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The chemical composition of mineral Brinrobertsite is indicated by the formula (Na,K,Ca)x(Al,Fe,Mg)4(Si,Al)8O20(OH)4•3.54(H2O) [x = 0.35, n = 3.54]. Brinrobertsite is commonly found in an Ordovician soapy mudtone (metabentonite). This mineral was altered from ash (glass) with pyrophyllite as the precursor mineral. Brinrobertsite is commonly found at its type of locality at Bangor, North Wales, England, Great Britain. Brinrobertsite was named for Brin Roberts, specialist for British clay minerals in the University of London. Mineral Brinrobertsite is known to crystallize in the monoclinic system of crystal formation. In optical mineralogy, the monoclinic system of crystal formation comprises crystals having three axes of unequal lengths. Two of which are usually found in a position that is oblique or not perpendicular to one another. However, both of which are commonly found perpendicular to the third axis. Brinrobertsite is also commonly showing perfect cleavage when viewed between crossed nicols of polarizing microscopes. Mineral Brinrobertsite is usually found colored gray, or yellowish gray in hand sample. Most crystals of mineral Brinrobertsite are usually found transparent in appearance.the crystal habit of mineral Brinrobertsite is usually found forming anhedral grains with granular minerals without the expression of crystal shapes. It is also sometimes found earthy to dull structure which shows clay-like texture with no visible crystalline affinities, like that of howlite. Sometimes it can be also found with microscopic crystals where crystals are visible only when viewed with the aid microscopes. Mineral Brinrobertsite is also commonly found with earthy to dull luster when viewed in reflected light of petrographic polarizing microscopes used in the field of optical mineralogy. This mineral is commonly found leeaving white streak when rubbed on white porcelain streak plate. Brinrobertsite is also commonly found showing biaxial figure when evaluated between crossed nicols of polarizing microscopes used in the field of optical mineralogy. Brinrobertsite is found to be not radioactive after chemical evaluation.



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Tuesday, March 29th, 2011 at 5:52 am
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Optical Mineralogy
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