Despite its iconic status and a long opal mining history, the geological processes behind the formation of opal are still largely unknown, however researchers believe they have uncovered opal’s secrets.
Experts at the University of Sydney say they can explain why the gemstone can be found almost exclusively in Australia from Coober Pedy in South Australia to Lightning Ridge in northern NSW. The area is situated in what geologists refer to as the Great Artesian Basin (GAB).
They report that the climactic conditions that created opal in the area roughly 100 million years ago closely resemble what scientists know about Mars.
According to University of Sydney associate professor Patrice Rey, the only other area where similar conditions have been observed is on the surface of our neighbouring planet.
Australia’s central landscape, once covered by sea, began drying out about 100 million years ago. As the GAB’s inland sea regressed an "extraordinary episode of acidic weathering" took place, caused by the exposed pyrite minerals, commonly referred to as fool's gold, releasing sulphuric acid.
As the surface cracked, silica-rich gel became trapped in the veins in the rock, which was later covered and protected from the weathering by secondary minerals, allowing the silica to solidify to form opals.
Scientists based much of their findings on a review of three decades of research on the evolution of the GAB and its rock formations, and research on weathering processes. They said it explains why many opal mines are preserved below extremely hard ground and not in overlying sandstone or siltstone formations.
Unfortunately, the research suggested that most of the precious opal had been transported and buried into the Ceduna Sub-basin, although it may not have survived erosion and transportation.
While scientists admitted that the type of acidic weathering was highly unusual at the Earth's surface, the same weathering conditions were documented in 2008 about the surface of Mars.
Life on Mars
The GAB and Mars also share a similar secondary mineralogy and even the same colour, hence there is a possibility that opal could be found on the Red Planet.
“Opal-like deposits have been detected on Mars, in several places,” professor of astrobiology at University of New South Wales (UNSW), Malcolm Walter told Jeweller. Prof Walter is a specialist on the prospects of life on Mars.
“These are spectroscopic observations from an orbiting satellite, NASA’s Mars Reconnaissance Orbiter. We would probably call it potch opal. There is no evidence of anything precious, except to scientists,” he added.
Prof Walter also believed that gold would exist on Mars because the planet has much the same composition as Earth.
“As far as I know, though, none has yet been detected.”
A colleague at UNSW, Professor Michael Archer from the School of Biological, Earth & Environmental Sciences Department, agreed that gold should exist on Mars.
He said previously-published speculation suggested that the amount of gold deposits found near the surface of the Earth – where they shouldn’t be, considering gold’s atomic weight – may have been plastered over the planet’s surface billions of years ago via “gold-laden meteorites”.
“Asteroids with high gold content are known to be ‘out there’ and all planets in our solar system were bombarded with masses of meteorites more than three billion years ago,” Prof Archer said.
The opal research is published in the Australian Journal of Earth Sciences.
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