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Sillimanite (Fribolite)
Current inventory:  0 gems
 

Chemistry:  Al₂SiO₅  [Aluminum Silicate]

Discovered in 1824;   IMA status: Valid (pre-IMA; Grandfathered).
Sillimanite is named after American minerologist Professor Benjamine Silliman (1779-1864) of Yale University, New Haven, Connecticut, USA, and founder of the American Journal of Science (Silliman's Journal). The name Fibrolite is in allusion to the fibrous nature of most material.

Classification

Mineral Classification:

Silicates

Strunz 8th Ed. ID:

8/B.02-10

Nickel-Strunz 10th Ed. ID:

9.AF.05

9 : SILICATES (Germanates)
A : Nesosilicates
F : Nesosilicates with additional anions; cations in [4], [5] and/or only [6] coordination

Crystal Data

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

Physical Properties

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

Optical Properties

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

Occurances

Geological Setting:

In high-grade metamorphic schists, gneisses, and hornfels; more rarely in pegmatites. Also as a detrital mineral.

Co-Type Localities:

• Vltava (Moldau), Sušice (Schüttenhofen), Plzen Region, Bohemia, Czech Republic;
• Chester, Middlesex County, Connecticut, USA

Year Discovered:

1824

View mineral photos:

Sillimanite Mineral Photos and Locations

More Information

Mindat.org
Webmineral.com

Sillimanite (also known as Fribolite) 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.
 

  
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