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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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