| Formula | (Fe,Mg)2(Al,Fe)9O6[SiO4]4(O,OH)2 |
small amounts of Mg or Al may substitute for Fe. |
| Crystal System | Monoclinic C2/m | Pseudo-orthorhombic |
| Gravity & Hardness | G=3.74-3.83, H=7.5 | |
| Crystal Habit | Euhedral prismatic crystals with 6 sided cross sections |
commonly contains inclusions, such as quartz |
| Cleavage | {010} moderate | conchoidal fracture |
| Color/Pleochroism | yellowish brown, pleochroism= colorless-yellow | X=colorless, Y=pale yellow, Z=yellow |
| Optic Sign | Biaxial (+) | |
| 2V | 2V=81 to 90 degrees |
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| Optic Orientation | Optic plane at right angle to (100) | a=7.83-7.95 b=16.50-16.82, c=5.55-5.71 Beta=90 degrees Gamma=90.12 degrees |
| Refractive Indices alpha = beta = gamma = delta = |
1.739-1.747 1.745-1.753 1.752-1.761 0.009-0.015 |
Increases with Fe content |
| Max Birefringence | 0.009-0.015 Angstroms | |
| Elongation | elongate parallel to c axis |
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| Dispersion | r >v , weak | |
| 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, and is used as an index mineral in metamorphic zoning. Staurolite may be found with garnet, cordierite, kyanite, muscovite, biotite and quartz. It is in the lower to middle amphibolite facies. | |
| Editors | Jezra Beaulieu (Hampshire 07), Nicole Collier (13), Elsie Eastman ('16) | |
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Photomicrograph of an elongate crystal of staurolite in plane polarized light (click for rollover image). |
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Photomicrograph of staurolite in plane polarized light (click for rollover image). Objective 1.5, Field of view 7750 microns x 5750 microns. |
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Photomicrograph of Staurolite. In PPL, it appears yellowish with a tiny bit of pleochroism, whereas in XPL it appears grayish blue. Click on the image to see a larger image with xpl rollover. |