Precious Opals Shine a Light on Mars
Precious opal is Australia’s national gemstone. Both precious opal and common opal are made of amorphous spheres of silica 150 to 400 nanometers or billionths of a meter wide, but in precious opal, these spheres are arranged in highly orderly arrays, resulting in scintillating colors.
The main source of the world's gem-quality opal is the red dirt of the Great Artesian Basin in central Australia, one of the largest continental basins on Earth. Precious opals have been mined there for more than a century, where they occur just within 165 feet (50 meters) of the surface.
It was long a mystery why precious opal formed at relatively shallow depths in central Australia, and why it could be found abundantly there yet almost nowhere else on Earth. Now geologist Patrice Rey at the University of Sydney in Australia finds precious opal in the red center of Australia may have formed due to conditions much like those seen on the surface of the red planet.
"The opal miner community was very helpful," Rey said. "Opal miners provided access to opal mines as well as countless samples, and this is a very keen audience, genuinely interested to learn more of opal."
Rey was a schoolboy in southern France when Australia appeared to him through his black and white TV, he recalled. The show was a documentary about opal mining Down Under, starting his fascination with Australia.
"I was 14 years old when I wrote to the Australian embassy in Paris to get documentation to migrate to Australia," Rey said.
"Given that the Great Artesian Basin is one of the largest intracontinental basin of Earth, and given that precious opal can be found anywhere on this basin and almost nowhere else on Earth, I asked, 'What is unique in the geology and the geological evolution of the Great Artesian Basin?'" Rey said.
The Great Artesian Basin was once filled with the Eromanga Sea, a shallow, cold, muddy, stagnant body of water that at its peak flooded 60 percent of Australia. Rey discovered the formation of Australian opal was apparently due to an extraordinary instance of acidic oxidative weathering during the drying out of the central Australian landscape that followed the retreat of the Eromanga Sea from the basin starting about 100 million years ago. Such acidic oxidative weathering over a large region is unusual on Earth — the planet's surface is usually loaded with carbonates that neutralize acid — explaining why opal is rare elsewhere near Earth's surface.
Acidic oxidative weathering has also been seen on the surface of Mars, Rey said. In addition, the red planet shares a number of other features with the Great Artesian Basin. "Similar rocks, similar flooding then drying out history, similar mineralogy, same color," Rey said. "This story shows that space exploration helps us to understand better our own planet."
All in all, the red center of Australia could be among the best matches on Earth for the surface of the red planet.
"It costs billions of dollars to send rovers and orbiters to Mars," Rey said. "Therefore, looking right here on Earth for ancient and modern analogues to Mars' environment is key to carry on research in greater detail and explore the role biology has in weathering processes."
The weathering Earth's surface experiences is affected by biological processes — for instance, by so-called "iron bacteria," which can live off iron and thrive in acidic conditions.
Over the past decade, opal mining has slowed considerably in Australia due in part to the lack of discovery of new opal fields, a slowdown that has accelerated the decline of many townships. These new findings regarding the geological origins of opals could help guide discovery of these precious stones.
"Precious opal is still part of the economic fabric and cultural identity of many remote townships in central Australia," Rey said. "The livelihood of thousands of families depend directly or indirectly on precious opal."
Rey detailed his findings online May 22 in the Australian Journal of Earth Sciences.