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 
Being less than of a rock jock than you, I risk great embarassment by naively stating:
1) A random distribution of the elements and minerals of our plant will ensure that there are areas rich and poor in each. A uniform distribution is not random.
2) Asking a geochemist would be a great idea as I highly suspect that they will not be able to answer what you truely seek. I am of the thought that geology (and all its subdisciplines) is a slow moving field (pun intended), with research dominated by practical applications, not the esoteric ideas that you are pursuing. You may need to ask quite a few geochemists before you find the right person and that will be part of the adventure. Or maybe I’m just spewing even more B.S. than usual?
1) True enough.
2) Actually, I have a special interest in Li, Be, & B. These elements are unusually scarce owing to the fact that they are spallation products and are the result of exposure of interstellar oxygen and other abundant heavy atoms to cosmic ray bombardment. What is of interest to me is how it came about that the elements Li, Be, & B find themselves concentrated in a few spots around the world.
It is an interesting exercise to trace the elements from industrial application through the mining stage and into the geochemistry of their migration. Some people collect stamps. This kind of thing interests me.
There is the possibility that the high concentrations are due to the impact of an asteroid composed of primordial material from before the heavier elements were synthesized.
The geometric shape that allows all of the mass to be as near the center of mass as possible is the sphere.
Locally… no. God is no damned good at anything. The shape of maximum surface gravitation for a homogeneous isotropic mass distribution (radius=R, spherical coordinates [R, theta, phi]):
Sphere, r(theta) = 2Rcos(theta)
Schmoo, r(theta) = 5^(1/3)Rsqrt[cos(theta)]
(6/5)(5/8)^(1/3) = 2.6% better
A point mass m_0 is the origin upon which a mass distribution’s surface is “resting.” The z-axis connects m_0 to the farthest point of the surface. A sphere would rest on the xy plane with the z-axis passing through its center.
Required is a surface of revolution about the z-axis for maximum gravitation on m0 at constant volume. If not, x,y-components divert the z-component. In spherical coordinates the surface is some function r = r(theta) independent of phi. Then, 50 lines re calculus of variations.
The moon is a treasure trove of minerals! Except it is all light elements and it is not differentiated.
Shit! I thought that was a pretty safe statement. How would you describe the other shape?
Interesting. So water on Earth is really what makes mining practical (in the sense of locating a mine where there are concentrations of materials of interest?) Does this mean that a body with no hydrothermal activity (e.g. the moon) won’t have mineral “deposits”, per se?
The big surprise for me (a non-geologist) is the extent to which hydrothermal action is implicated in a large variety of ore bodies. For instance, the big borax deposit in Boron, California, is an evaporite, meaning it is a deposit of precipitate resulting from the evaporation of ground water that pooled at the surface. Over time it was covered in sediment. Gold and silver veins in Colorado are thought to have been put in place by hydrothermal action.
It may be that there are non-aqueous processes that result in elemental concentration on dry bodies like the moon. But the reality check for space pioneers relying on mining on the moon is this- self sustainable mineral extraction off earth may be harder than you think.
Pegmatites are a curious group of minerals that are often associated with elemental anomolies, i.e., spodumene (lithium enriched). At present I do not know if hydrothermal action is part of their formation.
“It may be that there are non-aqueous processes that result in elemental concentration on dry bodies like the moon. But the reality check for space pioneers relying on mining on the moon is this- self sustainable mineral extraction off earth may be harder than you think. “
This seems to argue against my first point from yesterday, althoguh that doesn’t bother me at all.
Maybe it is a matter of scale. Even on the moon, at a fine enough scale, you don’t see a uniform distribution of elements and minerals. Perhaps the hydrothermal action (that I also was unaware of being critical to certain ore formations) is needed for a large scale disruption to uniformity. NASA has (literally) only scratched the surface of the moon so it is too early to tell much of its distributions.
The Wikipedia article on lunar geology describes a nonuniform surface and core, although it makes only qualitative statements.
Regardless of my babble (previous and appropriately described as bovine hindgut fermentation), I do find this quite interesting. Thanks for bringing it to my (our?) attention.
Hi John, I’ve no doubt at all that the lunar surface is non-uniform from the perspective of mineral content. But what defines an “ore body” is that it contains an economical concentration of some desired material. That is, the metal desired can be economically recovered at a profit. An occurance does not an ore body make.
While it may be physically possible to recover Iridium from the KT boundary- a type of “deposit”-, it hardly seems a profit making exercise. If you want to make a profit in Ir, you have to have access to something like the Bushveld Complex in South Africa where PGM’s are anomolously concentrated.
My point with the moon is that much of our earthly experience with mining is based on vein and placer deposits that are often the result of concentration by hydrothermal action. While the atoms of useful metals are surely present on the moon or an asteroid, the issue of recoverable concentrations seems very questionable for inhabitants hoping to be self-sufficient off earth. Somebody is just going to have to do a survey.
Got it. Sometimes I’m even more dense than the rocks.
On a not so unrelated vein, there’s this new report on the role of hdyrothermal water on the oxidation of iron in the mantle.
The shmoo (no “c” therein, my bad) is a bit apple-shaped. The sweet spot is the stem hole, but not depressed. Or like a cardioid without the dimple,
Moving some mass “up” a bit toward the sweet spot cancels radially. It does the magic orthogonal to the surface. The gain for being closer exceeds the loss for not being farther away.