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The Thermodynamic Sinks of this World

What would an elemental soup cook up to?

Roald Hoffmann

Is this a Geochemical Question?

When I first posed the question of the thermodynamic equilibrium at a research group meeting, I thought I’d find a ready analysis in any geochemistry text. I found some (see, for instance, the last section in Konrad Krauskopf’s older Introduction to Geochemistry), but not much. The reason is that my question is a theorist’s dream, and geochemists are practical people. Here is what J. Donald Rimstidt of Virginia Tech told me:

The phase or substance that is stable at a given temperature and pressure is not just controlled by its free energy of formation, but it is affected by the bulk composition of the system. As a result, geochemists are concerned with elemental abundances and the physical processes that affect abundances. Once the elemental abundances for a region have been established, then we draw a box around that region and use equilibrium thermodynamics to predict the phase or species distribution. This local equilibrium assumption works well for some cases (especially at high temperature and pressure). At earth-surface conditions, where chemical reaction rates as well as mass transport rates can be slow, metastable conditions can persist regardless of the scale of the model domain.

His last point, the persistence of metastable molecules and minerals, is one I made above. More important, geochemists don’t worry about my castle-in-the-sky equilibrium world, because … the Earth (and every object in the universe) has a history. There is no infinitely replenishing cornucopia of all elements; even if we chose our region as big as the universe, the abundance of the elements is constrained by the physics of nucleosynthesis in the moments after the Big Bang, and in the subsequent more drawn out formation of heavier elements in stars and supernovae.

Remember those fluorides, the most stable of compounds? Why isn’t the world full of fluorides? Because the abundance of the element fluorine in the universe, in the Solar System, and in the Earth is relatively small.

A beautiful account of the evolution of minerals has been given by Robert M. Hazen, of the Carnegie Institution, and his coworkers. To quote the abstract of their paper:

The stages of mineral evolution arise from three primary mechanisms: (1) the progressive separation and concentration of the elements from their original relatively uniform distribution in the pre-solar nebula; (2) an increase in range of intensive variables such as pressure, temperature, and the activities of H2O, CO2, and O2; and (3) the generation of far-from-equilibrium conditions by living systems.

The first stage led to the approximately 250 minerals found in unweathered meteorite samples, the second to about 1,500 other minerals, and the third, directly or indirectly, to most of the Earth’s 4,200 known mineral species.

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