GEOLOGY

Geology



Geology

Sillimanite

Sillimanite, AlzSiOs, occurs in three principal colored varieties-yellow, brown, and blue. The yellow and brown varieties are widespread whereas the blue variety is a mineralogical curiosity that has been well-documented from only four localities worldwide. Iron, chromium and titanium are the most abundant minor elements in sillimanite and the most likely to be responsible for the color. Grew and Rossman (1976a, b) and Grew (1976) had prevously communicated the results of a study of the optical absorption of a blue and a yellow sillimanite and observed that the absorption spectra of these varieties are fundamentally different. They suggested that the yellow color is largely due to Fe3* in tetrahedral coordination' On the other hand, Hilenius (1979) reported an absorption spectrum of a yellow sillimanite and concluded that iron in the sixcoordinate site was responsible for the color.

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.

 

            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 ...