Cordierite

Hand Specimen identification: Prismatic cleavage, color, and association aid identi-fication, but if it is not blue, this mineral is often dif-ficult to distinguish without a microscope. It may be easily confused with quartz in hand specimen and with plagioclase in thin section.

Chemical Composition : (Mg,Fe)2Al4Si5O18

Hardness : 7 to 71 2

Specific Gravity2.5 to 2.8

Cleavage/Fracture : fair prismatic (010), poor (100)/conchoidal

Luster/Transparency : vitreous/transparent to translucent

Color : indigo to gray-blue

Streak : white

Optical Properties : In thin section, cordierite may be clear or pale blue-violet. Lamellar twinning is common. Pleochroic halos around zircon inclusions are diagnostic. Cordierite often alters to a fine-grained mass called pinnite. It may be confused with quartz, feldspar, or nepheline. Biaxial 1+ or – 2, a = 1.54, b = 1.55, g = 1.56, d = 0.02, 2V = 65° to 105°.

Crystallography : Orthorhombic, a = 17.13, b = 9.80, c = 9.35, Z = 4; space group C2/c2/c2/m; point group 2/m2/m2/m.

Habit : Rare, euhedral crystals are short and prismatic and may appear pseudohexagonal. Twins are common. Cordierite is more typically granular, massive, or compact.

Structure and composition : Cordierite, like beryl, consists of 6-membered tetrahe-dral rings joined in a three-dimensional framework. Mg2+, Fe2+, and Al3+ link the rings to each other. Hollow channels, sometimes occupied by H2O or al-kalis, run parallel to the c-axis. Fe:Mg ratio is variable.

Occurrence and associations : Cordierite is found as a product of contact or re-gional metamorphism in high-grade metamorphosed aluminous rocks. Rare occurrences in igneous rocks have been reported. Associated minerals include gar-net, sillimanite, spinel, plagioclase, anthophyllite, and orthopyroxene.

Varieties : A high-temperature polymorph of cordierite, indi-alite, is isostructural with beryl.

Related Minerals : Cordierite is structurally similar to beryl, and is chemically and structurally similar to osumilite, 1K,Na21Fe,Mg221Al,Fe231Si,Al212O30 . H2O.

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