Andalusite
| Andalusite | |
|---|---|
 Andalusite, Tyrol Austria. | |
| General | |
| Category | Silicate mineral nesosilicate |
| Formula (repeating unit) | Al2SiO5 |
| Crystal system | orthorhombic |
| Space group | 2/m 2/m 2/m - Dipyramidal |
| Unit cell | a = 7.7980 Å b = 7.9031 Å c = 5.5566 Å; Z = 4 |
| Identification | |
| Color | Pink, violet, yellow, green, white, gray; in thin section, colorless to pink or green |
| Crystal habit | As euhedral crystals or columnar aggregates having nearly square cross sections; fibrous compact to massive |
| Twinning | On {101}, rare |
| Cleavage | Good on {110}, poor on {100} |
| Fracture | uneven to subconchoidal |
| Mohs scale hardness | 6.5 - 7.5 |
| Luster | Vitreous |
| Streak | White |
| Diaphaneity | Transparent to nearly opaque with inclusions |
| Specific gravity | 3.17 (+/- .04) |
| Optical properties | double refractive, biaxial negative; chiastolite has anomalous aggregate reaction.[1] |
| Refractive index | nα = 1.629 - 1.640 nβ = 1.633 - 1.644 nγ = 1.638 - 1.650 |
| Birefringence | δ = 0.009 - 0.010 |
| Pleochroism | Weak |
| 2V angle | 71 - 86° |
| Dispersion | r < v strong |
| Ultraviolet fluorescence | non-fluorescent |
| References | [1][2][3][4] |
Andalusite is an aluminium nesosilicate mineral with the chemical formula Al2SiO5.
The variety chiastolite commonly contains dark inclusions of carbon or clay which form a checker-board pattern when shown in cross-section.
A clear variety first found in Andalusia, Spain can be cut into an interesting gemstone.[5] Faceted andalusite stones give a play of red, green, and yellow colors that resembles a muted form of iridescence, although the colors are actually the result of unusually strong pleochroism.
It is associated with mica schist which increases alkali content in ultimate product and so it has not been exploited economically so far.
Occurrence
Andalusite is a common regional metamorphic mineral which forms under low pressure and low to high temperatures. The minerals kyanite and sillimanite are polymorphs of andalusite, each occurring under different temperature-pressure regimes and are therefore rarely found together in the same rock. Because of this the three minerals are a useful tool to help identify the pressure-temperature paths of the host rock in which they are found. An example rock includes hornfels.
It was first described and named after the type locality in the Ronda Massif, Málaga, Andalusia, Spain in 1789.[3][4]
See also
References
- ^ a b Gemological Institute of America, GIA Gem Reference Guide 1995, ISBN 0-87311-019-6
- ^ http://rruff.geo.arizona.edu/doclib/hom/andalusite.pdf Handbook of Mineralogy
- ^ a b http://www.mindat.org/show.php?id=217&ld=1&pho= Mindat.org
- ^ a b http://webmineral.com/data/Andalusite.shtml Webmineral data
- ^ International Colored Gem Association: Anadalusite