The chemical formula of mineral Sillimanite is indicated by Al2SiO5. Sillimanite is a Silicate mineral. It is known to crystallize in the orthorhombic system. In optical mineralogy, the orthorhombic crystal system comprises crystals having three mutually perpendicular axes, of which all are of different lengths. Sillimanite is a biaxial positive mineral. Its indices of refraction range from 1.653 to 1.683. The birefringence of Sillimanite is usually found within the range of 0.018 to 0.022. The optic angle of the mineral Sillimanite is usually about 20 degree to 30 degree. Sillimanite is commonly found exhibiting a high positive relief if viewed under several adjustments on the aperture diaphragm of the petrographic polarizing microscope.
Sillimanite is usually found having a hardness measure ranging from 6.5 to 7.5 when evaluated using the Mohs scale method. The specific gravity measure of the mineral is within the range of 3.23 grams per cubic centimeters to 3.27 grams per cubic centimeters. Sillimanite is commonly found colorless or white in hand sample but can also be found colored yellow, brown, or blue. When rubbed on a white porcelain streak plate, Sillimanite is commonly found leaving a white streak. It is usually exhibiting a vitreous luster when viewed in reflected light of polarizing microscope. The colored varieties of Sillimanite usually contain iron or chromium.
Sillimanite crystals are usually found colorless in thin section and in grain mount. The color of the colored varieties may appear patchy. Sillimanite is most commonly found as slender prismatic crystals or as fine fibrous crystals called fibrolite. This fibrolite commonly forms radiating, swirled, or matted aggregates that may be more splendidly exhibited under polarizing microscope for mineralogists. Cross sections of mineral Andalusite through crystals are usually more or less diamond shaped.
When Sillimanite is closely examined with the aid of the petrographic polarizing light microscope, it is commonly found having single good cleavage that is parallel to the length of the crystals. Elongate crystals of Sillimanite are commonly fractures across their length. The cleavage of the mineral Sillimanite is usually visible in basal sections that are parallel to one diagonal of the diamond most especially when evaluated using the petrographic polarizing microscope. There is no twinning reported. The elongate grains of Sillimanite show parallel extinction and they are usually length slow.
The indices and birefringence of mineral Sillimanite vary within a relatively narrow range. Sillimanite in thin section shows interference colors ranging up to lower second order. But most of the grains will show first-order colors. Fine fibers of mineral fibrolite rarely show higher than first order yellow colors when evaluated closely with the aid of the petrographic polarizing light microscope. If Sillimanite samples are large enough, its basal sections are usually found yielding a biaxial positive figure. It is usually found with optic angle at about 20 degree to 30 degree. The fragments lying on the cleavage plane yields flash figures that are more clearly exhibited when viewed with the aid of the polarizing microscope for mineralogists. The optic axis dispersion is usually strong.
Sillimanite is usually altered to sericite. Sericite is consisting predominantly of fine-grained white mica. Sillimanite is also involved in a variety of metamorphic mineral reactions and may possibly show reaction textures with associated minerals. Distinguishing features of Sillimanite include high relief, moderate birefringence, parallel extinction and crystal habit. Sillimanite is commonly found in medium- and high-grade mica schist, gneiss, hornfels, and other related rocks. Sillimanite is a common accessory mineral in granitic rocks. It is rarely found in pegmatites and quartz veins. Sillimanite can be also a locally important detrital mineral.
