Some acquaintances have asked about my new interest in geology. What’s the deal with rocks and mining?
What interests me is not so much the economic value and extravagant production of certain minerals and precious metals. What is of interest is the question of how it came about that there is such a thing as an ore body. An ore body is a geological formation which is defined by a localized concentration of certain substances. How does it happen that chemical elements can become concentrated from a more distributed condition?
Celebrity astronomers are often seen on cable channels pedantically nattering on about “Star Stuff”. OK, Dr. Skippy, what is star stuff and what does it do? What are the particulars about the local star stuff, ie., the earth? This is the realm of cosmochemistry and geochemistry- elective classes the TV glamour boys apparently skipped.
The nucleosynthesis of the heavy elements (C to U) and their subsequent ejection from exploding stars is an inherently dispersive process. Eventually, here and there, some heavy matter will aggregate to form a protoplanetary cloud which can then produce planetary bodies. Inevitably, some of the heavy matter is pulled into massive bodies dominated by the presence of thermonuclear fuels- that is, hydrogen and helium. Sufficiently large accumulations of these two highly abundant elements will compress and initiate a self-sustaining fusion reaction of hydrogen to form the (n+1)th generation of stars. All told, some heavy matter accumulates to form of planetary bodies while some of it siphons into the next generation of stars.
It is within the ability of gravity to concentrate matter into smaller volumes of space as a dense, bulk phase. The geometric shape that allows all of the mass to be as near the center of mass as possible is the sphere. This is why we don’t see planets shaped like cubes, pyramids, or ponies.
Once cooled well below incandescence, the matter in a sufficiently constituted and situated planet may begin to self-organize into chemical phases. Along the lines of the Three Bears allegory, Earth is parked in an orbit that is just right for the presence of liquid water. Irrespective of the needs of life, liquid water is critical for the eventual concentration of some elements into ore bodies.
Earth has a gas phase blanketing a liquid phase which wets much of the bulk rocky phase of the planet. A generous portion of water circulates in the maze of fractured recesses of the planetary crust. In the case of Earth, we know that our planet has a fluid core within a solid shell. This molten phase in the core energizes a kind of convective heat engine that will drive the shuffling motion of tectonic plates and episodic volcanic mass transfer on the surface.
Matter has gravitationally self-organized at the planetary scale on the basis of density. But what is perhaps most interesting to a chemist is the phase composition of the planetary solid matter. On cooling, a body of magma will sequentially produce precipitates representing different chemical substances. Over geological time this igneous rock may experience modification by the hydrothermal action of hot water under high pressure. Depending on its circumstances, parts of the formation may be depleted of soluble constituents or it may receive a deposit of new mineral species.
On the scale of planets, the earth has self-organized into bulk phases of matter- Solid, liquid, and gas. But at a much smaller scale, the earth self-organizes into domains of chemical substances. This is evident by simple inspection of a piece of granite. A piece of pink granite shows macroscopic chemical domains of potassium feldspar, quartz, and mica. While these three mineral components of granite are compounds and not pure elements, they nonetheless represent self-organization of species based on chemical properties.
The forces that drive chemical differentiation in mineral formation are ultimately thermochemical in nature. Large differences in Ksp lead to partitioning and phase separation of distinct substances. Subsurface formations may be approximately adiabatic on a short time scale, but over deep time they can slowly cool and equilibrate to yield a sequence of fractional crystallizations of metal carbonates, oxides, silicates, and aluminates giving rise to a complex bulk composition.
Speaking only for myself, coming to an understanding of how mineral deposits form is a kind of hobby. If I wanted immediate answers to specific questions, I suppose the most expedient thing would be to consult a geochemist. But where is the adventure in that? The answers are not the fun part. The real adventure is in the struggle to find the best questions. As it often happens, once you can frame the problem sufficiently, the answer falls out in front of you. Whoever dies with the greatest insight wins.
Lamentations on Chemistry 