Opal occurrence in Australia
The opal deposits in Australia are located in Queensland , New South Wales and South Australia . They extend over an area of 300 kilometers wide and about 1,000 kilometers in length in the state of Queensland alone. On July 27, 1993, the Australian government declared the opal a national symbol, thus a gemstone of Australia.
These deposits occupy the leading position internationally, with Australia being able to cover world trade needs between 2007 and 2008 with a share of 95 to 97 percent.
Opals
Opals are found in two rocks: in Australia mainly in sedimentary rocks and only in a few places in volcanic rocks .
Opal (SiO 2 • nH 2 O) is a water-containing compound of silicon and oxygen , which is present in amorphous form and is chemically comparable to quartz , which, in contrast, has a crystal lattice . Opals vary with a water content that is incorporated into the opal solids from 2 to 20 percent and the precious opals from 6 to 10 percent.
The difference between the common opal , called potch in Australia , and precious opal, which is processed into jewelry, lies in the lack of play of colors. The colors of the opal (black, white, gray, blue, green and orange) depend - regardless of the play of colors - on the chemical composition of the surrounding rocks and the trace elements they contain, such as iron , cobalt , copper , nickel , silver , etc.
The play of colors of the precious opal depends on the layers of myriads of tiny pebble balls and their diameter in the nanometer range (nm), which reflect the light.
Opal types and occurrences
Location map of major opal deposits in Australia |
The opal fields in Australia are extensive - for example, in Queensland they extend in a 300-kilometer-wide and 1,000-kilometer-long strip of deeply weathered Cretaceous rock , which extends northwest of Hungerford along the border of Queensland and New South Wales, west of Cunnamulla , Quilpie, Longreach and Winton to Kynuna.
In Australia there is the rare flaming black opal, dark and light opal, boulder opal, matrix opal, crystal opal, and yowah opal. Most of Australia's opals are of sedimentary origin and mostly around 18 million years old.
Black Opals are found in New South Wales, but only at Lightning Ridge , and other opal deposits are at White Cliffs and Coocoran .
The boulder opal is found in Queensland, which is embedded in fist-sized pieces of rock. There are also opals there called Yowah Nut Opal. These are enclosed precious opals, which are surrounded by nut-shaped iron-containing shells, the matrix opals. In Queensland the opal fields are in the Winton Formation . There is a specialty in Queensland, the Church Windows , which show a pattern like church windows. They are found only at a few sites in Yowah, Quilpie and Jundah. The Yowah , Koroit , Toompine , Quilpie , Kyabra-Eromanga , Bulgroo , Yaraka , Jundah , Opalton Mayneside and Kynuna fields are the most important opal deposits in Queensland. The Opal discovery site of Springsure is an exception in Australia, as the opals found there are of volcanic and not sedimentary origin.
Most opals are mined in South Australia. There are mainly the light opals, but also at Mintabie black opals. Coober Pedy is known as the "capital of opals" . Other large mining areas in South Australia are at Andamooka , Mintabie and Lambina .
There are also so-called wood opals , whereby opal was deposited in them during the silicification process of fossil woods. In Western Australia , cat-eye opals have been found in which asbestos minerals are deposited and produce a band of light that follows the eye of the beholder. Opal pineapples found at White Cliffs are rare . There are also opals in Australia, which are known as matrix opals, as they occur in the pore spaces of tuffs as well as in conglomerates of pebbles. In Australia, opals are called Opalpipe , which solidified in tube-like cavities of rocks.
The opal fields in Australia are mostly referred to in miles, such as Eight Mile Plain , which represents their distance from the nearest town.
Play of colors of a black opal, one of the largest Australian opals (2765 carats)
Boulder opal
Yowah nut opal
A common opal, a "potch", not a precious opal
Boulder opal found at Carisbrooke Station near Winton in Queensland
Emergence
The Australian opal deposits are located in a sedimentary basin , the Great Artesian Basin, from the Mesozoic Era , which covers about one fifth of Australia and consists of sedimentary sequences from the Jurassic and Cretaceous.
Opals arise from silica solutions that have changed into a solid consistency through slow water loss through a silica gel. During this process, silicic acid-containing liquid accumulations were created in cavities of all kinds. The evaporation process of the water involved and its remaining proportion determine the way in which the opal forms. The alternating cycles of dry and humid climatic periods as well as the weathering products of existing clay minerals represent further initial requirements .
The Opalvorkommen be regarded today 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. Here the opal often forms a filling of pore spaces in the granular sediment structures, which can also show up as cementation . In this way, existing lineaments of different origins, cavities in fault zones and other planar discontinuities in rocks were filled . These spaces could be flowed through by the groundwater, which deposited silica gels in them.
Most of Australia's opals are the result of complex weathering and silicification processes in the period between the Cretaceous and the Tertiary 17 million years ago. In addition to the sediment-formed opals, there are few opal deposits formed in volcanic contexts in Springsure , in the south of Brisbane and in Western Australia near Kalgoorlie . The geological conditions of Mintabie differ from this, as these opals can be found in rocks of the Paleozoic Era , which are therefore under the layers of the Early Cretaceous.
According to Jack Towensend, a distinction is made between three development phases: amorphous silica gel was formed in silica- rich groundwater and then silica spherulites , also called silica spheres, whose size is in the nanometer range (nm), formed. Amorphous silica gel and silica spheres settled at the bottom of the water and slowly filled cavities - also in fossils . Balls of different sizes continued to settle and formed the potch ( German common opal ), the opaque and non-glossy common opal and not a precious opal. When the movement of the water calmed down, tiny spheres of the same size formed, creating the play of colors. The so-called precious opals with the coveted play of colors were created through slow growth. The precious opals have spheres between 300 and 150 nm in size and are evenly arranged. The valuable black opals should have spheres with a diameter of less than 150 nm. These processes were never uniform. While the spheres no longer changed, the amorphous silica gel was able to harden further through loss of water.
The geologists and prospectors in Australia discuss three theories of opal formation: the Weathering Model explains them from chemical-mechanical processes, the Syntectonic Model from the interaction of mineral-rich waters and geological disturbances, and the Microbe Model from the effects of food and excreta of microbes .
history
The first European to discover a common opal in Australia in 1841 was the German geologist Johannes Menge . He found him 70 km northeast of Adelaide in South Australia at the Tarrawilla Ranch in the Flinders Range near Angaston . Little attention was paid to this find as it was not a precious opal.
There is written evidence of precious opal finds from 1872 at Adavale and Springsure. The first commercial mining began in Queensland at Kynuna, Opalton, Kyabra, Yowah and Koroit in the 1890s. In the early years of the 20th century, more opals were found in New South Wales' White Cliffs than in Queensland. In 1905 opals were discovered in Lightning Ridge, 1915 in Coober Pedy and 1930 in Andamooka.
During the Great Depression , the Opal funding declined and only in the 1940s, the mining increased by extensive finds, the most comprehensive Opalgewinnung in South Australia was made.
From 1900 to 1957, opal was mainly extracted in the Hayricks mine in Queensland near Quilpie. In the 1960s, demand from Europe and Asia increased. However, sales developed differently depending on the economic situation.
Dismantling
Exploration
Until 1987, the exploration of opal fields in Australia was carried out by manual digging or with Calweld drilling equipment and the rock and the drilling dust transported to the surface were examined for silicified crusts. After that, the more cost-effective and faster use of drills with a diameter of 230 mm became common, with the sandstone lying on top being drilled through and the resulting drill dust assessed. In recent years, drills with a diameter of 120 mm have been used. In 1986 the Geological Survey of New South Wales developed the SIROTEM geophysical method for exploring opal fields, measuring the electrical resistance of sandstone, claystone and geological faults. The advantage of this method is that large areas can be examined in relatively short periods of time. There are also attempts to explore opal fields from an airplane.
Mining methods
In the early years of quarrying, the mining fields were in the distant outback , which the prospectors invaded with horses, food and tools. Mining was done with hammer and chisel, shafts were driven and opals were transported in buckets to the surface, where they were sorted and split up. The shafts were driven by hand until the 1960s, and drilling rigs, mechanical winches and pneumatic hammers are now in use. To develop the deeper layers, Calweld drilling rigs are used, which are capable of drilling holes approximately one meter in diameter to a depth of 20 meters.
The first underground mine was built in Lightning Ridge in 1901. The vertical wells are to a depth drilled on which the opals are suspected. Opal seekers, tools and equipment are drained down to the mining level. The reduction follows the horizontally lying opal höffigen layers, whereby explosives may be used. A second shaft will later be created for air circulation and for the removal of the excavated material, which is called dirt . Dirt with the opals is carried to the surface of the earth and the opals contained are sorted with blowers or sorted out with water rinsing in large mixing plants, which are similar to concrete mixing plants.
In recent years, opal deposits have been increasingly developed in open-cast mining using excavators, bulldozers and other equipment, from which a better yield is expected.
In Lightning Ridge, claims can be acquired by anyone, including foreigners, for around 40 AUD in the size of 50 × 50 meters, up to a maximum of two per person. In 2008 there were 4,500 claims in Lightning Ridge.
Dreamtime stories about opals
In the dream time of the Aborigines , opals also found their precipitation. In one legend, the opal was a gift from heaven, from a rainbow that touched the earth, creating the color of this gemstone.
A dream time story tells of the mythical pigeon Mambi, who threw a stick of fire into the air and when it fell to the ground, it split with numerous sparks that sank into the rock near Coober Pedy and each spark formed a shiny opal.
Another oral tradition calls the "Great Opal", a creature that could decide the fate of people. After long unending wars between two Aboriginal tribes , they pelted each other with stones for lack of other weapons. An Aborigine threw a large rock into the air, which got stuck in the horizon and got bigger and bigger until it broke up. The "Great Opal" hurled lightning bolts out of the rock. When he saw the many dead warriors, he wept and his tears turned into a rain storm. When the sun fell on the raindrops, the first rainbow appeared. Since then, the Aborigines around Coober Pedy have assumed that a rainbow is a sign that tribal laws have been broken.
Famous Opals of Australia
A pliosaur that was 85 percent fossilized to opal , named Eric in the press , was a member of the genus Umoonasaurus that lived in the south of Lake Eromanga - today's Eromanga Basin - in the Early Cretaceous about 115 million years ago. The fossilized dinosaur was found in 1987 by a miner named Eric at Coober Pedy , who sold it to a businessman. This got into financial difficulties and so Eric could be bought again. After students collected money and a financial donation from Akrubra Hats , the Australian Museum was able to acquire the largest opalized skeleton of a vertebrate and has been exhibiting it since 1993.
The Olympic Australis Opal is a 23,609.88 carat (about 3.5 kg) opal and one of the largest on earth with a length of 180 mm, a height of 120 mm and a width of 115 mm, priced at AUD 2,500,000 was appreciated. It was found in 1956 at Coober Pedy. It's unpolished.
The largest opal that was found and made into a gemstone in August 1957 was the Americus Australis with 16,261.56 carats (1 carat = 0.2 grams), which was found in the Olympic opal field near Coober Pedy at a depth of about 10 m .
The Andamooka Opal was found at Stevens Creek near Andamooka in South Australia in 1946 and weighed 885.94 carats in its raw state.
Three opals that have been identified as Aurora Australis are red and black opals, the cut one found in 1938 weighing 180 carats and selling for $ 1 million.
The Galaxy Boulder Opal , which weighed about 700 grams (3,500 carats) unpolished, was found near Jundah in 1956 and valued at AUD 1,000,000.
The oval Flame Queen Opals are also known . These are eye-shaped embedded opals that are surrounded by opals of different colors. They were found in Lightning Ridge in 1914.
In 1907 in this opal area of Koroit a round shaped 15 cm opal was found by a single worker, also one of the largest opals ever found. That is why it was named Great Australian .
Others
New South Wales wants to proclaim the black opal a federal symbol and set up an Australian Opal Center in Lightning Ridge .
In 1973 an Australian 8 cent series of stamps depicting an opal was issued by the Australian Post, and in 1995 an AUD 1.0 stamp depicting a bright opal and another AUD 2.50 with a black opal.
On July 27, 1993, the opal was declared the Australian nation's official symbol for gemstones.
Lightning Ridge is home to the largest collection of Australian opals and opalized fossils, the Australian Opal Center . It is a national museum that has its collection housed in a two-story, 100-meter-long underground building. The collection is open to the public.
Web links
- Illustrations of Australian opals
- Images of large opals including Galaxy Opal , Olimpic Australis Opal
- Illustration of the opalized skeleton of the pliosaur Eric
- Illustration of the Flame Queen Opal
Individual evidence
- ↑ australia.gov.au : National symbols , in English, accessed February 14, 2012
- ↑ a b itsanhonour.gov.au (PDF; 74 kB): National Gemstone , in English, accessed on February 12, 2012
- ↑ Eduard Gübelin : Opal, the colorful harlequin - made of SiO 2 . In: Extra Lapis. No 10 (1996): Opal: The noblest fire in the mineral kingdom. ISSN 0945-8492
- ↑ a b pir.sa.gov.au : DMITRE Minerals , in English, accessed on February 14, 2012
- ↑ a b mines.industry.qld.gov.au ( Memento of the original from September 11, 2012 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. : Opal , in English, accessed February 15, 2012
- ^ William Cooper, Barry J. Neville: Queensland: Home of the Boulder Opal . P. 40. In: Extra-Lapis. No 10 (1996): Opal: The noblest fire in the mineral kingdom . ISSN 0945-8492
- ^ William Cooper, Barry J. Neville: Queensland: Home of the Boulder Opal . Pp. 48-49. In: Extra Lapis. No 10 (1996): Opal: The noblest fire in the mineral kingdom . ISSN 0945-8492
- ↑ australiangemmologist.com.au ( Memento of the original from February 21, 2011 in the Internet Archive ) Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice. : Roy Beattie: The Springsure deposit of precious Opal , accessed February 19, 2012
- ↑ pir.sa.gov.au ( Memento of the original from March 27, 2012 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. : Location map of opal mining in South Australia, accessed February 15, 2012
- ↑ Jack Towensend: The Formation of Opals in Australia . P. 30. In: Extra-Lapis. No 10 (1996): Opal: The noblest fire in the mineral kingdom . ISSN 0945-8492
- ^ State of New South Wales. Department of Primary Industries: About opal . at www.dpi.nsw.gov.au (English)
- ↑ Jack Towensend: The Formation of Opals in Australia . Pp. 32-35. In: Extra Lapis. No 10 (1996): Opal: The noblest fire in the mineral kingdom . ISSN 0945-8492
- ↑ dpi.nsw.gov.au : Formation of Opal , in English, accessed February 15, 2012
- ^ Allan W. Eckert: The World of Opals . 68. New York 1997. ISBN 0-471-13397-3 . Online on google books
- ^ William Cooper, Barry J. Neville: Queensland: Home of the Boulder Opal . Pp. 44-45. In: Extra Lapis. No 10 (1996): Opal: The noblest fire in the mineral kingdom . ISSN 0945-8492
- ↑ dpi.nsw.gov.au Opal Prospecting , in English, accessed February 14, 2012
- ^ A b William Cooper, Barry J. Neville: Queensland: Home of the Boulder Opal . Pp. 45-46. In: Extra Lapis. No 10 (1996): Opal: The noblest fire in the mineral kingdom . ISSN 0945-8492
- ↑ a b australianopalcentre.com ( Memento of the original from April 23, 2012 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. : Opal mining. The search for one the Earth's great treasures , in English, accessed February 18, 2012
- ↑ australia.gov.au ( Memento of the original from September 15, 2012 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. : Our national symbols , in English, accessed February 14, 2012
- ↑ Heike Ludwig: Stone of Peace . S: 2nd In: Extra Lapis. No 10 (1996): Opal: The noblest fire in the mineral kingdom . ISSN 0945-8492
- ^ Benjamin P. Kear, Natalie I. Schroeder, Michael SY Lee: An archaic crested plesiosaur in opal from the Lower Cretaceous high-latitude deposits of Australia. Biology letters 2 (4), December 2006; Pp. 615-619. PMC 1833998 (free full text)
- ↑ australianmuseum.net.au : Omoonasaurus demoscyllus , in English, accessed on February 13, 2012
- ↑ abc.net.au : Metthew Perkins: Opal dinosaur shows Australian character and avoids jewelery , August 5, 2001, in English, accessed February 12, 2012
- ↑ F. Leechman: The opal book , University of California Press, 1961
- ^ Allan W. Eckert: The World of Opals. 120. New York 1997. ISBN 0-471-13397-3 Online on Googlebooks
- ↑ grahamblackopal.com ( Memento of the original from March 12, 2012 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. : GALAXY OPAL FINEST BOULDER OPAL DISCOVERED IN THE WORLD, accessed February 17, 2012
- ^ Allan W. Eckert: The World of Opals. 115. New York 1997. ISBN 0-471-13397-3 . Online on google books
- ↑ 'Flame Queen' jckonline.com ( Memento of the original from September 25, 2015 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. : 'Flame Queen' Opal to be Sold at Auction , March 28, 2008, in English, accessed on February 12, 2012
- ^ Mineralienatlas : Koroit , accessed on February 12, 2012
- ↑ theridgenews.com.au : Recognition for Black Opal , December 1, 2011, in English, accessed February 14, 2012
- ↑ mineralstamps.com : Opal (image of the 8-cent stamp), accessed on February 27, 2012
- ↑ mineralstamps.com : Gem, Rock, and Mineral Postage Stamps From Australia (illustration of 9 cent, 1 dollar and 2 dollar stamps), accessed February 27, 2012
- ↑ australianopalcentre.com ( Memento of the original from March 14, 2012 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. : Opalised Fossils. Precios relics from the Age of Dinosaurs , accessed February 18, 2012
- ↑ australianopalcentre.com ( Memento of the original from March 29, 2012 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. : Welcome to the Australian Opal Center , accessed February 18, 2012
