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Minerals
Mineral Definition and Classification Differences Between Minerals and Rocks
Mineral Composition of Rocks Physical Properties of Minerals Chemical Classes of MineralsA mineral is a naturally occurring substance formed through geological processes that has a characteristic chemical composition, an ordered atomic structure and specific physical properties. A rock, by comparison, is an aggregate of minerals and need not have a specific chemical composition. Minerals range in composition from pure elements to very complex silicates with thousands of known forms. The study of minerals is called mineralogy.
Mineral Definition and Classification
To be classified as a true mineral, a substance must be a solid and have a crystalline structure. It must also be a naturally occurring, homogeneous substance with a defined chemical composition
A crystal structure is the orderly geometric spatial arrangement of atoms in the internal structure of a mineral. Minerals fall into one of the six basic crystal systems. This crystal structure is based on regular internal atomic arrangement that is often expressed in the geometric form that the crystal takes.
Chemistry and crystal structure together define a mineral. In fact, two or more minerals may have the same chemical composition, but differ in crystal structure (these are known as polymorphs). For example, Pyrite and Marcasite are both iron sulfide, but their arrangement of atoms differs. Similarly, some minerals have different chemical compositions, but the same crystal structure: for example, Halite (made from sodium and chlorine), Galena (made from lead and sulfur) both have the same cubic crystal structure.
Crystal structure greatly influences a mineral's physical properties. For example, though Diamond and Graphite have the same composition (both are pure carbon), graphite is very soft, while diamond is the hardest of all known minerals. This happens because the carbon atoms in graphite are arranged into sheets which can slide easily past each other, while the carbon atoms in diamond form a strong, interlocking three-dimensional network.
There are currently more than 4,000 known minerals. Of these, perhaps 100 can be called common and the rest are "rare" to "extremely rare."
Differences Between Minerals and Rocks
A mineral is a naturally occurring, inorganic solid with a definite chemical composition and a specific crystalline structure. A rock is an aggregate of one or more minerals. Some rocks are predominantly composed of just one mineral. For example, limestone is a sedimentary rock composed almost entirely of the mineral calcite. Other rocks contain many minerals and the specific minerals in a rock can vary widely. Some minerals, like quartz, mica or feldspar are common, while others have been found in only one or two locations worldwide. The vast majority of the rocks of the Earth's crust consist of quartz, feldspar, mica, chlorite, kaolin, calcite, epidote, olivine, augite, hornblende, magnetite, hematite and limonite.
Commercially valuable minerals and rocks are referred to as industrial minerals. Rocks from which minerals are mined for economic purposes are referred to as ores.
A main determining factor in the formation of minerals in a rock is the chemical composition of the rock, for a certain mineral can be formed only when the necessary elements are present in the rock. Calcite is most common in limestone, as these consist essentially of calcium carbonate; quartz is common in sandstones and in certain igneous rocks which contain a high percentage of silica.
Other factors are of equal importance in determining the natural of rock-forming minerals, principally the mode of origin of the rock and the stages through which it has passed in attaining its present condition. Two rock masses may have very much the same bulk composition and yet consist of entirely different assemblages of minerals. The tendency is always for those compounds to be formed which are stable under the conditions under which the rock originated. Granite is created by the consolidation of molten magma at high temperatures and great pressures and its component minerals are those stable under such conditions. Exposed to moisture, carbonic acid and other agents at the ordinary temperatures of the Earth's surface, some of these original minerals, such as quartz and white mica are relatively stable and remain unaffected; others weather or decay and are replaced by new combinations. These changes are accompanied by disintegration, and the rock falls into a loose, incoherent, earthy mass which may be regarded as a sand or soil. The materials thus formed may be washed away and deposited as sandstone or siltstone. The structure of the original rock is now replaced by a new one; the mineralogical constitution is profoundly altered; but the bulk chemical composition may not be very different. The sedimentary rock may again undergo metamorphism being recrystallized and converted into a rock not very different in mineral composition but radically different in structure to the granite which was its original state.
Physical Properties of Minerals
Classifying minerals can range from simple to very difficult. A mineral can be identified by several physical properties, some of them being sufficient for full identification. In other cases, minerals can only be classified by more complex chemical or X-ray analysis.
The physical properties commonly used in identifying minerals are:
Crystal structure and habit : A mineral may show good crystal habit or form, or it may be massive, granular or compact with only microscopically visible crystals.
Hardness: The physical hardness of a mineral is usually measured according to the Mohs scale. This scale is relative and goes from 1 to 10. Minerals with a given Mohs hardness can scratch the surface of any mineral that has a lower hardness than itself.
Mohs Hardness Scale:
. Talc Mg3Si4O10(OH)2
. Gypsum CaSO42H2O
. Calcite CaCO3
. Fluorite CaF2
. Apatite Ca5(PO4)3(OH,Cl,F)
. Orthoclase KAlSi3O8
. Quartz SiO2t
. Topaz Al2SiO4(OH,F)2
. Corundum Al2O3
. Diamond C (pure carbon)Lustre indicates the way a mineral's surface interacts with light and can range from dull to glassy (vitreous).
Color indicates the appearance of the mineral in reflected light or transmitted light for translucent minerals (i.e. what it looks like to the naked eye).
Streak refers to the color of the powder a mineral leaves after rubbing it on an unglazed porcelain streak plate. Note that this is not always the same color as the original mineral.
Cleavage describes the way a mineral may split apart along various planes. In thin sections, cleavage is visible as thin parallel lines across a mineral.
Fracture describes how a mineral breaks when broken contrary to its natural cleavage planes.
Specific gravity relates the mineral mass to the mass of an equal volume of water, namely the density of the material. While most minerals, including all the common rock-forming minerals, have a specific gravity of 2.5 - 3.5, a few are noticeably more or less dense, e.g. several sulfide minerals have high specific gravity compared to the common rock-forming minerals.
Other properties: fluorescence (response to ultraviolet light), magnetism, radioactivity, tenacity, piezoelectricity and reactivity to dilute acids.
Minerals may be classified according to chemical composition. The list below is in approximate order of their abundance in the Earth's crust.
Silicate class Carbonate class Sulfate class Halide class
Oxide class Sulfide class Phosphate class Element class
Silicate Class
The largest group of minerals by far are the silicates (most rocks are 95% silicates), which are composed largely of silicon and oxygen, with the addition of ions such as aluminium, magnesium, iron and calcium. Some important rock-forming silicates include the feldspars, quartz, olivines, pyroxenes, amphiboles, garnets, and micas.
Carbonate Class
The carbonate minerals include calcite and aragonite (both calcium carbonate), dolomite (magnesium/calcium carbonate) and siderite (iron carbonate). Carbonates are commonly deposited in marine settings when the shells of dead planktonic life settle and accumulate on the sea floor. Carbonates are also found in evaporitic settings (e.g. the Great Salt Lake, Utah) and also in karst regions, where the dissolution and reprecipitation of carbonates leads to the formation of caves, stalactites and stalagmites. The carbonate class also includes the nitrate and borate minerals.
Sulfate Class
Sulfates commonly form in evaporitic settings where highly saline waters slowly evaporate, allowing the formation of both sulfates and halides at the water-sediment interface. Sulfates also occur in hydrothermal vein systems as gangue minerals along with sulfide ore minerals. Another occurrence is as secondary oxidation products of original sulfide minerals. Common sulfates include anhydrite (calcium sulfate), celestite (strontium sulfate), barite (barium sulfate), and gypsum (hydrated calcium sulfate). The sulfate class also includes the chromate, molybdate, selenate and sulfite minerals.
Halide ClassThe halides are the group of minerals forming the natural salts and include fluorite (calcium fluoride), halite (sodium chloride) and sylvite (potassium chloride). Halides, like sulfates, are commonly found in evaporitic settings such as playa lakes and landlocked seas such as the Dead Sea and Great Salt Lake. The halide class includes the fluoride, chloride, bromide and iodide minerals.
Oxide Class
Oxides are extremely important in mining as they form many of the ores from which valuable metals can be extracted. They also carry the best record of changes in the Earth's magnetic field. They commonly occur as precipitates close to the Earth's surface, oxidation products of other minerals in the near surface weathering zone, and as accessory minerals in igneous rocks of the crust and mantle. Common oxides include hematite (iron oxide), magnetite (iron oxide), chromite (iron chromium oxide), spinel (magnesium aluminium oxide - a common component of the mantle), rutile (titanium dioxide), and ice (hydrogen oxide). The oxide class includes the oxide and the hydroxide minerals.
Sulfide Class
Many sulfide minerals are economically important as metal ores. Common sulfides include pyrite (iron sulfide - commonly known as fools' gold), chalcopyrite (copper iron sulfide) and galena (lead sulfide). The sulfide class also includes the selenides, the tellurides, the arsenides, the antimonides, the bismuthinides, and the sulfosalts (sulfur and a second anion such as arsenic).
Phosphate Class
The phosphate class includes the phosphate, arsenate, vanadate, and antimonate minerals. By far the most common phosphate is apatite which is an important biological mineral found in teeth and bones of many animals.
Element Class
The elemental group includes metals and semi-metals, and non-metals (gold, silver, copper, antimony, bismuth, graphite, sulfur). This group also includes natural alloys.References: Excerpts and general information were borrowed from Wikipedia and Encarta, as well as other sources.
Mineral Definition and Classification Differences Between Minerals and Rocks
Mineral Composition of Rocks Physical Properties of Minerals Chemical Classes of Minerals
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