| Formula | Fe2Al9O6(Si,Al)4O22(OH)2 |
small amounts of Mg or Zn may substitute for Fe. |
| Crystal System | Monoclinic C2/m | Pseudo-orthorhombic |
| Gravity & Hardness | G=3.74-3.83, H=7-7.5 | |
| Crystal Habit | Euhedral prismatic crystals with 6 sided cross sections |
commonly contains inclusions, such as quartz |
| Cleavage | no distinct cleavage,(010) poor | conchoidal fracture |
| Color/Pleochroism | Yellow and slightly pleochroic | X=colorless, Y=pale yellow, Z=yellow |
| Optic Sign | Biaxial (+) | |
| 2V | 2V=79 to 90 degrees |
|
| Optic Orientation | Z^c Y^b ~27-32 degrees X^a ~1-5 degrees Optic pane at right angle to (100) |
a=7.863-7.871, b=16.534-16.613, c=5.632-5.663 Beta=90-90.45 degrees Gamma=90.12 degrees |
| Refractive Indices alpha = beta = gamma = delta = |
1.736-1.747 1.740-1.754 1.745-1.762 0.009-0.015 |
Increases with Fe content |
| Max Birefringence | 0.033 - 0.052 Angstroms | |
| Elongation | elongate parallel to c axis |
|
| Extinction | parallel, max of 30 degrees on grains | Length slow |
| Dispersion | weak to moderate , r >v | |
| Distinguishing Features | Staurolite's yellow color, pleochroism, relief and habit make it distinguishing. It is vitreous and has a grey streak. Staurolite's hand sample has characteristic penetration twinning and unique crystal habit. The crystals are brown, red or yellow in color. May resemble tourmaline in thin section, but tourmaline is uniaxial. |
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| Occurrence | Staurolite is found in medium-grade pelitic metamorphic rock. Staurolite may be found with garnet, cordierite, aluminum silicate, muscovite, biotite and quartz. It is in the lower to middle amphibolite facies. | |
| Editors | Jezra Beaulieu (Hampshire 07) | |
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Photomicrograph of an elongate crystal of staurolite in plane polarized light (click for rollover image). |
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Six sided crystals in plane polarized light. it can be distinguished by its yellow coloration. |
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Six sided crystals in cross polarized light. |