Prismatic
brown crystals of Sillimanite
A close up view of the edge of the
above crystal showing fine striations parallel to the "C" axis.
CHEMISTRY - Al2SiO5
CRYSTALLOGRAPHY - Orthorhombic, prismatic
CRYSTAL GROWTH AND HABITS - Crystals are commonly acicular with roughly a square cross section. The crystals may also appear rounded and often have fine striations parallel the "C" axis. Good crystals of Sillimanite are rare and most crystals are poorly terminated. Sillimanite may also occur as crystal mats and only rarely as radiating groups.
COLOR AND OPTICAL PROPERTIES - The color of Sillimanite varies from colorless or white through shades of gray to brown. It may also be yellowish, grayish green or blue. It is transparent to translucent.
HARDNESS - 6.5 - 7.5
SPECIFIC GRAVITY - 3.23 - 3.24
LUSTER - Vitreous to subadamantine to dull
STREAK - White
BREAKABILITY - Sillimanite has perfect cleavage on {010} and has an uneven fracture.
OCCURRENCE - Sillimanite is a mineral found in high grade metamorphic rocks like gneisses, schists or hornfels. It is only rarely found in pegmatites. It may also be found as clasts in sedimentary rocks.
ASSOCIATED MINERALS - Sillimanite is associated with Andalusite, Kyanite, potassium feldspars, Almandine garnet, Cordierite, Biotite and Quartz. It is trimorphic with Kyanite and Andalusite.
MINERAL NAME - Sillimanite is named after the American Professor Benjamin Silliman. A Professor of chemistry and mineralogy at Yale University.