Categories: Australian Opal | Opal Types
I have read many many different articles on how Australian precious opal was formed but have had some doubts on the commonly accepted theory of water seeping through sandstone and dissolving silica ,which then formed a gel and seeped into cracks and crevices in the underlying clay. Silica is almost insoluble so how did it dissolve in the water passing through the sandstone? And why would it settle out into sub microscopic spheres? With these doubts in mind I did some more research on the internet ( Which I acknowledge is not always the best source of information) and eventually found a few sources which, in my opinion, much better explains the process of opal formation in Australia. This post presents the information I found and I hope will attract some comments from people who are much better qualified than I am. I am not a chemist or geologist but I do like a theory to be based on fact. So here goes.
It is commonly accepted that precious opal was formed in Australia around 100 million years ago.
Australia was a much different looking continent way back then.
In the period from 200 million years ago to 145 million years ago (Jurassic Period) Most of what we now know of Australia was above water and was joined onto Antarctica.
Due to Continental drift the continental masses moved continually until the end of the Cretaceous period (145 million to 65 million years ago )when the larger masses were relatively in the position they are today, although they are still moving.
During the Cretaceous period the temperature of the earth was raised. The ice caps melted and the sea levels rose until the entire inland of what we now know of Australia was covered by a large shallow inland sea called the Eromanga Sea.
The northern extremes of Australia during this time were quite hot and the southern areas extremely cold as they were still close to Antarctica at that time.
This vast sea was home to many strange marine creatures and plants which are now extinct as well as normal sea creatures that exist today.
Some examples of these creatures that were present at this time are:
Plesiosaurus - large whale like creatures with long necks.
Ichthysaurs - Dolphin like creatures.
Ammonites - Squid related animals living in distinctive shells of varying sizes.
Belemnites - Also squid related animals
Crinoids - Sea lilies.
The evidence of all this is confirmed by the many fossil finds in NSW and Queensland.
If you transpose the location of the Australian opal fields onto a map showing the Eromanga Sea at that time you will see that these areas were all under water at the time of the opal formation.
Coober Pedy, Andamooka, White Cliffs, Mintabie and the various Queensland Boulder Fields were all within this Inland sea area. It is also interesting to note that Lightning Ridge was at the eastern edge of the sea in what was swampy fresh water. Proof of this is in the opalised fossils found at Lightning Ridge. They are of freshwater species whereas fossils found at the other fields are of saltwater species.
What is Opal
Opal is a form of silica dioxide and is made up of trillions of submicroscopic spheres which can only be seen with an electron microscope. In precious opal these sphere are arranged in a regular pattern of like sized clumps with water interspersed between the spheres. Opal has a water content of between 6% -20%. In common opal these spheres are arranged randomly and there is no play of colour.
In precious opal the regularly spaced spheres act like a diffraction grating and break out the various wavelengths of visible light.
The smaller the sphere size the smaller the wavelength of light is broken up. Small spheres cause blue light to be broken out while larger spheres break out the greens, yellows oranges and reds. As the spheres are of varying sizes in any opal so to the colour patterns vary. That is why all opals are different.
The colours in opal are the result of white light entering the surface, being diffracted into the various colours by the array of spheres of molecules and re-emerging to the surface. That is why you should only view opal with light shining onto the surface. If you hold opal up to a light source you will only see the background colour of the opal and not a play of reflected colour. The background colour of the opal is a result of other chemicals within the silica. Aluminium, iron, carbon etc change the background colour but not the colour play caused by the diffraction process.
Opal is generally found in very distinct areas and at specific levels. At Lightning Ridge the opal is found in the upper layers of what is called the Finch clay facies which is a clay layer just below the sandstone. At Andamooka the opal is in a thin band of material just below the sandstone and above the clay layer.
For opal to form there had to be some specific factors in play. No one knows for sure how it happened but the main theory is that the following must have been present.
There must have been a source of Silica.
There must be a clay barrier to hold the silicon as it hardens.
There must be a change from an alkaline to an acidic environment.
The presence of aluminium oxide, ferric oxide or magnesium oxide is a requirement
.The presence of sodium chloride or sodium sulphate is a requirement.
There are some differing views on how opal was formed with the most common being that water seeped through the sandstone layer releasing the silicon and that this collected in faults decomposed fossils etc at the interface with the clay layer. While this explains most of the results there is still some major questions to be asked.
Why did the opal form these sub microscopic spheres?
How did silica, which is insoluble in anything but fluoric acid, dissolve in the sandstone?
Not long ago I came across an article by Richard Carew.
His explanation is as follows and I quote: "The formation of precious opal is caused by the interaction of an electrolyte and kaolinite clay. Kaolinite is a two layer clay that has one layer of silicone dioxide and one layer of mixed silica and aluminium and trace elements. Kaolinite comes from the mechanical and chemical weathering of plagioclase feldspars usually in the form of volcanic ash. In Australia, where most opal comes from, large pure beds of this mineral were laid down millions of years ago. When the layers were being formed Australia was dominated by a freshwater inland sea, so, as the feldspars were settling out, the very finest grains of it settled last forming a thin layer of what was to become "opal dirt" which is the term used by the miners to describe the thin layer of clay where opal is found.
This fine grained volcanic ash became kaolinite in a slow process of chemical and mechanical weathering.
Then the ground went through some upheavals causing cracks and faults in the ground. By this time the kaolinite was covered by sand-stone and the clay was protected and compressed.
When the fault lines appeared there was an up-welling of the remnants of the fresh water sea in the form of the great underground aquifer that exists to this day.. When this water moves through the ground it picks up dissolvable minerals like iron and sulphur creating an electrolyte which interacts with the kaolinitic clay dissolving the layer of aluminium and other trace metals like magnesium leaving behind the insoluble silica which rolls up and forms submicroscopic spheres.. This is happening in the finest layer of clay where the particles are minute to start with and then the fine particles are transported to cracks and crevices. An ion exchange happens with the clay when the sulphide solutions hit it and an opal gel begins to form. Depending on the fineness and regularity of the clay particle precious opal will form"
To me this theory makes sense and Len Cram, one of the experts on Australian opal has actually formed opal at his home in Lightning Ridge. Using a source of silicon, an electrolyte and water together with some opal dirt he has caused opal to form in a very short time. His opal is in layers up to 3mm thick and is apparently very similar to the opal found in nature. He keeps his exact formula a secret.
This indicates that the theory of ion exchange is possibly the way it happened so long ago and certainly goes a long way to explaining some of the anomalies of the "seepage through the sandstone" theory.
This theory written by Richard Carew makes a lot more sense to me and I hope, by sharing it, it will provoke some discussion.
Thanks,
Johno