Andalusite Al2SiO5

Optical Properties:

Biaxial -; very high 2V; usually colorless, may be red under crossed polarizers; first order gray interference colors; typically euhedral (idioblastic) crystals; 2 cleavages; parallel or symmetrical extinction. Carbonaceous inclusions may be arranged symmetrically to form a variety called chiastolite.

The photograph shows a porphyroblast of andalusite, with faint dark cross of carbonaceous specks, in a fine-grained hornfels.

Occurrence:

Common mineral in contact metamorphosed shales; also occurs in regional, high temperature, low pressure metamorphic rocks and in granite pegmatites.

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