Opal Formation
an introduction to opal formation
Let us first look at the both the science and conditions that will be needed to discuss the topic of how opal forms. Once we have looked at some of the basic components for opal formation we can move to considering specific varieties of opal and in particular the formation of precious opal (opal with a Play-of-colour, POC).
The way in which precious opal is formed in nature remains something that is not entirely well understood. Therefore as science continues to explore and postulate theories of opal formation it is likely that details about how opal forms will change over time.
There also remains a distinct division in the postulations regarding the actual production of the silica for opal formation. There is not yet agreement between the so called “biogenic” source of silica and the purely “chemical weathering” methods of silica production for opal formation. This remains a complex and unresolved process.
There have been many attempts to explain how a silica solution of uniformly sized silica spheres can be derived from the host rock geology, and deposited within the rock strata in such a way as to form a suitable solution of silica spheres that is precipitated by a suitable mechanism to allow formation in a regular array suitable for creating the diffraction effects seen.
It is interesting to note that most hypotheses for opal formation are examined and detailed with respect to Australian sedimentary type opal. Little examination has been given to other sources of opal, specifically the formation of opal in other geological environments of volcanic nature seen in overseas or foreign occurrences of opal. More recently a substantial amount of scientific discussion has been provided by Boris Chauvire who has conducted field work and examination of opal in Ethiopia.
A part of this discussion most likely should be considered with respect to the ‘commercial’ production in of precious opal in Australia, especially as the Australian opal fields have up until recently produced the major proportion of precious opal for sale to the international gemstone and jewellery markets. Literature suggests a period of time where Australian precious opal production suppled upwards of 95% of world gemstone opal production. Since the turn of the century, the market has changed with the discovery and mining of opal in Ethiopia.
An historical view of opal formation in Australia
Perhaps the most accepted historical theory (at least for the formation of Australian sedimentary opal) is that of the ‘Deep weathering theory’ discussed by Watkins (1984) and you can read this report here. This theory is based upon the chemical weathering of silica rich rocks to produce a suitable amount of silica in solution for formation into spheres for precious opal deposition and remains as an introduction to an accepted postulation or theory for precious opal formation. Perhaps one of the most important features from the discussion in this report is to identify the basic “needs” or components to be considered for precious opal formation.
For precious opal to form we need:
A source of silica
Water or a source of water
Terrestrial water or subterranean water
Appropriate geological or mineralogical conditions
The geological strata
The need for the correct chemical environment within these conditions
The chemistry in the environment that allows formation
A suitable place and geological environment for precious opal to form
Also related to the strata
A timetable for precious opal to form.
different ideas, theories, postulations, models and discussion.
Recently there have been several other hypotheses proposed for opal formation, along with some possible modifications to the deep weathering theory. Most of these hypotheses remain localised in the discussion of opal formation and prospecting in the Lightning Ridge district of NSW. Most of the research has been related to providing suitable prospecting information for the expansion of opal mining of what appears to be a limited resource. Whilst much of the observed information can be correlated to other Australian deposits of opal there remain differences in each field and not all of the observed features are necessarily applicable to the broader issue of opal formation in the Queensland boulder opal fields, South Australian opal fields or foreign opal occurrences.
