Epidote

minerals

magnetite

actinolite

limestone

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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
Common impurities: Al,Mg,Mn
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.

Localities

At Arendal, Aust-Agder, Norway, epidote occurs with scapolite (FM OP 167).

At the Raskoh mountains, Kharan, Balochistan, Pakistan, epidote pseudomorphs after magnetite have been found (KL p226).

At Croft Quarry, Croft, Blaby, Leicestershire, England, UK, the deposition of epidote precedes the formation of zeolites, and there is usually a lining of epidote on both flanks of the veins, associated with a little hematite. On a number of specimens epidote is associated with molybdenite (R&M 20.15).

At Granitethorpe quarry, Sapcote, Blaby, Leicestershire, England, UK, epidote occurred together with pyrite and some large crystals of pink feldspar; it seems likely that this is an occurrence of epidote in a pegmatite. Subsequently the quarry was flooded, but it is still possible to find minor amounts of epidote as granules and crystals completely enclosed within the tonalite. The deposition of epidote preceded that of the associated pyrite (R&M 20.15).

At Lane's Hill quarry, Stoney Stanton, Blaby, Leicestershire, England, UK, epidote occurred in granite pegmatite veins as radiating aggregates of epidote with large pink crystals of feldspar (JRS 20.15).

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

At the Dolgellau Gold-belt, Gwynedd, Wales, UK, epidote is widespread, together with clinozoisite, in alpine fissure-type quartz - chlorite - epidote - albite - calcite dominated veins and pods hosted by altered greenstone. Epidote generally occurs as sheaves of prismatic or fibrous crystals to 4 cm in length and free-standing crystals are very rare (MW).

At the Coed-y-Brenin deposit, Ganllwyd, Gwynedd, Wales, UK, milky quartz veins, carrying epidoteclinozoisite, variably accompanied by clinochlore, albite, ferroan dolomite and calcite, occur widely, exclusively hosted by intrusive rocks. (JRS 21.117-118).

At the Dinorwic Quarry, Llanberis, Gwynedd, Wales, UK, typical pistachio-green epidote occurs as a component of alpine fissure-type mineralisation in basalt dykes hosted by slate. Specimens typically comprise intergrown aggregates of prismatic crystals of epidote associated with quartz and chlorite (MW).

At Marloes Bay, Pembrokeshire, Wales, UK, well crystallised epidote occurs with quartz in veins hosted by basic volcanic igneous rocks (MW).

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.

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