Biaxial Birefringence
| Material | Crystal system | nα | nβ | nγ |
|---|---|---|---|---|
| borax Na2(B4O5)(OH)4·8(H2O) | Monoclinic | 1.447 | 1.469 | 1.472 |
| epsom salt MgSO4·7(H2O) | Monoclinic | 1.433 | 1.455 | 1.461 |
| mica, biotite K(Mg,Fe)3AlSi3O10(F,OH)2 | Monoclinic | 1.595 | 1.640 | 1.640 |
| mica, muscovite KAl2(AlSi3O10)(F,OH)2 | Monoclinic | 1.563 | 1.596 | 1.601 |
| olivine (Mg, Fe)2SiO4 | Orthorhombic | 1.640 | 1.660 | 1.680 |
| perovskite CaTiO3 | Orthorhombic | 2.300 | 2.340 | 2.380 |
| topaz Al2SiO4(F,OH)2 | Orthorhombic | 1.618 | 1.620 | 1.627 |
| ulexite NaCaB5O6(OH)6•5(H2O) | Triclinic | 1.490 | 1.510 | 1.520 |
Biaxial birefringence, also known as trirefringence, describes an anisotropic material that has more than one axis of anisotropy. For such a material, the refractive index tensor n, will in general have three distinct eigenvalues that can be labeled nα, nβ and nγ.
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