Uncovered: the truth about opal formation

By Amy Middleton 31 May 2013
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The secret of how opals formed in Australia’s Red Centre could shed light on the landscape on Mars.

NEW RESEARCH HAS EXPLAINED the mysterious formation of opals, found in abundance in Australia’s red centre, and the information could shed light on the environment on Mars.

Australia produces over 90 per cent of the world’s precious opals, but before now scientists have never been able to explain precisely how the gemstones formed.

“Before this we did not know [opal’s] origin, why it forms at such shallow depths or why it can be found in central Australia and almost nowhere else on Earth,” says lead researcher Professor Patrice Rey, a geologist at the University of Sydney.

Opals formed by acidic weathering

Patrice says the findings, published this week in the Australian Journal of Earth Sciences, reveal that opals formed during “an extraordinary episode of acidic weathering, during the drying out of the central Australian landscape.”

Mars-like landscape near Coober Pedy, SA. (Credit: University of Sydney)

Between 100 million and 97 million years ago, a vast sea that covered 60 per cent of Australia – from Coober Pedy in South Australia to the Gulf of Carpentaria in northern Australia – began retreating.

This drying out of Australia’s centre increased the acidity levels at shallow depth, releasing silica through the weathering of sandtone. Further weathering then lowered the acidity to a level at which precious opal can form in the silica-rich gel.

Australia’s Red Centre similar to Mars

Central Australia is believed to be the only place on earth where acidic weathering of this scale has ever taken place, although similar conditions have been observed on the surface of Mars.

Non-precious opal deposits were discovered on the Red Planet by NASA in 2008.

“If you look at Mars and the Red Centre, they share similar characteristics,” says Patrice. “Similar rocks went through similar weathering processes, so potentially precious opals might exist there.”

Patrice says central Australia offers a “unique natural laboratory”, where researchers can study biological processes that could potentially be present on Mars.

Mike Snow, a minerals expert at the South Australian Museum, says the findings are compelling, and may well provide a glimpse into the landscape on Mars.

“The landscapes of Mars and the [Red] Centre both have large amounts of red oxidised iron,” says Mike. “This is part of the opal story.”

“Perhaps opal may well occur on Mars if it is similar to the Great Artesian Basin.”