Neosilicates: Garnet Flashcards
Garnet General Formula
X3Y2(SiO4)3
2 Garnet Groupings
Pyralspites
Ugrandites
Pyralspites Members
Pyrope (Py) - Mg Al
Almandine (Al) - Fe Al
Spessartine (Sp) - Mn Al
Ugrandites Members
Uvarovite (Uv) - Ca, Cr
Grossular (Gr) - Ca, Al
Andradite (An) - Ca, Fe
Complete Solid Solutions in the Garnet Group
Py-Al, Al-Sp
Uv-Gr-An
Almost always found as solid solution
Almandine (Al)
Iron, Aluminum
Fe3Al2(SiO4)3
Pyrope (Py)
Mg, Al
Mg3Al2(SiO4)3
Grossular
Ca, Al
Ca3Al2(SiO4)3
Symmetry of Garnet crystallisation
Cubic
Most common Garnet
Almandine
Garnet colour in thin section
Colourless, Pale pink, Pale to dark brown, greenish-grey. Often zoned
Weak pleochroism
Garnet form in thin section
Prismatic commonly euhedral
Garnet Relief in thin section
High
Garnet Cleavage in thin section
None but fractures common
Garnet birefringence in thin section
Isotropic
Garnet extinction in thin section
Isotropic
Garnet twinning in thin section
Isotropic
Garnet distinguishing features
Well-formed euhedral crystals, isotropic, very high relief, frequent incursions
Garnet Paragenisis
Usually in medium-grade metamorphic rocks (Schists and Gneisses)
Garnet structure
Alternating SiO4 tetrahedra and YO6 octahedra share corners to form a three-dimensional network
