Sillimanite

Sillimaite (named after professor Sillman of Yale University, USA) is blue to green, with distinct pleochroism showing pale yellowish green, dark green, and blue from different angles. When crystals occurs in long slender prisms in parallel groups, resembling fibres, the material is often called fibrolite.

Sillimanite is found in metamorphic rocks and occasionally in pegmatites. Blue and violet stones are found in Burma; greenish grey stones in Sri Lanka; fibrolite in Idaho (USA). Other sites include Czechoslovakia, India, Italy, Germany and Brazil.

Cause of Color :

Iron

Chemical Composition :

Aluminium Silicate – Al2SiO5

Classification / Type :

Polymorphous with Andalusite and Kyanite.

Crystal System / Forms :

Orthorhombic System / Prismatic crystals with vertical striations on prism faces.

Cuts & Uses :

Facetted cuts, cabochons, beads, etc.

Dispersion : 0.015
Hardness : 7 - 7.5
Lustre : Vitreous
Magnification :

Very fine needles parallel to the cleavage direction, fingerprints and crystals.

Optic Character :

Anisotropic, D.R.; Biaxial positive.

Pleochroism : Varies with depth of color.
Refractive Index / Birefringence :

1.658 – 1.678 / 0.020

Simulants (with separation tests) :
Sources :

Myanmar (Mogok tract), Sri Lanka, India (Orissa, Andhra Pradesh, Tamil Nadu), Kenya, Canada.

Specific Tests :

Tendency to fracture due to easy cleavage.

Spectrum :

Narrow bands at 462nm, 411nm and 410nm characteristic of darker colored stones.

Reader Interactions

Leave a Reply