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Opal (SiO2.nH2O) is a form of silica chemically similar to quartz, but containing a variable amount of water within the mineral structure. Water content varies between 2 and 20%, with precious opal generally containing 6–10%.
Precious opal is composed of small spheres of amorphous silica packed in a regular array. White light is diffracted by these layers and broken up into the colours of the spectrum, causing the characteristic display of colours for which opal is so highly prized. The colour observed is dependent on the layer spacing, which is determined by sphere size. In opal which shows dominant red fire, the spheres are ~4000 Å (Angstrom units; 1 Å = 10-7 mm) in diameter, while green opal spheres are ~2500 Å. The body colour or background for the diffracted colour play may be milky white, grey, blue, black or colourless. Black opal is the most highly sought after variety as the dark background shows off the colour play best.
In common opal or potch, which shows no play of colours, the silica spheres are either of assorted sizes which do not produce the regular array required for colour diffraction or are too small to produce a visible play of colour.
Opal is used as a gemstone or ornamental stone in jewellery, carvings, inlay material and mosaics. In jewellery it is most often cut and polished as a cabochon, with a domed top, flat base and oval, round or free-form shape. Transparent or jelly opal and honey potch is occasionally faceted for jewellery.
Opal is found as two types: volcanic opal which infills vesicles and cracks in igneous rocks, and the more familiar sedimentary or sandstone type which is found in Australia’s main fields. Most deposits in other countries are volcanic opal, much of which is prone to crazing.
More than 95% of the world’s precious opal comes from South Australia, New South Wales and Queensland. During the 1990s, the value of South Australia’s annual production of rough opal averaged ~$40 million, about half of the total Australian production.
Most of South Australia’s opal is sold in the rough on the opal fields and exported to Hong Kong, Taiwan, China and other overseas countries for cutting and marketing. However, much of the State’s top quality opal is cut in Australia, generally increasing its value four to five fold, and in some cases by up to 10 times.
During the Tertiary Period, Cretaceous sediments in the Great Artesian Basin, and Palaeozoic and older rocks near its margins, underwent deep weathering and alteration to kaolin. Soluble silica released during this process percolated downwards in groundwater through the rock mass via faults, joints, fractures and other planar discontinuities. It precipitated as a gel composed of silica spheres which hardened and cemented together during a slow drying process in a zone of fluctuating watertable levels. In some areas, impermeable clay lenses or fossils formed favourable sites for the deposition of opal. Most of the opal is believed to have been emplaced 15–20 million years ago.
The simplest and earliest form of mining was pit or shaft sinking with pick and shovel, and a hand-operated winch to remove mullock, but the industry gradually became more mechanised. Bulldozers are now commonly used to expose the opal level when it is at relatively shallow depths, then the opal is carefully removed by pick. In deeper areas, shafts are generally sunk by large diameter bucket drills (Calweld drills), then reamed out to allow tunnelling machines, bobcats or boggers to be lowered.
A variety of methods are used to remove the mullock from underground workings:
Self unloaders haul a bucket to the surface on two parallel rails which are curved above the ground such that the bucket empties a few metres away from the shaft. An automatic reversing switch is tripped to return the bucket underground.
Blowers are truck-mounted machines similar to large vacuum cleaners. A fan or blower driven by a stationary diesel engine draws mullock out of the shaft through connected metal pipes to collect in a bin at the surface, which empties automatically when full.
In South Australia, precious opal was discovered at Coober Pedy in 1915, at Mintabie during 1921–22 and at Andamooka in 1930 (Fig. 1). Numerous other smaller deposits were discovered after the larger fields, including Lambina (1930s) and Stuart Creek (1947).
Opal at Andamooka occurs in the shallow marine Bulldog Shale, part of the Marree Subgroup of Early Cretaceous age, which overlies Algebuckina Sandstone or laps directly onto pre-Mesozoic rocks (Fig. 2). The top sub-unit of Bulldog Shale, called kopi by the miners, consists of highly weathered white sandy clay with scattered, large erratic boulders. At the base of the kopi there is an extensive sandy boulder bed, called the concrete or conglomerate band, which contains numerous pebbles, cobbles and boulders of pre-Mesozoic rocks, chiefly Arcoona Quartzite. Beneath the conglomerate band is a pale brown, grey or yellow claystone with a low sand content referred to as the mud.
Opal at Andamooka occurs predominantly at one horizon, referred to by miners as the level, at the contact of the conglomerate band and the mud. Other sub-levels (squibby levels) occur above the main level but none are as prominent, continuous or productive.
The main opal varieties produced are crystal opal (transparent to translucent), white opal, and some black opal. Painted ladies are boulders, generally of quartzite, which split along a fracture to reveal a coating of opal. Matrix opal is cloudy stone with flashes of colour, which is thought to form by replacement of limestone boulders in the conglomerate band. Opalised sandstone or opalstone forms by deposition of opal in the spaces between the quartz grains in sandstone boulders. Both matrix and opal sandstone can be treated by soaking in sugar solution and boiling in sulphuric acid to darken the body colour and enhance the play of colour.
As at Andamooka, opal occurs in Early Cretaceous Bulldog Shale (Fig. 3). The weathered rock, termed sandstone by the miners, is ~50 m thick, the base approximating the base of the Stuart Range escarpment. In this weathered zone the rock is bleached white or varicoloured, silty or sandy claystone, with kaolinite as the dominant clay mineral. The nature of the weathered profile varies, and opal miners consider even subtle variations important in finding opal. Below the sandstone, the profile changes to denser, less porous, mauve, grey or brown claystone, sometimes called mud by the miners. Opal is found in the sandstone in sub-horizontal to sub-vertical veins infilling cracks and joints up to 100 mm thick, but generally less than 10 mm. It also fills spaces left by dissolution of the calcareous parts of fossil animals, generally bivalves (shells), and a few examples of opalised vertebrae of marine, fish-like reptiles (plesiosaurs and icthyosaurs) have been found. Most opal is found in levels, several of which can occur at any one locality. Levels are not consistent, even for short distances within fields, and disruption by slides (steeply dipping faults or joints) adds to the difficulty of correlation. Although opal occurs throughout the sandstone, the most productive zone is usually from ~5 m above to 1 m below the change to darker coloured, denser claystone (mud).
Predominantly light, opaque to translucent opal is produced at Coober Pedy, along with some crystal and traces of black.
The geology at Mintabie differs from other major opal producing fields in Australia. Mintabie is the only field producing opal from Palaeozoic rocks. The Mintabie beds in which the opal is found underlie Early Cretaceous sediments, and are considered to be of Ordovician age. They comprise well-sorted, kaolinitic, white sandstone with minor claystone interbeds, and exhibit large-scale cross-bedding which suggests deposition in a fluviatile environment. They crop out extensively along the local escarpment, and in the vicinity of the workings dip south to southwest at 5–10º.
Precious opal and potch, as in other fields, are found infilling horizontal structures and vertical and inclined joints (Fig. 4). In addition, opal is often found along curved bedding planes within cross-beds. Thicker opal has frequently been found at intersections of two or more of these structures.
There appear to be no preferred depths for opal at Mintabie, and it has been found from near the surface to depths of ~30 m.
Varieties include black opal, transparent crystal and white opaque opal. Mintabie opal often exhibits distinct darker and lighter bands, with colour plays generally in the lighter bands. This type of opal can be cut so that a band with a play of colour is backed by darker opal or black potch, forming a natural doublet.
Opal in South Australia. 2004. Minerals and Energy Handbook - Available from the PIRSA, Minerals and Energy Customer Services Centre. See here for contact information.