Mineralogy (World of Earth Science)
Mineralogy is the study of minerals. Rocks in the earth's crust are composed of one or more minerals. A mineral in the geologic sense is a naturally occurring, inorganic, crystalline solid. A particular mineral has a specific chemical composition. Each mineral has its own physical properties such as color, hardness, and density.
Most minerals are chemical compounds that are made of two or more different elements. The composition of a mineral is shown by its chemical formula, which states each of the chemical elements present in the mineral as well as the ratios of each element. For example, the mineral quartz has the chemical formula SiO2. This means that quartz is made of the elements silicon (Si) and oxygen (O). The formula also shows that for every one silicon atom, two oxygen atoms are present. The mineral orthoclase has the chemical formula KAlSi3O8. A molecule of orthoclase contains one potassium (K) atom, one aluminum (Al) atom, three silicon atoms, and eight oxygen atoms. Some minerals always have the same chemical formula. Quartz always is composed of SiO2 and halite is always made of sodium (Na) and chlorine (Cl), with the chemical formula NaCl. Some minerals can have more than one chemical formula, depending on their composition. Sometimes an element can substitute for another in a mineral. This occurs when the atoms of two elements are the same charge and close to the same size. For example, an iron (Fe) atom and a magnesium (Mg) atom are both about the same size, so they can substitute for each other. The chemical formula for the mineral olivine is (Mg,Fe)2SiO4. The (Mg,Fe) indicates that either magnesium, iron, or a combination of the two may be present in an olivine sample.
Native elements are minerals that are composed of only one element. These are the substances that dietitians call minerals. Examples of native elements include gold (Au), silver (Ag), and platinum (Pt). Two other native elements are graphite and diamond, both of which are entirely made of carbon (C).
All minerals are crystalline solids. A crystalline solid is a solid consisting of atoms arranged in an orderly three-dimensional matrix. This matrix is called a crystal lattice. A crystalline solid is composed of molecules with a large amount of order. The molecules in a crystalline solid occupy a specific place in the arrangement of the solid and do not move. The molecules not only occupy a certain place in the solid, but they are also oriented in a specific manner. The molecules in a crystalline solid vibrate a bit, but they maintain this highly ordered arrangement. The molecules in a crystalline solid can be thought of as balls connected with springs. The balls can vibrate due to the contractions and expansions of the springs between them, but overall they stay in the same place with the same orientation. It is not easy to deform a crystalline solid because of the strong attractive forces at work within the structure. Crystalline solids tend to be hard, highly ordered, and very stable.
Under ideal conditions, mineral crystals will grow and form perfect crystals. An ideal condition would be in a place where the crystals are allowed to grow slowly without disturbances, such as in a cavity. A perfect crystal has crystal faces (planar surfaces), sharp corners, and straight edges. The external crystal form is controlled by the internal structure. When the atoms in a crystal are arranged in a perfectly orderly fashion, the crystal will also be formed in a perfect orderly fashion. Even if a perfect crystal is not formed, the internal crystalline structure can be shown. Many minerals exhibit a property called cleavage. A mineral that has cleavage will break or split along planes. If the internal structure is formed in an orderly crystal arrangement, then the breaks will occur along the planes of the internal crystal structure.
There have been over 3,500 minerals identified and described. Only about two dozen of these are actually common. There are a limited number of minerals, mainly because there are only a certain number of chemical elements that can combine to form chemical compounds. Some combinations of elements are unstable, such as a potassium-sodium or a silicon-iron compound. In addition, only eight elements are found abundantly in the earth's crust, where minerals are formed. Oxygen and silicon alone account for more than 74% of the earth's crust. These factors place a limit on the number of possible minerals.
The minerals that have been discovered and studied can be placed into one of five groups. These groups are the silicate minerals, carbonate minerals, oxides, sulfides, and halides. The silicate minerals are those that contain silica, a combination of silicon and oxygen. Examples of silicates include quartz, orthoclase, and olivine. The silicate minerals are the most common, making up approximately one-third of all known minerals. They are composed of building blocks called the silica tetrahedron. A silica tetrahedron is one silicon atom and four oxygen atoms. The atoms are arranged in a four-faced pyramidal structure (the tetrahedron) with the silicon atom in the center. The silicon atom has a +4 charge, and each of the four oxygen atoms have a charge. As a result, a silicon tetrahedron has a net charge of . Because of this charge, it does not occur in isolation in nature. A silica tetrahedron is always bound to other atoms or molecules.
There are two types of silicate minerals, the ferromagnesian and the nonferromagnesian silicates. Ferromagnesian silicates are those containing iron, magnesium, or both. These minerals tend to be dark colored and more dense than the nonferromagnesian silicates. An example of a ferromagnesian silicate is olivine. Nonferromagnesian silicates do not have iron and magnesium. These minerals are light colored and less dense. The most common nonferromagnesian silicates are the feldspars.
The carbonate minerals contain the carbonate ion, (CO3)2/sup>. Calcite (CaCO3), the main component of limestone, is an example of a carbonate mineral. The oxides are minerals that contain an element combined with oxygen. An example of an oxide is hematite, Fe2O3. The sulfides contain a cation combined with sulfur (S2/sup>). An example of a sulfide is galena (PbS), which is lead (Pb) combined with sulfur. The halides all contain halogen elements, such as chlorine and fluorine (F). Examples of halite minerals include halite (NaCl) and fluorite (CaF2).
All minerals posses specific physical properties such as color, luster, crystal form, cleavage, fracture, hardness, and specific gravity. The physical characteristics of a mineral depend on its internal structure and chemical composition. The physical properties of minerals can be used for identification purposes by mineralogists. Color is the least reliable of the physical properties. Many minerals display a variety of colors due to impurities. Some generalizations can be made, however. Ferromagnesian silicates, for example, are usually black, brown, or dark green. The luster of a mineral refers to the way in which light is reflected off the mineral. Two types of luster can be displayed: metallic or nonmetallic.
The crystal form of a mineral is also a physical property specific for the type of mineral being observed. This property is most easily observed when the mineral has formed a perfect crystal. Cleavage is the tendency of a mineral to break or split along planes. There are different types of cleavage that correspond to the different internal crystal structures that make up individual minerals. Fracture occurs when a mineral does not break along smooth planes, rather along irregular surfaces. Some minerals display cleavage, others display fracture.
The hardness of a mineral is its resistance to being scratched. The Mohs hardness scale can be used to determine how hard a mineral is by determining what will scratch its surface. The specific gravity of a mineral is the ratio of its density to the density of water. For example, a mineral with a specific gravity of 4.0 is four times as dense as water, meaning that an certain volume of the mineral would weigh four times as much as an equal volume of water. The specific gravity of a mineral is determined by its composition and structure.
Mineralogy is an interesting science that studies the nature of mineralsaturally occurring, inorganic crystalline solids. There are many different minerals, each with its own properties determined by its chemical composition. Mineralogists continue to search for new, useful minerals in the earth's crust.
See also Chemical bonds and physical properties; Crystals and crystallography; Ferromagnetic; Mohs' scale