What could have been: a rare mineral from the Cranbourne meteorites

It may come as a surprise that new discoveries can be made in the most unlikely places, even in meteorites that have been studied for decades.

Scientists at the Museums Victoria Research Institute have found a rare mineral, appearing as small green patches on some rusty fragments of the Cranbourne iron-nickel meteorites.

‘If I'd been astute enough to look at these patches five years ago, perhaps we could have had a new mineral found first in Victoria. We’d have probably called it cranbournite,’ says Dr Bill Birch, Geosciences curator emeritus.

Alas, a team in Europe beat him to it by just three years.

‘This is the third occurrence of this mineral anywhere in the world, which was first described from a meteorite in 2021,’ he says. 

This meteorite came from a place called Muonionalusta, in northern Sweden, which gives the mineral its name.

The second find, only a year later, was on a museum specimen from an abandoned nickel mine in Tasmania.

‘There was a little photo of it, just this green patch,’ says Bill.

‘I thought, I think I've seen that sort of thing on rusty fragments from the Cranbourne meteorites, so I got curious and found these pieces in our collection.

‘A bit more testing, and it turned out we had the same mineral.’

The research has now been published in the Australian Journal of Mineralogy.

But the Cranbourne meteorites have been in the museum’s collection for decades, and had been studied in detail by scientists in other institutions in the 1940s, so why only now?

‘The early descriptions do mention green crusts on some samples, but the researchers didn't have the methods that we have now to fully describe them and name them,’ says Bill.

Modern methods

The first step was to carefully select a pure sample of the mineral under the microscope before crushing it to a powder to test in the museum’s X-ray diffraction equipment.

‘This test gave what's called the X-ray diffraction pattern, which is like a two-dimensional fingerprint of the crystal structure of the mineral,’ explains Bill.

As the X-ray diffraction pattern of muonionalustaite was already known, it was a quick process, working with Oskar Lindenmayer, the museum’s Geosciences collection manager, to identify the substance.

‘Our pattern was a perfect match for muonionalustaite. I think it took Oskar and I about 20 minutes to confirm it,’ says Bill.

‘We just looked at each other and went, wow, there you go—there's a new one for Victoria,’ says Oskar.

Then it was time to bring this mineral to light.

‘Minerals that don’t immediately catch your eye like this one can look amazing when you actually see them under high magnification,’ says Bill.

‘It’s a new world.’

Oskar and Bill used a scanning electron microscope (SEM) at up to 600 times magnification to photograph the mineral’s structure.

‘You can see that surface is made up of these tiny little lathe-shaped crystals,’ explains Oskar.

‘They're a really beautiful texture.’

The scanning electron microscope also revealed the elements present in the mineral and their proportions, enabling a chemical formula to be calculated.

‘Having the combination of the X-ray diffraction and the scanning electron microscope with the ability check the chemistry makes all the difference,’ says Bill.

How does this new mineral form?

There are 13 pieces of the Cranbourne meteorite found southeast of Melbourne, in Victoria. They likely sat in the soil for thousands of years before they were excavated from the mid 1800s onwards.

‘In that time, the groundwater has seeped into the meteorites, reacted with the iron and nickel, and formed this new mineral, which consists of nickel and chlorine with water,’ explains Bill.

‘It can form presumably wherever there is a nickel deposit, or an iron-nickel meteorite, exposed to slightly salty groundwater.

‘It's not really Nobel Prize-winning stuff, but it gives us some indication of the processes that can go on at the Earth's surface once something extraterrestrial lands.’

And the way this mineral was found may open the door to new discoveries.

‘This was hidden in our collection for many years,’ says Bill.

‘But there is always hidden material in big geological collections like the one we have in our museum. I’m sure we could find new minerals, if we looked hard enough.

‘That’s the significance of having a mineral collection like ours.’