The Evolution of Minerals

The Earth's Minerals have evolved over time, with perhaps two thirds of all minerals owing their existence to the effects of living organisms.

Robert Hazen (et al)¹ describe the evolution of the various minerals from a few initial minerals to the thousands known today as conditions have changed over the Earth's history.

In the beginning of our solar system, only a few minerals existed, perhaps only about 60, all found today in the most primitive asteroids and comets.

A dozen of these have been identified in Pre-solar grains which condensed from solar nebulae (ours or the outflows from other stars). Minerals include several elements (graphite, diamond, iron), and simple oxides (magnetite, corundum), sulfides (pyrite), nitrides, and even carbides (silicon carbide, titanium carbide), phosphides (such as schreibersite (Fe,Ni)₃P), and silicides (such as xifengite, Fe₅Si₃).

CAIs (Calcium-Aluminum-rich Inclusions) also predate the birth of our solar system by perhaps 2 million years. These contain additional minerals, including anorthite (CaAl₂Si₂O₈), hibonite ((Ca,Ce)(Al,Ti,Mg)₁₂O₁₉), melilite (Ca₂Al₂SiO₇), perovskite (CaTiO₃), pyroxenes such as hedenbergite(CaFeSi₂O₆) and diopside (CaMgSi₂O₆), spinel (MgAl₂O₄), and additional forsterite-rich olivine ((Mg, Fe)₂SiO₄, but very little iron is present). CAIs are somewhat equivalent to the chondrules that comprise the bulk of many asteroids, but they are high in aluminum and calcium, and quite low in iron, with distinctive isotopic signatures which imply a different supernova source than the bulk of our solar system's materials.

Chondrules (millimeter-size spherical grains characteristic of asteroids called chondrites) likely condensed from within our own solar nebula, and show evidence of flash heating to a liquid state followed by rapid cooling. This supported the formation of additional minerals (primarily silicates), raising the total to perhaps 60 minerals. Type 1 chondrules appear to have formed under reducing conditions and are mostly fosterite (Mg₂SiO₄) and enstatite (MgSiO₃) with metallic iron or iron sulfides such as troilite (FeS). Type 2 chondrules formed under oxidizing conditions and are composed primarily of olivine ((Mg,Fe)₂SiO₄) and hypersthene ((Mg,Fe)SiO₃), with iron oxides such as magnetite (Fe₃O₄). In both types, additional minerals (generally silicates) are found. The chondrules are cemented together by a mixture of additional nebular material which also incorporated pre-solar grains.

Additional compounds undoubtedly existed, but conditions did not favor the differentiation/isolation of compounds, let alone the growth of crystals.

As these chondrules (etc) accumulated into larger bodies, interactions between them expanded the range of minerals to include several hundred minerals, including many additional silicates, and possibly some hydroxides. These are found in the asteroids known as achondrites.

As bodies grew large enough to retain heat of impact and/or from radioactive decay, additional metamorphosis of minerals became possible. Planetary embryos are large enough to support differentiation, where dense components such as metals gravitationally migrate to the core, leaving lighter minerals (often silicates) as a planetary crust and intermediate density minerals (such as olivine) in the middle. The process implies enough heat to melt silicates into a liquid magma which might slowly cool forming primitive granites. These bodies may have also supported liquid water in their interiors for extended times, allowing the formation of hydrated minerals such as clays. The net effect is that several hundred mineral species were formed as the conditions allowed the slow growth of additional minerals as magma bodies cooled as granites, or as wet conditions supported the solution of primordial minerals and subsequent crystallization of new minerals. Mars and Venus appear to contain 500 or more distinct minerals, and may have only reached this level of mineralogical evolution.

On the Earth, the development of plate tectonics and the presence of large volumes of liquid water produced conditions allowing even more minerals: the metamorphic minerals and sedimentary minerals that crystallize from a water solution. These require conditions of reprocessing of minerals under intense heat and pressure, and include metamorphic minerals such as the garnets. The early Earth thus supported the development of more than a thousand different mineral species, possibly as many as 1500.

And then, life evolved. Life processes allow conditions very far from equilibrium to exist, including the accumulation of free oxygen in the atmosphere. Fully two-thirds of the minerals we recognize are oxides formed during the weathering of other minerals, or sulfates, phosphates, and carbonates that are favored under oxidizing conditions. But some minerals directly owe their existence to life, including aragonite.

At present, mineralogists have identified (and recognize) approximately 4,500 minerals on Earth. But there will be more in the future. Perhaps new minerals will be recognized that owe their existence to humanity, as we often create conditions even further from equilibrium in support of the industrial and agricultural processes than green algae did several billion years ago. I've already thought of a few names to propose: trashheapite and landfillite, or perhaps polyesterite!

In Summary:

> 4.56 Ga: 60 species of minerals (mostly as chondrules & pre-solar grains - materials found in chondrite meteorites)

4.56 - 4.55 Ga: 250 species of mineral (achondrites and iron)

4.55 - 4.0 Ga: 350 species of minerals (igneous rock)

4.0 - 3.2 Ga: 1000 species of minerals (granitoids, primitive granites)

3.2 - 2.8 Ga: 1500 species of minerals (new sulfate ores, due to plate tectonics and first life)

2.5 - 1.9 Ga: 4000 species of minerals (great oxidation)

0.54 > now: 4500 species of minerals (calcites, dolomite, opal and clays)

¹ Robert M. Hazen, Dominic Papineau, Wouter Bleeker, Robert T. Downs, John M. Ferry, Timothy J. McCoy, Dimitri Sverjensky and Hexiong Yang. Mineral evolution. American Mineralogist, 2008