Epidote

sorosilicate

limestone

Formula: Ca2(Al2Fe3+)[Si2O7] [SiO4]O(OH)
Sorosilicate (Si2O7 groups) epidote group
Specific gravity: 3.38 to 3.49
Hardness: 6
Streak: White
Colour: Yellowish-green, green, brownish-green, black
Solubility: Slightly soluble in hydrochloric acid; insoluble in sulphuric and nitric acid
Environments:

Pegmatites
Metamorphic environments (typical)
Basaltic cavities

Epidote is a widespread mineral, found in veins and joint fillings in some granitic rocks, in pegmatites, and in contact and regional metamorphic environments. It is a low temperature mineral formed by metamorphism of limestone with calcium-rich garnet, diopside, vesuvianite and calcite.
Epidote may be found in gneiss and hornfels.
It is characteristic of the albite-epidote-hornfels facies and it is also a mineral of the prehnite-pumpellyite, greenschist, amphibolite and blueschist facies.

At Buddon Wood quarry, Mountsorrel, Leicestershire, England, UK, epidote occurs with chlorite and quartz on granodiorite (RES p191).

Alteration

Epidote forms as a reaction product of plagioclase feldspar, pyroxene and amphibole.

aegirine, epidote and CO2 to albite, hematite, quartz, calcite and H2O
4NaFe3+Si2O6 + 2Ca2(Al2Fe3+ [Si2O7](SiO4)O(OH) + 4CO2 → 4Na(AlSi3O8) + 3Fe2O3 + 2SiO2 + 4CaCO3 + H2O (DHZ 2A p511)

Ca-Fe amphibole, anorthite and H2O to chlorite, epidote and quartz
CaFe5Al2Si7O22(OH)2 + 3CaAl2Si2O8 + 4H2O → Fe5Al2Si3O10(OH)8 + 2Ca2Al3Si3O12(OH) + 4SiO2 (JVW p363)

chlorite (clinichlore), actinolite and albite to glaucophane, iron-poor epidote, SiO2 and H2O
9Mg5Al(AlSi3O10)(OH)8 + 6☐Ca2Mg5Si8O22(OH)2 + 50Na(AlSi3O8) → 25☐Na2(Mg3Al2)Si8O22(OH)2 + 6Ca2Al3[Si2O7][SiO4]O(OH) + 7SiO2 + 14H2O
This is a metamorphic reaction (DHZ 3 p156).

chlorite (clinochlore), iron-poor epidote and SiO2 to amphibole (tschermakite), anorthite and H2O
3Mg5Al(AlSi3O10)(OH)8 + 6Ca2(Al2Fe3+)[Si2O7][SiO4]O(OH) + 7SiO2 → 5☐Ca2(Mg3Al2)(Si6Al2)O22(OH)2 + 2Ca(Al2Si2O8) + 10H2O
This reaction occurs at a fairly high metamorphic grade (DHZ 3 p154).

epidote and chlorite to hornblende and anorthite
6Ca2Al3(SiO4)3(OH) + Mg5Al2Si3O18(OH)8 → Ca2Mg5Si8O22(OH)2 + 10CaAl2Si2O8
This reaction represents changes when the metamorphic grade increases from the greenschist facies to the amphibolite facies (KB p429 diagram p430).

epidote and quartz to anorthite, grossular and H2O
4Ca2Al3(SiO4)3(OH) + SiO2 → 5CaAl2Si2O8 + Ca3Al2(SiO4)3 + 2H2O
This reaction occurs as the degree of metamorphism increases.

Common impurities: Al,Mg,Mn

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