Zirconia: The World’s Hottest Rock

Zirconia: The World’s Hottest Rock

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.

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.

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.

When a city-sized asteroid hits the ground at 15 km/second, an enormous amount of energy is released, like "a billion hydrogen bombs worth of energy,.

This produces a lot of heat – so much heat, you could vaporize rocks. The rapid cooling that follows impact 'freezes' in place whatever is inside the rock. 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 research team 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.