Sillimanite is the high-temperature aluminosilicate polymorph, characteristic of medium to high-grade (much of amphibolite to granulite facies) regionally metamorphosed pelites and some high-temperature contact metamorphic rocks.  The sillimanite in A occurs as colourless high positive relief prismatic grains in the upper left, with pronounced parting perpendicular to the crystal length, and long axes aligned parallel to the foliation defined by the biotite (the “c” axes of sillimanite are approximately parallel to the stage in the upper left).  The relief of sillimanite is higher than that of the nearby quartz and plagioclase but lower than coexisting garnet.  In the lower right corner of A, square cross-sections of sillimanite, cut perpendicular to “c”, are apparent.  In B the interference colours range to second order blue,  the maximum colour. Extinction is parallel to the long axis. Sillimanite in C and D occurs as high relief fine-grained acicular grains, aggregates of which are known as fibrolite.  Samples are from unknown location. All fields of view are 2.2 mm across.  A and C ppl, B and D x-nicols.