Introduction to Mineral
Mineral, In a strict sense, a mineral is a naturally occurring solid substance that has a definite, uniform chemical composition and that was formed by an inorganic process—that is, by a process not involving living things. The study of such substances is called mineralogy. In a much broader sense, a mineral is any substance in the mineral kingdom—the realm of nature made up of nonliving things.
Substances that are obtained from the earth for their commercial value are commonly referred to as mineral resources. (Some of them, such as asphalt, chalk, coal, and petroleum, are not minerals in the strict sense of the word because they lack a definite chemical composition.) People, have always depended on various mineral resources to help provide for many basic needs.
Minerals usually occur in combination with each other or with other substances.
Rock is a mixture of minerals.
Ore is rock that contains economically important amounts of one or more minerals. The most important ores are those that contain metals.
Gems are rocks that are highly prized for their beauty.
Dietary Minerals are certain chemical elements that occur in foods and that are essential to life.
Composition and Properties
Minerals are composed of chemical elements. A few minerals, such as diamond and sulfur, contain only one element. Most, however, are compounds of elements—very complex compounds, in some cases. A mineral may contain impurities (mere traces of elements) that may affect its color. However, impurities do not alter the basic chemical composition of a mineral.
Many minerals have more than one physical form, each with distinct properties. Quartz, for example, may consist of visible crystals that appear six-sided; or it may occur as a mass of microscopic crystals.
The physical characteristics used by mineralogists to identify minerals include:
1. Crystal Structure.
2. Cleavage, the way in which a mineral splits when broken.
3. Fracture, the texture of a mineral's freshly broken surface.
4. Solubility, in water or acid.
5. Luster and Color.
6. Streak, the color of the mark that a mineral makes on an unglazed white tile.
7. Hardness.
Formation and Occurrence
Some minerals are crystallized in the cooling of molten rock from the earth's interior. Others are formed by the action of heat on solid rock. Many minerals result from secondary processes. An example is the replacement of one chemical element in a mineral by another element (thus forming a different mineral) as gases or solutions come in contact with rock.
Minerals usually have the form of tiny crystals spread more or less evenly through rock. However, concentrations of large crystals often occur, most commonly in veins in rock. Minerals also are found attached to rock surfaces, as in caves.
More than 1,500 kinds of minerals occur in the earth's crust. About 200 kinds are relatively common. Minerals also occur in meteorites (rocks that reach the earth from outer space), on the moon, and on planets and their moons.
Classification of Minerals
Minerals are classified primarily by their chemical composition. A widely accepted system places minerals in the following families:
Native Elementsare chemical elements occurring in nature in the native, or free, state, uncombined with other elements. (An element that is sometimes found natively may occur also, under different conditions, in a chemical combination belonging elsewhere in the classification system.) The native elements are not numerous but they have high economic value. Most are metallic or semimetallic elements.
Sulfides and Sulfosaltscontain sulfur as a main element. Sulfides are simple compounds of sulfur and a metal. Sulfosalts are compounds of sulfide and a metallic or semimetallic element—usually antimony arsenic, or bismuth. All minerals of this family are heavy. Most are metallic looking and relatively soft, and their crystals can be shattered into a dark powder. A typical sulfosalt is proustite (silver, sulfur, and arsenic).
Oxides and Hydroxidesare compounds of oxygen, or oxygen plus hydrogen, and one or more metals. These minerals vary greatly in physical characteristics.
Halidesare compounds of one or more metals and one or more of the following elements: fluorine, chlorine, bromine, and iodine. Many halides are very soluble in water. All are soft, and light-colored when freshly exposed.
Carbonateare compounds of one or more metals and carbon plus oxygen. All carbonates are soft, light-colored, and translucent or transparent. All are soluble in acid, giving off bubbles of carbon dioxide gas.
Nitratesare chemically similar to carbonates but have nitrogen in place of carbon.
Boratesare compounds of one or more metals, and boron plus oxygen, with water. Most borates are soft and very soluble.
Sulfatesare compounds of one or more metals, and sulfur plus oxygen, with (in some cases) water. Most sulfates are very light-colored, fairly heavy, and translucent or transparent. They may be soluble either in water or in acid.
Phosphates, Arsenates, and Vanadatescontain oxides of phosphorus, arsenic, and vanadium. These minerals are rather soft and heavy, and may be highly colored.
Tungstates and Molybdatescontain oxides of tungsten and molybdenum. This is a small family, typified by scheelite (calcium tungstate).
Silicatesare complex minerals containing silicon as a main element. About half of the known minerals belong to this family. None of the silicates is very heavy. Most have a glassy luster. Silicates are hard, but may be crushed to a light-colored powder even though the sample is dark-colored. Most silicates strongly resist melting, and are not easily dissolved. Among the silicates are a number of gemstones and industrial minerals.