity measures are particularly important for extremely dense materials, such as lead or gold.
In addition to these specifics, others may be used for identifying some kinds of minerals. Magnetite and a few other minerals, for instance, are magnetic, while minerals containing uranium and other elements with a high atomic number may be radioactive, or subject to the spontaneous emission of high-energy particles. Still others are fluorescent, meaning that they glow when viewed under ultraviolet light, or phosphorescent, meaning that they continue to glow after being exposed to visible light for a short period of time.
Minerals are classified into eight basic groups:
The first group, native elements, includes metallic elements that appear in pure form somewhere on Earth; certain metallic alloys, alluded to earlier; and native nonmetals, semi-metals, and minerals with metallic and nonmetallic elements. Sulfides include the most important ores of copper, lead, and silver, while halides are typically soft and transparent minerals containing at least one element from the halogens family: fluorine, chlorine, iodine, and bromine. (The most well known halide, table salt, is a good example of an unconsolidated mineral.)
Oxides are noncomplex minerals that contain either oxygen or hydroxide (OH). Included in the oxide class are such well-known materials as magnetite and corundum, widely used in industry. Other nonsilicates (a term that stresses the importance of silicates among mineral classes) include carbonates, or carbon-based minerals, as well as phosphates and sulfates. The latter are distinguished from sulfides by virtue of the fact that they include a complex anion (a negatively charged atom or group of atoms) in which an atom of sulfur, chromium, tungsten, selenium, tellurium, or molybdenum (or a combination of these) is attached to four oxygen atoms.
There are two other somewhat questionable classes of nonsilicate that might be included in a listing of minerals—organics and mineraloids. Though they have organic components, organics—for example, amber—originated in a geologic and not a biological setting. Mineraloids, among them, opal and obsidian, are not minerals because they lack the necessary crystalline structure, but they can be listed under the more loosely defined heading of "rocks."
Only a few abundant or important minerals are nonsilicates, for example, the iron oxides hematite, magnetite, and goethite; the carbonates calcite and dolomite; the sulfides pyrite, sphalerite, galena, and chalcopyrite; and the sulfate gypsum. The vast majority of minerals, including the most abundant ones, belong to a single class, that of silicates, which accounts for 30% of all minerals. As their name implies, they are built around the element silicon, which bonds to four oxygen atoms to form what are called silica tetrahedra.
Silicon, which lies just below carbon on the periodic table of elements, is noted, like carbon, for its ability to form long strings of atoms. Carbon-hydrogen formations, or hydrocarbons, are the foundation of organic chemistry, while formations of oxygen and silicon—the two most abundant elements on Earth—provide the basis for a vast array of geologic materials. There is silica, for instance, better known as sand, which consists of silicon bonded to two carbon atoms.
Then there are the silicates, which are grouped according to structure into six subclasses. Among these subclasses, discussed in the Minerals essay, are smaller groupings that include a number of well-known mineral types: garnet, zircon, kaolinite, talc, mica, and the two most abundant minerals on Earth, feldspar and quartz. The name feldspar comes from the Swedish words feld ("field") and spar ("mineral"), because Swedish miners tended to come across the same rocks that Swedish farmers found themselves extracting from their fields.
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