Chrysoberyl
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| Chrysoberyl | |
|---|---|
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| General | |
| Category | Oxide minerals |
| Formula (repeating unit) |
BeAl2O4 |
| Strunz classification | 04.BA.05 |
| Crystal symmetry | Orthorhombic 2/m2/m2/m dipyramidal |
| Unit cell | a = 5.481 Å, b = 9.415 Å, c = 4.428 Å; Z = 8 |
| Identification | |
| Color | Various shades of green, yellow, brownish to greenish black, may be raspberry-red under incandescent light when chromian; colorless, pale shades of yellow, green, or red in transmitted light |
| Crystal habit | Crystals tabular or short prismatic, prominently striated |
| Crystal system | Orthorhombic |
| Twinning | Contact and penetration twins common, often repeated forming rosette structures |
| Cleavage | Distinct on (110), imperfect on (010), poor on {001} |
| Fracture | Conchoidal to uneven |
| Tenacity | Brittle |
| Mohs scale hardness | 8.5 |
| Luster | Vitreous |
| Streak | White |
| Specific gravity | 3.5 - 3.84 |
| Optical properties | Biaxial (+) |
| Refractive index | nα=1.745 nβ=1.748 nγ=1.754 |
| Pleochroism | X = red; Y = yellow-orange; Z = emerald-green |
| 2V angle | Measured: 70° |
| References | [1][2][3] |
| Major varieties | |
| Alexandrite | Color change; green to red |
| Cymophane | Chatoyant |
An interesting feature of its crystals 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" oriented at 120° to its neighbors and taking up 120° of the cyclic trilling. If only two of the three possible twin orientations are present, a "V"-shaped twin results.
Ordinary chrysoberyl is yellowish-green and transparent to translucent. When the mineral exhibits good pale green to yellow color and is transparent, then it is used as a gemstone. The three main varieties of chrysoberyl are: ordinary yellow-to-green chrysoberyl, cat's eye or cymophane, and alexandrite. Yellow-green chrysoberyl was referred to as "chrysolite" during the Victorian and Edwardian eras, which caused confusion since that name has also been used for the mineral olivine ("peridot" as a gemstone); that name is no longer used in the gemological nomenclature.
Alexandrite, a strongly pleochroic (trichroic) gem, will exhibit emerald green, red and orange-yellow colors depending on viewing direction in partially polarised light. However, its most distinctive property is that it also changes color in artificial (tungsten/halogen) light compared to daylight. The color change from red to green is due to strong absorption of light in a narrow yellow portion of the spectrum, while allowing large bands of blue-greener and red wavelengths to be transmitted. Which of these prevails to give the perceived hue depends on the spectral balance of the illumination. Typically, alexandrite has an emerald-green color in daylight (relatively blue illumination of high color temperature) but exhibits a raspberry-red color in incandescent light (relatively yellow illumination).
Cymophane is popularly known as "cat's eye". This variety exhibits pleasing chatoyancy or opalescence that reminds one of an eye of a cat. When cut to produce a cabochon, the mineral forms a light-green specimen with a silky band of light extending across the surface of the stone.
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Occurrence
Chrysoberyl forms as a result of pegmatitic processes. Melting in the Earth's crust produces relatively low-density molten magma which can rise upwards towards the surface. As the main magma body cools, water originally present in low concentrations became more concentrated in the molten rock because it could not be incorporated into the crystallization of solid minerals. The remnant magma thus becomes richer in water, and also in rare elements that similarly do not fit in the crystal structures of major rock-forming minerals. The water extends the temperature range downwards before the magma becomes completely solid, allowing concentration of rare elements to proceed so far that they produce their own distinctive minerals. The resulting rock, igneous in appearance but formed at a low temperature from a water-rich melt, with large crystals of the common minerals such as quartz and feldspar, but also with elevated concentrations of rare elements such as beryllium, lithium, or niobium, often forming their own minerals, is called a pegmatite. The high water content of the magma made it possible for the crystals to grow quickly, so pegmatite crystals are often quite large, which increases the likelihood of gem specimens forming.Chrysoberyl can also grow in the country rocks near to pegmatites, when Be- and Al-rich fluids from the pegmatite react with surrounding minerals. Hence, it can be found in mica schists and in contact with metamorphic deposits of dolomitic marble. Because it is a hard, dense mineral that is resistant to chemical alteration, it can be weathered out of rocks and deposited in river sands and gravels in alluvial deposits with other gem minerals such as diamond, corundum, topaz, spinel, garnet, and tourmaline. When found in such placers, it will have rounded edges instead of sharp, wedge-shape forms. Much of the chrysoberyl mined in Brazil and Sri Lanka is recovered from placers, as the host rocks have been intensely weathered and eroded.
If the pegmatite fluid is rich in beryllium, crystals of beryl or chrysoberyl could form. Beryl has a high ratio of beryllium to aluminium, while the opposite is true for chrysoberyl. Both are stable with the common mineral quartz. For alexandrite to form, some chromium would also have had to be present. However, beryllium and chromium do not tend to occur in the same types of rock. Chromium is commonest in mafic and ultramafic rocks in which beryllium is extremely rare. Beryllium becomes concentrated in felsic pegmatites in which chromium is almost absent. Therefore, the only situation where an alexandrite can grow is when Be-rich pegmatitic fluids react with Cr-rich country rock. This unusual requirement explains the rarity of this chrysoberyl variety.
Alexandrite
The alexandrite variety displays a color change (alexandrite effect) dependent upon the nature of ambient lighting. Alexandrite effect is the phenomenon of an observed color change from greenish to reddish with a change in source illumination due physiological response of the human eye in a particular part of the visible spectrum. This color change is independent of any change of hue with viewing direction through the crystal that would arise from pleochroism. Alexandrite results from small scale replacement of aluminium by chromium ions in the crystal structure, which causes intense absorption of light over a narrow range of wavelengths in the yellow region of the spectrum.[citation needed]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 a columbine or raspberry red by incandescent light.
Alexandrite up to 5 carats (1,000 mg) and larger were traditionally thought to be found only in the Ural Mountains, but have since been found in larger sizes in Brazil. Other deposits are located in India (Andhra Pradesh), Madagascar, and Sri Lanka. Alexandrite in sizes over three carats are very rare.
Some gemstones described as lab-grown (synthetic) alexandrite are actually corundum laced with trace elements (e.g., vanadium) or color-change spinel and are not actually chrysoberyl. As a result, they would be more accurately described as simulated alexandrite rather than synthetic, but are often called Czochralski alexandrite after the process that grows the crystals. [6]
Cymophane
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.
Gems lacking the silky inclusions required to produce the cat's eye effect are usually faceted. An alexandrite cat's eye is a chrysoberyl cat's eye that changes color. "Milk and honey" is a term commonly used to describe the color of the best cat's eyes. The effect refers to the sharp milky ray of white light normally crossing the cabochon as a center line along its length and overlying the honey colored background. The honey color is considered to be top-grade by many gemologists but the lemon yellow colors are also popular and attractive. Cat's eye material is found as a small percentage of the overall chrysoberyl production wherever chrysoberyl is found.
Cat's eye really became popular by the end of the 19th century when the Duke of Connaught gave a ring with a cat's eye as an engagement token; this was sufficient to make the stone more popular and increase its value greatly. Until that time, cat's eye had predominantly been present in gem and mineral collections. The increased demand in turn created an intensified search for it in Sri Lanka. Early 20th century prices could go up as high as $8000 for a cut stone.[8]
See also
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Wikimedia Commons has media related to: Alexandrite |
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Wikimedia Commons has media related to: Chrysoberyl |
References
- ^ Handbook of Mineralogy
- ^ Mindat.org
- ^ a b Webmineral data
- ^ Klein, Cornelis; and Cornelius S. Hurlbut, Jr. (1985). Manual of Mineralogy (20th ed. ed.). New York: Wiley. ISBN 0-471-80580-7.
- ^ ""Alexandrite or Diaphanite?". In Alexandrite Tsarstone Collectors Guide.". 2006. Retrieved 2007-07-09.
- ^ "Alexandrite Synthetics and Imitations, In Alexandrite Tsarstone Collectors Guide.". 2006. Retrieved 2007-07-09.
- ^ "Mitchell, T. E. and Marder, J. M., "Precipitation in Cat's-Eye Chrysoberyl," Electron Microscopy Soc. Proceedings, 1982.
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