Staurolite

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Staurolite
Staurolite-26463.jpg
Staurolite from Pestsovye Keivy, Keivy Mountains, Kola Peninsula, Murmanskaja Oblast', Northern Region, Russia, 2.5 x 2.2 x 1 cm
General
Category Nesosilicate
Formula
(repeating unit)
Fe2+2Al9O6(SiO4)4(O,OH)2[1]
Strunz classification 09.AF.30
Crystal symmetry Monoclinic prismatic
H-M symbol: (2/m)
Space group: C 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
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 X = colorless; Y = pale yellow; Z = golden yellow
2V angle Measured: 88°, Calculated: 84° to 88°
Dispersion r > v; weak
References [2][3][4]

Staurolite is a red 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: Fe2+2Al9O6(SiO4)4(O,OH)2. Magnesium, zinc and manganese substitute in the iron site and trivalent iron can substitute for aluminium.[1]

Properties[edit]

Staurolite from Madagascar

A special property of staurolite is that it often occurs twinned in a characteristic cross-shape, called penetration twinning. 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.

Name[edit]

The name is derived from the Greek, stauros for cross and lithos for stone in reference to the common twinning.

Occurrence[edit]

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.[5]

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.[6] Samples are also found in Island Park, Idaho, near henrys lake. Taos, New Mexico, near Blanchard Dam in Minnesota and Selbu, Norway.

Use[edit]

Staurolite is one of the index minerals that are used to estimate the temperature, depth, and pressure at which a rock undergoes metamorphism.

References[edit]

  1. ^ a b Klein, Cornelis and Cornelius S. Hurlbut, Jr., Manual of Mineralogy, Wiley. 20th ed., 1985, p. 382 - 3 ISBN 0-471-80580-7
  2. ^ http://rruff.geo.arizona.edu/doclib/hom/staurolite.pdf Handbook of Mineralogy
  3. ^ http://www.mindat.org/show.php?id=3753&ld=1&pho= Mindat.org
  4. ^ http://webmineral.com/data/Staurolite.shtml Webmineral data
  5. ^ The Audubon Society Field Guide to North American Rocks and Minerals, Chesterman and knopf.
  6. ^ Virginia State Parks