Minerals
Minerals are the basic materials which make up the earth's crust. Most rocks are combinations of minerals. A mineral can be either a chemical element or a chemical compound and is defined as a natural, inorganic, crystalline solid.
Each mineral has its own characteristic chemical composition and can be identified by its properties. These properties, which include hardness, crystalline structure, luster, and color, are a result of the way the atoms in a particular mineral are held together.
Over 3,000 different minerals are known to exist on earth. Of these, only about 20 are considered common. These are the "rock-forming" minerals. Ten of these minerals alone make up 90% of the mass of the earth's crust. These ten minerals are quartz, orthoclase, plagioclase, muscovite, biotite, calcite, dolomite, halite, gypsum, and the ferromagnesian minerals. Minerals that are not rock-forming are known as "accessory" minerals.
Minerals can be divided into two main groups based on their chemical composition: silicate and non-silicate. The silicates contain the elements silicon (Si) and oxygen (O). These minerals constitute 96% of the earth's crust. Quartz is a silicate which contains only silicon and oxygen. Other silicates, such as feldspars, contain metals which combine with silicon and oxygen. Quartz and the feldspars alone make up over 50% of the earth's crust. The silicates also occur in a very specific crystal structure known as the silicon-oxygen tetrahedron. This is a four-sided structure which occurs when four oxygen atoms combine in a pyramid with one silicon atom in the center. These tetrahedra can then combine with each other to form a single chain (as in pyroxene), a double chain (like the amphibole minerals), or sheets (such as is found in the micas).
The non-silicate minerals do not contain the element silicon. These minerals can be subdivided into six groups; the carbonates, halides, native elements, oxides, sulfates, and sulfides. The carbonates contain a CO3 group. The halides contain either the element chlorine (Cl) or fluorine (F) in combination with sodium (Na), potassium (K), or calcium (Ca). The native elements are pure elements which occur in nature, for example silver (Ag) or copper (Cu). The oxides contain oxygen in combination with any other element except silicon. The sulfates contain a SO4 group, and the sulfides contain the element sulfur (S).
With so many different minerals one would think that identification would be problematic. Mineralogists, scientists who study minerals, have devised seven simple tests which help identify many minerals. These tests are color, luster, streak, cleavage and fracture, hardness, crystal shape, and density. Some minerals also have special properties which can be used for definitive identification.
The color of a mineral can be used for identification. While color alone is not conclusive evidence, it can be used in combination with the other tests to determine the identity of a mineral. Many minerals have a very specific characteristic color. Examples include: sulfur, which is yellow; or cinnabar, which is red.
The way in which light is reflected from the surface of a mineral determines its luster. Luster can be described as metallic or nonmetallic. If a mineral has a metallic luster, it reflects light much like polished metal. If a mineral does not have a metallic luster, it is said to be nonmetallic and can be described as glassy, waxy, pearly, brilliant, or dull.
The streak test for mineral identification involves rubbing the mineral on an unglazed ceramic tile (called a streak plate). The color of the mineral in powdered form is the color of the streak. The streak color may be different from the color of the mineral itself.
The way the atoms in a mineral combine to form its crystalline structure determines how it will appear when broken. Cleavage refers to a mineral's ability to split easily along certain flat surfaces, or planes. If a mineral fractures when broken, it breaks unevenly. Uneven fractures can be either fibrous (resembling strands of fibers) or conchoidal (resembling chipped glass).
Another useful test is the hardness test. Hardness refers to the ability of a mineral to resist scratches and is determined by the strength of the bonds in its atomic structure. The test is performed by scraping a mineral with different materials and noting which will produce a scratch on the mineral. The mineral is then compared with a scale developed in 1822 by a German mineralogist named Friedrich Mohs, appropriately named the "Mohs hardness scale." This rates a mineral's hardness on a scale from one to ten, with ten being the hardest. Examples of materials on the Mohs scale include talc with a hardness of 1, a fingernail with a hardness of 2.5, a copper penny with a hardness of 3, glass with a hardness of 6, steel with a hardness of 6.5, quartz with a hardness of 7, and diamond with a hardness of 10. If a mineral can be scratched by a material found on the scale, its hardness must be less than that of the scratching material. If a mineral can scratch a material found on the scale, it must be harder than that material.
The last two tests which can be used to identify a mineral are crystal shape and density. A mineral is formed in a specific crystalline shape which is determined by how the atoms in a mineral combine. A mineral's density, or its mass divided by its volume, is determined by the kinds of atoms it has as well as how closely these atoms are packed together. These tests, combined with those described above, can be used to identify an unknown mineral sample.
Some minerals have special properties besides those described above. For example, a mineral might be magnetic. Some minerals display fluorescence or phosphorescence. Some are radioactive. Others may display double refraction, the bending of light shining through a crystal to form a double image. These specific characteristics can be used to further identify a mineral.
Minerals are important natural resources as well as items of beauty. Steel is made from the iron (Fe) mined from iron-rich minerals. Glass is made from silicon which is found in many minerals. Other metals that come from mineral ores include titanium (Ti) and molybdenum (Mo). Diamond is used for cutting and grinding instruments. Many technological advances have been made as a result of scientific study of the properties and characteristics of the minerals found in the earth's crust.
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