Garnet

Garnet
General
CategoryNesosilicate
Formula
(repeating unit)
The general formula X3Y2(SiO4)3
IMA symbolGrt[1]
Crystal systemIsometric
Crystal class
Space groupIa3d
Identification
Colorvirtually all colors, blue is rare
Crystal habitRhombic dodecahedron or cubic
CleavageIndistinct
Fractureconchoidal to uneven
Mohs scale hardness6.5–7.5
Lustervitreous to resinous
StreakWhite
DiaphaneityCan form with any diaphaneity, translucent is common
Specific gravity3.1–4.3
Polish lustervitreous to subadamantine[2]
Optical propertiesSingle refractive, often anomalous double refractive[2]
Refractive index1.72–1.94
BirefringenceNone
PleochroismNone
Ultraviolet fluorescencevariable
Other characteristicsvariable magnetic attraction
Major varieties
PyropeMg3Al2Si3O12
AlmandineFe3Al2Si3O12
SpessartineMn3Al2Si3O12
AndraditeCa3Fe2Si3O12
GrossularCa3Al2Si3O12
UvaroviteCa3Cr2Si3O12

Garnets ( /ˈɡɑːrnɪt/) are a group of silicate minerals that have been used since the Bronze Age as gemstones and abrasives.

All species of garnets possess similar physical properties and crystal forms, but differ in chemical composition. The different species are pyrope, almandine, spessartine, grossular (varieties of which are hessonite or cinnamon-stone and tsavorite), uvarovite and andradite. The garnets make up two solid solution series: pyrope-almandine-spessartine (pyralspite), with the composition range [Mg,Fe,Mn]3Al2(SiO4)3; and uvarovite-grossular-andradite (ugrandite), with the composition range Ca3[Cr,Al,Fe]2(SiO4)3.

  1. ^ Warr, L. N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
  2. ^ a b Cite error: The named reference GRG was invoked but never defined (see the help page).