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The chemical formula of mineral Alunite is indicated by KAl3(SO4)2(OH)6, or Potassium Aluminum Sulfate Hydroxide. Alunite is actually a Sulfate mineral. Alunite mineral is a common source of the chemical called alum and is also known as alum stone. In a process known as alunitization, the sulfuric acids act upon the potassium rich feldspars, thus leading to the formation of the mineral Alunite. Hydrothermal solutions, usually rich in certain ore metals are accompanied by the sulfuric acids to form large quantity of Alunite, more than enough to make Alunite a rock-forming mineral. Limestone is one mineral often Alunite is mistaken of. The same is true for the massive rock forming Dolomite. But as a distinguishing factor, it is known that Alunite does not bubble even when powdered, as identified through an acid test. By alterations of the Alkali feldspar, the mineral Alunite is produced.

 

Mineral Alunite is known to crystallize in the trigonal division or hexagonal crystal system, which can be seen clearly visible when the mineral is viewed with the aid of petrographic polarizing light microscopes. In the field of optical mineralogy, 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. Alunite crystals appear to be flattened with cubic rhombohedrons resemblance when viewed under polarized light microscope used in the field of optical mineralogy. Combinations of two trigonal pyramids are known as rhombohedrons. They actually have variations of tabular to flattened rhombohedral looking crystal inclusions that are usually visible when viewed under polarizing microscope used in optical mineralogy. However, this mineral rarely forms crystals. They are more commonly formed as small and usually lining the fissures in the rock-formed mineral. They are most commonly found as earthly masses, maybe films or crusts, or sometimes botryoidal and granular formations that can be seen clearly exhibited when viewed with the aid of polarized light microscopes used in the field of optical mineralogy. Crystals are usually transparent to translucent in appearance. The extinctions shown under petrographic polarizing light microscope are commonly parallel or sometimes may appear symmetrical in most sections. Basal sections are found dark in almost all positions when the stage of polarizing light microscope for mineralogists is rotated.

 

 Alunite commonly exhibits colors under polarizing microscopes used in the field of optical mineralogy that may vary from white or gray to reddish. They are usually colorless in thin sections or grain mounts. Alunite mineral commonly exhibits vitreous to pearly luster in reflected light of petrographic polarizing microscope for mineralogists. It exhibits a fair relief as the stage of the polarizing light microscope used in the field of optical mineralogy is rotated. When the mineral is evaluated between crossed nicols of polarized light microscope, basal sections and cleavage flakes yield a uniaxial positive optic axis figure. The fracture found exhibited is commonly conchoidal to uneven when the mineral is examined under petrographic polarizing light microscopes used in optical mineralogy. The hardness measure of Alunite using the Mohs scale method is usually found ranging from 3.5 to 4 giving an average of 3.75. The specific gravity measure of mineral Alunite is approximately 2.7 grams per cubic centimeters to 2.8 grams per cubic centimeters, which is considered average for translucent minerals. Alunite has fair cleavage found in one direction that is usually visible under polarizing microscopes for mineralogists with minor adjustments on the aperture diaphragm, but only in large crystal formations.   When mineral Alunite is rubbed on a white porcelain streak plate, it will leave a white streak. It has a rather strong birefringence with interference colors ranging up to second-order blue when viewed in plane-polarized light of petrographic polarizing light microscope used in the field of optical mineralogy.

 

Alunite minerals are commonly formed at volcanic fumaroles. It occupies the pockets or seams in volcanic rocks where its formation is presumed through their chemical reaction with the escaping sulfurous vapors. They can also be found in the oxidizing and weathering zones of hydrothermal sulfide deposits.



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Saturday, November 28th, 2009 at 10:24 am
Category:
The Sulfates Mineral Class
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