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Mineral Alunite has the formula KAl3(SO4)2(OH)6 or Potassium Aluminum Sulfate Hydroxide. Alunite is one of the Sulfate minerals is also known as alum stone. This Sulfate mineral Alunite is a common source of the alum chemical. This mineral is most commonly formed in a process known as alunitization. In this process, the sulfuric acids act upon the potassium rich feldspars, thus leading to the formation of the mineral Alunite. Hydrothermal solutions that are usually rich in certain ore metals are most commonly accompanied by the sulfuric acids to form large quantity of mineral Alunite. This is more than enough to make Alunite as a rock-forming mineral. Most mineral Alunite contains significant amount of sodium and there are scientific researches about the formation of extensive solid solution between mineral Alunite and natroalunite whoe chemical formula is indicated by NaAl3(SO4)2(OH)6.

 

Alunite has moderate positive relief found when it is evaluated plane light of petrographic polarizing light microscope. 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 2.6 grams per cubic centimeters to 2.9 grams per cubic centimeters, which is considered average for translucent minerals. Alunite is usually found white or grayish in hand sample. It is usually colorless in thin sections or grain mounts. Alunite may also appear yellowish, reddish, or brownish. Mineral Alunite commonly exhibits vitreous to pearly luster in reflected light of petrographic polarizing microscope.

 

Rare crystals of Alunite are usually tabular parallel to plane {0001} or sometimes rhombohedral yielding square or diamond-shaped cross sections. The angle between the faces is almost 90.8-degree. Thus, the rhombohedrons are cubelike. Mineral Alunite is most commonly found as granular, plumose, or flaky aggregates and as disseminated grains. The size of the grains is often too fine to obtain reliable optical data. The mineral has basal cleavage found on plane {0001} and it is usually distinct. The basal cleavage usually controls the grain orientation in grain mounts to some degree. There is also another rhombohedral cleavage found on {1012}. There is no twinning reported.

 

The fracture found exhibited is commonly conchoidal to uneven when the mineral is examined under petrographic polarizing light microscopes. When mineral Alunite is rubbed on a white porcelain streak plate, it will leave a white streak. The extinction of mineral Alunite is parallel to the trace of the basal cleavage {0001} in all sections and the fast ray vibrates parallel to the cleavage. The fast ray is identified as length fast. It has a rather strong birefringence with interference colors ranging up to second-order blue when viewed petrographic polarizing light microscope. But there are some sample with lower birefringence producing only first-order colors. The indices said to vary irregularly with composition. The index of the ordinary ray of relatively pure Alunite is 1.572, while the index of its extraordinary ray is 1.592. Alunite with some iron components is usually having higher indices. Natroalunite has the same range of indices as Alunite. In grain mount, it is quite difficult to measure the index of extraordinary ray. This is because cleavage strongly influences grain orientations. Basal sections and cleavage flakes of mineral Alunite yield a uniaxial positive optic axis figure when evaluated between crossed nicols of the polarizing light microscopes.

 

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. Alunite is often difficult to identify because it is most commonly found as fine-grained material. X-ray diffraction and other techniques are usually used in this type of mineral identification process. Alunite is relatively found as a common mineral from altered volcanic rocks having intermediate and felsic composition. The mineral is usually produced through the action of fluids that are sulfur-bearing found in feldspars. They can also be found in the oxidizing and weathering zones of hydrothermal sulfide deposits. Alunite is commonly associated with clay minerals and other feldspar alteration products. It is also associated with the fine-grained quartz.



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Monday, March 24th, 2008 at 4:26 am
Category:
The Sulfates Mineral Class
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