| Crystal System |
Orthorhomic |
| Chemical Comp.. |
Al2SiO5 |
| Color |
Colorless |
| Pleochroism |
Pale
yellow-brown in thick sections |
| Cleavage |
One
good |
| Relief |
High |
| Birefringence |
Usually first order up to low second
order |
| 2V | 21-30˚ |
| Optic Sign | Positive |
| Twinning |
None |
Shown
in the photomicrographs are fibrous aggregates of sillimanite.