grossularandradite

Grossularandradite is a garnet mineral comprising the solid-solution series between grossular, Ca3Al2Si3O12, and andradite, Ca3Fe2Si3O12. In this series, calcium occupies the X site and aluminum or iron occupies the Y site of the garnet structure, giving the simplified composition Ca3(Al,Fe)2Si3O12. The endmembers represent the aluminum-rich and iron-rich extremes, respectively.

Grossularandradite garnets occur in metamorphic rocks such as skarns and contact metamorphic zones, and in some igneous rocks where calcium- and iron-rich fluids crystallize garnet. The series forms a continuous solid solution, with composition varying from grossular toward andradite as Al is replaced by Fe3+ (and accompanying substitutions in the X and Y sites). Trace components such as Mn, Cr, and V can occur.

Gemstone varieties arise from this series. Green grossular garnet, known as tsavorite, is prized as a gemstone; green andradite varieties include demantoid. Color in the series is controlled by trace elements such as Cr, V, and Fe; pure endmembers are seldom found in nature.

Physical properties of garnets in this series reflect their isometric crystal structure. They crystallize in the cubic system and commonly form dodecahedra or trapezohedra, with a vitreous luster and transparency ranging from transparent to translucent. Hardness on the Mohs scale is typically about 6.5–7.5, and specific gravity is roughly 3.6–4.0, increasing with iron content.