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Nomenclature of garnets

All species of garnets possess similar physical properties and crystal forms, but differ in chemical composition. The species names are pyrope, almandine, spessartine, grossular, uvarovite and andradite.

Garnets are isomorphic members of the silicate group of minerals. Their general formula is X3Y2(Si O4)3. The X site is usually occupied elements like Ca2+, Mg2+, and Fe2+ while the Y site is occupied by Al3+, Fe3+, or Cr3+ ions. As most of these cations are similar in size and electrical charge, they are interchangeable in the crystal matrix and it is because of these substitutions which occurred during the process of crystallization, that we see such a large range of colors in garnets. It also explains why there is a considerable variation in the refractive indices as well as the specific gravities. The latter is very important to remember when testing garnets, as no value will be the same and values will fall within a range.

Garnets can be divided into two main groups, the Pyralspite group and the Ugrandite group. The species names represent end positions in a solid solution series where they describe a specific ideal chemistry. Within the series there exists a nearly limitless range of intermediary colors that correspond to the ion substitutions. In general, the Pyralspite group garnets occur in some shade of red, while the Ugrandite garnets are usually some shade of green.

Pyralspite group:

  • Almandine - iron aluminum silicate
  • Pyrope - magnesium aluminum silicate
  • Spessartine - manganese aluminum silicate

Ugrandite goup:

  • Andradite calcium iron silicate
  • Grossularite - calcium aluminum silicate
  • Uvaravite - calcium chromium silicate

The end member gemological designations are based on chemistry but as so many of the stones are of mixed compositions and varying colors, a host of trade and varietal names have evolved to describe these other garnets. These intermediary garnets may have some exotic trade names, but a strict definition of boundaries between the groups and the sub groups does not exist.

The list below includes the most important varieties, species, and trade names for garnet:

Alamandite	      Fe2+3Al2(SiO4)3
Andradite	      Ca3Fe3+2(SiO4)3
Color Change Garnet   [Mg3 + Mn3]AL2(SIO4) + Vanadium
Demantoid	      Ca3Fe3+2(SiO4)3
Grossularite	      Ca3Al2(SiO4)3
Hessonite	      Ca3Al2Si3O12
Malaia Garnet	      [Mg3 + Mn3]AL2(SIO4)
Mali Garnet	      Ca3Al2(SiO4)3
Pyrope	              Mg3Al2(SiO4)3
Rhodolite 	      Al2(SiO4)3
Spessartite   	      Mn2+3Al2(SiO4)3
Tsavorite	      Ca3Al2(SiO4)3

and some lesser known species:

Calderite	      Mn+23Fe+32(SiO4)3
Goldmanite	      Ca3V3+2(SiO4)3
Henritermierite	      Ca3Mn3+2(SiO4)2(OH)4
Hibschite	      Ca3Al2(SiO4)(3-x)(OH)4x (x= 0.21.5)
Katoite	              Ca3Al2(SiO4)(3-x)(OH)4x (x= 1.5-3)
Kerimasite	      Ca3Zr2(Fe+3O4)2(SiO4)
Kimzeyite	      Ca3Zr2(Al+3O4)2(SiO4)
Knorringite	      Mg3Cr2(SiO4)3
Majorite	      Mg3(Fe2+Si)(SiO4)3
Menzerite-(Y)	      Y2CaMg2(SiO4)3
Momoiite	      Mn2+3V3+2(SiO4)3
Morimotoite	      Ca3(Fe2+Ti4+)(SiO4)3
Schorlomite	      Ca3Ti4+2(Fe3+O4)2(SiO4)
Spessartine	      Mn2+3Al2(SiO4)3
Toturite	      Ca3Sn2(Fe3+O4)2(SiO4)
Uvaravite	      Ca3Cr2(SiO4)3
Calderite	      Mn+23Fe+32(SiO4)3
Goldmanite	      Ca3V3+2(SiO4)3
Henritermierite	      Ca3Mn3+2(SiO4)2(OH)4
Hibschite	      Ca3Al2(SiO4)(3-x)(OH)4x (x= 0.21.5)
Katoite	              Ca3Al2(SiO4)(3-x)(OH)4x (x= 1.5-3)
Kerimasite	      Ca3Zr2(Fe+3O4)2(SiO4)
Kimzeyite	      Ca3Zr2(Al+3O4)2(SiO4)
Knorringite	      Mg3Cr2(SiO4)3
Majorite	      Mg3(Fe2+Si)(SiO4)3
Melanite	      Ca3Fe3+2(SiO4)3 + Titanium
Menzerite-(Y)	      Y2CaMg2(SiO4)3
Momoiite	      Mn2+3V3+2(SiO4)3
Morimotoite	      Ca3(Fe2+Ti4+)(SiO4)3
Schorlomite	      Ca3Ti4+2(Fe3+O4)2(SiO4)
Spessartine	      Mn2+3Al2(SiO4)3
Toturite	      Ca3Sn2(Fe3+O4)2(SiO4)
Uvaravite	      Ca3Cr2(SiO4)3

Many garnets cannot be identified by color and refractive index alone and some form of spectral analysis is required.

Overview

Chemistry

Related

See also


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