The largest crystal here is ~ 1 cm in diameter, which is pretty good for sillimanite. SILLIMANITE
In hand sample, sillimanite is typically white, but can be very difficult to see owing to its fine grain size.
In thin section, sillimanite is colorless in plane light, although mats of fibrous sillimanite (fibrolite) can be brownish/tan. Under crossed polars, sillimanite has second order interference colors.
Images of sillimanite; some of these link to larger versions of the image.
The largest crystal here is ~ 1 cm in diameter, which is pretty good for sillimanite.
 Thin section, plane light: Prismatic sillimanite cross-cutting intergrown andalusite (pinkish, mostly near top of photo) and sillimanite, Hamadan, Iran. Field of view ~ 4 mm. |
 Thin section, crossed polars: Prismatic sillimanite (bright colors) in andalusite (gray/pale yellow), Hamadan, Iran. Field of view ~ 3 mm. |
 Thin section, plane light: Fibrous sillimanite within fractured andalusite in a boudinaged quartz vein, Nigde Massif, Turkey. Field of view ~ 4 mm. |
 Thin section, plane light: Randomly oriented fibrous sillimanite (tan) in garnet (lower right) + quartz + mica schist, Chebeague Island, Maine. Field of view ~ 2 mm. |
 Thin section, plane light: Almost everything in this photo is sillimanite. On the right is prismatic sillimanite (view down the c-axes of prismatic crystals); on the left is fibrous sillimanite, from a Sil-Qz vein, Hamadan, Iran. |
 Thin section, crossed polars: Same view as sample to left. Some of the brighter colors in with the fibrous sillimanite are muscovite crystals. Field of view ~ 4 mm. |
 Thin section, plane light: Mats of fibrous sillimanite in an upper staurolite facies schist, Dutchess County, NY; Whitney et al. (1998a, b). |
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 Thin section, plane light: Fibrolite (thin wispy crystals oriented E-W in photo) in quartz-feldspar-biotite-garnet schist, Nigde Massif, Turkey. Note elongate garnet near largest biotite crystal. Whitney & Dilek, 1998. |
 Thin section, plane light: Prismatic sillimanite that has completely replaced (pseudomorphed), surrounded by spinel (dark green) and cordierite (colorless) in gedrite-biotite gneiss, Thor-Odin dome, BC. Norlander et al. (2002). |
 Thin section, plane light: Staurolite crystal (yellow) containing sillimanite inclusions. Staurolite is in andalusite (pinkish) + kyanite (top right) in quartz vein, Hamadan, Iran. |
 Thin section, plane light: Fibrous sillimanite (needles) in biotite-garnet schist, Chebeaque Island, Maine. Field of view ~ 1.5 mm. |
 Thin section, plane light: Most of the Al2SiO5 here is kyanite, excelt the top right of the top kyanite grain, and part of the right side just below that. The upper edge has a rim of spinel; the colorless, low relief mineral is cordierite. At top: biotite. Thor-Odin dome, BC. |
 Thin section, plane light: Garnets in matrix of fibrous sillimanite (tan) + plagioclase, Hamadan, Iran. The garnet at right is an atoll garnet. Field of view ~ 4 mm. Sepahi et al. (2004) |
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 Thin section, crossed polars: Same sample as in row above, far right. |
