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The chemical formula of Calcite is indicated by CaCO3 or Calcium Carbonate. Calcite is a Carbonates mineral. It is commonly found having moderate negative to high positive relief and the change is marked during the rotation of the petrographic polarizing light microscope. Most of the Calcite minerals that are found forming particularly in the sedimentary environment consist of almost pure CaCO3. However, there have been some scientific studies conducted proving that under elevated temperatures calcium can be substituted by substantial amounts of Mg, Fe, Mn, or Zn. The most common Calcite mineral is the Magnesium-bearing Calcite. Calcium may also be possibly substituted by some small amounts of large cations such as Sr or Ba.


Mineral Calcite is considered most common carbonate mineral. When viewed carefully under a petrographic polarizing light microscope, Calcite commonly exhibits wonderful and splendid images. Mineral Calcite is actually considered one of the most common mineral species on Earth. About almost 4 percent by weight of the Earths crust is composed of Calcite and these are usually formed in many different geological settings.


            The name of the mineral species Calcite was derived from the Greek word chalix, which means lime. It was also relevant to the Latin word calx, which also means lime. This is allusion to the common color exhibited by the mineral. Mineral Calcite is actually a polymorph of aragonite and vaterite. Calcite is also isostructural with mineral otavite.


            Mineral Calcite can be found so varied. These is may be due to the fact that Calcite is so widely distribute and so very abundant. A thousand of different shapes can be formed literally by Calcite crystals. The crystal habit of Calcite usually consists of combinations of rhombohedrons and scalenohedrons. These can be done by combining the basic forms of the negative and positive rhombohedrons, various scalahedrons, steeply, moderately and slightly inclined rhombohedrons, pinacoid and prism, which are only a cited examples of the most common forms. Calcite has been identified having more than 300 forms, which when combined will produce a thousand of different variations of Calcite mineral crystals. In most rocks, however, calcite forms anhedral grains or aggregates of grains. Fossil shells and thin veins may be fibrous or columnar. There are several Calcite varieties that are very much known in the public. One of these is the Iceland Spar, which is considered distinctive due to its double refraction display. Mineral Calcite is sometimes mistaken as quartz but the distinction can be made through hardness test because Calcite has the hardness of 3 while quartz has 7.


            Mineral Calcite has an extremely variable color but generally it appears white or maybe colorless in hand samples. Sometimes it can be seen with light shades of orange, green, yellow, pink, black, blue, red, gray and brown that adds up to the splendid way color behaves in the sample as exhibited by the mineral when it is evaluated under polarizing light microscopes. Calcite is most commonly found colorless in thin section and grain mount. Occasionally, mineral Calcite crystals can be also found iridescent when viewed under polarized microscopes. Calcite commonly exhibits a vitreous to resinous to dull luster if in massive form or chalk variety when it is examined closely in reflected light of polarizing light microscopes for geologists. The specific gravity measure of the mineral Calcite usually gives an approximate value of 2.6 grams per cubic centimeters to 2.8 grams per cubic centimeters. This may actually go higher with substantial substitution of Fe, Mg, or Zn for Ca. The hardness measure of mineral Calcite when it is examined using the Mohs scale method is 3. When specimen of Calcite is rubbed on a white porcelain streak, it commonly leaves a white streak. Calcite commonly effervesces vigorously in cold dilute HCl.  Mineral Calcite is commonly found having perfect cleavage in one direction when it is evaluated between crossed nicols of geological polarizing light microscopes. Calcite has perfect rhombohedral cleavage in one direction and the fragments are usually found lying flat on cleavage surfaces. The angle between cleavages is about 74.5-degrees. The fracture that is found when Calcite is evaluated under several adjustments on the aperture diaphragm of the geological microscopes is usually irregular or uneven and mostly steplike.


            Calcite is commonly found having lamellar twins on the negative rhombohedron as clearly found when the sample is evaluated with the aid of the polarizing light microscope. With close observation, it can be found that the lamellae are usually parallel to one edge of the cleavage rhomb or maybe along the long diagonal of the rhomb. There can be also possible occurrence of simple and rare twins. The lamellar twinning is often the result of the deformation. It may also sometimes be used to determine the orientation of the stresses that produces the deformation. There are also some cases wherein a sample is caused to twin because of mishandling in preparing the sample in thin section or in grain mount. Twin varieties of Calcite is actually among the favorites of many mineral collectors and this mineral These many crystal varieties of mineral Calcite are the included crystals, phantoms, color varieties, unique association and pseudomorph that are commonly found exhibiting a splendid microscope images under polarizing microscopes for geologists. Calcite has been regarded having endless varieties on the face of the Earth. Each Calcite variety is commonly found exhibiting a nice, fascinating and interesting microscope appearance when viewed under polarized light microscopes.


            Mineral Calcite is known to crystallize in the trigonal division and in hexagonal system, which can be seen clearly when evaluated under geological microscopes. The hexagonal system of crystallization comprises crystals having four axes. Three of which are positioned in a single plane with equal length and are symmetrically spaced. The fourth axis is found to be perpendicular to the other three axes. Calcite mineral crystals are usually found transparent to translucent in appearance. The crystal habit of mineral Calcite commonly include the extremely variable with almost any trigonal form possible that can be found having an interesting appearance when viewed under polarizing microscopes for geologists. As mentioned, the most common form among Calcite crystals is the scalahedrons, hexagonal prisms, pinacoid and rhombohedrons. When combined with more than 300 other forms, a multitude of crystal shapes can be produced but mostly trigonal or pseudohexagonal. Most Calcite crystals can be easily cleaved to form perfect rhombohedrons. Mineral Calcite is commonly associated with several other interesting minerals such as quartz, analcime, dolomite, sulfides, mica and several other interesting ore minerals.


            Mineral Calcite has uniaxial negative figure and this can be seen more clearly exhibited under petrographic polarizing microscopes. It can be found with numerous isochromes and thin, well-defined isogyres. However, there are some Calcite samples that are biaxial negative. The extinction angle is inclined or symmetrical to cleavage traces. The direction of the fast ray vibration is parallel to the short diagonal of the rhombohedral faces. The angle between the trace of the twin lamellae and the direction of the fast ray vibration is about 55-degrees. The refractive indices of Calcite are usually found ranging from 1.640 to 1.486 but this may be marked changed to negative with stage rotation of polarized microscope. The interference colors found is usually extreme creamy high-order colors when viewed under polarizing microscopes. The maximum birefringence of mineral Calcite is commonly found ranging from 0.154 to 0.174 that are seen clearly when viewed under polarizing light microscopes for geologists. Mineral Calcite is commonly found fluorescent in ultraviolet light and usually exhibiting several light colors, which depend on local high double refraction. Mineral Calcite is not pleochroic even when evaluated between crossed nicols of geological polarizing light microscope. Mineral Calcite is not radioactive.


            Mineral Calcite may be altered to Dolomite through diagenetic processes. It may also be replaced by quartz, iron, opal, manganese oxides, and many other minerals that are considered pseudomorph of Calcite. Calcite is actually soluble in many natural water and it can be easily removed from a rock sample through solution.    Mineral Calcite is considered as a very abundant mineral species. It has been found in lava flows and more often, it is associated with hydrothermal veins. Mineral Calcite is actually considered as the principal component of marbles, limestones and several other sedimentary rocks that are considered calcareous. In limestones, Calcite can actually originate as precipitates or maybe as an accumulation of shells. Calcite is also very common in hydrothermal veins most especially as a gauge mineral. Mineral Calcite is found mostly in most geologic settings. It can be also found as a replacement mineral in most other environments in one from or another. Mineral Calcite commonly forms a chemical deposits as limestone and it can be also found as regionally or contact metamorphosed into marbles. Mineral Calcite is rarely found as carbonatites as igneous rocks. Calcite is also considered as a typical sedimentary mineral that formed by precipitation through evaporation. Under high carbon dioxide pressure, Calcite is commonly found stable through most phases of metamorphosis. Mineral Calcite can actually dissociate into a variety of complex calcium silicates when the pressure is reduced. The best field indicators of mineral Calcite usually include cleavage, crystal habit, hardness, abundance, reaction to acid and double refraction. Mineral Calcite notably occur at some famous mineral localities like some areas in the United States such as Franklin, New Jersey, Missouri, Kansas, Wisconsin and the Oklahoma localities, Elmwood, Tennessee, Brush Creek, Pugh Quarry, Ohio, Rosiclaire, Illinois, Andreasburg, Harz Mountain and Saxony, Germany, Guanajuato, Mexico, Brazil, Cornwall, Lancashire, Durham, England, Bombay area in India, many African localities, Eskifjord, Iceland, as well as some other localities around the world with their own unique varieties.

Monday, March 24th, 2008 at 4:29 am
The Carbonates and Borates Mineral Class
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