Structure | Crystal axes and Angles | Optic Character | Refractive Indexes | Optic Sign | Pleochroism | Examples |
---|---|---|---|---|---|---|
Amorphous | No orderNo axes | IsotropicSingly refractive | 1 RI | None | None | Opal, Amber, Glass, Plastic |
Isometric | 1 axis lengthAll at 90° | IsotropicSingly refractive | 1 RI | None | None | Diamond, Spinel, Garnet |
Tetragonal | 2 axis lengths All at 90° | AnisotropicDoubly refractive | 2 RI’s | Uniaxial | May be dichroic | Zircon |
Hexagonal | 2 axis lengths | AnisotropicDoubly refractive | 2 RI’s | Uniaxial | May be dichroic | Quartz, Beryl, Apatite, Corundum, Tourmaline |
Orthorhombic | 3 axis lengthsAll at 90° | AnisotropicDoubly refractive | 3 RI’s | Biaxial | May be trichroic | Topaz, Zoisite, Peridot |
Monoclinic | 3 axis lengths 2 axes at 90°; 1 oblique | AnisotropicDoubly refractive | 3 RI’s | Biaxial | May be trichroic | Orthoclase, Spodumene |
Triclinic | 3 axis lengths all axes oblique | AnisotropicDoubly refractive | 3 RI’s | Biaxial | May be trichroic | Labradorite, Axenite |
GEM STRUCTURE CHART
Compliments ofThe International Gem Society
Minerals that form in the tetragonal system form one of these three basic shapes. “Tetragonal System” by Donald Clark.