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Encyclopedia > Chrysoberyl
Chrysoberyl

General
Category Mineral
Chemical formula Beryllium aluminium oxide, BeAl2O4
Identification
Color Various shades of green and yellow; brownish, reddish; rarely, blue
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 [110] Distinct, [010] Imperfect
Fracture Conchoidal to uneven
Mohs Scale hardness 8.5
Luster Vitreous
Refractive index Biaxial (+) nα=1.745 nβ=1.748 nγ=1.754
Pleochroism Strong in alexandrite
Streak White
Specific gravity 3.5 - 3.84
Major varieties
Alexandrite Color change; green to red
Cymophane Chatoyant

The mineral or gemstone chrysoberyl, not to be confused with beryl, is an aluminate of beryllium with the formula BeAl2O4.[1] The name chrysoberyl is derived from the Greek words chrysos and beryllos, meaning "golden" and "gem crystal". Despite the similarity of their names, chrysoberyl and beryl are two completely different gemstones. Chrysoberyl is the third hardest naturally occurring gemstone and lies between corundum and topaz on the hardness scale. Chrysoberyl is a mineral consisting of ordinary colorless or yellow transparent chrysoberyl, cymophane (chrysoberyl cat's eye), and alexandrite. [2] Image File history File links Size of this preview: 688 × 599 pixels Full resolution (930 × 810 pixel, file size: 505 KB, MIME type: image/jpeg) Chrysoberyl Origin: São João Grande, Espírito Santo, Brazil Description = Four crystals. ... Minerals are natural compounds formed through geological processes. ... A chemical formula (also called molecular formula) is a concise way of expressing information about the atoms that constitute a particular chemical compound. ... General Name, Symbol, Number beryllium, Be, 4 Chemical series alkaline earth metals Group, Period, Block 2, 2, s Appearance white-gray metallic Standard atomic weight 9. ... General Name, Symbol, Number aluminium, Al, 13 Chemical series poor metals Group, Period, Block 13, 3, p Appearance silvery Standard atomic weight 26. ... An oxide is a chemical compound containing an oxygen atom and other elements. ... In mineralogy, shape and size give rise to descriptive terms applied to the typical appearance, or habit of crystals. ... A crystal system is a category of space groups, which characterize symmetry of structures in three dimensions with translational symmetry in three directions, having a discrete symmetry group. ... In crystallography, the orthorhombic crystal system is one of the 7 lattice point groups. ... It has been suggested that twin boundary be merged into this article or section. ... Cleavage, in mineralogy, is the tendency of crystalline materials to split along definite planes, creating smooth surfaces, of which there are several named types: Basal cleavage: cleavage parallel to the base of a crystal, or to the plane of the lateral axes. ... For fractures in geologic formations, see Rock fracture. ... Mohs scale of mineral hardness characterizes the scratch resistance of various minerals through the ability of a harder material to scratch a softer. ... Lustre (American English: luster) is a description of the way light interacts with the surface of a crystal, rock or mineral. ... The refractive index (or index of refraction) of a medium is a measure for how much the speed of light (or other waves such as sound waves) is reduced inside the medium. ... Pleochroism is an optical phenomenon where due to double refraction of light by a colored gem or crystal, the light is divided into two paths which are polarized at a 90° angle to each other. ... The streak (also called powder color) of a mineral is the color of the powder produced when it is dragged across a unweathered surface. ... Relative density (also known as specific gravity) is a measure of the density of a material. ... Minerals are natural compounds formed through geological processes. ... A selection of gemstone pebbles made by tumbling rough rock with abrasive grit, in a rotating drum. ... Three varieties of beryl: Morganite, Aquamarine, and Heliodor The mineral beryl is a beryllium aluminium cyclosilicate with the chemical formula Be3Al2(SiO3)6. ... General Name, Symbol, Number aluminium, Al, 13 Chemical series poor metals Group, Period, Block 13, 3, p Appearance silvery Standard atomic weight 26. ... General Name, Symbol, Number beryllium, Be, 4 Chemical series alkaline earth metals Group, Period, Block 2, 2, s Appearance white-gray metallic Standard atomic weight 9. ...


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. Crystal twinning occurs when two separate crystals share some of the same crystal lattice points in a symmetrical manner. ...


There are three main varieties of chrysoberyl; ordinary yellow chrysoberyl, cat´s eye or cymophane, and alexandrite. Although yellow chrysoberyl was referred to as chrysolite during the Victorian and Edwardian eras, that name is no longer used in the gemological nomenclature.


Ordinary chrysoberyl is a yellowish-green, transparent to translucent chrysoberyl and has often been referred to in the literature as chrysolite due to the common olive color of many of its gems, but that name is no longer used in the gemological nomenclature. When the mineral exhibits good pale green to yellow color and is transparent, then it is used as a gemstone.


Alexandrite, a strongly pleochroic (dichroic) gem, will exhibit emerald green, red and orange-yellow colors and tend to change color in artificial light compared to daylight. The color change from red to green is due to strong absorption of light in the yellow and blue portions of the spectrum. Typically, alexandrite has an emerald green color in daylight but exhibit a raspberry red color in incandescent light.


Cymophane is popularly known as cat's eye. This variety exhibits pleasing chatoyant 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. Tiger eye In gemology, chatoyancy (or chatoyance) is an optical reflectance effect seen in certain gemstones. ...

Contents

Occurrence

Chrysoberyl was formed as a result of pegmatitic processes that occurred at least 250 million years ago. High temperatures and pressures from the outer layers of the earth´s mantle forced molten magma towards the surface. As the main magma body cooled, water originally present in low concentrations became more concentrated in the molten rock because it could not be incorporated into the crystallization of the localized minerals. Consequently, the remaining portion of the molten magma was water rich. It was also rich in rare elements and silica that still had not solidified. When this water-rich magma was expelled in the final stages of the crystallization, it solidified in cracks and crevasses to form a pegmatite. Magma is molten rock located beneath the surface of the Earth (or any other terrestrial planet), and which often collects in a magma chamber. ... Frost crystallization on a shrub. ... The chemical compound silicon dioxide, also known as silica, is the oxide of silicon, chemical formula SiO2. ... This article or section does not cite its references or sources. ...


If the pegmatite magma was rich in beryllium, crystals of beryl and chrysoberyl could form but for alexandrite to form, some chromium would also have had to be present. Since beryllium and chromium are extremely rare elements in rocks, this is only process which could have concentrated these unusual elements in an environment where crystallization could occur. General Name, Symbol, Number beryllium, Be, 4 Chemical series alkaline earth metals Group, Period, Block 2, 2, s Appearance white-gray metallic Standard atomic weight 9. ... Three varieties of beryl: Morganite, Aquamarine, and Heliodor The mineral beryl is a beryllium aluminium cyclosilicate with the chemical formula Be3Al2(SiO3)6. ... General Name, Symbol, Number chromium, Cr, 24 Chemical series transition metals Group, Period, Block 6, 4, d Appearance silvery metallic Atomic mass 51. ...


The high water content of the magma made it possible for the crystals to grow quickly, so pegmatite crystals are often quite large and this is of course important for gem specimens. Chrysoberyl is always accompanied by quartz. It occurs in granite pegmatites and mica schists and in contact with metamorphic deposits of dolomitic marble. It is also recovered from river sands and gravels in alluvial deposits with corundum, spinel, garnet and tourmaline. Quarrying granite for the Mormon Temple, Utah Territory. ... Rock with mica Mica sheet Mica flakes The mica group of sheet silicate minerals includes several closely related materials having highly perfect basal cleavage. ... Categories: Mineral stubs | Metamorphic rocks ... Dolomite crystals from Touissite, Morocco Dolomite is the name of both a carbonate rock and a mineral (formula: CaMg(CO3)2) consisting of a calcium magnesium carbonate found in crystals. ... Corundum (from Tamil kurundam) is a crystalline form of aluminium oxide and one of the rock-forming minerals. ... The spinels are any of a class of minerals which crystallize in the isometric system with an octahedral habit. ... Garnet is a group of minerals that have been used since the Bronze Age as gemstones and abrasives. ... The tourmaline mineral group is chemically one of the most complicated groups of silicate minerals. ...


Chrysoberyl has high enough specific gravity that it will concentrate with black sands in active or paleoplacer stream deposits and concentrate with other relatively heavy minerals such as cassiterite, diamond, corundum, topaz and garnet. When found in 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. Cassiterite is a tin oxide mineral, SnO2. ... This article is about the gemstone. ... Corundum (from Tamil kurundam) is a crystalline form of aluminium oxide and one of the rock-forming minerals. ... Topaz is a silicate mineral of aluminium and fluorine with the chemical formula Al2SiO4(F,OH)2. ... Garnet is a group of minerals that have been used since the Bronze Age as gemstones and abrasives. ...


Chrysoberyl deposits can be divided into three types, excluding rare dolomitic hosts. These include chrysoberyl in pegmatites intruded into ultramafic rocks, chrysoberyl hosted by pegmatites intruded into aluminous rocks, and chrysoberyl found as a primary mineral in REE-pegmatites. Dolomite crystals from Touissite, Morocco Dolomite is the name of both a carbonate rock and a mineral consisting of calcium magnesium carbonate (formula: CaMg(CO3)2) found in crystals. ...


There have been very few research projects on the genesis of chrysoberyl due to its rarity in primary host rocks since most chrysoberyl is recovered from placers. However, it may be hypothesized that in order to produce chrysoberyl, metamorphic overprint of some beryllium- and aluminum-rich pegmatites may be necessary. .[3]


Chrysoberyl

Yellow chrysoberyl gemstone featuring oval step cut.
Yellow chrysoberyl gemstone featuring oval step cut.

Chrysoberyl was discovered in 1789 and described and named by Abraham Gottlob Werner, in 1790. Werner worked at the Freiberg School of Mining from 1790-1793 and was well known as one of the most outstanding geologists of his time. He is best known today as the loser in the battle of the Neptunists and Vulcanists that raged in the 1780s.[4] Image File history File linksMetadata No higher resolution available. ... Image File history File linksMetadata No higher resolution available. ... Abraham Gottlob Werner Abraham Gottlob Werner (1749 or 1750 - 1817), was born in Wehrau, a city in Prussian Silesia, southeastern Germany. ...


Chrysoberyl is normally yellow, yellow-green, or brownish with its color being caused by the presence of iron. Spectroscopic analysis will usually reveal a strong band where the violet takes over from the blue. As the color darkens from bright yellowish-green to golden-yellow to brown, this band increases in strength. When the stone has a strong color, two additional bands can be seen in the green-blue. The most common inclusions are liquid-filled cavities containing three-phase inclusions. Stepped twin planes may be apparent in some cases. Some very rare minty bluish-green chrysoberyls from Tanzania owe their color to the presence of Vanadium.


Despite the similarity of their names, chrysoberyl and beryl are two completely different gemstones. Members of the beryl group include emerald, aquamarine, and morganite while members of the chrysoberyl group include chrysoberyl, cymaphane (cat´s eye) and alexandrite. Beryl is a silicate and chrysoberyl is an oxide and although both beryl and chrysoberyl contain beryllium, they are separate gemstone species unrelated in any other way. Because of the confusion between chrysoberyl and beryl, chrysoberyl is relatively unknown in its own right and the alexandrite variety is much more widely recognized. The only natural stones harder than chrysoberyl are corundum and diamond. Three varieties of beryl: Morganite, Aquamarine, and Heliodor The mineral beryl is a beryllium aluminium cyclosilicate with the chemical formula Be3Al2(SiO3)6. ... Three varieties of beryl: Morganite, Aquamarine, and Heliodor The mineral beryl is a beryllium aluminium cyclosilicate with the chemical formula Be3Al2(SiO3)6. ... General Name, Symbol, Number beryllium, Be, 4 Chemical series alkaline earth metals Group, Period, Block 2, 2, s Appearance white-gray metallic Standard atomic weight 9. ...


Alexandrite

Alexandrite step cut cushion, 26.75 cts. Alexandrites this large are extremely rare.
Alexandrite step cut cushion, 26.75 cts. Alexandrites this large are extremely rare.

The alexandrite variety displays a color change (alexandrite effect) dependent upon light, along with strong pleochroism. Alexandrite results from small scale replacement of aluminium by chromium oxide, which is responsible for alexandrite's characteristic green to red color change. 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. The optimum or "ideal" color change would be fine emerald green to fine purplish red, but this is exceedingly rare. Because of their rarity and the color change capability, "ideal" alexandrite gems are some of the most expensive in the world. Image File history File linksMetadata Size of this preview: 800 × 377 pixels Full resolution (1365 × 643 pixel, file size: 625 KB, MIME type: image/jpeg) // Alexandrite Cushion, 26. ... Image File history File linksMetadata Size of this preview: 800 × 377 pixels Full resolution (1365 × 643 pixel, file size: 625 KB, MIME type: image/jpeg) // Alexandrite Cushion, 26. ... Pleochroism is an optical phenomenon where due to double refraction of light by a colored gem or crystal, the light is divided into two paths which are polarized at a 90° angle to each other. ... General Name, Symbol, Number chromium, Cr, 24 Chemical series transition metals Group, Period, Block 6, 4, d Appearance silvery metallic Atomic mass 51. ... Map of the Ural Mountains The Ural Mountains (Russian: , Uralskiye gory) (also known as the Urals, the Riphean Mountains in Greco-Roman antiquity, and known as the Stone Belt) are a mountain range that runs roughly north and south through western Russia. ... For other things of this name, see Columbine (disambiguation). ... Binomial name Rubus idaeus L. The Raspberry or Red Raspberry (Rubus idaeus) is a plant that produces a tart, sweet, red composite fruit in summer or early autumn. ...


According to a widely popular but controversial story, alexandrite was discovered by the Finnish mineralogist Nils Gustaf Nordenskiöld, (1792 -1866) on the tsarevitch Alexanders sixteenth birthday on April 17, 1834 and named alexandrite in honor of the future Tsar Alexander II of Russia. Sometimes, Nils Gustaf Nordenskiöld is confused with his son, Adolf Erik Nordenskjöld (1832–1901), also a famous Finnish geologist, mineralogist and arctic explorer who accompanied his father to the Ural Mountains to study the iron and copper mines at Tagilsk in 1853. However, Adolf Erik Nordenskjöld was only two years old when Alexandrite was discovered and only ten years old when a description of the stone was published under the name of Alexandrite for the first time. Nils Gustaf Nordenskiöld (October 12, 1792 — February 2, 1866) was a Finnish mineralogist and a traveller. ... It has been suggested that Tsaritsa be merged into this article or section. ... Alexander (Aleksandr) II Nikolaevich (Russian: Александр II Николаевич) (born 17 April 1818 in Moscow; died 13 March 1881 in St. ... Nils Gustaf Nordenskiöld (October 12, 1792 — February 2, 1866) was a Finnish mineralogist and a traveller. ... Nils Adolf Erik Nordenskiöld Nils Adolf Erik Nordenskiöld by Axel Jungstedt 1902 Nils Adolf Erik Nordenskiöld with the Vega by Georg von Rosen Adolf Erik Nordenskiöld Baron (Nils) Adolf Erik Nordenskiöld [IPA: [nuːrdenʃɶld]], also known as A. E. Nordenskioeld (November 18, 1832, Helsinki...


Although it was Nordenskjold who discovered alexandrite, he could not possibly have discovered and named it on Alexander's birthday. Nordensljold's initial discovery occurred as a result an examination of a newly found mineral sample he had received from Perovskii, which he identified it as emerald at first. Confused with the high hardness, he decided to continue his examinations. Later that evening, while looking at the specimen under candlelight, he was surprised to see that the color of the stone had changed to raspberry-red instead of green. Later, he confirmed the discovery of a new variety of chrysoberyl, and suggested the name "diaphanite"(from the Greek "di" two and "aphanes", unseen or "phan", to appear, or show).[2]


The name of the first person to actually find this stone has been lost in the mists of time. However, the first person to bring it to public attention, and ensure that it would be forever associated with the Imperial family was Count Lev Alekseevich Perovskii (1792-1856.)[5]


The finest alexandrites were found in the Ural Mountains, the largest cut stones being in the 30 carat (6 g) range, though many fine examples have been discovered in Sri Lanka (up to 65 cts.), India (Andhra Pradesh), Brazil, Myanmar, and especially Zimbabwe (small stones usually under 1 carat (200 mg) but with intense color change). Overall, stones from any locale over 5 carats (1 g) would be considered extremely rare, especially gems with fine color change. Alexandrite is both hard and tough, making it very well suited to wear in jewelry. Andhra Pradesh  : (Telugu: ఆంధ్ర ప్రదేశ్, Urdu: آندھرا پردیش, IPA: ), is a state in South India. ...


The gem has given rise to the adjective "alexandritic", meaning any transparent gem or material which shows a noted change in color between natural and incandescent light. Some other gem varieties of which alexandritic specimens have been found include sapphire, garnet, and spinel. Sapphire (from Hebrew: ספּיר Sapir) is the single-crystal form of aluminium oxide (Al2O3), a mineral known as corundum. ... Garnet is a group of minerals that have been used since the Bronze Age as gemstones and abrasives. ... The spinels are any of a class of minerals which crystallize in the isometric system with an octahedral habit. ...


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. General Name, Symbol, Number vanadium, V, 23 Chemical series transition metals Group, Period, Block 5, 4, d Appearance silver-grey metal Atomic mass 50. ... The spinels are any of a class of minerals which crystallize in the isometric system with an octahedral habit. ...


Synthetic alexandrite is used as an active laser medium. Alexandrite laser crystals tend to be round, with a pale brown tint. Synthetic alexandrite has been created by the floating zone method. ... Within a laser, the active laser medium is the material that exhibits optical gain. ...


Cymophane

Fine color Cymophane with a sharp and centered eye.
Fine color Cymophane with a sharp and centered eye.

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 chatoyancy 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. Image File history File linksMetadata No higher resolution available. ... Image File history File linksMetadata No higher resolution available. ... Tiger eye In gemology, chatoyancy (or chatoyance) is an optical reflectance effect seen in certain gemstones. ... Rutile in trellis texture characteristic of secondary rutile. ... A cabochon or cabouchon is a gemstone which has been shaped and polished as opposed to facetted. ...


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. The tourmaline mineral group is chemically one of the most complicated groups of silicate minerals. ... Scapolite (Gr. ... Corundum (from Tamil kurundam) is a crystalline form of aluminium oxide and one of the rock-forming minerals. ... The spinels are any of a class of minerals which crystallize in the isometric system with an octahedral habit. ... Quartz is one of the most common minerals in the Earths continental crust. ...


Lacking the silky inclusions required to produce the cat´s eye effect is 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 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.


See also

Gem animals. ... Synthetic alexandrite has been created by the floating zone method. ...

References

  1. ^ Chrysoberyl In Webmineral. Retrieved online 08:20, January 25, 2005
  2. ^ a b Klein, Cornelis; and Cornelius S. Hurlbut, Jr. (1985). Manual of Mineralogy, 20th ed., New York: Wiley. ISBN 0-471-80580-7. 
  3. ^ Chapter 6: World Occurrences. (2006, February 07). In Alexandrite.net, Tsarstone collectors guide. Retrieved online 07:32, February 26, 2007
  4. ^ Chapter 3: Species and variety. (2006, February 07). In Alexandrite.net, Tsarstone collectors guide. Retrieved online 06:45, February 26, 2007
  5. ^ Chapter 2: Diaphanite or Alexandrite? (2006, December 07). In Alexandrite.net, Tsarstone collectors guide. Retrieved online 08:20, January 25, 2007

External links

  • Alexandrite guide
  • http://www.gemstone.org/gem-by-gem/english/catseye.html
  • http://mineral.galleries.com/minerals/oxides/chrysobe/chrysobe.htm
  • http://www.mindat.org/min-1039.html
  • http://www.webmineral.com/data/Chrysoberyl.shtml
  • http://www.mindat.org/min-109.html
  • http://mineral.galleries.com/minerals/gemstone/alexandr/alexandr.htm
  • http://www.gemstone.org/gem-by-gem/english/alex.html

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Chrysoberyl is the most abundant constituent of the pegmatite aside from quartz and feldspar.
chrysoberyl: Definition and Much More From Answers.com (1488 words)
Chrysoberyl is a rare mineral found most commonly in pegmatite dikes and occasionally in granitic rocks and mica schists.
Chrysoberyl is transparent to translucent and sometimes chatoyant.
Chrysoberyl occurs in granitic rocks, pegmatites and mica schists; often it is found in alluvial deposits.
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