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Alexandrite (variety of Chrysoberyl)

Discovered in 1842;   IMA status: Not Approved (a variety of Chrysoberyl)

Chemistry

Chemical Formula:

BeAl₂O₄ 

Beryllium Aluminum Oxide + traces of Chromium

Molecular Weight:

126.97 gm

Composition:

Beryllium

7.10 %

Be

19.70 %

BeO

Aluminum

42.50 %

Al

80.30 %

Al₂O₃

Oxygen

50.40 %

O

100.00 %

100.00 %

= TOTAL OXIDE

Classification

Mineral Classification:

Oxides

Strunz 8th Ed. ID:

4/B.07-10

Nickel-Strunz 10th Ed. ID:

4.BA.05

4 : OXIDES (Hydroxides, V[5,6] vanadates, arsenites, antimonites, bismuthites, sulfites, selenites, tellurites, iodates)
B : Metal: Oxygen = 3:4 and similar
A : With small and medium-sized cations

Related to:

n/a

Varieties:

Alexandrite, Chrysolite Cat's Eye, Cymophane

Synonyms:

Aluminate of Glucina, Alumoberyl, Chrysberil, Chrysoberil, Chrysopal (of Delamétherie), Crisoberilo, Diaphanite, Krisoberil, Krysoberril, Oriental Chrysolite

Crystal Data

Crystallography:

Orthorhombic - Dipyramidal

Crystal Habit:

Crystals are tabular or short prismatic, to 22 cm; prominently striated

Twinning:

Common, producing either flattened heart-shaped or pseudohexagonal multiple contact and penetration twins.

Physical Properties

Cleavage:

Distinct on [110] (though rarely observed), Imperfect on [010], Poor on [001]

Fracture:

Irregular/Uneven, Conchoidal

Tenacity:

Brittle

Moh's Hardness:

8.5

Density:

3.75 (g/cm³)

Luminescence:

Weak red in SW and LW UV

Radioactivity:

Not Radioactive

Other:

Electrical conductivity increases with temperature

Optical Properties

Color:

Green shades, emerald-green, greenish white, yellowish green, greenish brown, yellow, blue, may be violet-red to raspberry-red under incandescent light

Transparency:

Transparent to Translucent; may be opaque and chatoyant with oriented inclusions (cat's eye effect)

Luster:

Vitreous

Refractive Index:

1.745 - 1.756  Biaxial (+)

Birefringence:

0.008 - 0.012

Dispersion:

r > v; 0.015 (low)

Pleochroism:

X = red; Y = yellow-orange; Z = emerald-green

Occurances

Geological Setting:

Characteristic of some granite pegmatites associated with high-grade mica schists or reaction zones in ultramafic rocks; also detrital in placers.

Common Associations:

Quartz, Muscovite, Albite, Beryl, Columbite, Tourmaline, Topaz, Kyanite, Staurolite (granite pegmatites); Phenakite, Apatite, Tourmaline, Fluorite (reaction zone pegmatites)

Common Impurities:

Fe, Cr, Ti

Type Locality:

Emerald mines (Izumrudnye Kopi; Malyshevskoye deposit), Malyshevo, Ekaterinburg (Yekaterinburg; Sverdlovsk), Sverdlovskaya Oblast', Urals Region, Russia

Year Discovered:

1842

View mineral photos:

Alexandrite Mineral Photos and Locations

More Information

Mindat.org (Alexandrite)
Mindat.org (Chrysoberyl)
Webmineral.com (Chrysoberyl)

Alexandrite is a rare green (chromium-bearing) gem variety of Chrysoberyl known for its color change effect. Its color depends on the light hitting it; green, blue-green or pale green in daylight; mauve, violet to purplish or red in incandescent light. This distinct pleochroism and its rarity accounts for its desirability and high value. Alexandrite is also available as a cat's eye gem with the same color change effect.

Alexandrite is named after Czar Alexander II (1818 - 1881) of Russia 1855 - 1881. The name Chrysoberyl is from the Greek word chrysos meaning golden, in allusion to the minerals color, and beryl.

Alexandrite was originally reported from Emerald mines (Izumrudnye Kopi; Malyshevskoye deposit), Malyshevo, Ekaterinburg (Yekaterinburg; Sverdlovsk), Sverdlovskaya Oblast', Urals Region, Russia. It is also known from sources in Australia, Brazil, India, Madagascar, Myanmar (Burma) Sri Lanka, Tanzania, New Mexico, USA and Zimbabwe. The rarest and most highly prized gems come from the Urals Region of Russia.

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