University:
Massachusetts Institute of Technology
Course:
12.109 | Petrology
Academic year:

2019
Views:

306

Pages:

1
Author:

Sasha Delacruz

Garnet Group X3Y2(SiO4)3 The garnet structure consists of isolated silica tetrahedra bonded laterally through cations in 6-fold and 8-fold coordination (see figure 16-5). In terms of composition, natural garnets can be divided into two groups – Pyralspite and Grandite. The Pyralspites (pyrope, almandine, and spessartine) all have Al3+ in the Y site and extensive substitution in the X site by Fe2+, Mg, and Mn. The grandite group contains grossular and andradite (and if uvarovite is included it becomes the ugrandite group). The grandites contain Ca2+ in the X site. Identification: Hand Sample: Thin Section: Plane Light: Very high positive relief. The color in thin section is usually colorless or a pale version of the hand sample color. No pleochroism. Garnets do not have cleavage and usually display fracturing. Crystals are usually euhedral to subhedral dodecahedrons or trapezohedrons. Crossed Polars: Isotropic. Members of the Garnet group: Type Formula Pyrope Mg3Al2(SiO4)3 Almandine Fe3Al2(SiO4)3 Spessartine Mn3Al2(SiO4)3 Grossular Ca3Al2(SiO4)3 Andradite Ca3Fe2(SiO4)3 Uvarovite Ca3Cr2(SiO4)3 Color in Hand Sample Deep red to nearly black Fine, deep red Brownish to red White, green, yellow, to pale brown Various shades of yellow and green Emerald-green