Garnet is the name of a group of silicate minerals with the general formula A3B2(SiO4)3, the six common garnets being almandine (Fe,Al), grossular (Ca,Al), pyrope (Mg,Al), spessartine (Mn,Al), andradite (Ca,Fe), and uvarovite (Ca,Cr). All garnets are hard (6.5–7.5 on the Mohs scale), resistant to physical abrasion and chemical attack, and have relatively high melting points. Some varieties are used as semi-precious stones, but the principal commercial use is as an abrasive in sandblasting, abrasive wheels, sandpaper, and polishing grits and powders. The relatively high SG of 3.7–4.2 makes garnet a useful medium for water filtration. The preferred commercial variety is almandine although andradite is also used.
World production in 2013 was 1.7 Mt with Australia producing approximatley 260,000 tonne.
South Australia has had no significant garnet production even though it is a common accessory mineral in metamorphic rocks throughout the State, particularly in Cambrian Kanmantoo Group metasediments of the eastern Mount Lofty Ranges. Occurrences of almandine have been investigated in shear zones within Willyama Supergroup metasediments near Mutooroo homestead in the Olary Province. It is found in uneconomic concentrations in some mineral sand deposits, including sediments along the western coast of Lake Alexandrina which have been a recent exploration target.
Andradite garnet specimen
In the operating Kanmantoo copper mine Hillgrove resources ltd reported potential for recovery of garnet form host garnetiferous metasediment with >12% garnet conent.
A small deposit containing discontinuous lenses rich in dark brown andradite garnet has been drilled at Milendella, 15 km northwest of Mannum. Here, impure calcareous sediments of Cambrian age have been converted to andradite–pyroxene–magnetite skarn by intrusions of Late Cambrian to Early Ordovician granites. 13 lensoid andradite garnet bodies to 100 m long x 6 m wide occur within a 1 km distance. No resource figure has been determined.