Garnets may come directly from an igneous melt, and may be found in granites (Fe, Mn), and basalt flows (Mg, some Fe). They can also form from the gas phase at depth. A final mode of formation is in metamorphic rocks, where they may form in regional metamorphic environments composed of aluminum rich sediments (almandine), or in siliceous limestone where a magma and carbonate have come into contact (grossular).
Garnets can be made in the lab by powdering MgO, Al2O3, and SiO2. The powder is then placed inside
gold or platinum tubes which are welded shut. (Gold and platinum are used since they do not melt and corrode at high
temperatures and do not suffer oxidation). The capsule in then placed in a hydraulic press and brought to a pressure of
80-50,000 atm. An electrical current is run through the sample to attain temperatures of 1200 to 1400
. This
simulates conditions 150 km below the surface.
| name | formula | color (ions) | locations |
| almandine | Fe3Al2(SiO4)3 | dark red (Fe+2), purple | Colorado, Alaska, Nepal |
| andradite | Ca3Fe2(SiO4)3 | lime, yellow green (Cr+3) | Italy |
| grossular | Ca3Al2(SiO4)3 | pink (Fe+2), colorless (pure), | |
| pale green, orange-brown (Mn+2) | Mexico, Tanzania, Canada | ||
| pyrope | Mg3Al2(SiO4)3 | colorless (Mg, Al), red | Alps |
| spessartite | Mn3Al2(SiO4)3 | orange (Mn+2), dark red orange | &; |
| uvarovite | Ca3Cr2(SiO4)3 | dark green |
Synthetic garnets include the following,
| Abbr. | Name | Formula | Use |
| GGG | gadolinium gallium garnet | Gd3Ga2(GaO4)3 | Used in magnetic bubble memories |
| YAG | yttrium aluminum garnet | Y3Al2(AlO4)3 | Synthetic gemstones |
When doped with Nd+3, YAGs are useful as the lasing medium in lasers.
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