Difference between revisions of "Sillimanite"
m (→Refractometer) |
(→Diagnostics) |
||
Line 43: | Line 43: | ||
Sillimanite is biaxial with a positive optic sign.<br /> | Sillimanite is biaxial with a positive optic sign.<br /> | ||
Due to the small 2V value (2V<sub>z</sub> = 21-30°) a diagnostic interference pattern should be easily obtained. | Due to the small 2V value (2V<sub>z</sub> = 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== | ==Phenomena== |
Revision as of 13:23, 27 August 2007
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 |
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.
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.
Treatments
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 |
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 |
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
- Introduction to Optical Mineralogy 2004 - William D. Nesse ISBN 0195149106