Studying the ways in which minerals change over time could shine a light on unanswered questions about the history of the Earth
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It started at a Christmas party on 6 December 2006, at about 8.15pm. Robert Hazen, a mineralogist and astrobiologist at the Carnegie Institution’s Geophysical Laboratory, had just been asked a question that would inspire an entirely new way of thinking about mineralogy. ‘It was naive, it was brilliant. It was a question that no mineralogist would ever have asked,’ he says.
The question was: ‘Were there any clay minerals in the Hadean [the geological period that began with the formation of the Earth about 4.6 billion years ago and ended four billion years ago]?’ But what Hazen heard was a much deeper question about whether the mineralogy of our planet has changed through time and, if so, why? The answer required many months of pouring over thousands and thousands of scientific papers, culminating in a 15-year study that categorises 5,659 recognised mineral species based on every known process by which they form.
Minerals owe their origins to an array of different processes – 57 in total, according to Hazen and his colleague Shaunna Morrison. There are minerals that are created by freezing and condensation, as well as minerals made by lightning, meteorite strikes and even penguin poo. ‘Crazy stuff, but these are the ways that minerals form,’ Hazen says.
The International Mineralogical Association (IMA) classifies minerals according to their chemical composition and physical structure, but many minerals occur in more than one form. Calcium carbonate is found both in calcite crystals in caves and in the calcite eyes of trilobites (extinct marine arthropods), but they are different kinds of calcite. Once these different forms are factored in, there are 10,556 different kinds of mineral in total.
Grouping these minerals by genesis – when and how they form – builds on the IMA’s system in a way that actually tells us something about the planet, or even other planets. For example, the high mineral diversity on Earth, compared to the Moon or Mars, is down to the essential role of water in forming 80 per cent of all known mineral species. ‘All the questions that geologists have been thinking about – how our oceans have changed, when plate tectonics started, how our planet has evolved over its four and a half billion year span – all these questions are folded into minerals,’ says Hazen. ‘They’re time capsules.’
The new system of classification does leave room for ambiguity; how the groups are split is somewhat subjective. Stellar diamonds, formed in ancient stars, are categorically different to mantle diamonds, formed by the high pressures and temperatures deep in the Earth. In contrast, pyrite (also called fool’s gold) can form under a ‘sliding scale’ of conditions: low, medium, high or higher temperatures, with no, some or lots of water. Judgement calls have to be made and Hazen openly welcomes input from scientists who think he’s got it wrong. ‘That’s the way science advances,’ he says.
Hazen’s real hope is that the work will mark the start of a new chapter in the field of mineral evolution. ‘Modern mineralogy based on physics and chemistry requires million-dollar equipment, which really limits mineralogy to a very small subset of privileged people,’ he says. Working with the sort of mineral data now being compiled, however, only requires a laptop and internet access. Describing his research group at the Carnegie Institution as the most diverse he’s ever seen in science, Hazen adds: ‘This approach has changed the literal face of mineralogy.’
