Staurolite

Staurolite
Staurolite from Pestsovye Keivy, Keivy Mountains, Kola Peninsula, Murmanskaja Oblast', Northern Region, Russia, 2.5 × 2.2 × 1 cm
General
CategoryNesosilicate
Formula
(repeating unit)
Fe2+2Al9O6(SiO4)4(O,OH)2[1]
IMA symbolSt[2]
Strunz classification9.AF.30
Crystal systemMonoclinic
Crystal classPrismatic (2/m)
(same H-M symbol)
Space groupC2/m
Unit cella = 7.86 Å, b = 16.6 Å
c = 5.65 Å; β = 90.45°; Z = 2
Identification
ColorDark reddish brown to blackish brown, yellowish brown, rarely blue; pale golden yellow in thin section
Crystal habitCommonly in prismatic crystals
TwinningCommonly as 60° twins, less common as 90° cruciform twins
CleavageDistinct on {010}
FractureSubconchoidal
TenacityBrittle
Mohs scale hardness7 – 7.5
LusterSubvitreous to resinous
StreakWhite to grayish
DiaphaneityTransparent to opaque
Specific gravity3.74 – 3.83 meas. 3.686 calc.
Optical propertiesBiaxial (+)
Refractive indexnα = 1.736 – 1.747 nβ = 1.740 – 1.754 nγ = 1.745 – 1.762
Birefringenceδ = 0.009 – 0.015
PleochroismX = colorless; Y = pale yellow; Z = golden yellow
2V angleMeasured: 88°, Calculated: 84° to 88°
Dispersionr > 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: Fe2+2Al9O6(SiO4)4(O,OH)2. Magnesium, zinc and manganese substitute in the iron site and trivalent iron can substitute for aluminium.[1]

  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. ^ 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.
  3. ^ http://rruff.geo.arizona.edu/doclib/hom/staurolite.pdf Handbook of Mineralogy
  4. ^ http://www.mindat.org/show.php?id=3753&ld=1&pho= Mindat.org
  5. ^ http://webmineral.com/data/Staurolite.shtml Webmineral data