The garnet group is a group of nesosilicate (insular SiO4 groups) minerals including almandine, pyrope, spessartine, andradite, grossular and uvarovite.

Plutonic igneous environments
Sedimentary environments
Placer deposits
Metamorphic environments (typical)
Hydrothermal environments

Garnet is a common and widely distributed mineral occurring abundantly in some metamorphic rocks, and as an accessory constituent in some igneous rocks. It is also found in sedimentary environments including placers, and in hydrothermal replacement lodes.
Garnet may be found in granite, diorite, skarn, schist and gneiss.
Its most characteristic occurrence is in mica schist, hornblende schist and gneiss.
It is characteristic of the amphibolite facies, and it is also a mineral of the greenschist, granulite, blueschist and eclogite facies.
Almandine and spessartine are the typical garnets in granite and granite pegmatites, and andradite typically occurs in metamorphic rocks (Lauf p98-101).

Garnets are insoluble in water, hydrochloric, nitric and sulphuric acid


At San Shek Wan, Lantau Island, Islands District, New Territories, Hong Kong, China, it is necessary to visit the site at low tide, when the outcrop on the beach is exposed. The outcrop is a sequence of metamorphosed sandstone, metamorphosed siltstone and granite, and a small skarn body was also identified with abundant magnetite and quartz veins. Little red crystals of garnet were found in the magnetite (Geological Society of Hong Kong newsletter 16.1.2, Mineralogy Society of Hong Kong field trip).

At Sha Lo Wan, Lantau Island, Islands District, New Territories, Hong Kong, China, the exposed skarn zone is about 5 m wide, and is composed mainly of garnet, vesuvianite, diopside and epidote, with scattered magnetite (Hong Kong Minerals (1991). Peng, C J. Hong Kong Urban Council).

The Ma On Shan Mine, Ma On Shan, Sha Tin District, New Territories, Hong Kong, China, is an abandoned iron mine, with both underground and open cast workings. The iron ores contain magnetite as the ore mineral and occur predominantly as masses of all sizes enclosed in a large skarn body formed by contact metasomatism of dolomitic limestone at the margins of a granite intrusion. In parts of the underground workings magnetite is also found in marble in contact with the granite. The skarn rocks consist mainly of tremolite, actinolite, diopside and garnet (Hong Kong Minerals (1991). Peng, C J. Hong Kong Urban Council).

At the Urucum mine, Minas Gerais, Brazil, muscovite pseudomorphs after garnet have been found (KL p236).


albite, diopside and magnetite to aegirine, garnet and quartz
2Na(AlSi3O8) + CaMgSi2O6 + Fe2+Fe3+2O4 ⇌ 2NaFe3+Si2O6 + Si2O6 + CaMgFe2+Al2(SiO4)3 + SiO2
This reaction may occur in blueschist facies rocks in Japan (DHZ 2A p512).

calcium-iron amphibole and anorthite to garnet (grossular and almandine), clinozoisite and quartz
Ca2Fe3Si8O22(OH)2 + 6Ca(Al2Si2O8) ⇌ 4/3Ca3Al2(SiO4)3 + 5/3Fe3Al2(SiO4)3 + 2Ca2Al3[SiO7][SiO4]O(OH) + 5SiO2
(MM 48.206)

amphibole, chlorite, paragonite, ilmenite, quartz and calcite to garnet, omphacite, rutile, H2O and CO2
NaCa2(Fe2Mg3)(AlSi7)O22(OH)2 + Mg5Al(AlSi3O10)(OH)8 + 3NaAl2(Si3Al)O10(OH)2 + 4Fe2+Ti4+O3 + 9SiO2 + 4CaCO3 → 2(CaMg2Fe3)Al4(SiO4)6 + 4NaCaMgAl(Si2O6)2 + 4TiO2 + 8H2O + 4CO2 In low-grade rocks relatively rich in calcite the garnet-omphacite association may be due to reactions such as the above (DHZ 2A p453).

amphibole, clinozoisite, chlorite, albite, ilmenite and quartz to garnet, omphacite, rutile and H2O
NaCa2(Fe2Mg3)(AlSi7)O22(OH)2 + 2Ca2Al3[Si2o7][SiO4]O(OH) + Mg5Al(AlSi3O10)(OH)8 + 3NaAlSi3O8 + 4Fe2+Ti4+O3 + 3SiO2 → 2(CaMg2Fe3)Al4(SiO4)6 + 4NaCaMgAl(Si2O6)2 + 4TiO2 + 6H2O In low-grade rocks relatively poor in calcite the garnet-omphacite association may be developed by the above reaction (DHZ 2A p453).

augite, albite, pyroxene, anorthite and ilmenite to omphacite, garnet, quartz and rutile
2MgFe2+Si2O6 + Na(AlSi3O8) + Ca2Mg2Fe2+Fe3+AlSi5O18 + 2Ca(Al2Si2O8) + 2Fe2+Ti4+O3 → NaCa2MgFe2+Al(Si2O6)3 + (Ca2Mg3Fe2+4)(Fe3+Al5)(SiO4)9 + SiO2 + 2TiO2 This reaction occurs at high temperature and pressure (DHZ 2A p449).

enstatite-ferrosilite, Fe-rich diopside and Fe, Cr-rich spinel to garnet and olivine
2(Mg,Fe2+)SiO3 + Ca(Mg,Fe)Si2O6 + (Mg,Fe)(Al,Cr)2O4 ⇌ Ca(Mg,Fe)2(Al,Cr)2(SiO4)3 + (Mg,Fe)2SiO4
(DHZ 2A p258)

enstatite-ferrosilite, augite and Fe and Cr-rich spinel to garnet and olivine
2(Mg,Fe)SiO3 + Ca(Mg,Fe)Si2O6 + (Mg,Fe)(Al,Cr)2O4 ⇌ Ca(Mg,Fe)2(Al,Cr)2(SiO4)3 + (Mg,Fe)2SiO4
(DHZ 2A p258)

labradorite, albite, forsterite and diopside to omphacite, garnet and quartz
3CaAl2Si2O8 + 2Na(AlSi3O8) + 3Mg2SiO4 + nCaMgSi2O6 → (2NaAlSi2O6 + nCaMgSi2O6) + 3(CaMg2)Al2(SiO4)3 + 2SiO2
This reaction occurs at high temperature and pressure (DHZ 2A p449).

muscovite, biotite and SiO2 to K-feldspar, garnet and H2O
KAl2(AlSi3O10)(OH)2 + K(Fe2+,Mg)3(AlSi3O10)(OH)2 + 3SiO2 → 2KAlSi3O8 + (Fe2+,Mg)3Al2(SiO4)3 + 2H2O
(DHZ 3 p23)

muscovite and garnet to biotite, sillimanite and quartz
KAl2(AlSi3O10)(OH)2 + (Fe2+,Mg)3Al2(SiO4)3 → K(Fe2+,Mg)3(AlSi3O10)(OH)2 + 2Al2SiO5 + SiO2
Muscovite is unstable in combination with garnet (DHZ 3 p24).

meionite (scapolite series) and augite to garnet, calcite and quartz
Ca4Al6O24(CO3) + 3Ca(Mg,Fe2+)Si2O6 ⇌ 3Ca2(Mg,Fe2+)Al2(SiO4)3 + CaCO3 + 3SiO2
(DHZ 4 p334)

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