sill i manite
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Sillimanite
Identification
Sillimanite is a metamorphic mineral found in high grade aluminous schists and gneisses. It is a polymorph
ofandalusite and kyanite, all having formula Al2SiO5.
Keys to identification are high relief, needle-like, fibrous or bladed habit, characteristic square cross sections
with one diagonal cleavage. It is clear and shows upper 2nd order interference colors.
Important Properties:
Colour Clear
Interference Colours Upper 2nd
order
Relief High Relief
Cleavage? Classic sillimanite forms needles with square cross sections that showone diagonal cleavage.
It may also appear as blades or as fine fibrous mats (calledfibrolite).
Images
Sillimanite, Quartz and Biotite in a Pelitic Gneiss
This sample contains needles of sillimanite (high relief and somewhat fractured), quartz, biotite and
magnetite. The sillimanite and quartz are colorless, the biotite is brown, and the magnetite is opaque (PP). In
the XP view, the quartz shows low order white-gray interference colors, while the biotite and
sillimanite higher order interference colors. The largest needle of sillimanite is about 2 mm long and shows
upper second order purple-red interference colors. Note the high reliefof the sillimanite compared to quartz.
http://www.und.nodak.edu/instruct/mineral/320petrology/opticalmin/andalusite.htmhttp://www.und.nodak.edu/instruct/mineral/320petrology/opticalmin/kyanite.htmhttp://www.und.nodak.edu/instruct/mineral/320petrology/opticalmin/relief.htmhttp://www.und.nodak.edu/instruct/mineral/320petrology/opticalmin/cleavage.htmhttp://www.und.nodak.edu/instruct/mineral/320petrology/opticalmin/birefringence.htmhttp://www.und.nodak.edu/instruct/mineral/320petrology/opticalmin/cleavage.htmhttp://www.und.nodak.edu/instruct/mineral/320petrology/opticalmin/relief.htmhttp://www.und.nodak.edu/instruct/mineral/320petrology/opticalmin/quartz.htmhttp://www.und.nodak.edu/instruct/mineral/320petrology/opticalmin/biotite.htmhttp://www.und.nodak.edu/instruct/mineral/320petrology/opticalmin/quartz.htmhttp://www.und.nodak.edu/instruct/mineral/320petrology/opticalmin/biotite.htmhttp://www.und.nodak.edu/instruct/mineral/320petrology/opticalmin/quartz.htmhttp://www.und.nodak.edu/instruct/mineral/320petrology/opticalmin/birefringence.htmhttp://www.und.nodak.edu/instruct/mineral/320petrology/opticalmin/biotite.htmhttp://www.und.nodak.edu/instruct/mineral/320petrology/opticalmin/birefringence.htmhttp://www.und.nodak.edu/instruct/mineral/320petrology/opticalmin/birefringence.htmhttp://www.und.nodak.edu/instruct/mineral/320petrology/opticalmin/relief.htmhttp://www.und.nodak.edu/instruct/mineral/320petrology/opticalmin/quartz.htmhttp://www.und.nodak.edu/instruct/mineral/320petrology/opticalmin/quartz.htmhttp://www.und.nodak.edu/instruct/mineral/320petrology/opticalmin/relief.htmhttp://www.und.nodak.edu/instruct/mineral/320petrology/opticalmin/birefringence.htmhttp://www.und.nodak.edu/instruct/mineral/320petrology/opticalmin/birefringence.htmhttp://www.und.nodak.edu/instruct/mineral/320petrology/opticalmin/biotite.htmhttp://www.und.nodak.edu/instruct/mineral/320petrology/opticalmin/birefringence.htmhttp://www.und.nodak.edu/instruct/mineral/320petrology/opticalmin/quartz.htmhttp://www.und.nodak.edu/instruct/mineral/320petrology/opticalmin/biotite.htmhttp://www.und.nodak.edu/instruct/mineral/320petrology/opticalmin/quartz.htmhttp://www.und.nodak.edu/instruct/mineral/320petrology/opticalmin/biotite.htmhttp://www.und.nodak.edu/instruct/mineral/320petrology/opticalmin/quartz.htmhttp://www.und.nodak.edu/instruct/mineral/320petrology/opticalmin/relief.htmhttp://www.und.nodak.edu/instruct/mineral/320petrology/opticalmin/cleavage.htmhttp://www.und.nodak.edu/instruct/mineral/320petrology/opticalmin/birefringence.htmhttp://www.und.nodak.edu/instruct/mineral/320petrology/opticalmin/cleavage.htmhttp://www.und.nodak.edu/instruct/mineral/320petrology/opticalmin/relief.htmhttp://www.und.nodak.edu/instruct/mineral/320petrology/opticalmin/kyanite.htmhttp://www.und.nodak.edu/instruct/mineral/320petrology/opticalmin/andalusite.htm -
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Staurolite with Fibrous Sillimanite (Fibrolite)
This sample, from near Poughkeepsie, New York, contains a large yellow mass ofstaurolite on the left hand
side (PP). The material to the right of the staurolite is mostly a mix of granular quartz and finefibrolite (fibrous
sillimanite, often forming mats or patches). Some of the sillimanite shows upper second order interference
colors but most is so fine grained that interference colors are hard to see. Minor muscovite is also present.
Quartz Adjacent to Biotite and Sillimanite
These photos are dominated by a large grain ofquartz. It is clear (PP), slightly fracture, and displays first order
gray interference colors. Other, smaller, quartz crystals are scattered around the periphery. A few small brown
(PP) biotite flakes are also present but are lost in the XP view. At the bottom, mostly on the left side, the
blocky high-reliefcrystals are sillimanite. Although sillimanite typically forms needles, here we are looking
down the axis of the needles so we see a square cross-section. Note that there is one
diagonal cleavage cutting across the square sillimanite end sections.
http://www.und.nodak.edu/instruct/mineral/320petrology/opticalmin/staurolite.htmhttp://www.und.nodak.edu/instruct/mineral/320petrology/opticalmin/staurolite.htmhttp://www.und.nodak.edu/instruct/mineral/320petrology/opticalmin/quartz.htmhttp://www.und.nodak.edu/instruct/mineral/320petrology/opticalmin/birefringence.htmhttp://www.und.nodak.edu/instruct/mineral/320petrology/opticalmin/birefringence.htmhttp://www.und.nodak.edu/instruct/mineral/320petrology/opticalmin/birefringence.htmhttp://www.und.nodak.edu/instruct/mineral/320petrology/opticalmin/muscovite.htmhttp://www.und.nodak.edu/instruct/mineral/320petrology/opticalmin/quartz.htmhttp://www.und.nodak.edu/instruct/mineral/320petrology/opticalmin/birefringence.htmhttp://www.und.nodak.edu/instruct/mineral/320petrology/opticalmin/quartz.htmhttp://www.und.nodak.edu/instruct/mineral/320petrology/opticalmin/biotite.htmhttp://www.und.nodak.edu/instruct/mineral/320petrology/opticalmin/relief.htmhttp://www.und.nodak.edu/instruct/mineral/320petrology/opticalmin/cleavage.htmhttp://www.und.nodak.edu/instruct/mineral/320petrology/opticalmin/cleavage.htmhttp://www.und.nodak.edu/instruct/mineral/320petrology/opticalmin/relief.htmhttp://www.und.nodak.edu/instruct/mineral/320petrology/opticalmin/biotite.htmhttp://www.und.nodak.edu/instruct/mineral/320petrology/opticalmin/quartz.htmhttp://www.und.nodak.edu/instruct/mineral/320petrology/opticalmin/birefringence.htmhttp://www.und.nodak.edu/instruct/mineral/320petrology/opticalmin/quartz.htmhttp://www.und.nodak.edu/instruct/mineral/320petrology/opticalmin/muscovite.htmhttp://www.und.nodak.edu/instruct/mineral/320petrology/opticalmin/birefringence.htmhttp://www.und.nodak.edu/instruct/mineral/320petrology/opticalmin/birefringence.htmhttp://www.und.nodak.edu/instruct/mineral/320petrology/opticalmin/birefringence.htmhttp://www.und.nodak.edu/instruct/mineral/320petrology/opticalmin/quartz.htmhttp://www.und.nodak.edu/instruct/mineral/320petrology/opticalmin/staurolite.htmhttp://www.und.nodak.edu/instruct/mineral/320petrology/opticalmin/staurolite.htm -
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Biotite-Sillimanite Schist with Graphite, from the Beartooth Mountains, Montana
This photomicrograph, about 2 mm across, is of a biotite-sillimanite schist. The biotite is brown and
shows second order interference colors in the XP view. Sillimanite in the upper left part of the photo is
relatively fine grained with high relief. Two other, coarser sillimanite grains are along the bottom edge. They
show blotchy interference colors ranging up to second order blue (XP). Also present is opaque
(black) graphite, flakey and intimately associated with the biotite. The clear (lower relief) material that is not
sillimanite, is quartz and K-feldspar. However, it is difficult to pick out the K-feldspar in these views, and some
of the quartz appears isotropic (although it is not).
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