Difference between revisions of "Sillimanite"
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{{sillimanite}} | {{sillimanite}} | ||
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+ | [[Image:Sillimanite2hy8.jpg|thumb|left|240px|A brown sillimanite. By Chaman Golecha, Gem Testing Laboratory, Jaipur.]] | ||
+ | <br clear="left" /> | ||
+ | |||
+ | {{images}} | ||
+ | |||
+ | |||
+ | ==Chemical composition== | ||
+ | Aluminium silicate Al<sub>2</sub>SiO<sub>5</sub>; 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. | ||
+ | |||
+ | ===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. | ||
+ | |||
+ | ==Phenomena== | ||
+ | |||
+ | Chatoyancy is common (blue-green stones) in Sri Lankan stones and six-pointed stars. | ||
+ | |||
+ | ==Treatments== | ||
+ | |||
+ | [[Image:Sillimanitefibrousdyessf1.jpg|left|thumb|240px|A fibrous sillimanite dyed red simulating a ruby. Such sillimanites are dyed in different colours to simulate various stones like ruby, sapphire, emerald and others.<br />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. | ||
+ | <br clear="all" /> | ||
+ | |||
+ | ==Simulants== | ||
+ | |||
+ | {| {{table}} style="margin-left:0;" width="70%" | ||
+ | |- | ||
+ | !colspan="2" style="background-color:white;"|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 | ||
+ | |} | ||
+ | |||
+ | {| {{table}} style="margin-left:0;" width="70%" | ||
+ | |- | ||
+ | !colspan="2" style="background-color:white;"|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. |
Revision as of 11:11, 27 August 2007
Sillimanite | |
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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 |
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.
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.
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.