Chrysoberyl

chrysoberyl

tourmaline

gahnite

beryllium

Images

Formula: BeAl2O4
Multiple oxide, beryllium-bearing mineral

Varieties

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

Properties

Crystal System: Orthorhombic
Specific gravity: 3.75 measured, 3.69 calculated
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).

Localities

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

At Colatina, Espírito Santo, Brazil, near the town of Colatina, a flat plain more than 3 km long separates low hills; a chrysoberyl crystal which weighed more than 2 kg and which produced cat’s-eye gemstones was once found in the alluvium of the plain, and in 1986 beautiful chrysoberyl specimens showing crystals to 5 cm, commonly twinned, were found (Minrec 54.6.716-729).

Itaguaçu, Espírito Santo, Brazil. One of the most interesting mineralogical aspects about chrysoberyl from Espírito Santo is the presence of sixling twinned crystals to several centimeters in diameter; these occur in the vicinity of Itaguaçu (Minrec 54.6.716-729).

At Pancas, Espírito Santo, Brazil, rounded fragments of green chrysoberyl are recovered in the alluvial deposits of the Pancas Field, together with aquamarine. Beautiful yellow-green euhedral chrysoberyl crystals occur in small pegmatites within the host granites, and V-twinned crystals to several centimeters are common.
One extraordinary V-twinned chrysoberyl from the Pancas Field is a floater measuring 18 x 22 cm, a flat, perfectly developed fishtail twin, dark honey-brown tending towards golden, and gemmy at the wingtips though the main mass is an opaque glassy brown.
Around 1990, the Fazenda Santa Isabel pegmatite near Pancas began to produce very fine specimens of chrysoberyl, in single, untwinned crystals, flattened V-twins, and (rarely) complete sixling twins. The crystals are sharp, fairly lustrous and translucent, and range in colour from brown through yellow-brown, honey-yellow and deep golden yellow. Most of the loose, floater crystals and twins are of thumbnail size, but some reach 10 cm, and one amazing V-twin measures 25 cm. In June 1995, a clay-filled pocket yielded hundreds of floater chrysoberyls, mostly V-twins but a few sixling twins as well, with a deep olive-green colour and up to 8 cm in size (Minrec 54.6.716-729).

At Santa Teresa, Espírito Santo, Brazil, V-twinned crystals of chrysoberyl to several centimetres are common. The sharp, brownish to grey-green, sixling twins are lustrous, mostly thumbnail-sized floaters which are gemmy in small areas. (Minrec 54.6.716-729).

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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