University of Colorado
Mineral Structure Data
Garnet
The garnet group is composed of minerals of the general formula
X3Y2Z3O12
where X = Mg ,Fe2+, or Ca; Y = Al, Cr, or Fe3+;
and Z = Si. The X-site is 8 coordinated, the Y -site is 6-coordinated
(octahedral) and the Z-site is 4-coordinated (tetrahedral).
The crystal structure of garnet showing its framework like structure of
octahedra and tetrahedra. In the cubic structures, all oxygens are
identical and are each bonded to one tetrahedron and one octahedron and
to two of the divalent dodecahedral sites (circles). Oxygen lies in a general position (point symemtry 1), and there are 96 per unit cell.
Pyrope Almandine Spessartine Grossular Andradite Uvarovite
Mg3Al2Si3O12 Fe3Al2Si3O12 Mn3Al2Si3O12 Ca3Al2Si3O12 Ca3Fe2Si3O12 Ca3Cr2Si3O12
Form.Wt. 403.15 497.76 495.03 403.15 508.19 500.48
Z 8 8 8 8 8 8
CrystalSystem Cub Cub Cub Cub Cub Cub
PointGroup 4/m32/m 4/m32/m 4/m32/m 4/m32/m 4/m32/m 4/m32/m
SpaceGroup Ia3d Ia3d Ia3d Ia3d Ia3d Ia3d
UnitCell
a(Å) 11.452 11.531 11.612 11.845 12.058 11.988
Vol 1501.9 1533.2 1565.7 1661.9 1753.2 1722.8
MolarVol 113.08 115.43 117.88 125.12 131.99 129.71
Density 3.565 4.312 4.199 3.600 3.850 3.859
Thermal Expansion(Volumetric)
alpha 19.9 15.8 17.2 16.4 20.6
a0 0.2311 0.1776 0.2927 0.1951 0.2103
a1 0.5956 1.2140 0.2726 0.8089 0.6839
a2 -0.4538 -0.5071 -1.1560 -0.4972 -0.2245
Elastic Parameters
Ks(GPa) 177 178.8 168.4 157
G(GPa) 92.0 96.3 108.9 90.
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Garnet Group X-Sites.
End-Member Pyrope Almandine Spessartine Grossular Andradite Uvarovite
C.N. 8 8 8 8 8 8
Occupant Mg Fe Mn Ca Ca Ca
Point Sym. 222 222 222 222 222 222
Wyckoff Not. 24c 24c 24c 24c 24c 24c
Frac.Coord.
x 1/8 1/8 1/8 1/8 1/8 1/8
y 0 0 0 0 0 0
z 1/4 1/4 1/4 1/4 1/4 1/4
Distances
1(4) 2.197 2.220 2.245 2.319 2.365 2.360
2(4) 2.343 2.378 2.406 2.490 2.500 2.499
Mean 2.270 2.299 2.326 2.405 2.433 2.429
sigma .078 .084 .086 .091 .072 .075
Poly.Vol. 20.14 20.93 21.65 23.88 24.55 24.48
Site Energy -1126. -1101. -1080. -1022. -1015. -1010.
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Garnet Group Y-Sites.
End-Member Pyrope Almandine Spessartine Grossular Andradite Uvarovite
C.N. 6 6 6 6 6 6
Occupant Al Al Al Al Fe+3 Cr+3
Point Sym. -3 -3 -3 -3 -3 -3
Wyckoff Not. 16a 16a 16a 16a 16a 16a
Frac.Coord.
x 0 0 0 0 0 0
y 0 0 0 0 0 0
z 0 0 0 0 0 0
Distances
O(6) 1.887 1.896 1.901 1.924 2.024 1.985
Poly.Vol. 8.937 9.086 9.155 9.491 11.046 10.413
Q.E. 1.0014 1.0004 1.0001 1.0007 1.0004 1.0007
Ang.Var. 4.93 1.38 0.30 2.33 1.36 2.64
Site Energy -2666. -2655. -2658. -2640. -2455. -2527.
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Garnet Group Z-Sites.
End-Member Pyrope Almandine Spessartine Grossular Andradite Uvarovite
C.N. 4 4 4 4 4 4
Occupant Si Si Si Si Si Si
Point Sym. -4 -4 -4 -4 -4 -4
Wyckoff Not. 24d 24d 24d 24d 24d 24d
Frac.Coord.
x 3/8 3/8 3/8 3/8 3/8 3/8
y 0 0 0 0 0 0
z 1/4 1/4 1/4 1/4 1/4 1/4
Distances
O(4) 1.635 1.628 1.636 1.645 1.643 1.643
Poly.Vol. 2.192 2.172 2.206 2.261 2.250 2.257
Q.E. 1.0150 1.0134 1.0117 1.0073 1.0071 1.0058
Ang.Var. 61.6 55.2 48.0 29.8 28.9 23.9
Site Energy -4380. -4413. -4400. -4402. -4432. -4426.
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Mineral Data Base Index
GEOL 1010 Syllabus
GEOL 1010 Syllabus
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