Smith College - Geology 222b - Petrology
Petrographic Data File



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-yellowX=colorless, Y=pale yellow, Z=yellow
Optic Sign Biaxial (+)  
2V 2V=81 to 90 degrees


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 = 

Increases with Fe content
Max Birefringence 0.009-0.015 Angstroms  

elongate parallel to c axis

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.

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)

Photomicrograph of an elongate crystal of staurolite in plane polarized light (click for rollover image).
Photomicrograph of staurolite in plane polarized light (click for rollover image). Objective 1.5, Field of view 7750 microns x 5750 microns.
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.

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