GrossularGrossular is a calcium-aluminium garnet with the formula Ca3Al2(SiO4)3, though the calcium may in part be replaced by ferrous iron and the aluminium by ferric iron. The name grossular is derived from the botanical name for the gooseberry, grossularia, in reference to the green garnet of this composition that is found in Siberia. Other shades […]
Pyrope Pyrope , from the Latin pyropos, means similar to fire. It is red in color and chemically a magnesium aluminium silicate with the formula Mg 3Al 2(SiO 4) 3, though the magnesium can be replaced in part by calcium and ferrous iron. The color of pyrope varies from deep red to almost black. Transparent […]
Spessartite or spessartine is manganese aluminium garnet, Mn3Al2(SiO4)3. Its name is derived from Spessart in Bavaria. It occurs most often in granite pegmatite and allied rock types and in certain low grade metamorphic phyllites. Spessartite of a beautiful orange-yellow is found in Madagascar (see Mandarin garnet). Violet-red spessartites are found in rhyolites in Colorado and […]
Hardness Because the chemical composition of garnet varies, the atomic bonds in some varieties are stronger than in others. As a result, the mineral shows a range of hardness on the Mohs Scale of about 6.5 to 7.5. The harder varieties, like almandite, are often used for abrasive purposes.Garnet group endmembersPyralspite garnets - Al in […]
Crystal Structure
Garnets are nesosilicates having the general formula X 3Y 2(SiO 4) 3. The X site is usually occupied by divalent cations (Ca 2+, Mg 2+, Fe 2+) and the Y site by trivalent cations (Al 3+, Fe 3+, Cr 3+) in an octahedral/tetrahedral framework with [SiO 4] 4- providing the tetrahedra. Garnets are most […]
Physical Properties
Appearance
Garnets are found in many colors including red, orange, yellow, green, blue, purple, brown, black, and colorless. The rarest of these is the blue garnet, discovered in the late 1990s in Bekily, Madagascar. It is also found in some parts of the United States and Russia. It changes color from blue-green in the […]