When speaking of Garnets most people think of the red gem that has already been highly appreciated by the ancient romans. However, garnets are much more. Instead of being only one particular mineral the Garnets represent an entire group of minerals that occus in many differnet colors, shapes, and sizes.
Garnets: Almandine, Andradite, Demantoide, Grossular, Hessonite, Pyrope, Rhodolithe, Spessartine, Tsavorite, Uvarovite, .....
The group of garnet minerals belongs to the group of complex silicate minerals. Chemically, all garnets have the formula A3B2[CO4]3. Every variabel (A, B, C) in this formula stands for certain chemical element. For example, position a can be occupied by two-fold positively charged ions (cations), such as Ca2+, Mg2+, Fe2+, or Mn2+. Position B, in contrast, is filled with a cation with charge +3 (Al3+, Fe3+, Cr3+, Ti3+, Mn3+, V3+, Zr3+). In most cases, position C is filled by silicium, Si4+, which forms together with four oxigen atoms a [SiO4]4- ion, which is an anion. In few cases an [SiO4]4--ion can be replaced by four hydroxyl groups [OH]-, forming the group of so-called hydro-garnets. Depending on the combination of elements in the formula several different Garnets can be formed, which have many different colors and other properties.
Due to their nature, i.e. their size and their charge, the elements are arranged in a certain geometry, which is repeated over and over again in a lattice (crystal-lattice) finally forming the macroscpopic crystal. The crystals of garnets belong to the cubic crystal system (For experts, they belong to the crystal class m3m).
The most common habitus for garnets is the rhombododekaeder consisiting of 12 rhombic faces (a rhombic face has four edges of equal length which have no right angle, right picture). Another common form of the garnet the ikositetraeder, which has 24 faces that have edges of different length (left picture).
Of the many theoretically possible combinations of chemical elements only some occur naturally. The most important are listed below:
Almandine
Formula: Fe3Al2[SiO4]3
Hardness: 7 to 7.5
Density: 4.32 g/cm3
Refractive index: nD= 1.83
Color: deep red
Size: up to several cm
Occurence: very abundant
Grossular
Formula: Ca3Al2[SiO4]3
Hardness: 6.5 to 7
Density: 3.6 g/cm3
Refractive index: nD= 1.734
Color: colorless, green, brown, yellow, orangered
Size: up to several cm
Occurence: abundant
Variant: Tsavorite (Tsavolithe): mint green to emerald green (color from Vanadium and Chromium)
Variant: Hessonite: yellow to red-brown (color from Fe3+)
Variant: Leukogarnet: colorless variant of grossular
The color may vary within grossulars due to changes in chemistry during crystall-growth:
Spessartine or Spessartite
Formula: Mn3Al2[SiO4]3
Hardness: 7 to 7.25
Density: 4.19 g/cm3
Refractive index: nD= 1.8
Color: organge to orange-red, brown, red, black
Size: up to few cm
Occurence: relatively abundant
Andradite
Formula: Ca3Fe2[SiO4]3
Hardness: 6.5 to 7
Density: 3.86 g/cm3
Refractive index: nD= 1.887
Color: brown, yellow, green, red, black
Size: up to several cm
Variant: Demantoide - colored green by chromium
Variant: Topazolith - yellowish to sherry-brown (similar to Topaz)
Occurence: abundant
Pyrope
Formula: Mg3Al2[SiO4]3
Hardness: 7 to 7.5
Density: 3.58 g/cm3
Refractive index: nD= 1.714
Color: blood red
Size: usually small (mm to cm), often mixed crystalls with Almandine (Rhodolithe, several cm)
Occurence: quite rare
Uvarovite
Formula: Ca3Cr2[SiO4]3
Hardness: 7 to 7.5
Density: 3.83 g/cm3
Refractive index: nD= 1.865
Color: green to emerald-green
Size: few mm, in rare cases 1 to 2 cm
Occurence: very rare
Melanite
Ca-Fe-Ti Garnet with less than 3 Si-atoms per formula-unit
Hydrogrossulare
Formula: Ca3Fe2[SiO4]3 - Does not form crystalls
Katoite
Formula: Ca3Al2{[SiO4]1-1.5[OH]8-6}
Schorlomite
Formula: Ca3(Ti,Fe3+)2[(Si,Fe3+)O4]3
Demantoide
(Chromium variant of Andradite)
Topazolith (Variety of Andradite)
Tsavorite (Vanadium-chromium variant of Grossular)
Color can vary from mint green to a deep emerald green.
Hessonite (Iron(III)-variant of Grossular)
Leukogarnet (colorless or white variant of Grossular)
Rhodolithe (Mixed-crystal between Almandine and Pyrope)
Graphic-Garnet (Eutectic mixture of quartz and garnet)
A eutectic mixure is a mixture of two components that crystalizes at a lower temperature than each of the two individual components. This means that during crystallization, i.e. when the heat in the pegmatite drops, a mixure of quartz and garnet stays liquid below the crystallization conditions of pure quartz and pure garnet. Then, at the eutectic temperature both components suddenly crystallize forming an "intergrown" mixture of microscopic regims of quartz and garnet.
Reference and Book Tip:
extra Lapis No. 9: Granat, Christian Wiese Publishers, Munich (ISBN 3-921656-35-4)
A very good introduction as well as a lot of very advanced information for experts - excellent pictures.