Chrysoberyl

chrysoberyl

tourmaline

gahnite

beryllium

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Formula: BeAl2O4
Multiple oxide

Varieties

Alexandrite is a green chromium-bearing variety of chrysoberyl that exhibits a colour change in natural and artificial light

Properties

Specific gravity: 3.75
Hardness: 8½
Streak: White
Colour: Green shades, emerald-green, greenish white, yellowish green, greenish brown, yellow, blue
Common impurities: Fe,Cr,Ti
Environments:

Pegmatites
Metamorphic environments
Hydrothermal environments

Chrysoberyl is normally found in pegmatites, and rarely in some fluorite-rich veins. The colour-change variety alexandrite is usually found in mica schist (Dana).

At Carnaiba, Brazil, chrysoberyl variety alexandrite occurs in schist associated with beryl variety emerald (Dana).

Near St Gothard, Switzerland, chrysoberyl occurs in dolostone with corundum (Dana).

At many localities in Maine, USA, chrysoberyl occurs with columbite, tourmaline, gahnite and beryl (Dana).

At New Hampshire, USA, chrysoberyl is considered rare and is only recorded as occurring in the Wasau Abrasives mine in Wilmot, the Ruggles and Summit mines in Grafton, the Iron Mountain mine in Bartlett, and the Ham and Weeks mine in Wakefield. The crystals are thin tabular and prominently striated, often twinned forming flattened heart shapes or pseudohexagonal contact twins. Chrysoberyl forms at high temperatures but is stable at lower temperatures when beryl forms, and crystals are found with beryl in New Hampshire. It appears that chrysoberyl occurs in many of the scattered pegmatites throughout the forest in Grafton county that never have been worked for minerals (R&M 97.3.218-220).

At the Ham and Weeks Mine, Wakefield, Carroll County, New Hampshire, USA, chrysoberyl occurs within the quartz matrix in the pegmatite, often with blue beryl, as crystals up to 2 cm, though most are much smaller (R&M 97.3.219).

Alteration

The assemblage (chrysoberyl and quartz) is stable only at relatively high temperatures, well above 400oC (AM 71.277-300).

beryl to chrysoberyl, phenakite and silica (dry)
Be3Al2Si6O18 to BeAl2O4 + Be2(SiO4) + 5SiO2 (dry)
Increasing temperature and pressure favours the forward reaction. At a pressure of 4 kbar the equilibrium temperatute is 1300 deg C (AM 71.277-300).

beryl and aluminium silicate to chrysoberyl and silica (water saturated)
Be3Al2Si6O18Be + 2Al2OSiO4 to 3BeAl2O4 + 8SiO2 (water saturated)
At high pressure, above 8 kbar, the aluminium silicate phase is kyanite. Increasing temperature and decreasing pressure favours the forward reaction. At a pressure of 16 kbar the equilibrium temperature is about 850oC (AM 71.277-300).

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 chrysoberyl, beryl and H2O
4BeAlSiO4(OH) + 2SiO2 ⇌ BeAl2O4 + Be3Al2Si6O18 + 2H2O
Increasing temperature favours the forward reaction (AM 63.664-676).

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