Petrified wood

[Sandta]

A couple of days ago a new member introduced himself... Jimmyjamesc... I commented on his cool avatar...
It is a pic of opalised petrified wood, i had said i too have a piece but is packed away in storage shed...
Today, at lunchtime , I stopped at the park where i found that piece. Within 15 mins i had found half a dozen pieces of petrified wood

.... Amazingly one of them also has some opalising in it .

As I am in Lake Macquarie , a bloody long way from any opal fields , i guess its not true opal ...
It sure is pretty , very vivid blue ( sorry the crap photos dont show it very well)

 

[Heatho]

Actually Sandta, it just may be Opal, there are more Opal deposits in NSW than you may think.

 

[slipped disc]

I would imagine Opal can occur anywhere given the right conditions.

 

[outboard]

I would imagine Opal can occur anywhere given the right conditions.

link
NEW RESEARCH HAS EXPLAINED the mysterious formation of opals, found in abundance in Australias 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.

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.

Australias 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.