Staurolite 2Al2SiO5.Fe(OH)2
Optical Properties:
Biaxial +; very high 2V; colorless to yellow to yellow-brown
pleochroism; high relief; maximum interference color first order red; parallel
or symmetrical extinction; prismatic crystals. Distinguishing features are
color and pleochroism, and the typical presence of numerous quartz inclusions.
The photos above (crossed polarizers on left, plane polarized light on right)
show typical inclusion-rich staurolite and characteristic interference color
and color in plane light. The picture below, under crossed polarizers, shows
a large staurolite grain in a schist,
surrounded by brightly colored muscovite and
some quartz.
Occurrence
Staurolite occurs in regional dynamothermal metamorphosed shales,
as porphyroblasts associated with garnet,
biotite, muscovite,
and possibly kyanite.