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Chrysoberyl

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Chrysoberyl
Cyclic trilling of chrysoberyl
General
CategoryOxide minerals
Formula
(repeating unit)		BeAl2O4
IMA symbolCbrl[1]
Strunz classification4.BA.05
Crystal systemOrthorhombic
Crystal classDipyramidal (mmm)
H-M symbol:(2/m 2/m 2/m)
Space groupPbnm
Unit cella = 5.481 Å,
b = 9.415 Å,
c = 4.428 Å; Z = 4
Identification
ColorVarious shades of green, emerald-green yellow, blue, 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 habitCrystals tabular or short prismatic, prominently striated
TwinningContact and penetration twins common, often repeated forming rosette structures
CleavageDistinct on {110}, imperfect on {010}, poor on {001}
FractureConchoidal to uneven
TenacityBrittle
Mohs scalehardness8.5
LusterVitreous
StreakWhite
Specific gravity3.5–3.84
Optical propertiesBiaxial (+)
Refractive indexnα=1.745 nβ=1.748 nγ=1.754
PleochroismX = red; Y = yellow-orange; Z = emerald-green
2V angleMeasured: 70°
References[2][3][4][5]
Major varieties
AlexandriteColor change; green to red
CymophaneChatoyant
Main chrysoberyl producing countries

The mineral orgemstonechrysoberylis analuminateofberylliumwith the formulaBeAl2O4.[5][6]The name chrysoberyl is derived from theGreekwords χρυσόςchrysosand βήρυλλοςberyllos,meaning "a gold-white spar". Despite the similarity of their names, chrysoberyl andberylare two completely different gemstones, although they both contain beryllium. Chrysoberyl is the third-hardest frequently encountered natural gemstone and lies at 8.5 on theMohs scale of mineral hardness,betweencorundum(9) andtopaz(8).[7]

An interesting feature of its crystals are thecyclic twinscalledtrillings.These twinned crystals have ahexagonalappearance, 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 totranslucent.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 orcymophane,andalexandrite.Yellow-green chrysoberyl was referred to as "chrysolite" during theVictorianand Edwardian eras, which caused confusion since that name has also been used for the mineralolivine( "peridot"as a gemstone); that name is no longer used in thegemologicalnomenclature.

Alexandrite, a stronglypleochroic(trichroic) gem, will exhibitemeraldgreen, red and orange-yellow colors depending on viewing direction in partiallypolarised 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 more blue-green and redwavelengthsto be transmitted. Which of these prevails to give the perceived hue depends on the spectral balance of the illumination. Fine-quality alexandrite has a green to bluish-green color in daylight (relatively blue illumination of highcolor temperature), changing to a red to purplish-red color inincandescent light(relatively yellow illumination).[8]However, fine-color material is extremely rare. Less-desirable stones may have daylight colors of yellowish-green and incandescent colors of brownish red.[8]

Cymophane is popularly known as "cat's eye". This variety exhibits pleasingchatoyancyor opalescence that reminds one of theeye of a cat.[9]When cut to produce acabochon,the mineral forms a light-green specimen with a silky band of light extending across the surface of the stone.

Occurrence[edit]

Chrysoberyl forms as a result ofpegmatiticprocesses. Melting in theEarth's crustproduces relatively low-density moltenmagmawhich 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 thecrystallizationof 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 is igneous in appearance but formed at a low temperature from a water-rich melt, with large crystals of the common minerals such asquartzandfeldspar,but also with elevated concentrations of rare elements such as beryllium,lithium,orniobium,often forming their own minerals; this is called apegmatite.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 thecountry rocksnear to pegmatites, when Be- and Al-rich fluids from the pegmatite react with surrounding minerals. Hence, it can be found inmicaschistsand in contact with metamorphic deposits ofdolomiticmarble. 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 suchplacers,it will have rounded edges instead of sharp, wedge-shape forms. Much of the chrysoberyl mined inBrazilandSri Lankais 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, somechromiumwould also have had to be present. However, beryllium and chromium do not tend to occur in the same types of rock. Chromium is most common inmaficandultramaficrocks in which beryllium is extremely rare. Beryllium becomes concentrated infelsicpegmatites 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[edit]

The alexandrite variety displays a color change dependent upon the nature of ambient lighting. 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 (520–620nm)[10][11]of the visible light spectrum.[10]Becausehuman visionis most sensitive to green light and least sensitive to red light, alexandrite appears greenish in daylight where the full spectrum of visible light is present, and reddish in incandescent light which emits less green and blue light.[10][12]This color change is independent of any change of hue with viewing direction through the crystal that would arise frompleochroism.[10]

Alexandrite from theUral Mountainsin Russia can be green by daylight and red by incandescent light. Other varieties of alexandrite may be yellowish or pink in daylight and acolumbineorraspberryred by incandescent light.[citation needed]

Stones that show a dramatic color change and strong colors (e.g., red-to-green) are rare and sought-after,[8]but stones that show less distinct colors (e.g. yellowish green changing to brownish yellow) may also be considered "alexandrite" by gem labs such as the Gemological Institute of America.[13][14]

According to a popular but controversial story, alexandrite was discovered by the Finnish mineralogistNils Gustaf Nordenskiöld(1792–1866), and named alexandrite in honor of the futureEmperor of All RussiaAlexander II Romanov.Nordenskiöld's initial discovery occurred as a result of an examination of a newly found mineral sample he had received from Perovskii, which he identified as emerald at first.[15][16]The first emerald mine had been opened in 1831. However, recent research suggests that the stone was discovered by Yakov Kokovin.[17]

Alexandrite 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 inIndia(Andhra Pradesh),Madagascar,Tanzania and Sri Lanka. Alexandrite in sizes over three carats are very rare. Today, several labs can producesyntheticlab-grown stones with the same chemical and physical properties as natural alexandrite. Several methods can produce flux-grown alexandrite, Czochralski (or pulled) alexandrite, and hydrothermally-produced alexandrite. Flux-grown gems are fairly difficult to distinguish from natural alexandrite as they contain inclusions that seem natural. Czochralski or pulled alexandrite is easier to identify because it is very clean and contains curved striations visible under magnification. Although the color change in pulled stones can be from blue to red, the color change does not truly resemble that of natural alexandrite from any deposit. Hydrothermal lab-grown alexandrite has identical physical and chemical properties to real alexandrite.[18]

Some gemstones falsely described as lab-grownsynthetic alexandriteare actuallycorundumlaced with trace elements (e.g.,vanadium) or color-changespineland are not actually chrysoberyl. As a result, they would be more accurately described assimulated alexandriterather than "synthetic". This alexandrite-likesapphirematerial has been around for almost 100 years and shows a characteristic purple-mauve colour change, which does not really look like alexandrite because there is never any green.[19]

  • Alexandrite
  • Rough alexandrite from Brazil
    Rough alexandrite from Brazil
  • Rough alexandrite under UV light
    Rough alexandrite underUV light
  • Alexandrite step cut cushion, 26.75 cts. This stone is bluish green in daylight and purple red under incandescent light
    Alexandrite step cut cushion, 26.75 cts. This stone is bluish green in daylight and purple red under incandescent light
  • An alexandrite ring with diamonds
    An alexandrite ring with diamonds

History Of Alexandrite Gemstones[edit]

Named after Czar Alexander II, who is noted for emancipating Russia’s serfs, alexandrite had largely been depleted in Russia by the 1890s due to extensive mining. A global rediscovery sparked renewed interest in this remarkable gem, often referred to as a “second-chance stone.”[citation needed]

The color of alexandrite can vary by origin and sometimes helps identify imitations. Generally, the highest quality stones, predominantly green, are found in Africa. Indian alexandrite typically exhibits blue or grassy-green hues in daylight, whereas Sri Lankan stones lean towards yellowish-green. Sri Lankan alexandrites are often larger but tend to have a weaker color change, displaying a softer purple at night.[citation needed]

Given the rarity of the true “alexandrite effect” today, choosing a stone often boils down to personal preference between its colors in different lights—whether one favors the “emerald by day” or “ruby by night” appearance. For instance, Madagascan alexandrite might appear more brownish than purplish in incandescent light. Ultimately, the beauty ofalexandriteis subjective, emphasizing that its allure truly lies in the eye of the beholder.[citation needed]

Cymophane[edit]

Fine-color cymophane with a sharp and centered eye
Fine-color cymophane with a sharp and centered eye

Translucent yellowishchatoyantchrysoberyl is calledcymophaneorcat's eye.Cymophane has its derivation also from the Greek words meaning 'wave' and 'appearance', in reference to the haziness that visually distorts what would normally be viewed as a well defined surface of a cabochon. This effect may be combined with a cat eye effect. In this variety, microscopic tubelike cavities or needle-like inclusions[20]ofrutileoccur 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 forms perpendicular to the c-axis. The color in yellow chrysoberyl is due to Fe3+impurities.[citation needed]

Although other minerals such astourmaline,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.[citation needed]

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.[citation needed]

Cat's eye became significantly more popular by the end of the 19th century when theDuke of Connaughtgave 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.[21]

See also[edit]

References[edit]

  1. ^Warr, L.N. (2021)."IMA–CNMNC approved mineral symbols".Mineralogical Magazine.85(3): 291–320.Bibcode:2021MinM...85..291W.doi:10.1180/mgm.2021.43.S2CID235729616.
  2. ^"Mineralienatlas – Fossilienatlas".Archivedfrom the original on 2 February 2017.Retrieved20 January2017.
  3. ^Handbook of MineralogyArchived2011-12-15 at theWayback Machine
  4. ^"Chrysoberyl: Chrysoberyl mineral information and data".Archivedfrom the original on 13 January 2017.Retrieved20 January2017.
  5. ^abBarthelmy, Dave."Chrysoberyl Mineral Data".Archivedfrom the original on 19 February 2017.Retrieved20 January2017.
  6. ^Rudler, Frederick William(1911)."Chrysoberyl".InChisholm, Hugh(ed.).Encyclopædia Britannica.Vol. 6 (11th ed.). Cambridge University Press. p. 320.
  7. ^Klein, Cornelis; Cornelius S. Hurlbut Jr. (1985).Manual of Mineralogy(20th ed.). New York: Wiley.ISBN0-471-80580-7.
  8. ^abc"Alexandrite Quality Factors".Archivedfrom the original on 18 December 2016.Retrieved20 January2017.
  9. ^Rudler, Frederick William(1911)."Cat's-Eye".Encyclopædia Britannica.Vol. 5 (11th ed.). p. 537.
  10. ^abcd"Why does alexandrite appear to change color in sunlight and artificial light?".Archivedfrom the original on 14 July 2014.Retrieved6 June2014.
  11. ^Xie, Fei; Cao, Yu; Ranchon, Cindy; Hart, Alan; Hansen, Robin; Post, Jeffrey E.; Whitney, Coralyn W.; Dawson-Tarr, Emma; Drew, Alan J.; Dunstan, David J. (April 9, 2020)."Explanation of the Colour Change in Alexandrites".Scientific Reports.10(1): 6130.Bibcode:2020NatSR..10.6130X.doi:10.1038/s41598-020-62707-3.PMC7145866.PMID32273534.
  12. ^Xie, Fei; Cao, Yu; Ranchon, Cindy; Hart, Alan; Hansen, Robin; Post, Jeffrey E.; Whitney, Coralyn W.; Dawson-Tarr, Emma; Drew, Alan J.; Dunstan, David J. (April 9, 2020)."Explanation of the Colour Change in Alexandrites".Scientific Reports.10(1): 6130.Bibcode:2020NatSR..10.6130X.doi:10.1038/s41598-020-62707-3.PMC7145866.PMID32273534.
  13. ^"Bonhams: Alexandrite and Platinum and Diamond Ring".Archivedfrom the original on 7 March 2016.Retrieved20 January2017.
  14. ^"GIA – Report Check".Archivedfrom the original on 12 October 2016.Retrieved20 January2017.
  15. ^""Alexandrite or Diaphanite?". In Alexandrite Tsarstone Collectors Guide ".2006.Archivedfrom the original on 2007-10-12.Retrieved2007-07-09.
  16. ^Nordenskiöld N.Alexandrit oder Ural Chrysoberyll // Schriften der St.-Petersburg geschrifteten Russisch-Kaiserlichen Gesellschaft fuer die gesammte Mineralogie. 1842. Bd 1. S. 116–127.
  17. ^Wise, Richard W. (2016).Secrets Of the Gem trade(Second ed.). Brunswick House Press. p. 93.ISBN9780972822329.
  18. ^Clark, Donald."Is Synthetic Alexandrite Real Alexandrite?".International Gem Society.Archivedfrom the original on 2016-07-07.Retrieved2016-09-21.
  19. ^"Alexandrite Synthetics and Imitations, In Alexandrite Tsarstone Collectors Guide".2006.Archivedfrom the original on 2008-05-30.Retrieved2007-07-09.
  20. ^"Mitchell, T. E. and Marder, J. M.," Precipitation in Cat's-Eye Chrysoberyl, "Electron Microscopy Soc. Proceedings, 1982.
  21. ^"U.S. Geological Survey, 1887, George Frederick Kunz, Cymophane, Cat's Eye as gemstone".1887.Archivedfrom the original on 2007-07-11.Retrieved2007-07-09.

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