Topaz General Category Silicate mineral Formula (repeating unit) Al 2SiO 4(F,OH) 2 Crystal symmetry Orthorhombic dipyramidal H-M symbol: (2/m 2/m 2/m) Space group: Pbnm Unit cell a = 4.65 Å, b = 8.8 Å, c = 8.4 Å; Z = 4 Identification Color Colorless (if no impurities), blue, brown, orange,