Formula: Ca2(Al2Fe3+)[Si2O7] [SiO4]O(OH)
Sorosilicate (Si2O7 groups), epidote group, epidote supergroup


Tawmawite is a chromium-bearing variety of epidote

Properties of Epidote

Crystal System: Monoclinic
Specific gravity: 3.38 to 3.49 measured, 3.43 calculated
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

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.


The Two Mile and Three Mile deposits, Paddy's River, Paddys River District, Australian Capital Territory, Australia, are skarn deposits at the contact between granodiorite and volcanic rocks. Epidote is a primary silicate that is common in porous silicate skarn, associated with grossular and hornblende. It also a common alteration product in the adjacent granite and dacitic volcanics, partially replacing plagioclase feldspars (AJM 22.1.35).

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.
Epidote has been found here, in association with vesuvianite or diopside (Hong Kong Minerals (1991). Peng, C J. Hong Kong Urban Council)

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 the Welkom goldfield, Lejweleputswa District, Free State, South Africa, epidote is one of the rare secondary minerals; a fine specimen has been found associated with minor small clear quartz crystals (R&M).

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).

The Cliff Mine, Phoenix, Keweenaw county, Michigan, USA, is situated at the base of a roughly 70-metre basalt cliff. A curious feature of the impressive thickness of the greenstone flow here is that it contains zones of “pegmatoid”: areas where slow cooling in the core of the lava flow allowed for large feldspar crystals exceeding 1 cm to grow. Such features are normally only observed in intrusive igneous rocks and are almost unheard of in basalt flows.
The Cliff mine primarily exploited rich copper mineralisation in the Cliff fissure (vein). Although mineralised with copper to some extent along its entire length, the part of the vein just below the greenstone flow carried the richest copper mineralisation by far. A significant amount of the copper recovered at the Cliff mine came from amygdaloids in the tops of 13 basalt flows which were cut by the Cliff vein. The discovery and mining of this vein proved that the veins were the source of the large masses of float copper that were already well known, and proved that the primary ore mineral in the district was native copper, not sulphides, as had been suspected earlier.
Epidote is a common alteration mineral in Lake Superior District basalts. At the Cliff mine, it occurs in typical pistachio-green masses or dark green crystals to 2.5 cm lining amygdules in the basalt or vugs in the vein. Typical associated species include calcite, pumpellyite, prehnite, quartz and rarely copper (MinRec 54.1.25-49).

At the Orogrande District, Otero county, New Mexico, USA, large numbers of fine epidote pseudomorphs after Carlsbad law orthoclase twins have been found. Associated minerals include kaersutite and albite (R&M 96.6.502-511).


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.

Back to Minerals