Sillimanite
Sillimanite (also known as Fibrolite) is a polymorph with Andalusite and Kyanite. A polymorph is a mineral that shares the same chemistry with another, or other, minerals but has a different crystal structure. Sillimanite is the rarest of the three trimorphs. Sillimanite is mostly known as a catseye gem when in its fibrous, massive form, but beautiful, clean faceted gems are also available although faceted gems are rare because clean Sillimanite is scarce and difficult to cut.
Sillimanite is found in many locations worldwide but gem quality crystals are quite rare. The only sources of gem quality Sillimanite crystals are Orissa, India; Mogok, Myanmar (Burma); Ratnapura, Sabaragamuwa Province, Sri Lanka.
| Category: | Nesosilicate |
| Formula: | Al2SiO5 |
| Aluminosilicate | |
| Crystallography: | Orthorhombic – Dipyramidal |
| Crystal Habit: | Crystals prismatic, may be acicular, with square cross sections, rounded and striated, poorly terminated, to 2 cm; as fibrous mats; rarely radiating. |
| Twinning: | None |
| Cleavage: | [010] Perfect |
| Fracture: | Uneven |
| Tenacity: | Tough in aggregate |
| Hardness (Mohs): | 6.5 – 7.5 |
| Density: | 3.23 – 3.24 (g/cm3) |
| Luminescence: | None |
| Radioactivity: | Not Radioactive |
| Color: | Colorless or White to Gray, also Brown, Yellow, Yellow-Green, Gray-Green, Blue-Green, Blue |
| Transparency: | Transparent, Translucent |
| Luster: | Vitreous to Subadamantine, Silky |
| Refractive Index: | 1.653 – 1.684 Biaxial ( + ) |
| Birefringence: | 0.0160 – 0.0230 |
| Dispersion: | Strong; r > v; 0.015 |
| Pleochroism: | If colored, X = pale brown or pale yellow; Y = brown or green; Z = dark brown or blue |
