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Formula: BeAlSiO4(OH)
Nesosilicate (insular SiO4 groups), beryllium-bearing mineral
Crystal System: Monoclinic
Specific gravity: 2.99 to 3.1 measured, 3.115 calculated
Hardness: 7½
Streak: White
Colour: Colourless, white, pale green to deep yellowish green, greenish blue, pale blue to deep blue
Solubility: Insoluble in acids
Common impurities: Zn,F,Ca,Mg,Fe,Na
Environments:
Pegmatites
Metamorphic environments
Hydrothermal environments
Euclase is a low-temperature hydrothermal mineral in granite pegmatites
and Alpine veins. It
is also found in chlorite schist and
phyllite, and as a detrital mineral (Dana). In pegmatites it is a
product of
decomposition of beryl (HOM). Associated minerals include
feldspar, quartz,
topaz, beryl,
mica,
calcite, ankerite and
chlorite
(HOM).
Localities
At Itacambira, Minas Gerais, Brazil, about 3 to 5 kg of bluish grey euclase were produced in the 1990s, mostly
as collection specimens. Soon thereafter the area was filled in for commercial planting of eucalyptus.
Quartzites also crop out in the area. The euclase crystals
were extracted from a colluvial deposit that covered weathered
quartz veins. The crystals found measure between 5 mm and 1.2 cm; very few of
them are terminated. The bluish grey colour appears to be caused by black
tourmaline microinclusions. Central bands in these crystals are often
filled with cryptocrystalline quartz.
Due to the planting of eucalyptus it is difficult now to find the exact location of the occurrence
(Minrec 56.3.261).
At Olhos-d'Água, Minas Gerais, Brazil, in the deposit área, hydrothermal quartz
veins cutting through quartzite are hosts to euclase crystals.
The crystals are euhedral and subhedral prisms, medium to pale beige to yellow in colour, rarely terminated.
Transparent, gem-quality specimens are common. The majority of the crystals measure between 1 and 2 cm. Unusual
inclusions of acicular rutile crystals have been noted
(Minrec 56.3.259-261).
At the the type locality, Ouro Preto, Minas Gerais, Brazil, euclase was discovered around 1772, in the
imperial topaz mines. Euclase is much rarer in the mines than
topaz, although the two species are probably cogenetic. The greatest numbers
of topaz and euclase crystals are collected in the immediate vicinity of
the village of Rodrigo Silva. This area also yields exceptional rutile
crystals as well as fine quartz crystals,
hematite rosettes and goethite
botryoids; small octahedral crystals of magnetite and/or
hematite pseudomorphs after
magnetite are found in weathered deposits.
Euclase crystals from the Ouro Preto region are most commonly blue, bluish green or yellowish green; only
rarely are they colourless. Almost all of the beautiful cut euclase gems from Brazil were made from crystals
which came from the Ouro Preto deposits
(MinRec 56.3.250-259).
Euclase from Ouro Preto - Image
The Chiá mine, São José da Safira, Minas Gerais, Brazil, is located in the important Serra do Cruzeiro Pegmatite
Field, a district which has been mined, mainly for beryl,
quartz and micas, since the 1940s. In
the Serra do Cruzeiro Field, only the Chiá pegmatite contains
euclase mineralisation. Other minerals recovered include tourmaline,
beryl (aquamarine, morganite and goshenite), pink
spodumene (kunzite), and “green gold” or “lemon”
quartz.
Mining takes place in several pegmatite bodies, or tunnels;
the Safirinha tunnel, noted particularly for its aquamarine, is the only one
that has produced euclase. Mining here is currently (2025) very active
(MinRec 56.3.276).
Euclase from the Chiá Mine - Image
At Santana do Encoberto, São Sebastião do Maranhão, Minas Gerais, Brazil, euclase crystals were discovered in
1968, and shortly thereafter the deposit became the most important producer of euclase in all of Brazil. The
geology of the area includes a belt of
quartz-mica
schists, in which
pegmatites are hosted, above
biotite gneisses. A large,
heterogeneous pegmatite, between 10 and 12 meters thick, has
a thick micaceous zone close to the quartz
core, where small replacement bodies contain euclase as well as albite,
calcite, green tourmaline,
spessartine, fluorapatite
and sulphides.
The euclase is glassy or milky; most of the crystals are terminated and smaller than 6 cm. Small euclase
crystal clusters on muscovite and
albite also occur. Currently the deposit is entirely abandoned and covered by
thick vegetation
(MinRec 56.3.266-274).
Euclase from Santana do Encoberto -
Image
The Seridó region in the Alto Mamões, Parelhas, Rio Grande do Norte, Brazil, contains Brazil’s largest concentration
of pegmatite deposits containing euclase. The region
has also produced the most beautiful euclase specimens for collectors after those of the Ouro Preto deposits.
The crystals are colourless, milky white or, more commonly, colourless with a long central blue stripe or belt; this
blue stripe is characteristic of euclase from this region. Beautiful crystal clusters to many centimetres
across have also been found here
(MinRec 56.3.266).
Euclase from Rio Grande do Norte -
Image
The Chandler Mine, Raymond, Rockingham County, New Hampshire, USA. Euclase is exceedingly rare in New
Hampshire and has been reported only from the
lithium-cesium-tantalum
(LCT) pegmatite at the Chandler mine, collected
in the 1970s. Only a few specimens are known from one pocket in a side cut where the euclase occurs as
colourless to pale blue crystals less than 1 mm in size. This was the first reported occurrence of euclase in
a North American pegmatite
(R&M 97.3.220)
At the Lost Hope mine, Miami, Karoi district, Mashonaland West, Zimbabwe, euclase
pseudomorphs after beryl have been found
(KL p220).
Alteration
Euclase is stable over a broad rangeof pressure and temperature, but it is stable only in environments with unusually high
alumina activities, thus accounting for its relative rarity
(AM 71.277-300).
bertrandite, euclase and quartz to
beryl and H2O
Be4Si2O7(OH)2 + 8BeAlSiO4(OH) + 14SiO2 ⇌
4Be3Al2Si6O18 + 5H2O
Increasing temperature favours the forward reaction
(AM 63.664-676).
bertrandite and kaolinite to euclase,
beryl and H2O
4Be4Si2O7(OH)2 + 7Al2Si2O5(OH)4 ⇌
10BeAlSiO4(OH) + 2Be3Al2Si6O18 + 13H2O
Increasing temperature favours the forward reaction
(AM 63.664-676).
bertrandite and kaolinite to euclase,
quartz and H2O
Be4Si2O7(OH)2 + 2Al2Si2O5(OH)4 ⇌
4BeAlSiO4(OH) + 2SiO2 + 3H2O
Increasing temperature favours the forward reaction
(AM 63.664-676).
chrysoberyl, bertrandite and
kaolinite to euclase and H2O
2BeAl2O4 + 2Be4Si2O7(OH)2 +
3Al2Si2O5(OH)4 ⇌ 10BeAlSiO4(OH) + 3H2O
Increasing temperature favours the forward reaction
(AM 63.664-676).
euclase to bertrandite, chrysoberyl,
quartz and H2O
8BeAlSiO4(OH) ⇌ Be4Si2O7(OH)2 + 4BeAl2O4 +
6SiO2
+ 3H2O
Increasing temperature favours the forward reaction
(AM 63.664-676).
euclase to beryl,
chrysoberyl, phenakite and H2O
20Eu to 3Be3Al2Si6O18 + 7BeAl2O4 + 2Be2(SiO4) +
10H2O
Increasing temperature and decreasing pressure favours the forward reaction. At a pressure of 6 kbar the equilibrium temperature is about
500oC, in the absence of impurities which might be incorporated in the beryl
(AM 71.277-300).
euclase to phenakite, chrysoberyl,
beryl and H2O
20BeAlSiO4(OH) ⇌ 2Be2(SiO4) + 7BeAl2O4 +
3Be3Al2Si6O18 + 10H2O
Increasing temperature favours the forward reaction
(AM 63.664-676).
euclase and kaolinite to chrysoberyl,
quartz and H2O
2BeAlSiO4(OH) + Al2Si2O5(OH)4 ⇌ 2BeAl2O4 +
4SiO2 + 3H2O
Increasing temperature favours the forward reaction
(AM 63.664-676).
euclase and silica to beryl,
chrysoberyl and H2O
4Eu + 2SiO2 to Be3Al2Si6O18 + BeAl2O4 + 2H2O
Increasing temperature and decreasing pressure favours the forward reaction. At a pressure of 8 kbar the equilibrium temperature is about
500oC, in the absence of impurities which might be incorporated in the beryl
(AM 71.277-300).
euclase and quartz to beryl,
kaolinite and H2O
6BeAlSiO4(OH) + 8SiO2 ⇌ 2Be3Al2Si6O18 +
Al2Si2O5(OH)4 + H2O
Increasing temperature favours the forward reaction
(AM 63.664-676).
euclase and quartz to chrysoberyl,
beryl and H2O
4BeAlSiO4(OH) + 2SiO2 ⇌ BeAl2O4 +
Be3Al2Si6O18
+ 2H2O
Increasing temperature favours the forward reaction
(AM 63.664-676).
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