Joanna Thompson writes:
A new origins-based system for classifying minerals reveals the huge geochemical imprint that life has left on Earth. It could help us identify other worlds with life too.
The impact of Earth’s geology on life is easy to see, with organisms adapting to environments as different as deserts, mountains, forests, and oceans. The full impact of life on geology, however, can be easy to miss.
A comprehensive new survey of our planet’s minerals now corrects that omission. Among its findings is evidence that about half of all mineral diversity is the direct or indirect result of living things and their byproducts. It’s a discovery that could provide valuable insights to scientists piecing together Earth’s complex geological history—and also to those searching for evidence of life beyond this world.
In a pair of papers published on July 1, 2022 in American Mineralogist, researchers Robert Hazen, Shaunna Morrison and their collaborators outline a new taxonomic system for classifying minerals, one that places importance on precisely how minerals form, not just how they look. In so doing, their system acknowledges how Earth’s geological development and the evolution of life influence each other.
Take, for example, pyrite crystals (commonly known as fool’s gold). “Pyrite forms in 21 fundamentally different ways,” Hazen said. Some pyrite crystals form when chloride-rich iron deposits heat up deep underground over millions of years. Others form in cold ocean sediments as a byproduct of bacteria that break down organic matter on the seafloor. Still others are associated with volcanic activity, groundwater seepage, or coal mines.
“Each one of those kinds of pyrite is telling us something different about our planet, its origin, about life, and how it’s changed through time,” said Hazen.
For that reason, the new papers classify minerals by “kind,” a term that Hazen and Morrison define as a combination of the mineral species with its mechanism of origin (think volcanic pyrite versus microbial pyrite). Using machine learning analysis, they scoured data from thousands of scientific papers and identified 10,556 distinct mineral kinds.
Morrison and Hazen also identified 57 processes that individually or in combination created all known minerals. These processes included various types of weathering, chemical precipitations, metamorphic transformation inside the mantle, lightning strikes, radiation, oxidation, massive impacts during Earth’s formation, and even condensations in interstellar space before the planet formed. They confirmed that the biggest single factor in mineral diversity on Earth is water, which through a variety of chemical and physical processes helps to generate more than 80 percent of minerals.
But they also found that life is a key player: One-third of all mineral kinds form exclusively as parts or byproducts of living things—such as bits of bones, teeth, coral, and kidney stones (which are all rich in mineral content) or feces, wood, microbial mats, and other organic materials that over geologic time can absorb elements from their surroundings and transform into something more like rock. Thousands of minerals are shaped by life’s activity in other ways, such as germanium compounds that form in industrial coal fires. Including substances created through interactions with byproducts of life, such as the oxygen produced in photosynthesis, life’s fingerprints are on about half of all minerals.
One implication of Hazen and Morrison’s findings is that our watery, living planet is probably much richer in mineral diversity than other rocky bodies in the solar system. “There are many minerals that simply couldn’t form on Mars,” said Hazen. “It doesn’t have penguins pooping on clay minerals, it doesn’t have bats in caves, it doesn’t have cactuses that are decaying or things like that.”
Hazen believes that the new taxonomy might even help with detecting life on planets around distant stars. Light from exoplanets detected by the James Webb Space Telescope and other sophisticated instruments could be analyzed to determine the chemical composition of their atmospheres; based on the measurable oxygen content, the presence or absence of water vapor, relative carbon concentrations and other data, researchers could try to predict what kinds of minerals would be likely to form from light-years away.
“In a really zoomed-out, broad-scale way, we are understanding not just our planet [but also] our entire solar system, and potentially solar systems beyond,” Morrison said. “That’s really incredible.”Nautilus