Euclase

euclase

beryllium

beryl

bertrandite

Images
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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