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