opal
| opal | |
|---|---|
 |
|
| General and classification | |
| chemical formula | SiO 2 • nH 2 O |
|
Mineral class (and possibly department) |
Oxides and hydroxides |
|
System no. to Strunz and to Dana |
4.DA.10 ( 8th edition : IV / D.01) 75.02.01.01 |
| Crystallographic Data | |
| Crystal system | X-ray amorphous / light crystalline |
| Physical Properties | |
| Mohs hardness | 5.5 to 6.5 (depending on the water content: the more water, the softer) |
| Density (g / cm 3 ) | 2.0 to 2.2 (depending on the water content) |
| Cleavage | no |
| Break ; Tenacity | shell-like, uneven, splintery |
| colour | extremely diverse, colorless or milky, gray, brown, red, yellow |
| Line color | White |
| transparency | translucent to opaque |
| shine | Greasy shine |
| Other properties | |
| Special features | opalescent play of colors |
The opal is a frequently occurring mineral from the mineral class of " oxides and hydroxides ". As an amorphous solid, opal (like glass ) does not have a crystal structure and usually appears as a massive vein filling or bulky.
Opals are only processed into gemstones .
Etymology and history
The term opal was taken from the Latin opalus or the Greek ὀπάλλιος opallios for 'precious stone' and probably comes from the Sanskrit upala . However, this is doubted by Allan W. Eckert in his work The World of Opals , since this word applies to all rocks, while the early Magyars / Hungarians called opal mines as Opalbanya .
Opals were already considered particularly valuable gemstones in ancient times , some of which were even rated higher than diamonds . Pliny the Elder wrote: "[...] he has a fire, finer that in the carbunculus, he has the purple spark of the amethyst and the sea-green of the emerald and an unbelievable mixture of light."
classification
In the meanwhile outdated, but still in use 8th edition of the mineral classification according to Strunz , the opal belonged to the mineral class of "oxides and hydroxides" and there to the department of "oxides with the molar ratio of metal: oxygen = 1: 2", where it was used together with Coesite , Cristobalite , melanophlogite , mogánite , quartz , stishovite , tridymite formed the "quartz group".
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 assigns the opal to the class of "oxides and hydroxides" and there into the department of "metal: oxygen = 1: 2 and comparable "a. However, this section is further subdivided according to the size of the cations involved and the relationship between the minerals and the crystal structure, so that the mineral can be found in the sub-section “With small cations: Silica family”, where it is only combined forms the unnamed group 4.DA.10 with tridymite .
In contrast to Strunz's systematics, the systematics of minerals according to Dana , which is mainly used in the English-speaking world, assigns the opal to the class of "silicates and germanates" and there in the department of " framework silicate minerals ". Here he is to be found as the only member of the unnamed group 75.02.01 within the subdivision “ Framework silicates: tetrahedral Si lattice, SiO 2 with H 2 O and organic molecules ”.
properties
As one of the few amorphous minerals, opal consists of hydrated silica gel with the general chemical composition SiO 2 · nH 2 O. The water content is usually between 4 and 9 percent, but can reach around 20 percent.
The outstanding property that makes the opal so desirable as a gem is the colorful, shimmering play of colors, the so-called opalizing of the precious opal. It is created by the reflection and interference of the light rays between the spheres of silica gel, which are between 150 and 400 nanometers in size. In the case of precious opals, these silica gel spheres are roughly the same size and are regularly arranged and densely packed.
The similar-sounding and often misleading term opalescence , on the other hand, describes a milky-bluish, pearlescent optical effect of common opals.
Opals can be dissolved by hydrofluoric acid and potassium hydroxide . Opal does not melt when heated over an open flame, but becomes dull and crackles.
Varieties
The silica gel itself is colorless or at most weakly colored. Colorless or water-clear varieties are correspondingly referred to as glass or milk opal. However, due to different colored impurities, the opal can still take on all possible darker basic colors, also known as body colors . There are gray, amber, red, brown and, more rarely, black varieties. In the jewelry industry , a distinction is made between precious opals and common opals.
Angel-Skin-Opal , on the other hand, is a misleading name for an opal-like mineral called palygorskite, which is white to rosy in color.
A spectacular but extremely rare variety are the so-called "Pineapple Opals" (also Opal Pineapple ), hedgehog or pineapple-shaped pseudomorphoses from Opal to Ikait , which have so far only been found in opal fields near White Cliffs and Andamooka in Australia.
Precious opal
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In general, opals with a lively, opalescent play of colors in gem quality are referred to as precious opals. In contrast, common opals , called potch in Australia , have no play of colors. The fire opals from Mexico are a specialty; they show a uniform, transparent color without any play of colors and are classed as precious opals.
Trade names
The main occurrence of the rare black opal is Lightning Ridge ( New South Wales in Australia ), especially in the opal fields Coocoran , Gravin and Glengarry. Other locations in Australia are the Mintabie in South Australia , as well as Mexico , Nevada in the USA and Indonesia . In addition to the black opal, a distinction is made between the black crystal opal , which is transparent to a certain extent. There is also the dark opal , which remains dark even in transmitted light, and the light gray opal . The occurrence of these varieties is mainly in the Mintabie, they are found where black opals occur.
Light opals used to be called white opals . The deposits are in South Australia near Coober Pedy , Mintabie , White Cliffs and also in Brazil. The light precious opals are divided into light crystal opal and jelly opal .
A specialty are the opals in Queensland , Australia , which are grouped under boulder opal . There are boulder matrix opals that bond with the limonitic rock and yowah nut opals that are enclosed in a ferrous shell. Lighter , darker and black boulders have the corresponding play of colors in the rock. There is also the boulder split , an opal split in the direction of storage with an identical color pattern.
The fire opals from Mexico ( Jalisco near Magdalena ) and Brazil ( Rio Grande do Sul near Campos Burgos and Piauí near Pedro II ) are available with and without play of colors; those with a play of colors are mostly faceted for retail.
The dendrite opals , also called moss opals , are characterized by a variety of colors. They are found in Lower Austria ( Doberberg ), the Czech Republic ( Český Krumlov ), Zambia ( Lake Kariba ), Peru and in Western Australia near Norseman . If there is a play of colors, they are classed as precious opals.
The harlequin opal with its strong, segmented play of colors with a transparent to translucent basic substance is very popular .
Often, opals are named after where they were found, for example the Lightning Ridge Black Opal, the Mexican Black Opal, the Andamooka Crystal Opal or the Andean Opal.
Dark ( black ) opal, rough stone
Dark opal, cut
Light ( white ) opal, rough stone
Light ( white ) opal, cut
Harlequin opal
Opal matrix
Boulder opal
Fire Opal (Mexico)
Moss opal
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Common opal
The common opals, which are also referred to as "half opal", include hyalite (from the Greek hyalos = glass) or glass opal , which, as a simple, water-clear opal with grape-kidney to crust-shaped formation, shows no play of colors.
The hydrophane (from the Greek: hydor (water) and phanos (shine)) or milk opal, which is also assigned to the common opals, is created by age-related loss of water from precious opal and is milky white with only a matt sheen and weak opalescence. By absorbing water, it becomes transparent again for a short time and receives its full play of colors. The stone was also called the world eye (oculus mundi) by the ancient mineralogists . It is irrelevant for use as jewelry , but it can be used in a liquid as an ornament .
Finally, wood opal was created through silicification of wood and is yellowish to brownish in color.
Further varieties belonging to the common opals are the banded agate opal , the reddish to brown jasper opal due to iron compounds , the yellowish honey opal , the porcelain-like, pearly Kascholong (cacholong, porcelain opal), the moss opal with its dendritic structures, the green and often as an imitation for the Chrysoprase used Prasopal (Chrysopal) and the yellowish-brown, wax- glossy wax opal .
As siliceous sinter or Geyserit loose are crusts Opal (siliceous sinter) called fine-grained, which is characterized by the activity of hot springs and geysers settle.
Kieselguhr is the only technically used variety. As a loose aggregate with fine pores, it is very absorbent and heat-insulating and is therefore used in construction .
Education and Locations
education
Opals are formed by the accumulation of silicic acid in different rocks. They arise either in sedimentary rocks or hydrothermally in volcanic rocks such as tuff , but also through sedimentation in organic material, which among other things creates wood opal. The accompanying mineral is chalcedony .
Sedimentary opals
In sediments and sedimentary rocks , a slow loss of water forms a silica gel that changes to a solid consistency. In the course of the evaporation of the water involved, a residual proportion remains. These processes determine the way in which sedimentary opals form. Alternating cycles of dry and humid climatic periods as well as the weathering products of existing clay minerals are further prerequisites for opal formation. The great Australian Opalvorkommen that in a sedimentary basin emerged that a fifth of Australia covered are now seen as a result of weathering processes where silica solutions in clays , sands, gravels Cretaceous rivers and sandstones and in fine-grained, clay-rich layers of marl and coarse conglomerate bands entered. Opal was able to fill pore spaces in granular sedimentary structures, which can also show up as cementation . In this way, existing lineaments , voids in fault zones and other planar discontinuities in rocks were also filled . Since these spaces were flowed through by the groundwater and the groundwater levels rose and fell during climatic periods, the silica gels in them could deposit and solidify.
The colors of the opal (black, white, gray, blue, green and orange) - regardless of the play of colors, also depend on the chemical composition of the surrounding rocks and the trace elements they contain, such as iron , cobalt , copper , nickel , silver and others .
The play of colors of the precious opal is determined by the layers of myriads of the smallest pebble spheres and their diameter in the nanometer range (nm), which are formed in the creation processes and reflect the light.
Volcanically formed opals
Opals also occur in volcanic rocks, for example the fire opal from Mexico in a rhyolite deposit . The opals found in volcanic rocks are the result of hydrothermal processes in which heat and pressure play an essential role. In studies by Russian scientists it was found that the hydrothermal formation of opals differs from the sedimentary in two ways: The silica spheres in the nano range are not stored in parallel, as in the formation of sedimentary opals, but in a chaotic manner. In contrast to the three-dimensional spheres of the sedimented opals, two-dimensional patchy photonic bands arise in the chaotic opal matrix. Block-like structures and thin films are responsible for the spectral color and the iridescence of volcanically formed opals.
A specialty is the yew tree opal near Eibenstock in Saxony , which was stored in the shape of a vein in igneous rock in layers of granite . This also applies to the forcherite in Austria near Ingering in Styria , which occurs in fissures of gneiss , a metamorphic rock .
Locations
In total, over 4000 locations for opal are known to date (as of 2018). With around 95% of the precious opals traded worldwide, however, Australia is the most important exporter . Mexico accounts for around 4% of world trade and the rest of around 1% comes from other parts of the world.
Australia and Oceania
- Australia in the states of New South Wales ( Lightning Ridge , White Cliffs ), South Australia ( Andamooka , Coober Pedy , Mintabie , Lambina ) and Queensland ( Yowah -, Koroit -, Toompine -, Quilpie -, Kyabra-Eromanga -, Bulgroo -, Yaraka , Jundah , Opalton-Mayneside and Kynuna ), also at Springsure (opals of volcanic origin) and dendritic opals at Norseman in Western Australia
- Indonesia to Java and Sumatra
- New Zealand in the Coromandel Valley ( Geyserite )
Africa
- Ethiopia (opal in rhyolite)
- Mali near Nioro du Sahel , Djudigui (opals in volcanic rocks)
- Zambia at Lake Kariba ( Dendrite Opal )
America
North America
In addition to Mexico, opal has been found in North America in the United States , Canada and Honduras .
- Honduras near Intibukat , Erandique , Tambla and Guayoca
- USA: Arizona near Tucson , California in the Mojave Desert , Idaho in Yellow Stone National Park (occurrence: Geyirit and in volcanic rocks), Louisiana near Lafayette , Nevada in the Virgin Valley , Oregon in the Blue Mountains and Utah near Milford
South America
The only precious opal deposits in South America are in Brazil , in Peru blue and pink opals are mined, which are sold under the trade name “Andean opals”.
The largest Brazilian find area is the area around the place Pedro II in the state of Piauí . The millimeter to centimeter thick opal dikes fill cracks between sedimentary rocks from the Devonian and Jurassic dolerite . Due to the low water content, the opals from Pedro II are among the hardest and most stable in the world. They have been mined commercially since the early 1970s, the most important mine was and has been the Boi Morto mine again since the mid-2000s. This was built and operated by Australians, explosives and heavy machinery were used for dismantling. From the end of the 1970s, the production quantities of up to 50,000 kilograms per year fell, in 1985 the last mine in Pedro II closed. Local garimpeiros continued to search for opals by hand on the surface and repeatedly found smaller deposits with extracted quantities of a few kilograms. In the mid-2000s, an Australian opal dealer reopened the Boi Morto mine and has been mining it since then. Brazilian fire opal was found in Castelo do Piauí and in the state of Rio Grande do Sul , there in the municipalities of Espumoso and Campos Borges .
Peruvian opals are collectively referred to as "Andean opals", but the two variants differ significantly from each other. The blue opals (sometimes also blue-green, blue-gray and turquoise) are extracted from a layer about five centimeters thick in the Acarí copper mine near the city of Nazca . This is mostly interspersed with brown and black dendrites , only individual opals are of a clear blue color. The color is created by microscopic chrysocolla inclusions; the opal contains up to one percent copper. Pink opal comes from the Monte Rosa mine near Ica and is not a pure opal, but a mixture of opal, palygorskite and chalcedony . The color is created by the smallest amounts of quinones , organic compounds that are contained in vegetable dyes, among other things.
Asia
- Kazakhstan near Voznjesjenskoje and Aleksejewskoje
- Turkey , western Anatolia near Karamandja near Simav and in western Turkey dendrite opals
Europe
Dubník in Slovakia was probably already known as a mining area for opals in Roman times . However, opal mining there is only guaranteed from the 15th to the 19th century. In 1920 he was discontinued. One of the largest precious opals in Europe was found there around 1670. The 594 gram opal has been in the imperial treasury in Vienna since around 1672 and is considered the most valuable gem in the Vienna collection ( Natural History Museum in Vienna ).
Sites:
- France between Tours and Bourges ( Quincyt , named after the place Quincy ) and in the Plateau Central and in the Paris basin (the latter is the dark brown liver opal )
- Germany in Saxony near Gröppendorf and Eibenstock (historical site)
- Iceland (geyserite)
- Austria near Ingering in Styria ( forcherite , a rare opal found in crevices of gneiss )
- Slovakia near Dubník (historical location of the so-called Hungarian opals , since Dubník was in Hungary until 1920 )
- Czech Republic near Červenka (historical location of the Hungarian opals , since Červenka was in Hungary until 1920) and Český Krumlov (dendrite opals ).
Outside the earth
The NASA announced in 2008 that on Mars are containing Opal large areas. From the presence of opal it is concluded that there once existed conditions that could have made the emergence of life forms possible.
Use as a gem stone
Gem-quality opals are extremely rare, so industrial mining only takes place in a few places in the world. Opal is found on all continents, but around 95 percent of all opals come from Australia . The deposits around the city of Coober Pedy provide the majority of the world's production there.
In order to bring their dazzling play of colors to their full development, opals are ground into cabochons . An exception is the fire opal, in which the red shining shine is enhanced with a facet cut .
Opal assessment
In international trade, the AGIA Body Tone Chart of the Australian jewelry industry is used to assess opals .
After this system have natural opals (English: Natural Opal ) neither saw marks still polishes. There are three types: Type 1 has a homogeneous chemical composition and Type 2 is the typical boulder opal from Queensland , which is still connected to the rock in which it was formed and has a different chemical composition. Its exterior can also be opalised. Type 3 originated in bands of rock or in cavities or it has tinsel-like deposits in the rock and is called matrix opal.
Other criteria are body color and transparency.
The body color varies from relative darkness to lightness. An assessment ignores the play of colors and is based on the AGIA Body Tone Chart . Black opals can be clear, see-through or opaque. According to the scale, opals are further sorted into dark opals and light opals. Opals with a distinct yellow, orange, red or brown base color are classified as either black, dark or light opal and according to the AGIA Body Tone Chart .
Opals are differently transparent to opaque. This is determined in transmitted light. Transparent to slightly transparent opals are known as crystal opals . Crystal only means a name here, because opals are amorphous.
Composite Opals, Synthetic Opals, Others
Composite opals are made of natural opal laminates that are hand-cemented onto other materials. There are three types, doublets, triplets and marquetry.
Since the black opals are rare and expensive, doublets and triplets are made. Opal doublets consist of calibrated precious opals in the millimeter range on a dark background, usually on dark chalcedony or potch . Opal triplets are made up of three layers, one layer of common opal as a base, on top of which a millimeter-thin precious opal and, in the past, a transparent rock crystal to protect it, today hard glass or lead glass . The opal doublets and triplets imitate black opals . Inlay work is made up of small precious opals that are cemented onto a base to form symbols, patterns and motifs.
Natural opals can be changed in their appearance by colors, heat, dyeing, substrates, adhesives, varnishes, waxes, oils or the use of chemicals. There are also synthetic opals, which are artificially produced, but have an identical composition to the precious opals, and also made of plastic, resin and glass.
Esoteric
Even in ancient Rome , the opal was considered the stone of love and hope. It is widely considered to be the talisman of thieves and spies.
For fans of esotericism , the opal is used as a healing stone to increase joie de vivre and optimism. In addition, it is said to alleviate physical ailments such as sore throats or to act against inflammation in general. The opal is also said to have beneficial effects in the case of heart and kidney problems. The increase in joie de vivre should also be able to turn into the opposite, since the opal supposedly intensifies every feeling. Instead of joie de vivre, any tiredness of life can be intensified. However, there is no scientific evidence for these alleged physical and psychological effects.
See also
literature
- Archie Kalokerinos: Opal - gemstone of a thousand colors . Kosmos Society for Friends of Nature, Franckh'sche Verlagshandlung, Stuttgart 1981, ISBN 3-440-05021-1 .
- Opal: The noblest fire of the mineral kingdom (= Christian Weise [Hrsg.]: ExtraLapis . Volume 10 ). Christian Weise Verlag, 1996, ISBN 3-921656-37-0 , ISSN 0945-8492 .
- Walter Schumann: Precious stones and gemstones. All kinds and varieties. 1900 unique pieces . 16th revised edition. BLV Verlag, Munich 2014, ISBN 978-3-8354-1171-5 , pp. 168 .
Web links
- Mineral Atlas: Opal (Wiki)
- David Barthelmy: Opal Mineral Data. In: webmineral.com. Retrieved December 27, 2018 .
- Roy Goldberg: precious opal - black opal, milk opal, fire opal, crystal opal. Optical properties of opal - star stones and birefringence. In: pinfire.de. Pinfire - Gems & Colloids, May 17, 2018, accessed December 27, 2018 .
Individual evidence
- ^ R. Goldberg, HJ Schöpe: X-ray amorphous and light crystalline - optical properties of opaline materials . In: Journal of the German Gemological Society . tape 59 , no. 1–2 , 2010, pp. 19-34 .
- ↑ a b c d Martin Okrusch, Siegfried Matthes: Mineralogie. An introduction to special mineralogy, petrology and geology . 7th, completely revised and updated edition. Springer, Berlin [a. a.] 2005, ISBN 3-540-23812-3 , pp. 116 .
- ^ Allan W. Eckert: The World of Opals . New York 1997, ISBN 0-471-13397-3 , pp. 57 f. and 177 (English, limited preview in Google Book Search).
- ↑ a b Jaroslav Bauer, Vladimír Bouška: Gemstone Guide . Verlag Werner Dausien, Hanau / Main 1993, ISBN 3-7684-2206-2 , p. 138-141 .
- ^ Walter Schumann: Precious stones and gemstones. All kinds and varieties. 1900 unique pieces . 16th revised edition. BLV Verlag, Munich 2014, ISBN 978-3-8354-1171-5 , pp. 54, 166 .
- ↑ Pineapple Opal. In: whitecliffsopal.com. White Cliffs Opal - Mining and Trading, August 13, 2004, accessed December 27, 2018 .
- ↑ Pineapple Opal. In: mindat.org. Hudson Institute of Mineralogy, accessed December 27, 2018 .
- ↑ Jürgen Schütz, Manfred Szykora: precious opals: naming and classification . In: Opal: The noblest fire of the mineral kingdom (= Christian Weise [Hrsg.]: ExtraLapis . Volume 10 ). Christian Weise Verlag, 1996, ISBN 3-921656-37-0 , ISSN 0945-8492 .
- ↑ Collective of authors: Small Opal Encyclopedia. References and technical terms . In: Opal: The noblest fire of the mineral kingdom (= Christian Weise [Hrsg.]: ExtraLapis . Volume 10 ). Christian Weise Verlag, 1996, ISBN 3-921656-37-0 , ISSN 0945-8492 , p. 18 .
- ^ A b c Walter Schumann: Precious stones and gemstones. All kinds and varieties. 1900 unique pieces . 16th revised edition. BLV Verlag, Munich 2014, ISBN 978-3-8354-1171-5 , pp. 168 .
- ↑ Jaspopal. In: brockhaus.de. Brockhaus, accessed December 27, 2018 .
- ↑ About opal. In: resourcesandgeoscience.nsw.gov.au. NSW Department of Planning and Environment, Division of Resources and Geoscience, November 5, 1010, accessed December 27, 2018 .
- ↑ Eduard Gübelin : Opal, the colorful harlequin - made of SiO 2 . In: Opal: The noblest fire of the mineral kingdom (= Christian Weise [Hrsg.]: ExtraLapis . Volume 10 ). Christian Weise Verlag, 1996, ISBN 3-921656-37-0 , ISSN 0945-8492 .
- ↑ Opal. In: energymining.sa.gov.au. Government of South Australia. Department for Energy and Mining, accessed December 27, 2018 .
- Jump up ↑ SV Vysotskiy, Nikolay Gennadievich Galkin, AV Barkar, Evgeniy Anatoljevich Chusovitin, AA Karabtsov: Hydrothermal precious opals of the Raduzhnoe deposit, north Primorye: The nature of the opalescence . In: Russian Journal of Pacific Geology . tape 4 , no. 4 , August 2010, p. 347-354 , doi : 10.1134 / S1819714010040068 (English).
- ↑ Number of locations for opal. In: mindat.org. Hudson Institute of Mineralogy, accessed December 27, 2018 .
- ↑ a b Opal: The noblest fire of the mineral kingdom (= Christian Weise [Hrsg.]: ExtraLapis . Volume 10 ). Christian Weise Verlag, 1996, ISBN 3-921656-37-0 , ISSN 0945-8492 .
- ↑ Torsten Purle: Andenopal - Properties, origin and use. In: steine-und-minerale.de. May 31, 2019, accessed June 14, 2020 .
- ↑ Jochen Knigge: Bamburro - digging for the big dream: Brazil's opals . In: Opal: The noblest fire of the mineral kingdom (= Christian Weise [Hrsg.]: ExtraLapis . Volume 10 ). Christian Weise Verlag, 1996, ISBN 3-921656-37-0 , ISSN 0945-8492 , p. 62-67 .
- ↑ Bob Farrar: Brazilian Opal and the Boi Morto Mine . In: Opal - The Phenomenal Gemstone (= Christian Weise [Ed.]: ExtraLapis English . Volume 10 ). Christian Weise Verlag, Munich 1996, p. 70-74 .
- ↑ Jaroslav Hyršl: Peruvian Opal . In: Opal - The Phenomenal Gemstone (= Christian Weise [Ed.]: ExtraLapis English . Volume 10 ). Christian Weise Verlag, Munich 1996, p. 79 .
- ↑ Profiles Top 50: Noble Opal. (PDF 2.1 MB) In: nhm-wien.ac.at. NHM Vienna, April 1, 2011, accessed on December 27, 2018 (precious opal from p. 5).
- ↑ Guy Webster, Dwayne Brown, Jennifer Huergo: NASA Orbiter Reveals Details of a Weather Mars. In: jpl.nasa.gov. Jet Propulsion Laboratory, October 28, 2008, accessed December 27, 2018 .
- ^ Opal Industry Nomenclature. In: opal.asn.au. Opal Association, accessed December 27, 2018 .