Sillimanite Al2SiO5

Optical Properties:

Biaxial +; small 2V (20-30 degrees); slender, needle-like habit; high relief; up to middle second order interference colors; parallel extinction.

The photos above show the needle-like form and birefringence of sillimanite (crossed polarizers on left, plane light on right).

The lower pictures (of two different sillimanite schists) show somewhat finer- grained needles of sillimanite (crossed polarizers on left, plane light on right).

Occurrence:

Occurs principally in medium to high grade regional and contact metamorphosed shales.

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