Sillimanite

Sillimanite is a rare stone that is named after Professor Silliman of Yale University, USA. It is blue to green, with distinct pleochroism showing pale yellowish-green, dark green, and blue from different angles. When crystals occur in long slender prisms in parallel groups, resembling fibers, the material is often called fibrolite.

Occurrence

Sillimanite occurs in metamorphic rocks and occasionally in pegmatites. Blue and violet stones are found in Myanmar (Burma). Greenish grey stones occur in Sri Lanka. On the other hand, fibrolite exists in Idaho (USA). Brazil, Italy, Germany, India, and Czech Republic/Slovakia are other important sources of sillimanite.

Sillimanite Properties

• Specific Gravity: 3.25
• Refractive Index: 1.66-1.68
• Birefringence (DR): 0.019
• Luster: Vitreous
• Crystal Structure: Orthorhombic
• Composition: Aluminum silicate
• Hardness: 7½

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Crystals Directory

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Birthstone

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