Chrysoberyl
| Chrysoberyl | |
|---|---|
 |
|
| General and classification | |
| chemical formula | BeAl 2 O 4 |
|
Mineral class (and possibly department) |
Oxides and hydroxides |
|
System no. to Strunz and to Dana |
4.BA.05 ( 8th edition : IV / B.07) 02/07/09/01 |
| Similar minerals | Andalusite , brasilianite , golden beryl and others (see use as gemstone ) |
| Crystallographic Data | |
| Crystal system | Orthorhombic |
| Crystal class ; symbol | orthorhombic-dipyramidal; 2 / m 2 / m 2 / m |
| Space group | Pbnm (No. 62, position 3) |
| Lattice parameters | a = 4.43 Å ; b = 9.40 Å; c = 5.47 Å |
| Formula units | Z = 4 |
| Frequent crystal faces | {010}, {001}, {101}, {012}, {111} |
| Twinning | cyclic penetration triplets at alexandrite |
| Physical Properties | |
| Mohs hardness | 8.5 |
| Density (g / cm 3 ) | measured: 3.75 (1); calculated: 3.69 |
| Cleavage | imperfect after {010}, clear after {110}, indistinct after {001} |
| Break ; Tenacity | shell-like to uneven |
| colour | colorless, yellow to golden yellow, brown, green, blue-green |
| Line color | White |
| transparency | transparent to translucent |
| shine | Glass gloss, fat gloss |
| Crystal optics | |
| Refractive indices |
n α = 1.746 n β = 1.748 n γ = 1.756 |
| Birefringence | δ = 0.010 |
| Optical character | biaxial positive |
| Axis angle | 2V = 70 ° (measured); 72 ° (calculated) |
| Pleochroism | visible: X = c = red-violet (similar to the Akeleien , engl .: columbine ) |
Chrysoberyl is a rarely occurring mineral from the mineral class of oxides and hydroxides . It crystallizes in the orthorhombic crystal system with the chemical formula BeAl 2 O 4 , is so chemically seen a beryllium - aluminate .
Chrysoberyl usually develops thick tabular to short prismatic crystals , which can be up to 22 centimeters in size and are mostly striped parallel to the c-axis. Its cyclical formation of twins with a pseudohexagonal-dipyramidal habit is characteristic.
In its pure form, chrysoberyl is colorless and transparent with a glass-like sheen on the surfaces. However, due to multiple refraction due to lattice construction defects or polycrystalline training, it can also appear white and, due to foreign admixtures of chromium and iron, take on a golden-yellow, green-yellow to blue-green or brownish color, with the transparency decreasing accordingly.
With a Mohs hardness of 8.5, chrysoberyl is the fourth hardest mineral after diamond (10), the very rare moissanite (9.5) and corundum (9).
Well-known gemstone varieties are the color-changing “alexandrite” and the silky shimmering (chrysoberyl) “cat's eye” with the optical effect of the same name .
Etymology and history
The name Chrysoberyl, from the Greek χρυσοβήρυλλος [chrysobḗryllos], is composed of the words χρυσός [chrysós] for "gold" and βήρυλλος [bḗryllos] for " beryl ".
The chrysoberyl is one of the 20 or so precious stones described by the Roman writer Pliny (23–79 AD) in his "Naturalis historia". Pliny wrongly saw chrysoberyl as a subspecies of beryl, as the golden brother of aquamarine (blue) and emerald (green), to which it does not belong and differs from them in chemical composition, structure and hardness. Nevertheless, chrysoberyl was only listed as an independent mineral ( Krisoberil ) in the mineral system of Abraham Gottlob Werner in 1789 .
classification
In the meanwhile outdated, but still in use 8th edition of the mineral systematics according to Strunz , the chrysoberyl belonged to the general department of "oxides with the molar ratio metal: oxygen = 3: 4", where together with swedenborgite it gave its name to the chrysoberyl-swedenborgite group “With the system no. IV / B.07 and the other members Ferrotaaffeite and Magnesiotaaffeit formed.
The 9th edition of Strunz's mineral systematics , which has been in effect since 2001 and is used by the International Mineralogical Association (IMA), also classifies chrysoberyl into the department of "Oxides with the molar ratio of metal: oxygen = 3: 4 (and comparable)". However, this section is further subdivided according to the relative size of the cations involved , so that the mineral can be found according to its composition in the sub-section “With small and medium-sized cations”, where it is the only member of the unnamed group 4.BA.05 .
The systematics of minerals according to Dana , which is mainly used in the English-speaking area , also assigns chrysoberyl to the class of "oxides and hydroxides", but there it is in the category of "multiple oxides". Here he is to be found as the only member of the unnamed group 07.02.09 within the subsection of " Multiple oxides with the general formula (A + B 2+ ) 2 X 4 , spinel group ".
Crystal structure
Chrysoberyl crystallizes orthorhombically in the space group Pbnm (space group no. 62, position 3) with the lattice parameters a = 4.43 Å ; b = 9.40 Å and c = 5.47 Å and 4 formula units per unit cell .
The crystal structure is similar to that of olivine , but in contrast to it consists of [BeO 4 ] tetrahedra , the corners of which are linked by octahedrally coordinated Al 3+ . The crystal chemical structural formula can therefore be described as Al 2 [BeO 4 ], analogously to olivine .
properties
Chrysoberyl is very sensitive to various alkalis and potassium hydrogen sulfate ( potassium bisulfate ) and is broken down by them. However , it is not changed before the soldering tube and by acids.
Varieties and modifications
Alexandrite , a very rare and valuable variety (Chrysoberyl - Mariinskit mixed series ), shimmers green to bluish green in daylight and red to purple in artificial light. This color change, also known as the shimmering or alexandrite effect , is caused by its chromium content . The cause is two spectral ranges with low absorption (high light transmission) and a range in between with strong absorption together with the different spectral brightness maximum of daylight and artificial light. In daylight, which contains a larger proportion of green light, it therefore appears green. In the light of an incandescent lamp or in candlelight, the red part of which is much stronger than the green, it appears bright red. Alexandrite also shows direction-dependent color changes - pleochroism .
The name alexandrite goes back to the later Russian tsar Alexander II (ruled 1855-1881), on the occasion of whose age-old declaration Lev Alexejewitsch Perowski named the stone in his honor. He was previously examined by Nils Gustaf Nordenskiöld . The main colors of the Russian army at that time were green and red.
Another variety is the chrysoberyl cat's eye or short cat's eye (outdated and no longer common synonyms Cymophan or Kymophan ) that the coveted cat eye effect shows. Only this variety can bear the sole designation cat's eye. All other minerals with the cat's eye effect must be identified by adding the appropriate mineral name. The billowing, silver-white light strip is created by refraction of light in the fine, parallel hollow channels.
Education and Locations
Chrysoberyl forms magmatic in pegmatites or through contact metamorphosis in slate and is also found in soap deposits due to its resistance . The accompanying minerals include albite , apatite , beryl , columbite , fluorite , various garnets , kyanite , muscovite , phenacite , quartz , spinel , staurolite , topaz and tourmalines .
As a rather rare mineral formation, chrysoberyl can sometimes be abundantly available at various sites, but overall it is not very common. So far (as of 2012) around 300 sites are known to be known.
The most famous chrysoberyl sites include:
- Brazil: Well-developed crystals and twins up to 22 centimeters in size were found mainly in pancas in the state of Espírito Santo , but several centimeters of chrysoberyls were also found at several sites in Bahia , Minas Gerais and other regions.
- India and Sri Lanka are well-known sites for the coveted varieties of jewelry alexandrite and cat's eye, which were found mainly in the areas around Deobhog in Chhattisgarh and Orissa (India) as well as in Ratnapura and other areas of Sabaragamuwa (Sri Lanka).
- In Madagascar in the area around could Ambatondrazaka large crystals are found up to 10 centimeters and a known gemstone deposit is Ilakaka in the province of Fianarantsoa .
- In Russia, Malysheva in the Urals is one of the best-known sites where, in addition to emerald and phenakite , valuable alexandrites of up to eight centimeters in size were found.
- Crystals several centimeters in size also occurred in the vicinity of Mogok in the Mandalay Division of Myanmar ( Burma ), the Masvingo province in the southeast of Zimbabwe, at Magara near Lake Manyara in Tanzania and at Maršíkov ( marsh village ) in the Czech Republic Olomoucký kraj ( Olomouc ) region .
In Austria, the mineral was found near Rieding in Carinthia, in the Mieslingtal in the Lower Austrian municipality of Spitz and in the Felbertal and Habachtal in the Salzburg part of the Hohe Tauern, in Switzerland some sites are known in the cantons of Graubünden and Ticino . German sites are not yet known.
Other sites are in the Antarctic, Australia, Bulgaria, China, the Democratic Republic of the Congo, Finland, France, Italy, Japan, Mozambique, Namibia, Niger, Norway, Poland, Zambia, Sweden, Spain, South Africa, in the United Kingdom and the United States of America.
Use as a gem stone
Chrysoberyl and its varieties are primarily used as gemstones , but only a small part of the chrysoberyl crystals are clear and transparent, as they are needed for jewelry production and mostly only relatively small pieces of the crystal can be cut out and made clear, shiny and warm luminous "gemstones" are cut using different facet cuts. Cat eyes, on the other hand, are given the cabochon cut necessary for optimally highlighting the chatoyance .
Due to the similarities in color and shape, chrysoberyl can be confused with various other minerals, some of which are also processed into gemstones such as andalusite , brazilianite , golden beryl , hiddenite , peridot , sapphire , sinhalite , scapolite , spinel , topaz , tourmaline and zircon .
Famous chrysoberyls
The largest chrysoberyl ever found was found in Rio de Janeiro, Brazil, and weighed 16 pounds. Another very large stone with a weight of 1876 ct (≙ 375.2 g) was found in Sri Lanka.
The largest cut alexandrite known to date has a weight of 66 ct and is kept in the Smithsonian Institution in Washington (USA). The "Hope Chrysoberyl", a light green, faceted stone weighing 45 ct, kept in London , is also famous .
Manipulations and imitations
Since chrysoberyl and especially the extremely rare and expensive alexandrite is a rare and correspondingly expensive gemstone, it is often imitated by various methods:
- Alexandrite has also been produced synthetically since 1888 . These synthetic crystals can only be clearly distinguished from natural stones with the help of gemological tests. The inclusions play an important role in this.
- Similar, cheaper minerals such as cat's eye quartz are often used to imitate chrysoberyl. Other imitations are created with the help of glass, synthetic corundum or spinel . The synthetic corundum, preferably sapphire , is also used to imitate alexandrite, because it shows a similar change in color, although it tends to go from red to purple. The trade names Blue Alexandrite and Sri Lankan Alexandrite are actually sapphires.
- Very successful imitations of chrysoberyl are achieved by creating duplicates (assembled gemstones), with garnet or glass serving as the background.
- In order to enhance the natural chrysoberyls with less valuable color expression through color change or intensification, they have been radioactively irradiated since 1997 . However, since there is strong residual radiation, especially when irradiated with elementary particles , the stones treated in this way sometimes have to be in quarantine for a few years .
See also
literature
- Hans Jürgen Rösler : Textbook of Mineralogy . 4th revised and expanded edition. German publishing house for basic industry (VEB), Leipzig 1979, ISBN 3-342-00288-3 , p. 381 .
- Paul Ramdohr , Hugo Strunz : Klockmann's textbook of mineralogy . 16th edition. Ferdinand Enke Verlag, 1978, ISBN 3-432-82986-8 , pp. 508 .
- Bernhard Bruder: Beautified stones . Neue Erde Verlag, 1998, ISBN 3-89060-025-5 , p. 58 .
- Manuel Font Altaba, Giuseppe Tanelli: Mineralogie , Neuer Kaiser Verlag (1995), ISBN 3-7043-1220-7
Web links
- Mineral Atlas: Chrysoberyl u. Mineral Atlas: Mineral Portrait / Alexandrite (Wiki)
- realgems.org - Chrysoberyl (with pictures of various gemstone cuts )
- Mineral Lexicon - Chrysoberyl
- Gemstone Etiquette - Chrysoberyl
- Institute for Gemstone Testing (EPI) and EPI - Radiant Stones
Individual evidence
- ↑ a b c d e Hugo Strunz , Ernest H. Nickel: Strunz Mineralogical Tables . 9th edition. E. Schweizerbart'sche Verlagbuchhandlung (Nägele and Obermiller), Stuttgart 2001, ISBN 3-510-65188-X , p. 187 .
- ↑ Webmineral - Chrysoberyl
- ^ Helmut Schrätze, Karl-Ludwig Weiner: Mineralogie. A textbook on a systematic basis . de Gruyter, Berlin; New York 1981, ISBN 3-11-006823-0 , pp. 384-385 .
- ↑ a b John W. Anthony, Richard A. Bideaux, Kenneth W. Bladh, Monte C. Nichols: Chrysoberyl , in: Handbook of Mineralogy, Mineralogical Society of America , 2001 ( PDF 70.3 kB )
- ↑ a b c d e Mindat - Chrysoberyl (English)
- ↑ CAS Hoffmann: Mineralsystem of Mr. Inspector Werners , published with his permission by CAS Hoffmann, Bergmannisches Journal, Volume 1 (1789), p. 369-398 ( PDF 1.83 MB; p. 6 )
- ↑ Martin Okrusch, Siegfried Matthes: Mineralogie. An introduction to special mineralogy, petrology and geology . 7th fully revised and updated edition. Springer Verlag, Berlin et al. 2005, ISBN 3-540-23812-3 , pp. 51 .
- ^ Friedrich Klockmann : Klockmanns textbook of mineralogy . Ed .: Paul Ramdohr , Hugo Strunz . 16th edition. Enke , Stuttgart 1978, ISBN 3-432-82986-8 , pp. 509 (first edition: 1891).
- ↑ Mindat - Alexandrite
- ↑ E. Gübelin, K. Schmetzer: Gemstones with Alexandrite Effect In: Gems & Gemology Winter 1982, pp. 197–203 (PDF available online at gia.edu ; download PDF press / choose, in English, last accessed July 7, 2016 )
- ↑ a b c Petr Korbel, Milan Novák: Mineral Encyclopedia . Nebel Verlag GmbH, Eggolsheim 2002, ISBN 3-89555-076-0 , p. 80 ( Dörfler Natur ).
- ^ Mindat - localities for chrysoberyl
- ^ A b c Walter Schumann: Precious stones and gemstones. All species and varieties in the world. 1600 unique pieces . 13th revised and expanded edition. BLV Verlags-GmbH., Munich et al. 2002, ISBN 3-405-16332-3 , p. 114 .
- ^ Zeno.org: Chrysoberyl , in: Pierer's Universal-Lexikon