Difference between revisions of "Sillimanite"

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m (Fibrolite moved to Sillimanite)
((edited) addition from Chaman Golecha (id:89))
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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.]]
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<br clear="left" />
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{{images}}
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==Chemical composition==
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Aluminium silicate Al<sub>2</sub>SiO<sub>5</sub>; polymorphous with Andalusite and Kyanite
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==Crystallography==
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Orthorhombic; prismatic crystals sometimes. Mainly massive or fibrous masses; As long slender prisms without distinct terminations often in parallel groups.
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==Diagnostics==
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===Color===
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Sapphire blue, blue-green, colourless, white, gray, yellowish, brownish, greenish, bluish, violet-blue.
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===Diaphaneity===
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Transparent to opaque.
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===Spectra===
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Indistinct lines at 462, 441 and 410nm (mainly in Sri Lankan stones).
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===Magnification===
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Very fine needles parallel to the cleavage direction (opposite to topaz and apatite); fingerprints, crystals, sometimes three directional needles.
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==Phenomena==
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Chatoyancy is common (blue-green stones) in Sri Lankan stones and six-pointed stars.
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==Treatments==
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[[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]]
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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.
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<br clear="all" />
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==Simulants==
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{| {{table}} style="margin-left:0;" width="70%"
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|-
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!colspan="2" style="background-color:white;"|Transparent stones
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|-
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!Name
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!Seperation
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|-
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|Labradorite|| Lower S.G. (floats 2.88); play of colour may be seen; structure
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|-
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|Spodumene|| Magnification features, UV, pleochroism
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|-
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|Chrysoberyl|| Higher heft; life; spectrum
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|-
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|Beryl|| Lower heft (floats 2.88); magnification; lower R.I.; uniaxial
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|-
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|Scapolite|| Lower S.G. (floats 2.88); fluorescence; cleavage
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|}
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{| {{table}} style="margin-left:0;" width="70%"
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|-
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!colspan="2" style="background-color:white;"|Chatoyant stones
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|-
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!Name
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!Seperation
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|-
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|Moonstone|| Lower S.G (floats 2.88); R.I.; UV reaction
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|-
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|Quartz|| Lower S.G. (floats 2.88); R.I; D.R.
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|-
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|Apatite|| Duller luster; R.I. with D.R.; magnification
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|-
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|Chrysoberyl|| Higher heft; spectrum
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|-
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|Phenakite|| R.I. with D.R.; lower Heft
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|}
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==Occurrence==
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Geological occurrence:
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A mineral of metamorphic rocks such as shcissts and gneiss; also granites.
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Geographical locations:
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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
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. By Chaman Golecha, Gem Testing Laboratory, Jaipur.


Sillimanite image gallery


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

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.
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
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.