Occurrence and Compositon
Sillimanite is a metamorphic mineral found in high grade aluminous schists and gneisses. It is a polymorph of andalusite and kyanite, all having formula Al2SiO5.

Keys to identification are high relief, needle-like, fibrous or bladed habit, characteristic square cross sections with one diagonal cleavage. It is clear and shows upper 2nd order interference colors.

Important properties
 ·Appearance and habit - Classic sillimanite forms needles with square cross sections that show one diagonal cleavage. It may also appear as blades or as fine fibrous mats (called fibrolite).
 ·Color and relief - clear with high relief
 ·Interference colors - upper second order

Similar minerals:
 ·Similar to andalusite but sillimanite is length slow and has greater birefringence and smaller 2V
 ·Kyanite has greater relief and greater 2V.
 ·Zoisite has gerater relief and lower birefringence.

Sillimanite, Quartz and Biotite in a Pelitic Gneiss

This sample contains needles of sillimanite (high relief and somewhat fractured), quartz, biotite and magnetite. The sillimanite and quartz are colorless, the biotite is brown, and the magnetite is opaque (PP). In the XP view, the quartz shows low order white-gray interference colors, while the biotite and sillimanite higher order interference colors. The largest needle of sillimanite is about 2 mm long and shows upper second order purple-red interference colors. Note the high relief of the sillimanite compared to quartz.

This sample comes from near Kazabazua, Quebec.

Staurolite with Fibrous Sillimanite (Fibrolite)

This sample, from near Poughkeepsie, New York, contains a large yellow mass of staurolite on the left hand side (PP). The material to the right of the staurolite is mostly a mix of granular quartz and fine fibrolite (fibrous sillimanite, often forming mats or patches). Some of the sillimanite shows upper second order interference colors but most is so fine grained that interference colors are hard to see. Minor muscovite is also present.

The field of view is about 3.5 mm.

Quartz Adjacent to Biotite and Sillimanite

These photos are dominated by a large grain of quartz. It is clear (PP), slightly fracture, and displays first order gray interference colors. Other, smaller, quartz crystals are scattered around the periphyry. A few small brown (PP) biotite flakes are also present but are lost in the XP view. At the bottom, mostly on the left side, the blocky high-relief crystals are sillimanite. Although sillimanite typically forms needles, here we are looking down the axis of the needles so we see a square cross-section. Note that there is one diagonal cleavage cutting across the square sillimanite end sections.

This sample comes from Western Massachusetts; the field of view is about 1.5 mm.

Biotite-Sillimanite Schist with Graphite, from the Beartooth Mountains, Montana

This photomicrograph, about 2 mm across, is of a biotite-sillimanite schist. The biotite is brown and shows second order interference colors in the XP view. Sillimanite in the upper left part of the photo is relatively fine grained with high relief. Two other, coarser sillimanite grains are along the bottom edge. They show blotchy interference colors ranging up to second order blue (XP). Also present is opaque (black) graphite, flakey and intimately associated with the biotite. The clear (lower relief) material that is not sillimanite, is quartz and K-feldspar. However, it is difficult to pick out the K-feldspar in these views, and some of the quartz appears isotropic (although it is not). The field of view is about 2 mm.