Sillimanite

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From The Gemology Project
Sillimanite
Chemical composition Al2SiO5
Crystal system Orthorhombic
Habit Long slender prisms, fibrous
Cleavage Good, prismatic {010}
Fracture Uneven, brittle
Hardness 6 - 7.5
Optic nature Biaxial +
Refractive index 1.653 - 1.685
Birefringence 0.014 - 0.021
Dispersion Low, 0.015
Specific gravity 3.20 - 3.26
Lustre Vitreous to silky
Pleochroism Strongly trichroic
A brown sillimanite.
Photo courtesy of Chaman Golecha, Gem Testing Laboratory, Jaipur.


Sillimanite image gallery


Sillimanite (also known as "fibrolite") is named after Benjamin Silliman (1779-1864). Silliman was the founder of the American Journal of Science (Silliman's Journal).
It is polymorphous with Anadalusite and Kyanite.


Contents

Chemical composition

Aluminium silicate Al2SiO5; polymorphous with Andalusite and Kyanite.

Crystallography

Orthorhombic; prismatic crystals sometimes. Mainly massive or fibrous masses; As long slender prisms without distinct terminations often in parallel groups.

Diagnostics

Color

Sapphire blue, blue-green, colourless, white, gray, yellowish, brownish, greenish, bluish, violet-blue.

Diaphaneity

Transparent to opaque.

Refractometer

The full R.I. range of sillimanite is: nα = 1.653-1.661, nβ = 1.657-1.662 and nγ = 1.672-1.685. With a maximum birefringence ranging from 0.018 to 0.022.
General R.I.: 1.658-1.678, D.R.: 0.020

Spectra

Indistinct lines at 462, 441 and 410nm (mainly in Sri Lankan stones).

Magnification

Very fine needles parallel to the cleavage direction (opposite to topaz and apatite); fingerprints, crystals, sometimes three directional needles.


Fibrous inclusions
60X Magnification
By Barbra Voltaire






The fibrous inclusions pictured on the left were found in translucent, violet, chatoyant sillimanite cabochons.







Polariscope

Sillimanite is biaxial with a positive optic sign.
Due to the small 2V value (2Vz = 21-30°) a diagnostic interference pattern should be easily obtained.

Pleochroism

Strong (generally pale green / dark green and blue).

Luminescence

UV fluorescence: Sometimes weak reddish (Burmese stones).

Phenomena

Chatoyancy is common (blue-green stones) in Sri Lankan stones and six-pointed stars.

Chatoyant sillimanite cabochons
By Barbra Voltaire














Treatments

A fibrous sillimanite dyed red simulating a ruby.
By Chaman Golecha, Gem Testing Laboratory, Jaipur


Fibrous sillimanite is very often dyed to imitate various "precious" gemstones like ruby, emerald and others. This type of sillimanite has a massive fibrous like appearance under magnification and show color concentrations. It is easily identified by the 1.66-1.68 range of R.I. with D.R. of 0.020 and S.G. of around 3.2. Such dyed sillimanite is now very often encountered in India.

Simulants

Transparent stones
Name Seperation
Labradorite Lower S.G. (floats 2.88); play of colour may be seen; structure
Spodumene Magnification features, UV, pleochroism
Chrysoberyl Higher heft; life; spectrum
Beryl Lower heft (floats 2.88); magnification; lower R.I.; uniaxial
Scapolite Lower S.G. (floats 2.88); fluorescence; cleavage
Diaspore Higher S.G.; higher R.I. and birefringence
Chatoyant stones
Name Seperation
Moonstone Lower S.G (floats 2.88); R.I.; UV reaction
Quartz Lower S.G. (floats 2.88); R.I; D.R.
Apatite Duller luster; R.I. with D.R.; magnification
Chrysoberyl Higher heft; spectrum
Phenakite R.I. with D.R.; lower heft
Iolite Lower R.I and S.G
Diaspore Higher S.G.; higher R.I. and birefringence

Occurrence

Geological occurrence: A mineral of metamorphic rocks such as shcissts and gneiss; also granites.

Geographical locations: India (various colours, chatoyant and star varities); Mogok stone tract (blue, violet blue), Burma; Sri Lanka (grayish green, chatoyant); Kenya (colourless, bluish); Idaho, USA (water-worn, massive). Also S. Dakota; Oklahoma; Delaware; N.Carolina; New Jersey; Pennsylvania; S. Carolina; Canada; Ireland; Scotland; France; Germany; Czechoslovakia; Brazil; Madagascar; Korea; South Africa; Tanzania.

References

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