Fibrolite (Sillimanite)
Sillimanite (also referred to as Fibrolite) is a polymorph with Andalusite and Kyanite. A polymorph is a mineral that shares the exact same chemistry with another, or other, minerals but has a crystal structure that is different. Sillimanite is the rarest of the 3 trimorphs. Sillimanite is certainly caused by known as a Catseye gem when in its fibrous, massive form, but gorgeous, clean faceted gems are available although faceted gems are rare because clean Sillimanite is scarce and difficult to cut.
Sillimanite can be found in numerous locations worldwide but gem quality crystals are quite uncommon. The only sources of gem quality Sillimanite crystals are Orissa, India; Mogok, Myanmar (Burma); Ratnapura, Sabaragamuwa Province, Sri Lanka.
| Category: | Nesosilicate |
| Formula: | Al2SiO5 |
| 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 |
