Neosilicates: Garnet

Question 1

Garnet General Formula

Answer 1

X3Y2(SiO4)3

Question 2

2 Garnet Groupings

Answer 2

Pyralspites

Ugrandites

Question 3

Pyralspites Members

Answer 3

Pyrope (Py) - Mg Al
Almandine (Al) - Fe Al
Spessartine (Sp) - Mn Al

Question 4

Ugrandites Members

Answer 4

Uvarovite (Uv) - Ca, Cr
Grossular (Gr) - Ca, Al
Andradite (An) - Ca, Fe

Question 5

Complete Solid Solutions in the Garnet Group

Answer 5

Py-Al, Al-Sp
Uv-Gr-An
Almost always found as solid solution

Question 6

Almandine (Al)

Answer 6

Iron, Aluminum

Fe3Al2(SiO4)3

Question 7

Pyrope (Py)

Answer 7

Mg, Al

Mg3Al2(SiO4)3

Question 8

Grossular

Answer 8

Ca, Al

Ca3Al2(SiO4)3

Question 9

Symmetry of Garnet crystallisation

Answer 9

Cubic

Question 10

Most common Garnet

Answer 10

Almandine

Question 11

Garnet colour in thin section

Answer 11

Colourless, Pale pink, Pale to dark brown, greenish-grey. Often zoned
Weak pleochroism

Question 12

Garnet form in thin section

Answer 12

Prismatic commonly euhedral

Question 13

Garnet Relief in thin section

Answer 13

High

Question 14

Garnet Cleavage in thin section

Answer 14

None but fractures common

Question 15

Garnet birefringence in thin section

Answer 15

Isotropic

Question 16

Garnet extinction in thin section

Answer 16

Isotropic

Question 17

Garnet twinning in thin section

Answer 17

Isotropic

Question 18

Garnet distinguishing features

Answer 18

Well-formed euhedral crystals, isotropic, very high relief, frequent incursions

Question 19

Garnet Paragenisis

Answer 19

Usually in medium-grade metamorphic rocks (Schists and Gneisses)

Question 20

Garnet structure

Answer 20

Alternating SiO4 tetrahedra and YO6 octahedra share corners to form a three-dimensional network