Chrysoberyl
X = c = red-violet (similar to the columbines, English:. Columbine ) Y = b = Yellow Orange Z = a = Emerald Green
The 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 BeAl2O4, is so chemically seen a beryllium aluminate.
Chrysoberyl usually develops dicktafelige to short prismatic crystals, which can be up to 22 inches tall and predominantly parallel to the c - axis are striped. Another characteristic is its cyclic twinning with pseudohexagonal - dipyramidalem habit.
In pure form, chrysoberyl is colorless and transparent with glass-like sheen on the surfaces. But it can also appear white and by foreign admixtures of chromium and iron, a golden yellow, yellow-green to blue-green or brownish color accept, the transparency decreases accordingly. Through multiple refraction due to lattice defects or polycrystalline training
With a Mohs hardness of 8.5 chrysoberyl is after diamond (10 ), the very rare moissanite (9.5 ) and the corundum ( 9) is the fourth hardest mineral.
Known Schmucksteinvarietäten are the color changing alexandrite and the silky shimmering cats eye with the same visual effect.
Special Features
Chrysoberyl is very sensitive to various alkalis and potassium hydrogen sulfate ( potassium bisulfate ) and is decomposed by them. Before the blowpipe and of acids it is not changed.
Etymology and history
The name chrysoberyl, from the Greek χρυσοβήρυλλος [ chrysobḗryllos ] is composed of the words χρυσός [ Chrysos ] for " Gold" and βήρυλλος [ beryllos ] for " beryl ".
The chrysoberyl is one of the 20 gemstones that already by the Roman writer Pliny ( 23-79 AD) describes in his " Naturalis historia ". Pliny saw chrysoberyl mistakenly called a subspecies of beryls, as gold colored brother of aquamarine (blue) and Emerald (green), which, however, he does not belong and is different from them in chemical composition, structure and hardness. Nevertheless, chrysoberyl is performed only in 1789 in the mineral systematics of Abraham Gottlob Werner as an independent mineral ( Krisoberil ).
Classification
In the now outdated but still in use 8th edition of the mineral classification by Strunz the chrysoberyl for general ward which was part of " oxides with the molar ratio of metal: oxygen = 3: 4 ," where he along with Swedenborgit as named the " chrysoberyl Swedenborgit Group "with the system number. Formed IV/B.07 and the other members Ferrotaaffeit and Magnesiotaaffeit.
The 9th edition used since 2001 and valid by the International Mineralogical Association (IMA ) of the Strunz'schen Mineral classification assigns the chrysoberyl also in the department of " oxides with the molar ratio of metal: oxygen = 3: 4 (and similar ) " a. This division, however, is further divided according to the relative size of the cations involved, so that the mineral is found according to its composition in the subsection " Using small and medium-sized cations ", where it forms the unnamed group 4.BA.05 the only member.
The mainly common in English-speaking classification of minerals according to Dana assigns the chrysoberyl in the class of " oxides and hydroxides ," there but in the department of " multiple oxides " one. Here he is, " multiple oxides with the general formula ( A B2 ) 2X4 spinel group " to find the only member of the unnamed group 07.02.09 within the subdivision of.
Varieties and modifications
Alexandrite, a very rare and valuable variety, illuminates green to bluish green in daylight and in artificial light red to purple on. This color change, also known as iridescence or alexandrite effect is caused by its chromium content. The cause is the strong pleochroism and the different spectral brightness maximum of the day and the artificial light. The Alexandrite practically acts like a filter that passes only red or green light. In the daylight, which contains a greater proportion of green light, it therefore appears green. In the artificial lamp or candle light, the red component is much stronger than the green, on the other hand it appears bright red. The name Alexandrite goes back to the later Russian tsar Alexander II (1855-1881), at whose Großjährigkeitserklärung in 1830 the stone was first found. The main colors of the former Russian army were green and red.
Another variety is the chrysoberyl cat's eye cat's eye or short ( outdated and no longer common synonyms Cymophan or Kymophan ) showing the coveted cat's eye effect. Only this variety must bear the sole designation cat's eye. All other minerals with the cat eye effect must be identified by the addition of the corresponding mineral name. The surging, silver-white streak of light caused by light refraction in the fine, arranged in parallel hollow channels.
Education and Locations
Chrysoberyl forms magmatic in pegmatites or by contact metamorphism in schists and due to its resilience in placer deposits. Accompanying minerals occur in, among other things albite, apatite, beryl, columbite, fluorite, various grenade, kyanite, muscovite, Phenakit, quartz, spinel, staurolite, topaz and tourmaline.
As a rather rare mineral chrysoberyl education can indeed be abundant in part to different sites, overall it is not very common. So far (as of 2012) are regarded as known total of 300 localities.
Among the most famous sites of chrysoberyl include:
- Brazil: Good trained and up to 22 centimeters large crystals and twins came to light especially in Pancas in Espírito Santo state, but also at several sites in Bahia, Minas Gerais and other regions were several centimeters chrysoberyls be found.
- India and Sri Lanka are known localities for the coveted Schmuckvarietäten alexandrite and cat's eye, mainly in the areas around Deobhog in Chhattisgarh and Orissa ( India) and in Ratnapura and other areas of Sabaragamuwa (Sri Lanka) were found.
- In Madagascar, were up to 10 cm large crystals are found and a well-known gemstone deposit is Ilakaka in the province of Fianarantsoa in the area around Ambatondrazaka.
- In Russia, in the Urals Malysheva belongs to the most famous archaeological sites, where in addition to emerald and Phenakit also found valuable Alexandrite up to eight centimeters in size.
- Several inches of large crystals were including in the area of Mogok in Mandalay Division of Myanmar ( Burma), the Masvingo Province in southeastern Zimbabwe, at Magara near the Lake Manyara in Tanzania and in Maršíkov ( Marschendorf ) in the Czech region Olomouc Region on ( Olomouc).
In Austria, the mineral found among others in Rieding in Carinthia, in Mieslingtal in the Lower austrian town of Spitz, as well as in Felber and Habachtal in Salzburg part of the Hohe Tauern, in Switzerland some find points in the cantons of Grisons and Ticino are known. German localities are not yet known.
Other localities lie among other things in Antarctica, Australia, Bulgaria, China, the Democratic Republic of the Congo, Finland, France, Italy, Japan, Mozambique, Namibia, Niger, Norway, Poland, Zambia, Sweden, Spain, South Africa, the United Kingdom and the United States of America.
Crystal structure
Chrysoberyl crystallizes in the orthorhombic space group Pbnm ( Raumgruppen-Nr. 62) with the lattice parameters a = 4.43 Å; b = 9.40 Å and c = 5.47 Å and four formula units per unit cell.
The crystal structure similar to that of olivine, is, however, contrary to that of [ Beo4 ] tetrahedra whose corners are linked via octahedrally coordinated Al3. The crystal- chemical structural formula can therefore be described analogous to olivine with Al2 [ Beo4 ].
Use as a gemstone
Chrysoberyl and its varieties are used primarily as gemstones use, but only a small part of the chrysoberyl crystals is clear and transparent, as it is needed for jewelery making and usually can only relatively small pieces of crystal cut out and warm to clear, brilliant and glowing " gems " are ground, with various facets cuts are applied. The other hand, cats eyes get the necessary for optimum highlighting the chatoyancy cabochon cut.
Due to similarities in color and form chrysoberyl can be confused with various other minerals, which are also partly processed into semi-precious stones such as, among others, andalusite, brazilianite, golden beryl, Hiddenite, peridot, sapphire, Sinhalit, scapolite, spinel, topaz, tourmaline and zircon.
Famous chrysoberyls
The largest chrysoberyl found so far occurred in Rio de Janeiro (Brazil ) to light and had a weight of 16 pounds. Another weighing 1876 ct ( ≙ 375.2 g ) is also very large stone was found in Sri Lanka.
The largest known, polished alexandrite has a weight of 66 ct and is preserved in the Smithsonian Institution in Washington (USA). Also famous is the London- kept " Hope Chrysoberyl ", a light green faceted stone 45 ct weight.
Manipulation and imitation
Since Chrysoberyl and especially the extremely rare and expensive alexandrite is a rare and correspondingly expensive gemstone, it is often imitated by various methods:
- Already since 1888 Alexandrite is also produced synthetically. These are to be distinguished only by using gemological tests properly of natural stones. The inclusions play an important role.
- Similar, cheaper minerals such as Cat's Eye quartz are often used to mimic the chrysoberyl. Other imitations are produced using glass, synthetic corundum or spinel. The synthetic corundum, preferably sapphire, also used for imitation of alexandrite, because it shows a similar color change, however, is rather from red to violet. The trade name of Blue Alexandrite and Sri Lankan alexandrite are actually so sapphires.
- Very successful imitations of chrysoberyl be achieved by the generation of duplicates ( composite -precious stones), which serves as a base garnet or glass.
- To enhance the natural chrysoberyls less valuable color development by color change or intensification, they are irradiated radioactive since 1997. But especially on exposure to elementary strong residual radiation is generated, the treated stones must sometimes several years in quarantine.