Almandine

nesosilicate

metamorphic

Almandine is the commonest mineral of the garnet group.
Formula: Fe2+3Al2(SiO4)3 nesosilicate (insular SiO4 groups)
Specific gravity: 4.318
Hardness: 7 to 7½
Streak: White
Colour: Red, black
Solubility: Insoluble in water, hydrochloric, nitric and sulphuric acid
Environments:

Plutonic igneous environments
Pegmatites
Metamorphic environments (common)

Almandine is the common garnet in metamorphic rocks, typically occuring in mica schist, gneiss and amphibolite, resulting from the regional metamorphism of argillaceous (clay-rich) sediments. It also occurs in contact metamorphic hornfels, and occasionally in plutonic rocks such as diorite, granite and granite pegmatites (Lauf p107). It is stable over a wide range of pressure-temperature conditions. Metamorphic almandine is a mineral of the hornblende-hornfels, amphibolite, granulite, blueschist and eclogite facies.

Alteration

almandine and phlogopite to pyrope and annite
Fe2+3Al2(SiO4)3 + KMg3AlSi3O12(OH)2 ⇌ Mg3Al2Si3O12 + KFe3AlSi3O10(OH)2
Both temperature and pressure affect the equilibrium of this reaction, but temperature is more significant (JVW p 179). This assemblage is commonly formed during amphibolite facies metamorphism of pelitic rocks (KB p129).

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)

calcium amphibole, grossular and quartz to diopside- hedenbergite, anorthite, pyrope-almandine and H2O
2Ca2(Mg,Fe2+)3Al4Si6O22(OH)2 + Ca3Al2(SiO4)3 + SiO2 = 3Ca(Fe,Mg)Si2O6 + 4Ca(Al2Si2O8) + (Mg,Fe2+)3Al2(SiO4)3 + 2H2O
Diopside-hedenbergite occurs commonly in regionally metamorphosed calcium-rich sediments and basic igneous rocks belonging to the higher grades of the amphibolite facies, where it may form according to the above reaction (DHZ 2A p272).

chloritoid and quartz to staurolite, almandine and H2O
23Fe2+Al2O(SiO4)(OH)2 + 8SiO2 ⇌ 4Fe2+2Al9Si4O23(OH) + 5Fe2+3Al2(SiO4)3 + 21H2O (DHZ 1A p844)

hornblende, grossular and quartz to Fe-rich diopside, anorthite, almandine and H2O
2Ca2(Mg,Fe2+)3(Al4Si6)O22(OH)2 + Ca3Al2Si3O12 + 2SiO2 = 3Ca(Mg,Fe2+)Si2O6 + 4CaAl2Si2O8 + (Mg,Fe2+)Al2Si3O12 + 2H2O
Fe-rich diopside occurs commonly in regionally metamorphosed calcium-rich sediments and basic igneous rocks belonging to the higher grades of the amphibolite facies. The above reaction is typical (DHZ 2A p272).

muscovite, iron-rich biotite and SiO2 to orthoclase, almandine and H2O
KAl2(AlSi3O10)(OH)2 + KFe2+3(AlSi3O10)(OH)2 + 3SiO2 ⇌ 2KAlSi3O8 + Fe2+3Al2(SiO4)3 + 2H2O
Iron-rich biotite is likely to react at lower PT conditions than iron-poor biotite (DHZ 3 p72).

staurolite and quartz to almandine and sillimanite and H2O
62Fe2+2Al9Si4O23(OH) + 11SiO2 ⇌ 4Fe2+3Al2(SiO4)3 + 23Al2OSiO4 + 3H2O
Increasing temperature favours the forward reaction. At higher pressure kyanite replaces sillimanite in the above reaction (AM61.699-709).

Common impurities: Mg,Mn,Ca, replacing Fe2+ (Lauf p108)

Back to Minerals