From the copper-stained rocks of the Grand Canyon to the newly discovered 10-meter-long crystals of calcium sulfate under Naica Mountain in Mexico, the vast majority of Earth's minerals owe their existence to life, say researchers who have put together the first comprehensive chronology of what they call mineral evolution. Biological and mineral evolution have been inseparable since the planet's beginnings, the scientists argue, and understanding that connection should provide new insights in both fields and critical clues in the search for life on other planets.
When Earth was young, its surface sported all 94 of the natural elements in the periodic table but very few minerals. Today, Earth teems with thousands of kinds of minerals, from agates to zircons. What happened in between? Some minerals were forged by volcanic and tectonic forces that have ranged over the planet's 4.5-billion-year history. Mineralogists have long known that other minerals resulted from organisms, like apatite in teeth, but up to now no one had attempted to establish the scale of biological influence.
So geochemist Robert Hazen of the Carnegie Institution of Washington in Washington, D.C., and colleagues traced the origins of nearly 3000 common terrestrial minerals--such as turquoise, azurite, and malachite--to about a dozen ancient minerals. All of these precursors were used, either directly or indirectly, by living organisms to produce all the others, starting with the actions of photosynthetic microbes that first appeared about 2.5 billion years ago. "We argue that there are literally thousands of different minerals that formed only because life produced an oxygen-rich atmosphere," says Hazen, whose team reports its results today in American Mineralogist.
But biochemistry goes far beyond oxygen. "Life seems to exploit every possible chemical pathway to obtain energy and make carbon-carbon bonds," says Hazen. For example, a newly discovered mineral called hazenite, named for Hazen, forms only because of phosphate produced by a microorganism that lives in the acidic waters of Mono Lake in California. And life has sometimes received a leg up from minerals, too. Mineralogist Peter Heaney of Pennsylvania State University, State College, who was not involved in the research, notes that calcium carbonate and other minerals buffered the acidity of early oceans, helping to make them habitable. He says the paper provides "a novel way of considering minerals in the context of Earth's history."
The results are bound to help the search for life on other planets, because it might be possible to identify former--or even existing--habitats by detecting the telltale spectrographic signatures of these minerals in light from distant planets (ScienceNOW, 13 November).
"I am very excited about this paper, not only as a mineralogist but as a teacher and curator," says David Saja of the Cleveland Museum of Natural History in Ohio. Instead of learning minerals by rote memorization of chemical classes, he says, students can use the new scheme created by Hazen to "put them into a large-scale context that clearly shows how they are part of the history of the Earth and universe around us." Mineralogist Jeffrey Post of the Smithsonian Institution in Washington, D.C., agrees. The research provides a better framework in which to think about minerals--"a fresh perspective from the strictures of the traditional classification scheme," he says.