| General Formula: Al2SiO5 |
Smaple: PT-78(2) | System: Orthorhombic |
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| Sillimanite var. prismatic Euhedral prismatic sillimanite, displaying a single cleavage. This is one of two common forms of sillimanite, the other is fibrolite. Field of View 2.7 mm, plane light |
Acute Bisectrix Note the low, first order grey, interference colours. The grains are oriented such that the view is looking down the c crystallographic axis and the Z indicatrix axis - the acute bisectrix. The rock type is a high grade pelite from the Grenville Province. Field of View 2.7 mm, crossed polars |
Block diagram showing the relationship between the crystallographic axes and the indicatrix axes. |
Optical Properties
| Colour Pleochroism |
colourless, mats of fibrolite may be pale brown non pleochroic |
Form | commonly occurs as slender prismatic crystals or as fine fibrous cyrstals called fibrolite |
| Relief RI |
high positive nalpha= 1653-1.661 nbeta= 1.657-1.662 ngamma= 1.672-1.683 |
Cleavage | one single good cleavage {010} parallel to the length of crystals |
| Birefringence Interference Colours |
0.018-0.022 lower second order |
Twinning | none reported |
| Interference Figure Optic Sign 2V |
biaxial positive 20-30° |
Optic Orientation | elongate sections show parallel extinction and are length slow |
| Composition | relatively pure Al2SiO5 with minor Fe3+, Ti4+ or Cr3+ | Alteration | may alter to sericite. May be involved in a variety of metamorphic reactions and display reaction textures with andalusite, sillimanite, staurolite, cordierite and garnet. |
| Occurrence | common mineral in medium to high grade pelitic schist, gneiss and related metamoprhic rocks. Associated with kyanite, andalusite, staurolite, muscovite, biotite, K-feldpsar, cordierite, corundum, and garnet. | Distinguishing Features | higher relief, moderate birefringence, parallel extinction, and habit |
