Minerals evolved along with life

Think evolution is something that happens only to plants and animals? Think again, say scientists who point out that evolution — change through time — happens in the mineral kingdom as well. As our solar system has aged, the number of types of minerals it contains has burgeoned from only a dozen or so to well over 4,000.

And about two-thirds of today’s minerals either directly or indirectly evolved thanks to the presence of life on Earth.

Minerals don’t mutate, reproduce or compete with one another like living organisms. Nevertheless, the variety and relative abundance of minerals on Earth have changed dramatically throughout the planet’s 4.5-billion-year history, says Robert Hazen, a geophysicist at the Carnegie Institution for Science in Washington, D.C. He and his colleagues chronicle the long-term growth in Earth’s mineral complexity in the November-December American Mineralogist.

Not only does a mineral have a specific chemical formula, it also has a distinct crystalline structure, says Hazen. This arrangement of atoms can have a profound effect on a mineral’s properties: While diamond, one form of pure carbon, is the hardest-known naturally occurring mineral, graphite, which has another arrangement of carbon atoms, is one of the softest.

Billions of years ago, the solar system was nothing more than a cloud of gas and dust. Although that material contained all of the naturally occurring elements found in the periodic table, most of those elements were too rare or too widely dispersed to significantly affect the crystal structure of the first few minerals that coalesced, says Hazen. In all, the tiny particles of interstellar dust that populated our nascent solar system — like those that still drift through space today, where orbiting probes can snatch them and return them to Earth for analysis— contained only a dozen or so minerals.

These dust particles, primarily made of carbide, oxide, nitride and silicate minerals, were the raw materials of today’s planets, the researchers note.

As the dust clumped into larger masses and the sun ignited, the changing conditions spawned new minerals. In the billions-of-years-old, meteorite-sized masses that fall to Earth today — which presumably reflect the composition of those that formed early in the solar system’s history — scientists have identified about 60 minerals, says Hazen. About 20 of those are found only in crystals a few micrometers across or smaller, he notes. Some of these minerals probably derive from the intense heat and pressures generated when the objects crashed into each other in space.

Once protoplanets grew to diameters of 200 kilometers or larger, the radioactive elements trapped within those masses generated enough internal heat to melt minerals at the core. After that, says Hazen, minerals began to segregate, with lighter minerals rising to the surfaces of the planets and denser ones sinking into the cores. About 10 million years after it formed, Earth probably contained around 250 minerals, the researchers speculate. “Four billion years ago, the world’s minerals were radically different than they are today,” Hazen notes.

On small planets, mineral evolution grinds to a halt early. On planets with enough gravity to trap volatile substances like water and carbon dioxide, geological processes like plate tectonics or hydrothermal activity can provide a chemical crucible that creates hundreds more minerals. The first life to arise on Earth —probably methane-generating organisms — didn’t significantly increase the planet’s mineral diversity, because that gas was already common in the atmosphere, says Hazen. About 2.5 billion years ago, Earth likely sported a total of around 1,500 minerals.

Then came oxygen-generating cyanobacteria, and another spurt of mineralogical evolution. Today, scientists have identified about 4,300 types of minerals, and they discover dozens more each year. Of those minerals, around two-thirds are the result of oxidation brought about, either directly or indirectly, by processes such as photosynthesis. The presence of any such minerals on other planets could be a sign that life once flourished there, Hazen and his colleagues contend. Similarly, when the Opportunity rover discovered the mineral jarosite on Mars, scientists hailed the find as a strong clue that liquid water once flowed there (SN: 3/6/04, p. 147).

The researchers’ new approach goes beyond merely categorizing minerals — sulfides in one drawer and oxides in another, for example — and considers how minerals are related to each other. “Looking at minerals from the broad perspective, in terms of planetary geology, is an interesting way of thinking about them,” says Jeffrey Post, a geologist at the Smithsonian Institution’s National Museum of Natural History in Washington, D.C. “This will give students a different context of how minerals form and how they evolved, not just what they are,” he notes.

The team’s new paper “is a remarkable article,” says Gary Ernst, a geologist at Stanford University. “The scope and scan of this [paper] is what’s amazing,” he adds.

Carl A. Francis, curator of the Mineralogical Museum at Harvard University, agrees. The team’s new analysis is the result of “an innovation in thinking,” he adds. “This is the first change in the way that geologists look at minerals in more than two centuries…. The really neat thing is, it makes sense to someone for whom minerals aren’t fascinating and important.”