The basics behind Postulations and Theories or “models” for opal Formation
The principles surrounding the formation of opal have been some what controversial from time to time, perhaps no more than in recent times. It should be recognised that information reported in scientific literature by observation of the structure of opal has changed substantially over time. Perhaps the greatest leap forward in the understanding of precious opal structure, including an understanding of why precious or noble opal shows its distinctive Play-of-colour (POC) occurred with investigations and discoveries made by the Australian scientists at the Commonwealth Scientific and Industrial Research Organisation (CSIRO) in the 1960’s.
Investigations by the CSIRO with an electron microscope (in the ‘60s a fairly new piece of scientific technology) discovered a three dimensional array of minute (nanometre sized) spherical silica particles that were capable of diffracting white light into spectral colours.
Since that time scientists have postulated a number of methods for the formation of suitably sized silica spheres by natural processes, within the constraints of the geology (what rocks are available), along with suitable environment, a suitable process, and a with suitable depositional sites for opal to form. Whichever of the proposed models or postulations for opal is correct, there remain several basic requirements for opal formation. We will look at some basic requirements of opal formation before we delve much more deeply into scientific literature. It is fair to consider that to form precious or noble opal we need:
A source of silica
Water
A suitable chemical environment to mobilise the silica
Pathways for the silica rich solutions to travel within the rocks or the geological profile
Depositional sites, a place for the opal solutions to be deposited, and opal to form
Geologically stable terrain
Time of formation
A Source of silica
Where does the silica for opal formation come from? Is the silica obtained from weathered or re-worked rocks or host rock environment in which opal is mined? This is to consider is the silica a result of geological “weathering processes” (a popular theory). Are these weathering processes “ordinary”, the action of rain, wind and terrestrial processes, or is there a “biogenic” influence provided by active biological processes like the action of bacteria and microbes.
Or is the silica provided by subterranean waters of the Great Artesian Basin, having already been placed into solution in the underground environment?
In the sedimentary environment and the theory for a “weathering formation” this is has been usually considered that the sandstone is chemically weathered to produce suitable silica rich solutions suitable for opal formation. Perhaps originally it was considered that terrestrial water would percolate through the sandstone and washout or dissolve (or weather the silica from the rocks) for opal formation (Watkins 1980). It is true to say that in each of the Australian deposits where opal is found that there is a consistent “profile” known in the fields as a “weathered profile” where the sandstones of the strata have been bleached of silica.
In the volcanic environment there are often similar “profiles” and it remains to be determined if the silica is weathered from other high silica content volcanic rocks. Often in areas of the volcanic environments, there is considerable amounts of “volcani-clastic” rocks or to say another way volcanic rock that is deposited and forms a similar type of rock to a sandstone. An example of this type of volcani-clastic rock would be the rock and ash that buried Pompeii in AD79. Ash fall from volcanic eruptions in particular higher silica content acidic igneous rocks such as rhyolite and andesite, can provide suitable material. More difficult, although not unrelated are the weathered clays found between other lava flows that appear to be host rocks for opal such as found in Tintenbar NSW and related to the Mount Warning volcano. Or is the opal in solution provided from other silica rich hydrothermal solutions percolating through volcanic strata.
Water
Natural opal contains a percentage of water of various types as part of its chemical make-up. This water can have several sources, in the sedimentary environment it is possible the water is terrestrial, rain water or from river flows. In the volcanic environment it is possible the water is of hydrothermal or subterranean origin. Fundamentally it is a difference between “does the water (silica solution) come from above (i.e. terrestrial) or from below (i.e. hydrothermal)”. As silica does not seem to dissolve readily in a bucket of water (try it) at say “room” temperature and on the surface of the earth (normal pressure), something we scientists call STP or standard temperature and pressure. There must be something else that is required to make opal.
The Chemical Environment
So understanding that silica does not readily dissolve in water without some help, we need to look at and postulate how the silica gets into solution. We then need to consider how the silica spheres (mentioned in our preamble) can form and then how they can be deposited into the appropriate “three dimensional array” that is needed to produce the diffraction grating for revealing the colours of the rainbow seen in precious or noble opal. This part of the science is quite complex and so will be left to our discussion in the section dedicated to opal formation.
Pathways for solutions to travel
Whichever formational hypothesis is correct, the silica rich solutions need to have some ability to move throughout the strata, and for water to both dissolve silica and then for excess water evaporate or move out of the depositional sights. The geology is full of joints in the rocks, faults (slides) on a minor scale, say one to tens of metres, lineaments and bedding planes over larger scales, and even possibly larger more regional tectonic structures, may provide these pathways or influence opal formation. In the sedimentary environments these pathways are easily identified by miners observing vegetation, and observation of aerial photographs by determining lineaments. The influence of more regional tectonics remains to be investigated.
Perhaps in consideration of opal formation whilst the “regional” aspect for opal formation seems to be related to the more vigorous science of volcanoes and volcanic forces, structures and eruptions, opal does not form during these processes. Opal forms after most of the activity has gone, (even if it may be nearby and still occurring. So in the volcanic environments there should be similar structures identifiable although some may be more difficult to observe and the literature is not quite as evolved as it is in Australia.
Depositional sites
A requirement for opal formation is then a place where these solutions can stop, perhaps a permeability barrier that stops the flow of the silica rich solution, or similar place where the opal rich solutions can pond or fill a hole and allow the deposition of the silica spheres into an ordered array. Perhaps a cast that was once another item like a bone or a shell, somewhere the opal solution can relatively speaking compact, dry out and solidify. A note should be made here that when considering “depositional sites” there also needs to be a provision and process for the formation of “opalised fossils. Depositional sites will also effect other forms of opal observed in the field such as opal “veins” or “seams” and opal nodules “Nobbies”, in the sedimentary environment. In the volcanic environment there needs to be an explanation of opal infilling volcanic rocks, the filling of vesicular “gas holes” or voids in lava’s such as at Rocky Bridge Creek NSW, or filling of volcanic “geodes” as seen in the Idaho, USA and perhaps the Ethiopian deposits.
Geologically stable terrain
Before, during and after the the processes of opal formation and deposition it is a necessary to consider that the geological strata (or rocks) are stable after the formation process and not extensively deformed in a major way by further geological processes such as major orogenic tectonics such as substantial folding or large scale changes related to regional metamorphism, or substantial volcanism. Silica is very mobile in the environment, so major tectonic movements and metamorphism of this kind will mobilise and allow the silica to be reused and incorporated into other minerals by natural processes.
Recently it has been recognised that the dynamic nature of the Australian tectonic plate and its interaction at tectonic plate boundaries and sub lithospheric mantle geology may have played a role in opal formation in the Great Artesian Basin.
Time of formation
A major consideration for any opal formation hypothesis remains to answer the questions “when did it happen?”. “Is opal still forming today?”
Here we have another difficulty in our understanding. It is often difficult to think about geological time. Geologists talk about processes and history of millions of years. This is often difficult for us to comprehend when in general we humans only occupy the planet for say 70- 80 years on average.
There is little doubt that in the sedimentary environment associated with Australian opal deposits that are located within the Great Australian Basin (GAB), (sometimes called the Great Artesian Basin opal is associated with the Cretaceous period of geologically history from 66 – 144 million years ago (ma). This is a quite a large range of time, however is constrained by geological history, and is confirmed or dated by many available fossils, some have been “opalised”. Research which is always continuing is presented by the Australian opal centre (AOC) and opal people and (opal-holics) and palaeontologists to fascinate and influence the time of opal formation.
historical theories of formation
The historical discussion is presently under development at the Opal Academy
a more modern approach to postulations of opal formation.
We have a number of papers that discuss the formation of precious opal, all of which take into consideration the topics introduced above.
A weathering model
There are many discussion papers that talk about a theory for opal formation that involves the weathering of rocks in the preserved geological strata that we can view in the present day. The theory is a simple one. The rocks in the terrain in Lightning Ridge as can be seen have been chemically weathered by terrestrial water, the silica dissolved and then reprecipitated at the “opal level” in suitable (geological) conditions to allow precious opal to form
We have chosen to refer readers too one of the documents available.
John Watkins 1984 - You can read about this report here:
A biogenic model
The biogenic model for opal formation in some ways enhances a purely weathering model for the production of silica suitable for the formation of precious opal by discussing the possibility of bacteria and microbes interacting in the environment to produce the silica need for precious opal formation.
Watkins and Behr - read the report in the NSW geological survey notes here:
A tectonic or syntectonic hypothesis - Pecover
This hypothesis suggests that the formation of opal (both precious and potch ) is related to the tectonic “adjustments of the geological terrain in the Great Artesian Basin (GAB or otherwise named the Great Australian Basin). The author suggests that the water for opal formation is available from the subterainian GAB aquifers, and the tectonic movements of the terrain creates an environment for opal formation. This hypothesis is perhaps constrained to Lightning Ridge in NSW and also South Australian fields of Coober Pedy and Andamooka.
Natural Gamma radioactivity-Senior-chadderton
Whilst this paper is primarily concerned with an exploration model for precious opal using analysis of Gamma radiation techniques it does have a bearing on the formation of precious opal primarily in the Lightning Ridge district in New South Wales. This hypothesis suggests that the formation of the spherical opal spheres necessary for the formation of the three dimensional array for precious opal is associated with a “central radioactive catalyst core”, or in other words there is an aspect to be considered that the initial small particle at the nucleous or centre of an opal sphere has a relationship to a readiactive particle. This radioactivity can be measured in the geological environment. The detection of this is residual radioactivity by detection of Gamma radiation may provide a suitable method of exploration .
