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andalusite
Andalusite is an aluminium nesosilicate mineral with the chemical formula Al2SiO5. Andalusite is trimorphic with kyanite and sillimanite, being the lower pressure mid temperature polymorph. At higher temperatures and pressures, andalusite may convert to sillimanite. Thus, as with its other polymorphs, andalusite is an aluminosilicate index mineral, providing clues to depth and pressures involved in producing the host rock.
==Varieties==
The variety chiastolite commonly contains dark inclusions of carbon or clay which form a cruciform pattern when shown in cross-section.
A clear variety first found in Andalusia, Spain can be cut into a gemstone.〔(''International Colored Gem Association: Anadalusite'' )〕 Faceted andalusite stones give a play of red, green, and yellow colors that resembles a muted form of iridescence, although the colors are actually the result of unusually strong pleochroism.
It is associated with mica schist which increases alkali content in ultimate product and so it has not been exploited economically so far.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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