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The chemical formula of mineral Autunite is indicated by Ca(UO2)2(PO4)2-10H2O or Hydrated Calcium Uranyl Phosphate. Autunite is actually a Phosphate mineral. Autunite was first discovered in the year 1852 near Autun, France. Autunite was named after the locality Autun, Saone-et-Loire, France. Autunite has alternate names like Lime Uranite and Calcium Autunite. Autunite mineral has high uranium content causing its radioactivity property and thus making it as one primary ore of Uranium. Autunite is known to crystallize in tetragonal system of crystal formation. In optical mineralogy, this tetragonal system comprises crystals having three axes, which are all in a position perpendicular to one another. Two axes are usually found having the same or equal length. Autunite mineral is commonly used as a mineral specimen and is considered as a minor ore of uranium. It is considered as one of the most attractive and popular radioactive mineral. Radioactivity of the Autunite mineral is due to the traces of uranium present within, which is commonly found constituting its chemical structure and formula. For several serious mineral collectors, Autunite is probably considered as the most popular uranium mineral.

 

Autunite mineral chemical structure is composed of phosphate tetrahedrons that are commonly linked to uranium-oxygen groups forming a distorted octahedral, which can be noticed more clearly when specimen is viewed with the aid of petrographic polarizing microscope used in optical mineralogy. Autunite can be found exhibiting a structure wherein the phosphates and uranium groups lie in sheets that are usually weakly held together by molecules of water. Due to this structure, a tabular habit of Autunite crystals is formed with one perfect cleavage in one direction that is usually found more clearly visible when the mineral is evaluated in plane light of polarizing light microscopes used in optical mineralogy. It is also because of this structure why Autunite mineral is relatively soft with insignificant hardness.

 

            When Autunite mineral loss water in its chemical structure, it readily converts to a different kind of mineral called meta-autunite-I, a considered pseudomorph of mineral Autunite. When this type of mineral is heated, it can actually produce a meta-autunite-II mineral. These two different types of minerals are not found in appreciable abundance in nature. It has been chemically evaluated that the conversion process is actually irreversible and ongoing. While the dehydration process can be reversed, rehydrating back to Autunite, the fragments actually do not fuse back into a single crystal. When rehydration process is made, the Autunite mineral becomes a pseudomorph of itself as an effect. With the x-ray diffraction test, the pattern of rehydrated Autunite will appear very much alike to that of the original Autunite, but will somewhat be blurred because of the slight misalignment of the crystal fragments. Rehydration of meta-autunite will actually result for the pushing apart of the re-expanding crystals and the break up of the pseudomorph crystals if the process is not done carefully and slowly. There are collected specimens, which are of certain age that are found partially converted. The meta-autunite powder will eventually powder after many years, thus the specimen will be ruined. It is actually recommended that fine specimens of Autunite mineral should be stored in a closed container in order to avoid water loss. There have been several drastic measures attempted in order to thwart the conversion of fine museum quality Autunite specimens to meta-autunite, and this includes lacquering. Since Autunite mineral is known to be radioactive, it should be stored away from other minerals that may be prone to be affected by radioactivity. Human exposure should be limited. Unless sealed in glass, all specimens should be considered to have converted to meta-autunite within a few months of being exposed to ambient air.

 

            Meta-autunite minerals are commonly associated Autunite in fractures in uraniferous igneous rocks. This mineral species can be found at Daybreak Mine in Mt. Spokane, Washington, USA but not really in an appreciable abundance. It was named as the lower hydrate of mineral Autunite. Specimens are usually found in shades of yellow, greenish yellow, or sometimes yellowish green colors when viewed in plane light of petrographic polarizing microscope for mineralogists. The specific gravity of this type is usually ranging from 3.45 grams per cubic centimeters to 3.55 grams per cubic centimeters, which gives an average of 3.5 grams per cubic centimeters. Its crystals are commonly translucent to opaque in appearance. The hardness measure of this meta-autunite mineral is only 1 when evaluated using the Mohs scale method. Meta-autunite commonly exhibits pearly luster in reflected light of polarized microscope. This meta-autunite is commonly found having uniaxial negative figure when evaluated between crossed nicols of polarized light microscopes. This is also a radioactive mineral. 

 

            Autunite mineral specimens on the other hand, commonly occur in various shades of dark green to bright lemon yellow that could appear more fascinating when viewed in transmitted light of petrographic polarizing microscopes used in optical mineralogy. In one Autunite mineral specimen, a mixture of yellow and green colors is often found. The luster exhibited may vary from being vitreous to pearly on the main pinacoid face when specimen is viewed in reflected light of polarized microscope used in optical mineralogy. Luster always glistens on cleavages as exhibited clearly when specimen is viewed under polarizing light microscopes for mineralogists. Autunite is commonly found showing cleavages in three directions when it is viewed in plane light of polarized microscope used in optical mineralogy. Autunite has perfect cleavage found in one direction and poor cleavage in two other directions, all of which can be seen more clearly exhibited when specimen is viewed with the aid of polarizing microscope for mineralogists. The specific gravity measure of Autunite mineral specimen gives an approximate value ranging from 3.1 grams per cubic centimeters to 3.2 grams per cubic centimeters, which is considered slightly above average for translucent minerals. The hardness measure of Autunite when it is evaluated using the Mohs scale method is usually found ranging from 2.0 to 2.5, which is commonly considered soft and not suitable for gemstone purposes. Mineral collectors usually consider this softness of Autunite as fragile and insignificant. Autunite is also most commonly found exhibiting uneven fracture when evaluated with the aid of a petrographic polarizing light microscope used in optical mineralogy. Uneven and flat surfaces are found, which are fractured in uneven in an uneven pattern. Fracture describes how a mineral breaks when broken contrary to its natural cleavage planes. Autunite is most commonly found leaving a pale yellow and sometimes yellow streak when mineral specimen is rubbed on a white porcelain streak plate. Specimens of mineral Autunite that are exhibiting shades of yellow and green colors are commonly found splendidly glowing when viewed in plane light of petrographic polarizing microscope for mineralogists. These varieties of Autunite crystals are actually known as fluorescent mineral specimens in both long wave and short wave ultraviolet light. Its crystal aggregates are very distinctive and commonly look like inflated mica books.

 

            Autunite crystals may be found varying from being translucent to being opaque in appearance. Crystals of Autunite may appear transparent if fresh and usually become translucent when dry. The crystal habit of mineral Autunite as described in optical mineralogy commonly includes tabular square crystals, which are usually found dominated by two pinacoid faces and are most commonly clearly found when viewed with the aid of polarized light microscopes for mineralogists. Tabular crystal formations usually form dimensions that are thin in one direction. Crystals may often appear in parallel growths that often exhibits a fanned-out look or in rosetta clusters, which can be more splendidly exhibited when viewed in transmitted light of petrographic polarizing microscope used in optical mineralogy. Other habits exhibited may include foliated, where crystals display two-dimensional platy forms, and scaly aggregates, which can be splendidly exhibited under petrographic polarizing light microscope used in optical mineralogy. Autunite can be also found in crusts or in earthy masses formations. Thin crystals of Autunite mineral are commonly bendable. The same is true with the cleavage sheets of the Autunite mineral specimens. Crystals may appear in square plates and also in scattered thin flakes and solid micaceous crusts, with crystal standing on the edge. Micaceous formation actually exhibits crystals having platy texture with flexible plates. It can be noticed that the swelling sheaves on edge in crusts tend to exfoliate as desiccation proceeds.

 

            Autunite is most commonly found showing a biaxial negative figure when evaluated between crossed nicols of petrographic polarizing light microscope used in optical mineralogy. Autunite specimens are pleochroic minerals when viewed in plane-polarized light of polarizing microscope used in optical mineralogy. Autunite is also associated with fluorite and quartz. Autunite is also known as a non-magnetic mineral. Crystals of Autunite are probably found metamic. Autunite has no dispersion found when viewed in plane light of polarizing microscopes used in optical mineralogy. Autunite is also found showing low surface relief when it is closely evaluated under several adjustments on the aperture diaphragm of petrographic polarizing microscope used in optical mineralogy. Its fluorescence property and its square plates usually distinguished Autunite form all other minerals. There can be possible traces of calcium found by obtaining the calcium sulfate precipitate, which is produced when sulfuric acid is added to the nitric acid solution. Autunite mineral specimens are commonly not pleochroic even between crossed nicols of polarized light microscopes.

 

Autunite is most commonly found associated with minerals like torbernite, uraninite, uranocircite, meta-torbernite, uranophane, and other uranium minerals. Autunite is commonly found along with these minerals. The best field indicators of Autunite minerals include color, fluorescence, crystal habit, flexible crystals, radioactivity, and its awesome associations. Autunite minerals are considered as secondary mineral in weathered zones of uraninite and metal-ore deposits. They can be also found as scales on joints and seams in pegmatites and in granite quarries. Autunite minerals are commonly found in pegmatites and hypothermal veins. They are usually considered as secondary uranium mineral and as alteration product of uraninite. Autunite mineral specimens can be also found as crusts in hydrothermal veins and pegmatites. There are especially rich masses of Autunite that have been found at Spruce Pine in North Carolina. The rich greenish yellow shades of Autunite in crusts forms and some in square crystals were found in France. It has notable occurrences in types of localities that include Zaire, Africa; Portugal; Bergen; Katanga, Congo (Kinshasa); Germany; Autun, France; Cornwall, England; and some areas in the United States such as the Mitchell Co., North Carolina and Mt. Spokane, Washington.



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Monday, February 16th, 2009 at 2:41 am
Category:
The Phosphates Mineral Class
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