Staurolite
| Staurolite | |
|---|---|
 Staurolite from Pestsovye Keivy, Keivy Mountains, Kola Peninsula, Murmanskaja Oblast', Northern Region, Russia, 2.5 x 2.2 x 1 cm |
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| General | |
| Category | Silicate mineral |
| Chemical formula | (Fe2+,Mg,Zn)1.5-2Al9[O6|(OH,O)2|(SiO4)4] |
| Crystal symmetry | Monoclinic 2/m |
| Unit cell | a = 7.86 Å, b = 16.6 Å, c = 5.65 Å; β = 90.45°; Z=2 |
| Identification | |
| Color | Dark reddish brown to blackish brown, yellowish brown, rarely blue; pale golden yellow in thin section |
| Crystal habit | Commonly in prismatic crystals |
| Crystal system | Monoclinic - Prismatic; Pseudo-orthorhombic |
| Twinning | Commonly as 60° twins, less common as 90° cruciform twins |
| Cleavage | Distinct on {010} |
| Fracture | Subconchoidal |
| Tenacity | Brittle |
| Mohs scale hardness | 7 - 7.5 |
| Luster | Subvitreous to resinous |
| Streak | White to grayish |
| Diaphaneity | Transparent to opaque |
| Specific gravity | 3.74 - 3.83 meas. 3.686 calc. |
| Optical properties | Biaxial (+) |
| Refractive index | nα = 1.736 - 1.747 nβ = 1.740 - 1.754 nγ = 1.745 - 1.762 |
| Birefringence | δ = 0.009 - 0.015 |
| Pleochroism | Pleochroism: X = colorless; Y = pale yellow; Z = golden yellow |
| 2V angle | Measured: 88°, Calculated: 84° to 88° |
| Dispersion | r > v; weak |
| References | [1][2][3] |
Staurolite is a red brown to black, mostly opaque, nesosilicate mineral with a white streak.
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[edit] Properties
It crystallizes in the monoclinic crystal system, has a Mohs hardness of 7 to 7.5 and a rather complex chemical formula: (Fe,Mg,Zn)2Al9(Si,Al)4O20(OH)4. Iron, magnesium and zinc occur in variable ratios.
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A special property of staurolite is that it often occurs twinned in a characteristic cross-shape. In handsamples, macroscopically visible staurolite crystals are of prismatic shape. They are often larger than the surrounding minerals and are then called porphyroblasts.
In thin sections staurolite is commonly twinned and shows lower first order birefringence similar to quartz, with the twinning displaying optical continuity. It can be identified in metamorphic rocks by its swiss cheese appearance (with poikilitic quartz) and often mantled porphyroblastic character.
[edit] Name
The name is derived from the Greek, stauros for cross and lithos for stone in reference to the common twinning.
[edit] Occurrence
Staurolite is a regional metamorphic mineral of intermediate to high grade. It occurs with almandine garnet, micas, kyanite; as well as albite, biotite, and sillimanite in gneiss and schist of regional metamorphic rocks.[4]
It is the official state mineral of the U.S. state of Georgia and is also to be found in the Lepontine Alps in Switzerland.
Staurolite is also found in Fairy Stone State Park in Patrick County, Virginia. The park is named for a local name for staurolite from a legend in the area.[5] Samples are also found in Taos, New Mexico.
[edit] Use
Staurolite is one of the index minerals that are used to estimate the temperature, depth, and pressure at which a rock undergoes metamorphism.
[edit] References
- ^ http://rruff.geo.arizona.edu/doclib/hom/staurolite.pdf Handbook of Mineralogy
- ^ http://www.mindat.org/show.php?id=3753&ld=1&pho= Mindat.org
- ^ http://webmineral.com/data/Staurolite.shtml Webmineral data
- ^ The Audubon Society Field Guide to North American Rocks and Minerals, Chesterman and knopf.
- ^ Virginia State Parks
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