| General Formula: Fe2Al9O6(SiO4)4(OH)2 |
Sample: PT-104c | System: Monoclinic |
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| Staurolite Poikiloblast Euhedral staurolite poikiloblast, with inclusions of biotite and quartz, from a Meguma Group pelite. The long diagonal of the staurolite grain measures 7.5 mm, plane light |
Staurolite Porphyroblast Staurolite grain at extinction, where the diamond shape is clearly visible. The long axis of the staurolite grain measures 7.5 mm, crossed polars |
Block diagram showing the relationship between the crystallographic axes and the indicatrix axes. |
Optical Properties
| Colour Pleochroism |
pale honey yellow or brown distinct pelochroism X = colourless or pale yellow Y = pale yellow to yellowish brown Z = golden yellow to reddish brown |
Form | crystals are usually prismatic and elongated parallel to the c axis, basal sections are 6 sided, with {110} prism faces dominant. |
| Relief RI |
high positive nalpha = 1.736-1.747 nbeta = 1.740-1.654 ngamma = 1.745-1.762 |
Cleavage | one poor cleavage, not observed in thin section |
| Birefringence Interference Colours |
0.009 - 0.015 up to 1st order white or yellow |
Twinning | not obvious in thin section |
| Interference Figure Optic Sign 2V |
Biaxial positive 2VZ = 80-90° |
Optic Orientation | X = b Y = a Z = c optic plane = (100) parallel extinction in longitudinal sections, symmetrical in basal sections longitudinal sections are length slow |
| Composition | Mg, Al, Ti, Zn, and Si may substitute for Fe | Alteration | alters to sericite or chlorite |
| Occurrence | common in medium grade pelitic compositions may be associated with garnet, andalusite, sillimanite, kyanite, cordierite, chloritoid, chlorite, muscovite, and biotite |
Distinguishing Features | colour, pleochroism, relief, and habit |
