gems, mineralogy, silicates, garnet

The garnets are a diverse group of minerals chemically, physically and in mode of occurrence. This diversity is brought about, in part, by the ability of the garnet structure to accommodate cations with a wide range of sizes and valence states. This varied chemistry results in a succession of colors from white, through shades of red, brown, yellow and green, to black. Hardness varies from 6.5 to 7.3 and specific gravity from approximately 3.5 to 4.3. Despite variations in the physical properties of garnets, their crystal morphology is rather limited with the predominant crystal forms being the dodecahedron {110} and the trapezohedron {211} or combinations of the two. Garnets can occur as stable phases in a wide range of pressures, temperatures and chemical environments. Although they are most commonly associated with contact and regional metamorphic rocks, they are also found in igneous rocks ranging from granite to peridotite as well as in felsic volcanics and pegmatites. Because of their resistance to chemical and mechanical breakdown they occur as detrital grains in sedimentary rocks. In the following section the mineralogy and crystal chemistry of silicate garnets will be discussed. Although the synthetic, non-silicate garnets constitute an unique and important group of compounds, they are not considered in this volume.