Garnet Group ( X 3 Y 2 Z 3 O 12 ). Garnets are diverse compositionally, physically, and in mode of occurrence - the garnet structure accommodates cations.

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Presentation transcript:

Garnet Group ( X 3 Y 2 Z 3 O 12 )

Garnets are diverse compositionally, physically, and in mode of occurrence - the garnet structure accommodates cations with a wide range of sizes and valence states - cation ordering lowers symmetry (Takeuchi and Haga, 1976) The color ranges from white through shades of Red, brown, yellow and green, to black Stable phases in a wide range of pressures, temperatures, and chemical environments

Garnet Structure Crystal system = Isometric Point group/crystal class = 4/m –3 2/m Space group = Ia 3d a= Z=8

Garnet Structure Site XYZ C. N. 864 Fractional Coordinate x 1/803/8 y 00 0 z 1/401/4

Garnet End-Members (Winchell, 1933) Ugrandite Group Pyralspite Group Uvarovite Ca 3 Cr 2 (SiO 4 ) 3 Pyrope Mg 3 Al 2 (SiO 4 ) 3 Grossular Ca 3 Al 2 (SiO 4 ) 3 Almandine Fe 3 Al 2 (SiO 4 ) 3 Andradite Ca 3 Al 2 (SiO 4 ) 3 Spessartine Mn 3 Al 2 (SiO 4 ) 3

Garnet Occurrences Pyrope - mineral of the upper mantle - ultra-basic rocks Almandine - common garnet in metamorphic rocks - regional metamorphism of argillaceous sediments Spessartine - occurs in skarn deposits and Mn-rich deposits Grossular - contact or regional metamorphism of impure limestones Andradite - may be the result of metamorphism of impure siliceous environments similar to that of grossular Uvarovite - rarest of garnets - found in serpentine associated with chromite

- Compression of Garnet at 300K End MemberPyrope GrossularAndradite Pmax (Gpa) Site bulk moduli (GPa) x 130(10) 115(13) 160 y(Al) 220(50) 220(50) 330(33) z(Si) 300(100) 300(100) 200(20) Reference Hazen & Finger Hazen & Finger Hazen & Finger (1989) (1978) (1989)

Garnets are differentiated by slight variations in color, density, and index of refraction. - - They have high indices of refraction - - Range of beautiful colors - - Transparent - - Lack cleavage and are durable Thus, make good candidates for gemstones - - semi-precious - - abundant - not as hard as other gemstones