Mn3Al2Si3O12

Mn3Al2Si3O12, commonly known as spessartine garnet, is a mineral species in the garnet group. It represents the Mn-rich end-member of the garnet solid solution A3B2Si3O12, where A is a divalent cation such as Mn2+ and B is Al3+. The name spessartine derives from the Spessart region of Germany, a classical source of Mn-rich garnets.

Chemical composition and structure: The compound adopts the isometric garnet structure, with Mn2+ occupying the eight-coordinate A-site and Al occupying the six-coordinate B-site in the lattice of SiO4 tetrahedra. The end-member Mn3Al2Si3O12 occurs as orange to reddish-brown crystals. Spessartine garnet typically has a vitreous luster and a high refractive index characteristic of garnets.

Physical properties: Crystals are typically dodecahedra or trapezohedra in the cubic system. Hardness ranges from 6.5 to 7.5 on the Mohs scale, and specific gravity is commonly around 3.8 to 4.3. The color in cut gemstones ranges from bright orange to reddish orange, depending on impurities and Mn content.

Occurrence and formation: Spessartine occurs in high-grade metamorphic rocks such as granulites and schists, and in some granitic pegmatites. It commonly associates with quartz, feldspars, micas, kyanite, and tourmaline. Important historic and current sources include Mn-rich garnets from the Spessart region, as well as deposits in Namibia, Brazil, the United States, and other locales.

Uses and significance: As a gemstone, spessartine is valued for its vibrant orange to red colors and is cut into faceted stones and cabochons. Garnets, including spessartine, are also used as industrial abrasives due to their hardness and toughness.