7/27/2019 Sill i Manite

1/3

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

7/27/2019 Sill i Manite

2/3

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

7/27/2019 Sill i Manite

3/3

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).

http://www.und.nodak.edu/instruct/mineral/320petrology/opticalmin/biotite.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/relief.htmhttp://www.und.nodak.edu/instruct/mineral/320petrology/opticalmin/birefringence.htmhttp://www.und.nodak.edu/instruct/mineral/320petrology/opticalmin/graphite.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/quartz.htmhttp://www.und.nodak.edu/instruct/mineral/320petrology/opticalmin/kspar.htmhttp://www.und.nodak.edu/instruct/mineral/320petrology/opticalmin/kspar.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/kspar.htmhttp://www.und.nodak.edu/instruct/mineral/320petrology/opticalmin/kspar.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/biotite.htmhttp://www.und.nodak.edu/instruct/mineral/320petrology/opticalmin/graphite.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/birefringence.htmhttp://www.und.nodak.edu/instruct/mineral/320petrology/opticalmin/biotite.htmhttp://www.und.nodak.edu/instruct/mineral/320petrology/opticalmin/biotite.htm