Science Fair Project Encyclopedia
|Chemical formula (or Composition)||Beryllium aluminium oxide, BeAl2O4|
|Color||various shades of green and yellow|
|Crystal habit||slender prisms and tabular form, dimensions are thin in one direction.|
|Crystal system||Orthorhombic 2/m2/m2/m|
|Twinning||contact and penetration twins common, often repeated forming rosette structures|
|Cleavage|| Distinct,  Imperfect|
|Fracture||conchoidal to uneven|
|Mohs Scale hardness||8.5|
|Refractive index||Biaxial (+) nα=1.745 nβ=1.748 nγ=1.754|
|Pleochroism||pronounced in varieties|
|Specific gravity||3.5 - 3.84|
The mineral or gemstone chrysoberyl, not to be confused with beryl, is an aluminate of beryllium with the formula BeAl2O4. Chrysoberyl is transparent to translucent and sometimes opalescent. An interesting feature of uncut crystals of chyrsoberyl are the cyclic twins called trillings. These twinned crystals have a hexagonal appearance, but are the result of a triplet of twins with each "twin" taking up 120 degrees of the cyclic trilling. The word chrysoberyl is derived from the Greek words meaning golden and beryl.
Chrysoberyl occurs in granitic rocks, pegmatites and mica schists; often it is found in alluvial deposits. It has also been found in contact metamorphic deposits of dolomitic marble with corundum, and in fluorine bearing skarns. Most chrysoberyl is recovered from river sands and gravels.
The alexandrite variety displays pleochroism. Alexandrite from the Ural mountains in Russia is green by daylight and red by incandescent light. Other varieties of alexandrite may be yellowish or pink in daylight and yellow-orange or green by incandescent light. It results from small scale replacement of aluminium by chromium oxide, which is responsible for alexandrite's characteristic green to red color change. Alexandrite was first discovered in 1831 in an emerald mining region of the Ural Mountains in Russia, it is said on the very same day that the Russian czar Alexander II came of age. It was named "alexandrite" in his honor by the mineralogist Adolf Erik Nordenskiöld. It is an interesting coincidence that the Russian national colors are green and red.
Translucent yellowish chatoyant chyroberyl is called cymophane or cat's eye. Cymophane has its derivation also from the Greek words meaning wave and appearance, in reference to the opalescence sometimes exhibited. In this variety, microscopic tubelike cavities or needlelike inclusions of rutile occur in an orientation parallel to the c-axis producing a chatoyant effect visible as a single ray of light passing across the crystal. This effect is best seen in gemstones cut in cabochon form perpendicular to the c-axis. The color in yellow chrysoberyl is due to Fe3+ impurities.
Although other minerals such as tourmaline, scapolite, corundum, spinel and quartz can form "cat's eye" stones similar in appearance to cymophane, the jewelry industry designates these stones as "quartz cat's eyes", or "ruby cat's eyes" and only chrysoberyl can be referred to as "cat's eye" with no other designation.
References and external links
- Hurlbut, Cornelius S.; Klein, Cornelis, 1985, Manual of Mineralogy, 20th ed., ISBN 0471805807
- Weinstein, Michael, 1958, The World of Jewel Stones, Sheridan House, New York
- Mineral galleries
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