Cordierite

Category  :  Mineral
Hardness  :  7 - 7.5
Gravity  :  2.57 - 2.66
Reflection  :  1.54 / 1.53
Color  :  Blue, violet, yellow-brown; transparent to translucent
Chemical formula  :  (Mg,Fe)2Al4Si5O18
Crystal system  :  Orthorhombic
Major varieties  :  Cordierite (Or Water Sapphire)

Cordierite typically occurs in contact or regional metamorphism of argillaceous rocks. It is especially common in hornfels produced by contact metamorphism of pelitic rocks. Two common metamorphic mineral assemblages include sillimanite-cordierite-spinel and cordierite-spinel-plagioclase-orthopyroxene. Other associated minerals include garnet (cordierite-garnet-sillimanite gneisses) and anthophyllite.[3][4] Cordierite also occurs in some granites, pegmatites, and norites in gabbroic magmas. Alteration products include mica, chlorite, and talc. Cordierite occurs in the granite contact zone at Geevor Tin Mine in Cornwall.

Catalytic converters are commonly made from ceramics containing a large proportion of cordierite. The manufacturing process deliberately aligns the cordierite crystals to make use of the very low thermal expansion seen for one axis. This prevents thermal shock cracking from taking place when the catalytic converter is used.

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