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Staurolite thin section

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About Staurolite
Staurolite is a metamorphic mineral that is found almost exclusively in medium-grade schists. In general, it has an isolated tetrahedral structure related to that of kyanite. The name comes from the Greek for cross, in allusion to its common cruciform twins.
Staurolite hand-specimen
Formula: Fe22+Al9Si4O23(OH)
System: Monoclinic
Color: Dark brown, brownish-black, red-brown
Lustre: Sub-Vitreous, Resinous
Hardness: 7–7½
Density: 3.74–3.83
XPL
XPL
XPL
PPL
PPL
Staurolite #1 thin section (hFOV 2mm)
XPL
XPL
XPL
PPL
PPL
Staurolite #2 thin section (hFOV 2mm)
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Staurolite PPL properties
Relief: High positive
Habit/Form: Crystals are usually prismatic and elongate parallel to the c axis. Basal sections are usually six sided with the {110} prism faces dominant. Penetration twins forming a cross at either 90° or about 60° are common but may not be obvious in thin sections. It also forms anhedral masses. Porphyroblasts in metamorphic rocks commonly are riddled with numerous rounded inclusions of quartz or other minerals.
Color: Colorless to honey/golden-yellow; pale yellow-green (Cr), pale green (Cr), cobalt blue (Co), pale yellow, pale yellow-green (Cr), pale green (Cr), violet-blue (Co), yellow-green (Cr), green (Cr), violet (Co)
Pleochroism: Weak to moderate, from colorless to honey/golden yellow
Cleavage: Weak cleavage trace parallel to {010}.
Staurolite XPL properties
Isotropy/Anisotropy: Anisotropic
Interference color: Order I gray, white or yellow – usually showing as yellow, due to inherent mineral color; the maximum interference color is first-order yellow to red.
Extinction angle: Parallel in most sections – to prismatic sections and cleavage traces; symmetrical extinction to {110} faces in basal sections.
Twins: Can show twins oriented at 60° or 90°. Twinning is rarely noted in thin sections.
Uniaxial/Biaxial: Biaxial (+)
Optic axial angle (2V): 2V measured: 88°, calculated: 84 – 88°
Staurolite distinguishing features under the microscope
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References
  • Deer, W. A., Howie, R. A., & Zussman, J. (2013). An introduction to the rock-forming minerals (pp. 498). Mineralogical Society of Great Britain and Ireland, London.
  • mindat.org – The Mineral Database