garnet

garnet An important rock-forming mineral group, with the general formula X₃Y₂Si₃O₁₂, where X may be Ca, Mg, Fe²⁺, or Mn and Y may be Al, Fe³⁺, or Cr³⁺; the main minerals are grossular (X =Mg, Y = Al), pyrope (X = Mg, Y = Al), almandine (X = Fe²⁺, Y = Al), spessartine (X=Mn, Y=Al), andradite (X = Ca, Y = Fe³⁺), and uvarovite (X = Ca, Y = Cr³⁺), and there is continuous chemical variation in the group; an unusual variety called hydrogrossular Ca₃Al₂[SiO₄]₂[SiO₄]_{1 − m} (OH)_4m has hydroxyl ions in the structure and is found in the rare rock type rodingite; sp. gr. 3.6–4.3; hardness 7.0–7.5; colour very variable depending on its chemical composition, and can vary from shades of deep red-brown to almost black, green, white, yellow, and brown; usually vitreous lustre; crystals cubic, with the most common form being dodecahedra; no cleavage; found in high-grade metamorphic and igneous rocks, in beach sands, and alluvial placers. Transparent pyrope crystals may be used as gemstones, but garnet is more generally used as an abrasive.