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Play of colours
Light boulder opal
Matrix boulder opal
Seam & vein opal
Its chemical formula is SiO2.nH2O where the water content ranges from one to 21 percent, and between six and 10 percent for precious opal.
There are two main forms of opal recognised - precious opal and common opal. That division is based on the presence or absence of the play of colours.
Precious opal exhibits the characteristic play of spectral colours.
Common opal is mostly opaque and shows no play of colours. When common opal is found in association with precious opal it is known as potch.
Most opal is common - precious opal showing brilliant spectral colours is comparatively rare.
The mineral's physical attributes are of note: X-ray diffraction studies show that all except the volcanically derived forms of opal are amorphous, the diffraction pattern displaying as a broad diffuse band. This non-crystalline nature is unlike most other gemstones. Opal is really a hardened gel of hydrated silica.
The background or body colour of the opal ranges from transparent through translucent and almost colourless to milky white, pale coloured to dark greys and black. This colour is imparted by disseminated impurities in the silica - iron oxides for yellows and reds to darker colours, manganese oxides and organic carbon imparting the black to dark colours. Very fine gas filled cavities cause white to greyish light colours.
Black opal, with a body colour from a dark grey to black and the full play of colours, is the most valuable variety of all. Its occurrence in Australia is largely confined to the Lightning Ridge field in New South Wales. Some rare boulder opal approaches this quality.
Milk or white opal exhibits its play of colours within a white to cream body colour. Most notable is the white opal of south Australia, but a similar body colour is also known from some boulder opal.
The brilliant range of precious opal's play of colours is due to the mineral structure. Modern electron microscope studies have shown that opal is composed of numerous minute spheres from 0.0001 mm to 0.0005 mm in diameter packed in particular arrays. In common opal the spheres are irregularly packed, whereas in precious opal they are arranged in orderly rows and layers. Each sphere is made up of a central nucleus with one or more concentric shells of primary silica particles.
The regular packing in precious opal acts as a three-dimensional diffraction grating to transmitted light and gives the spectacular play of colours unique to this gem. Studies of opal have shown that the number of shells of primary silica particles comprising each sphere determines the colour of the gem.
Another unique feature of precious opal is the pattern of colours associated with the play of colours. A whole range of patterns differentiate the various types. Some of the more common are listed here.
Harlequin opal has a mosaic or chequered pattern of coloured patches. It is the rarest and the most sought after.
Fire opal shows flashes of predominantly red, orange, and yellow within transparent to translucent stones of pale yellow to red body colour.
Pin fire or pin opal is composed of closely spaced specks of brilliant colour.
In flame opal, the colour may be seen as red bands or streaks, whereas,
In flash opal it appears as sudden brilliant flashes.
In girasol the play of colours appears as a floating internal light.
Terms such as boulder opal, sandstone opal, matrix opal and pipe opal were somewhat ambiguous, there being considerable variation in the meaning of the different terms. Furthermore, the terminology of the opal miner is rather vague, with some words meaning different things to different people. The following definitions may allow some uniformity.
There are many types and varieties of precious opal, but western Queensland is renowned for boulder opal.
Boulder opal describes opal found within ironstone concretions of varying shape and size. The boulder shape is generally elongated or ellipsoidal with the long axis orientated horizontally, and the size ranges up to three metres in length and breadth and one metre in thickness. The direction of elongation usually parallels the bedding in the enclosing sandstone. The boulders may be confined to one or more zones known as boulder levels, or may be erratically distributed through the sandstone. Their composition ranges from sandstone types which consist of a rim or crust of ferruginised sandstone surrounding a sandstone core, and ironstone types which are composed almost totally of iron oxides. Commonly, these concretions have a concentrically banded structure and precious opal may fill concentric, radial, or random cracks, particularly on the underside of the concretion.
Only a small proportion of boulders contain precious opal of any economic value. Boulder opal is widely distributed throughout western Queensland and is the State's most significant source of precious opal.
Like other precious opal, there are many varieties of boulder opal defined on body colour, play of colour, and patterns. A specific nomenclature for the variations has been preposed by the Australian Gemmological Association to standardise the names for miners, gemstone buyers and all associated with the gemstone industry.
Black boulder opal. This rare, very valuable Queensland opal rivals Lightning Ridge black opal in both the uniform darkness of its black body colour, and the contrasting range of complete spectral hues visible in its play of colours.
Crystal boulder opal. These transparent boulder opals owe their dark body colour to the dark brown colour of the ferruginous sandstone or ironstone backing that may be observed through the polished surface of the opal
Light boulder opal - light coloured translucent to opaque opal with a ferruginous sandstone or ironstone backing.
Boulder matrix opal - an anastomosing network of precious opal veins within ferruginous sandstone or ironstone.
Yowah Nuts - small rounded opaliferous ironstone concretions that may host either solid opal or opal matrix.
The nuts are small ironstone boulders which are spherical to ellipsoidal in shape and up to five centimetres across. They may have hollow centres or be filled with powdery clay or a kernel of opal. Opal is found mainly between concentric layers on the underside of the nuts or as a network of thin veins through the ironstone concretion. The latter is frequently termed opal matrix.
The best development of this type of opal is at Yowah where the concretions form distinct nut bands which are commonly associated with layers of mudstone clasts or clay pellets.
Fun stones - attractive and comparatively inexpensive fee-form boulder opals of lower qualit>
Boulder splits - a matched pair of boulder opals formed by splitting rough boulder opal along a flat vein.
Seam and vein opal - very rare boulder opals cut from the thin irregular veins of opal that occasionally occur in the ferruginous seam (casing), or sandstone above the seam, or in the claystone below the seam.
The thin ironstone casings are up to five centimetres thick at the contact between sandstone and underlying fine-grained sediment. In some places, the upper surface of the seam has rounded, botryoidal protrusions (nobbies) up to several centimetres across, and both seam and nobbies commonly contain thin, horizontal veins and random flecks of brilliantly coloured precious opal. Immediately above the seam, opal dirt is commonly found.
Sandstone Opal - dark brown, ferruginous sandstone impregnated with pin points of precious opal.
The sandstone opal is formed by replacement of the matrix of the sandstone and is found impregnating the ferruginous sandstone 20 t 30 centimetres above the seam.
Pipe Opal - crystal opal cut from solid opal recovered from unique pipe-like structures. Sometimes sandstone is retained on the backs. >
The pipes may be up to several centimetres in diameter and may be hollow or opal filled. They may swell and thin or coalesce with others to form knotted masses. Their formation is possibly joint controlled, or they may represent zones of higher permeability within the sandstone which acted as channel-ways for descending waters.
Wood Opal - refers to opal which forms in tube-like shapes by the replacement of woody tissue, particularly in the opal dirt or in the mudstone underlying a seam.
Play of colours
Light boulder opal
Matrix boulder opal
Seam & vein opal
|HOW OPAL FORMED
The consensus among geologists is that almost all Australian gem opal has formed by precipitation of silica from very dilute silica solutions or colloidal suspensions, derived from the deep weathering of feldspathic sedimentary rocks under the action of percolating groundwater.
This deep weathering is a chemical alteration. The feldspar minerals are altered to kaolinite, releasing silica in an aqueous solution or suspension which may collect in traps or cavities in the rocks. These cavities may be open fissures, interstices between particles in a conglomerate, holes left by dissolution of shells, bones, wood or inorganic minerals (such as gypsum and calcite), or hollow cores and cracks in ironstone concretions.
The ironstone concretions may be localised along the base of palaeochannels, on the downthrown side of differential compaction faults, or along basal undulations of bedding interfaces. The silica solution or suspension may have been concentrated by evaporation through the overlying sediments, or concentration may have been effected by the action of clay beds which in many places underlie the opal-bearing horizons, as semi-permeable membranes allowing the water to pass through but retaining the silica.
Increased concentration my have caused the separation of discrete silica particles which aggregated by collision into equidimensional spheres. When these reached a particular size, they may have undergone undisturbed settling and hardening into an ordered arrangement which formed precious opal. More commonly, a disordered arrangement of spheres accumulated during settling forming common opal or 'potch'. Because water is retained in the opal structure, application of heat, or in some cases merely exposure to a dry atmosphere, can cause fractures to develop.
Opaline silica is common throughout the leached profile of the Cretaceous Winton Formation , which consists of feldspathic sandstone, siltstone, and mudstone. In some places, the matrix of the sandstone is replaced by opaline silica, which may also replace wood and gypsum.
Precious opal, though sometimes found in these modes, is most commonly associated with various forms of concretionary ironstone.
Distribution of opal deposits in Queensland is extremely wide and erratic in contrast to the relatively confined occurrences in New South Wales and South Australia.
Queensland's productive fields lie within a belt of Cretaceous sedimentary rocks - known as the Winton Formation - which extends from the New South Wales border at Hunderford in a northwesterly direction to Kynuna, a distance of about 1000 kilometres. The opal belt stretches to the west of the railheads at Cunnamulla, Quilpie, Longreach and winton, and encompasses the smaller centres of Eulo, Eromanga, Windorah and Jundah. Mining activity is concentrated mainly in the opal fields within the belt, although some scattered operations are outside these areas previously described.
Staff of the Bureau of Mineral Resources and the University of New South Wales have studied the geology of western Queensland in detail and have arrived at an explanation of the sequence of geological events which were crucial to the formation of opal.
During Cretaceous time, the sedimentary rocks of the Winton Formation were deposited in streams and lakes. A period of deep weathering affected these rocks during latest Cretaceous to early Eocene time (50-65 million years ago) causing the formation of the tri-layered Morney Profile to depths of up to 90 metres.
In this profile an upper siliceous zone overlies a varicoloured zone, which in turn overlies a basal ferruginous zone. Iron oxides leached down from the overlying rocks wee chemically precipitated as concretionary ironstone in the basal ferruginous zone. Development of drainage patterns saw sedimentation along river systems with fragmentation and minor erosion of the Morney Profile along interfluves.
A second weathering event in the late Oligocene (approximately 25 million years ago) formed the morphologically distinct Canaway Profile, consisting of an indurated crust and mottled zone grading down into varying thicknesses of the residual older profile and in places merging laterally into it. The depth of the indurated kaolinitic breccia which formed the crust extended down some 10 to 14 metes, with the full profile extending to about 40 metres depth. Silica was released during this second stage of weathering and migrated downwards as an aqueous sol.
Where erosion had removed sufficient of the older profile to bring he basal ferruginous zone to within 40 metres of the surface, this silica was able to collect and precipitate as opal within voids in the ironstone host rocks. The Canaway Profile apparently formed along interfluves and may have developed at the same time with surface silcrete across adjacent plains mantled by quartzose clastics. Opal deposition and silcrete formation were probably contemporaneous.
The crust and mottled zone of the Canaway cut across the former tri-layered arrangement of the parent (Morney) profile. The convergence of the basal ferruginous zone (of the Morney) with the indurated crust (of the Canaway) appears to be an important factor in opal formation. Where the crust and basal ferruginous zones are in close proximity, the host ironstone bodies were favourably located within a fluctuating groundwater table which permitted deposition and dehydration of siliceous material. Geological processes (folding, faulting, erosion) to the present day have either completely removed these weathered profiles or left remnants of them as flat-topped landforms (mesas) or low rises.
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