Robert Hazen was attending a Christmas party one December evening in 2006 when a friend and fellow biologist asked a simple question: “Were there clay minerals in the Hadean?”
The question came from an important place. The Hadean Aeon is what scientists call the first chapter of Earth’s history—the fiery and mythopoetic time from the formation of our planet to about 4 billion years ago. And clay minerals, commonly found in soils around the world today, play a key role in some of the many theories about how life began.
But according to Hazen, a mineralogist at the Carnegie Institution for Science in Washington, D.C., it wasn’t a question that his field was equipped to study a decade or two ago.
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Now he hopes that will change, thanks to a new mineral cataloging system that takes into account how and when a mineral formed. This is described in two articles published today in the journal American mineralogist. (This study may be the vanguard of more than 70 other studies.)
“I think this gives us the opportunity to answer almost an unlimited number of questions,” said Shona Morrison, a geoscientist at the Carnegie Institution and one of the paper’s authors.
Traditionally, mineralogists classified crystalline compounds by their chemical composition (what atoms are in a given mineral?) and their structure (if you zoom in, how would you see these atoms arranged?).
“The way mineralogists think about their field is: every mineral is an idealized chemical composition and crystal structure,” said Hazen, also one of the authors of the paper. “That’s how we define ‘mineral species’.”
The International Mineralogical Association (IMA), the world congress for the field of study, defines about 5,800 enumerated species: from pyrite and diamond to hydroxyapophyllite-(K) and ferro-ferri-fluoro-likeite. It’s a collection that scientists have been amassing for centuries.
This scheme is great for identifying minerals by their face, but it doesn’t say much about how a geological artifact might have formed. Pyrite, for example, can be traced to everything from hot water and volcanoes to meteorites and man-made mine fires. Without this additional knowledge, if you find pyrite, you will not understand the story it is trying to tell you. Other minerals are carried in extreme conditions by lightning strikes or directly from life on Earth, such as in bones or bird droppings. There are minerals that occur because of the oxygen that early bacteria pumped into Earth’s ancient atmosphere.
Hazen and Morrison wanted to create a next-level catalog that connected materials to their stories. “What we really wanted to do was bring context,” Morrison says.
Currently, there are several ways that researchers can find out where, when, and how a mineral formed. They can look at trace elements, which are extra bits of chemical and biological matter that are incorporated into a mineral from its surroundings. They could look at the ratio of different radioactive isotopes in a mineral, which, like carbon dating, can tell scientists how far back a given mineral can go. They may even think about the structure or color of a mineral; samples that are oxidized or rusted, for example, may change their appearance.
Armed with data science methods—commonly used today by biologists to analyze genomes and by sociologists to find groups of people in a social network—Morrison was able to correlate many of these factors and discover the formation histories of various minerals. It took her team 15 years to search thousands of minerals from around the planet and tag them with one of 57 different environments of formation, ranging from space minerals that predated Earth to minerals formed in human mines.
They have now transformed the IMA’s 5,800 species into more than 10,500 of what Morrison and Hazen call “mineral species.” A mineral can have many types if it has formed in several different ways.
Take a diamond for example. Chemically, it is one of the simplest minerals, made entirely of carbon atoms arranged in a cubic structure. But the new catalog lists nine different types of it: a diamond that was baked and pressed into the Earth’s mantle, a diamond that precipitated from a meteor impact, a diamond from carbon-rich stars before life ever existed, and many others.
In the revised Morrison and Hazen guide, about five-sixths of the IMA minerals come in only one or two types. But nine minerals actually branch into 15 types. And no mineral in the catalog has as many species as pyrite: 21.
In creating this scheme, Hazen and Morrison (both also of Curiosity The rover team) are looking far beyond Earth. If you find a mineral on another world and know where it formed, you can quickly figure out what kind of environment that planet had in ancient times. For example, if your mineral is among the 80 percent of species that originated in contact with water, then you may have evidence of a long-dead ocean.
And if your mineral is among the one-third of mineral types formed by biological processes, it could be a hint of long-extinct extraterrestrial life.
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“A new way of seeing minerals is emerging,” said Patrick Cordier, a mineralogist at the University of Lille in France, in a statement. “Minerals become witnesses, markers of the long history of matter.”
“You can hold a mineral that is hundreds of millions or billions of years old [old]. You can hold a meteorite that is 4.567 billion years old,” says Hazen. “There is no other tangible evidence for the earliest history of our solar system.”