GROUP 1 | GROUP 2 | SYSTEM | Isotropic | Cubic |
Anisotropic | Uniaxial(2RIs) | Hexagonal |
Tetragonal | ||
Biaxial(3RIs) | Orthorhombic | |
Monoclinic | ||
Triclinic |
Note: 'Group' refers to type of optical properites. First observe:
If yes - isotropic; If no - anisotropic (GROUP 1)
Shapes of unit cells are determined by the crystal system, with the following results:
CRYSTAL SYSTEM | AXES LENGTHS | AXIAL ANGLES | CUBIC | All equal | All 90 |
TETRAGONAL | a=b not = c | all 90 |
HEXAGONAL | two are equal | 2 are 90, one 120 |
ORTHORHOMBIC | all unequal | all are 90 |
MONOCLINIC | all unequal | two are 90 |
TRICLINIC | all unequal | none are 90 |
Example showing how optical properties that provide information about the crystal system can help distinguish similar looking gems!:
Blue sapphire - hexagonal - thus uniaxial.
Blue colored cubic zirconia (looks like sapphire) - cubic thus isotropic
Tanzanite (blue like sapphire) - orthorhombic thus biaxial.