Modern Australian

A disappointing earring, and the world's hottest rock: zirconia

  • Written by Nick Timms, Senior Lecturer, Curtin University

My favourite gem is an occasional series in which we ask a scientist to share the fascinating geological and social features of a beautiful rock.

Just last week, my partner handed me an earring that she had found in a park near her home more than seven years ago, and for which she’d had no luck finding the owner.

The earring had a gold setting that gripped a very large, transparent, beautifully cut gemstone. Given that I am a geologist, she asked me to check it out for her to find out more about the stone. After all, we thought, if it was a diamond, then it could fund a holiday to somewhere lovely, given its large size. Hawaii, perhaps?

Read more: Why we value diamond rings and other Valentine's Day gifts

Upon analysing the earring under an electron microscope, it was immediately apparent that the earring wasn’t a real diamond. It turned out to be cubic zirconia.

Funnily enough, zirconia is a topic very close to my heart and my professional expertise. In 2017 I wrote a research paper on the “world’s hottest rock”, and zirconia was part of that story.

A disappointing earring, and the world's hottest rock: zirconia A false colour image of a grain of zircon (yellow) that has formed a rim of zirconia (multicolours) at 2,370℃ when the surrounding rock melted during an ancient meteorite impact. This is the hottest known temperature to be achieved by any rock naturally at Earth’s surface. Nick Timms, Author provided

Crystal lattice gems

Zirconia is a mineral with a crystal structure made from the elements zirconium (Zr) and oxygen (O), with the chemical formula ZrO₂. It looks quite a lot like diamond, but is only worth a fraction of the value because it is manufactured and not a rare natural gem.

Cubic zirconia belongs to a family of zirconia minerals, each having a different configuration of atoms that give rise to different crystal lattice structures, called polymorphs. Similarly, graphite and diamond are polymorphs of carbon – each made entirely of carbon but with different structures.

Like diamond, cubic zirconia is stable at very high temperatures and pressures.

But unlike its sparkly friend diamond, cubic zirconia isn’t stable at Earth’s surface. In fact, it has never been found in natural rocks, which is why it hasn’t been given a proper mineral name by geologists.

Read more: What is a gem? And why painite from Myanmar can fetch US$60,000 per carat

When zirconia is manufactured for making jewellery, the manufacturers use additives to stabilise it. Impurities are incorporated into the mineral and act like atomic scaffolding, holding the structure together so that it resists transforming into one of the other zirconia polymorphs.

If it is pure, though, it readily morphs into one of its siblings – first to a form known as tetragonal zirconia, then to baddeleyite – a polymorph of zirconia that is stable at room temperature. Every time the atoms shift to transform into a different polymorph, they leave evidence behind within the crystal.

Earth’s hottest rocks

So what has this got to do with the hottest rocks on Earth’s crust?

I am interested in finding out how Earth responds when asteroids hit it. I have studied a few of the 192 meteorite impact craters that have been discovered on Earth, including the one that was ultimately responsible for ending the existence of the dinosaurs.

Australia has its fair share of craters – Wolfe Creek is particularly well known, made famous by a chilling horror movie.

Read more: Target Earth: how asteroids made an impact on Australia

Canada’s ancient landscape has also accumulated many scars of bombardment. One particular ancient impact structure – Mistastin Lake in Labrador – contains the solidified remnants of a lava flow near its centre that forms huge cliffs called Discovery Hill.

A disappointing earring, and the world's hottest rock: zirconia Mistastin Lake in Canada sits in a meteorite impact crater. Google Maps, Author provided

The energy released by the meteorite impact was enough to melt and even vaporise the rocks at ground zero, which then cooled quickly (quenched) to form the glassy black rocks of the cliffs.

A disappointing earring, and the world's hottest rock: zirconia The impact structure at Discovery Hill, Mistastin Lake – where the world’s hottest rock was found. CLICK TO VIEW. Mike Zanetti, Western University, Canada., Author provided

The rocks had the texture of syrup when I examined them under the microscope, and I could easily see that only a few fragments of the most physically robust minerals had escaped being completed melted.

Frozen in this quenched rock soup were a few little remnant grains of the mineral zircon (with the chemical formula ZrSiO₄). The zircon must have been present in the target rocks before the catastrophic event, and had taken a “hot bath” when they became immersed in the lava.

However, zircon is a tough cookie and doesn’t melt when it is heated. Instead, and at high enough temperatures, it decomposes to form tiny crystals of zirconia and liquid silica. The specs of zircon in this particular rock had begun to decompose, reacting within the hot magma and had become encrusted with a beautiful rind of zirconia (the baddeleyite form).

A disappointing earring, and the world's hottest rock: zirconia Rock collected from the top of Discovery Hill, on the southweastern edge of Mistastin Lake (shown in the photographs above). Mike Zanetti, Western University, Canada, Author provided

My research team could read the paper trail in the baddeleyite, detecting the former presence of cubic zirconia from which the baddeleyite had transformed. This meant that this rock had once been at a blistering 2,370℃ during the impact event around 38 million years ago. This is the hottest temperature ever recorded for a rock anywhere on Earth’s surface.

This finding made us wonder what might Earth have gone through early in its history, when it was being bombarded frequently by similar-sized or even bigger impacts, and its surface was subject to these extreme temperatures on a regular basis.

There’s an amazing sense of excitement and awe from making discoveries from studying tiny mineral fragments in rocks that spark such deep thoughts about events and timescales almost beyond comprehension. Luckily, this can offset the mild feeling of disappointment after analysing a found earring.

Another time, Hawaii, another time!

Read more: Life, death and politics in Hawaii: 125 years of colonial rule

Authors: Nick Timms, Senior Lecturer, Curtin University

Read more http://theconversation.com/a-disappointing-earring-and-the-worlds-hottest-rock-zirconia-97084

NEWS

As the dust of the election settles, Australia's wildlife still needs a pathway for recovery

The Darling River near Louth NSW, April 2019, in the midst of a drought compounded by upstream irrigation policies.Jaana Dielenberg, Author providedThe environment was a keyconcern in the recent federal...

The long and complicated history of Aboriginal involvement in football

Over the next two weekends, the Australian Football League celebrates the contribution of Indigenous peoples to the history of the game. At the same time, a new documentary will show...

Curious Kids: why are there waves?

Nina Maile Gordon/The Conversation, CC BY-NC-NDCurious Kids is a series for children. If you have a question you’d like an expert to answer, send it to curiouskids@theconversation.edu.au You might also...

Why Sydney residents use 30% more water per day than Melburnians

Melbourne's water supplies are running low after years of drought.shutterstockThis week Melbourne’s water storage dropped below 50%, a sign of the prolonged and deepening drought gripping eastern Australia. Sydney is...

Friday essay: YouTube apologies and reality TV revelations

A little over a year ago, former Australian cricket captain Steve Smith made a tearful confession and apology to the public, having been banned from cricket for 12 months for...

six ingredients of successful public policy

Australia’s national policy response to HIV/AIDS has been lauded as one of the best in the world.ShutterstockIn the lead up to the recent federal election, there was plenty of negative...

Population DNA testing for disease risk is coming. Here are five things to know

Screening millions of healthy people for their risk of disease can be cost-effective. But it raises ethical and regulatory concerns.from www.shutterstock.comDNA testing to predict disease risk has the potential to...

How the dangerous evolution of Pakistan’s national security state threatens domestic stability

Protests followed the terrorist attack that killed more than 40 Indian military personnel in the disputed state of Jammu and Kashmir. AAP/Jaipal Singh, CC BY-NDIn February, a terrorist attack by...

the tall buildings of Australia show why we need strong design guidelines

Towering canyons of concrete and glass are an increasingly dominant feature of fast-growing cities like Melbourne.ymgerman/ShutterstockPrivate enterprise has shaped the skylines of Australia’s cities, and the names of their highest...

If you think less immigration will solve Australia's problems, you're wrong; but neither will more

More by luck than design, recent recent levels of immigration seem to be in a 'goldilocks zone' that balances economic, social and environmental objectives.www.shutterstock.comAre we letting too many or too...

Let them play! Kids need freedom from play restrictions to develop

Playing in nature improves children's learning, social and emotional skills.MI PHAM/unsplashYou may have heard of play. It’s that thing children do – the diverse range of unstructured, spontaneous activities and...

Gamers use machine learning to navigate complex video games – but it's not free

Playing Dota 2? You can do better with a little help from machine learning.Shutterstock/hkhtt hj Some of the world’s most popular video games track your activity as you play –...

Popular articles from Modern Australian

EXYRA eyewearShould you get a hair transplant in Turkey if your hair is grey?Do You Need a Tummy Tuck or Just Liposuction?Best 4 Sassicaia Wine with Soothing Taste and AromaMarvelous Makeover - 5 Tips to Revitalize Your Look This SummerWhat to Expect When Recovering from Gynecomastia SurgeryClickClack Pantry Range | Helping Australians save time & moneyThe Gentleman’s Guide to Wearing Custom TiesGynecomastia – Understanding the Facts and Treatment OptionsIs Coffee Good for you10 Foods Which Reduce Blood Sugar Levels4 Unexpected Reasons Why You Could Be Losing Your HairWhat to Do When Traveling From Australia to USAWhat we should know about ‘nitric oxide’ and why we need more of itDownsizing: What Is Too Small?