By Larry Larason
In 2008 Robert Hazen and seven colleagues presented the idea of mineral evolution. Minerals don’t mate, mutate, or pass on their characteristics to progeny, so how can we say that they evolve? It’s not that minerals evolved, exactly, but that the world evolved, changing the habitat of mineral formation, so that different minerals were formed. Consider this: planets and asteroids formed at the same time in our solar system; the planets are just accreted blobs of the material left over from the formation of the sun. Meteors are odds and ends of the material that is still being swept up and added to the planets. In meteorites we find about 250 minerals. The same is true of the moon’s surface, which has not been eroded or chemically altered in any significant way since it formed. The primal surface of Earth was probably very similar, with the same suite of 250 minerals, yet today at least 4400 minerals have been identified on Earth, although many of them are rare. How did this come about? Hazen, et al., proposed ten stages in Earth’s history; I’ll discuss a few of the high points.
We know that Earth has changed significantly since it formed. For example, during Archean times both the mantle and the crust were hotter than now, maybe by as much as 500o C. A type of magnesium-rich lava called komatiite formed in those days and can still be seen in very old rocks at a few places around the globe. Komatiite was so hot when it erupted that it flowed like water. The minerals in komatiite aren’t so unusual, but one of them, olivine, appears in an unusual form; instead of occurring in its usual stubby crystals, in komatiite it appears as feathery dendrites. Very nearly all komatiite formed more than three billion years ago. It is considered to be an “extinct” rock type.
The Earth has more water than any of the other rocky planets, and water plays a part in mineral evolution. By dissolving minerals it provides an opportunity for elements to recombine in new ways, as new minerals. Since there is evidence that Mars had surface water in an early eon, Hazen estimates that 500 minerals may be found on that planet.
But probably the greatest influence on Earth’s mineral evolution was caused by the evolution of life. Life appeared about 3.4 billion years ago, but it didn’t do much until a billion years later, when new forms of single celled organisms using photosynthesis to split water, began to fill the oceans and atmosphere with free oxygen. There was oxygen on Earth before then, of course, but it was locked up in rocks, water, and atmospheric carbon dioxide. The cyanobacteria, aka blue green algae, lived in shallow water. When they began emitting oxygen the seas were full of dissolved iron. The iron captured much of the first free oxygen, became iron oxide [hematite] and fell to the floor of the ocean to create banded iron formations. These deposits were common around 2 billion years ago. They are the greatest source of iron ore in the modern world. The time when they formed is termed the Great Oxidation Event [GOE]. Of course, it wasn’t an event in the sense of something that happened in 24 hours or less. It took maybe half a billion years for oxygen in the atmosphere to approach one percent of what we now consider normal. It took that much time for the iron in the sea water to be depleted so that O2 could build up in the air. Ironically, oxygen was poisonous to many of the anaerobic cyanobacteria and drove them to extinction.
Free oxygen allowed many more combinations of elements to form as minerals. Some of the most colorful ones we know today are seen only in rocks formed after the GOE, including the copper minerals turquoise, azurite, and malachite. More importantly, it allowed life to move onto land after oxygen in the atmosphere formed ozone [O3], which blocks much of the ultraviolet radiation [UV] put out by the sun. UV causes sunburn and can disrupt DNA molecules in both plants and animals. Without the filtering effect of ozone in the stratosphere, life on land would be deadly.
About the time that banded iron formations ceased growing in the sea, a new type of rock began appearing on land. These rocks are called “red beds.” The sandstone or shale red beds contain iron which has been oxidized – rusted. During the Carboniferous Period oxygen in the atmosphere may have exceeded thirty percent. The red beds deposited during that time of abundant atmospheric oxygen and in the succeeding Permian are often bright red. Consider our Red Rock Park just east of Gallup; the red Jurassic sandstones, those beautiful cliffs, would have been dull gray without atmospheric oxygen to rust the particles of iron contained among the grains of sand.
What about Mars, the red planet? The amount of oxygen on Mars is miniscule, but water vapor in the atmosphere loses some of its hydrogen to space, leaving a bit of oxygen to rust the planet’s surface. Hazen proposes that Mars is red only in a thin veneer on the surface
Hazen admits that nothing is new in his rewrite of mineralogy; almost all of it was known previously, except the concept of mineral evolution which ties it all together. And the insight that life and geology go hand in hand in shaping our world, making it a suitable place for our kind of life to thrive.