Staurolite
Reddish brown to black nesosilicate mineral
Staurolite | |
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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 | Nesosilicate |
Formula
(repeating unit) |
Fe 2+ 2 Al 9 O 6 (SiO 4 ) 4 (O,OH) 2 [1] |
IMA symbol | St [2] |
Strunz classification | 9.AF.30 |
Crystal system | Monoclinic |
Crystal class |
Prismatic (2/m)
(same H-M symbol ) |
Space group | C2/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 |
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 | n α = 1.736 - 1.747 n β = 1.740 - 1.754 n γ = 1.745 - 1.762 |
Birefringence | δ = 0.009 - 0.015 |
Pleochroism | X = colorless; Y = pale yellow; Z = golden yellow |
2V angle | Measured: 88°, Calculated: 84° to 88° |
Dispersion | r > v; weak |
References | [3] [4] [5] |
Staurolite is a reddish brown to black, mostly opaque, nesosilicate mineral with a white streak. It crystallizes in the monoclinic crystal system, has a Mohs hardness of 7 to 7.5 and the chemical formula: Fe 2+ 2 Al 9 O 6 (SiO 4 ) 4 (O,OH) 2 . Magnesium , zinc and manganese substitute in the iron site and trivalent iron can substitute for aluminium. [1]
Properties
Staurolite often occurs twinned in a characteristic cross-shape, called cruciform penetration twinning. [6] In handsamples, macroscopically visible staurolite crystals are of prismatic shape. The mineral often forms 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.
Name
The name is derived from the Greek , stauros for cross and lithos for stone in reference to the common twinning.
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. [7]
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 most commonly found in Fannin County, Georgia . [8] It 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. [9] Samples are also found in Island Park, Idaho , near Henrys Lake ; Taos , New Mexico ; near Blanchard Dam in Minnesota ; and Selbu , Norway.
Use
Staurolite is one of the index minerals that are used to estimate the temperature, depth, and pressure at which a rock undergoes metamorphism.
References
- 1 2 Klein, Cornelis and Cornelius S. Hurlbut, Jr., Manual of Mineralogy, Wiley. 20th ed., 1985, p. 382 - 3 ISBN 0-471-80580-7
- ↑ Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols" . Mineralogical Magazine . 85 (3): 291–320. Bibcode : 2021MinM...85..291W . doi : 10.1180/mgm.2021.43 . S2CID 235729616 .
- ↑ 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
- ↑ Simpson, B. (1983). Rock & Minerals . Elsevier. p. 41. ISBN 9780080984117 . Retrieved 17 November 2017 .
- ↑ The Audubon Society Field Guide to North American Rocks and Minerals, Chesterman and knopf.
- ↑ Fannin County Archives
- ↑ Virginia State Parks
Authority control : National |
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