The Opal's Fire
Opals, a rainbow of fire locked in rock, are among the most wonderful of nature's gifts. Maggie Koerth-Baker returns with the light truth about weird silica. Opals are just silica, the same stuff that beach sand is made from. But they’re weird silica. Look at the opal in the video above. The first thing you notice is the colors -- green, blue, purple. It’s like a solidified rainbow. Color is the whole reason humans are interested in precious opals. It’s what makes the precious precious. But pure silica is white. In fact, common opals (the gemstone’s boring cousin) are dull, rather than shiny, and come in shades more appropriate for Patrick Bateman’s business cards than a Pride parade float.
Scientists know what makes the difference between precious opal, common opal, and regular silica. But why those differences exist is kind of a mystery. A new theory has an answer for that, while simultaneously explaining why most of the precious opals in the world come from a single place. The solution depends on one surprising fact. Turns out, Central Australia has a lot in common with the surface of Mars.
The preciousness of precious opal boils down to a difference in structure. The silica in sand is a crystal; quartz, to be precise. Its atoms are arranged in tidy, ordered patterns. Opals are amorphous solids, sort of the opposite of crystalline solids. That doesn’t mean, however, that the atoms in an opal are just thrown together willy-nilly. Instead, to get an opal, and particularly to get a precious opal, you need a very specific atomic structure.
“If you look at opal with a microscope, what you will see are small spheres,” says Frank Marlow, a materials scientist with the Max Plank Institute who has studied the structure of opals. All opals are made of spheres, but not all those spheres are created equal. In a precious opal, the spheres are all the same size. What’s more, they’re arranged in a pattern that, while not quite rising to the level of crystalline, is definitely ordered. Common opals, on the other hand, are more of a jumble of spheres in a wide variety of sizes.
Those differences affect the way opals reflect light, which affects the color we perceive when we look at them. The little balls that that make up precious opal are arranged in a pattern that just happens to match certain wavelengths of light, reflecting back the colors associated with those wavelengths. The opal in the video must be made up of particularly small spheres -- tinier than 150 nm -- because it appears blue and purple. Opals made of “large spheres” -- on the order of 350 nm -- appear more orange and red. It’s the same phenomenon that produces the shimmering colors on the back of a CD.
“A CD is made of stripes of information, and if the distance between those stripes is comparable to the wavelength of light, it becomes colorful,” Marlow said. “DVDs appear more blue because the stripes are more near to each other.” Common opals, with their irregular pattern of varying spheres, never hit that sweet spot, so we don’t see them as colorful.
That’s the easy bit, the part we can explain. What’s not well understood: Why opals form this way. Joel Brugger, a geophysicist, told me that there are many theories of how opals form and why they form the way they do, but, until recently, none of them really accounted for the fact that the vast majority of precious opals all form in the same place -- Central Australia.
More than 90 percent of the world’s precious opal comes from Australia’s Great Artesian Basin -- more than 600,000 square miles of red dirt sitting on top of a giant aquifer. The basic idea behind opal formation is that it happens in places where the climate alternates between wet and dry, and where silica-rich rocks are present, ready to be slowly broken down by weathering. That way, you get the mixture of silica and water that creates opal. This silica gel infiltrates gaps in the rock, and pores where rocks and fossil material have been previously weathered away. As it solidifies, it forms opal. Precious opal forms when the spheres get filtered out, leaving each layer of sediment with a majority of spheres of a single size, Brugger said. The idea is that each layer of sediment, with its standardized spheres, produces a little bit of opal which fills in a pore in a fossil or a hole in a rock.
But this basic theory leaves some pretty big gaps unfilled. For one thing, water and silica are extremely common worldwide. So are wet/dry cycles. And that leaves us without a good answer for why Central Australia has so many of these stones and the rest of the world so few. If the ingredients are everywhere, why is only one place baking the cake? You can see the other big problem when you look at opalized fossils, like the one in the video at the beginning of this post. If the sediment filtering theory is correct, then the opal that infiltrates fossils should have been formed at many different times. But that’s clearly not the case. Brugger has studied opalized fossils and says it’s common to see colors and banding patterns that are consistent across the entire fossil, something that wouldn’t happen if the fossil had been slowly filled in pore by pore. What’s more, while opals worldwide date to a wide range of ages, the opals of the Great Artesian Basin appear to be all around the same age -- dating to around 100 million years ago.
In May of 2013, geoscientist Patrice Rey proposed a variation on the weathering theory that explained why Central Australia was so special. Back in 2008, NASA rovers found opals on Mars. They were common opals, not precious ones, but the discovery got Rey thinking about what the Great Artesian Basin and the Red Planet had in common -- namely, all that red dirt. Could a similar geologic process be at work in both places? Rey thinks so.
In a paper published in the Australian Journal of Earth Sciences, Rey put together a theory that both Brugger and Marlow say is the most comprehensive explanation of opal formation that they’ve ever seen. The secret lies in changing ph conditions - something that happened both in Australia and on Mars.
To get the weathering that breaks down silica-rich rock, you need an acidic environment. But, in order to produce synthetic precious opal in the laboratory, you need an alkaline environment, Rey said. The high ph seems to create conditions that standardize the size of the silica spheres in a relatively short amount of time, thanks to a process called Ostwald ripening. This ripening is the tendency for particles in solution to grow in size. It’s easier to form little bitty spheres, but they’re also less stable. Unable to survive on their own, they either join up with larger spheres or shrink away. Eventually, the size of the spheres starts to standardize as little ones vanish and larger ones grow.
The ideal opal-making environment, then, is a place that transitions from wet and acidic to dry and alkaline. What makes the Great Artesian Basin special, Rey said, is that it represents a massively large area that went through just such a process all at the same time. In the late Cretaceous, the area was covered by a shallow sea. In most places, a sea like this wouldn’t get acidic enough for long enough, because the sea would be full of alkaline carbonate minerals. But the Great Artesian Basin was too muddy and too close to cold Antarctica for those carbonates to form. Instead, there were at least six different acid-producing conditions that would have been favored during this time, Rey said. Without the carbonates, those forces were left unchecked and the Great Artesian Basin became not just wet and acidic, but especially so.
But it didn’t stay that way forever. The ancient Basin also contained a lot of feldspar and clay. As these materials dissolved in the acidic environment, they broke down into alkaline materials that raised the ph, a process that happened as the inland sea, itself, dried up. Here, in this one location, you had the perfect conditions to make precious opal -- something that never happened on the same scale anywhere else on Earth.
And Mars? There volcanic gases interacted with the atmosphere to create highly acidic conditions during a period where the planet had water. But here’s the catch, Mars never had the period of massive alkalinization that the Great Artesian Basin did. The result: The Basin has lots of precious opal with its neat, similarly sized spheres of silica while the opal on Mars is of the common sort. “They’re not exactly the same,” Rey said. “But it turns out Australia is a pretty good model for the weathering we see on Mars.”
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