Fibrolite is an aluminosilicate mineral with the chemical formula Al2SiO5. Fibrolite is fibrous form the gemstone variety of sillimanite. Although fibrolite has priority over sillimanite, the two species were not ultimately recognized as the same species until about 1854 by James Dwight Dana.
Named in 1802 by Jacques Louis Comte de Bournon because of its fibrous habit. It is an Orthorhombic crystal system mineral with 7 hardness according to the Mohs hardness scale.
Mostly, It is found in colors such as colorless or white to grey, also brown, yellow, yellow-green, grey-green, blue-green and blue: colorless in thin section. Sometimes It has Sub Adamantine Luster. This Bi-Axial Mineral pleochroism is mostly Colourless to pale brown to yellow.
Natural sillimanite/fibrolite rocks cut into the required shape and size are used mainly in glass industries. Sillimanite is the best raw material for the manufacture of high alumina refractories or 55-60% alumina bricks.
It can be found in mainly Sri Lanka and Burma also, Japan, India, France, USA, Portugal, Poland, Spain.
• Refractive index: nα = 1.653 – 1.661
nβ = 1.654 – 1.670
nγ = 1.669 – 1.684
• Birefringence δ = 0.020 – 0.022
• Pleochroism: Colourless to pale brown to yellow
• Cleavage: perfect
• Fracture: Splintery
• Tenacity: Tough
• Mohs scale hardness: 7
• Luster: Vitreous to subadamantine, silky
• Streak: White
• Diaphaneity: Transparent to translucent
• Specific gravity: 3.24
• Optical properties: Biaxial (+)
• Crystal habit: Prismatic crystals, fibrous, acicular