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The garnet group is a group of nesosilicate (insular SiO4 groups) minerals including
almandine,
kimzeyite,
pyrope,
spessartine,
andradite,
grossular and
uvarovite.
Environments:
Plutonic igneous environments
Pegmatites
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
Localities
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).
Alteration
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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