Ca3Fe2Si3O12

Ca3Fe2Si3O12, commonly known as andradite, is a mineral in the garnet group. It represents the iron-rich end-member of garnet with calcium occupying the X site and ferric iron occupying the Y site, giving the ideal composition Ca3Fe2Si3O12 in the general garnet formula X3Y2(SiO4)3.

Andradite crystallizes in the isometric (cubic) system and typically forms well-shaped crystals such as dodecahedra and trapezohedra. It is isotropic in optical properties and has a vitreous to resinous luster. The color is variable, commonly olive-green to brown or black, reflecting iron content and oxidation state. The mineral is relatively hard and dense for a silicate garnet.

Formation and occurrence: andradite forms in high-temperature metamorphic environments and calc-silicate skarns where calcium-rich rocks interact with iron-bearing fluids. It commonly occurs with other garnets and minerals such as grossular, vesuvianite, diopside, calcite, and quartz. It is found worldwide in a range of geological settings, including contact metamorphic zones and skarn deposits, sometimes in association with emerald-bearing or chromian variants.

Nomenclature and significance: andradite is named for its composition and its place in the garnet group; the name is often linked to the mineralogist José Bonifácio de Andrada e Silva. As an indicator mineral, andradite helps characterize high-temperature, calcium-iron–rich metamorphic environments. Gem varieties exist in the broader andradite family, notably demantoid, a green chromium-rich form.