| Crystal System |
Triclinic |
| Chemical Comp.. |
NaAlSi3O8-CaAl2Si2O8 |
| Color |
Colorless |
| Pleochroism |
None |
| Cleavage |
Poor |
| Relief |
Low |
| Birefringence |
Gray/White |
| 2V | 77-78˚ |
| Optic Sign |
Na=Positive; Ca=Negative |
| Twinning |
Polysynthetic |
Plagioclase exhibits charactersitic lamellar (polysynthetic) twinning under cross polarized light. In metamorphic rocks, the polysynthetic twinning is less pronounced. Bi = Biotite; Plag = Plagioclase; Qz = Quartz; Zr = Zircon; Mu = Muscovite.
