One of the reasons gold is so valuable is that it has very low reactivity. When you make something with gold, it retains its shine. Even if it does react with other substances, gold is almost insoluble, and this combination makes it difficult to refine from other substances. This is one of the reasons why most of the gold we extract comes from deposits where the gold exists in large chunks, some of which can weigh hundreds of kilograms.
If you read the previous paragraph carefully, you might have noticed a problem here: If gold is so difficult to obtain in its pure form, how could natural processes produce such huge chunks? On Monday, a group of Australian researchers published their hypothesis and supporting evidence. They propose that the piezoelectric effect caused by earthquakes essentially electroplates the gold onto quartz crystals.
hypothesis
About 75 percent of all gold harvested by humanity comes from so-called orogenic gold deposits. Orogeny is the term used to describe the tectonic process that creates mountains, and orogenic gold deposits form in layers where two rocks move past one another. These areas are often filled with hot hydrothermal fluids, and the heat increases gold’s solubility from “almost nonexistent” to “very low,” typically less than one milligram per liter of water.
Another remarkable thing about these deposits is that they are commonly associated with the mineral quartz, a crystalline form of silicon dioxide. This fact became the basis for a new hypothesis that brought together several topics that are generally thought to be largely unrelated.
It turns out that quartz is the only abundant mineral that is piezoelectric; that is, it generates an electric charge when subjected to pressure. You don’t need to understand why this is the case to follow this hypothesis, but the researchers’ explanation of the piezoelectric effect is surprisingly convincing and clear, so I’ll quote it here for those who want to learn something from this study: “Quartz is the only common mineral that forms crystals that have no center of symmetry (asymmetric center). Asymmetric center crystals that are distorted under stress have an imbalance in their internal electrical configuration and generate an electric potential (or voltage) that is directly proportional to the mechanical force applied across the crystal.”
Because quartz is an insulator, this electrical potential does not easily dissipate by itself. However, the potential can be removed by transferring electrons to or from any substance (including liquids) that comes into contact with the quartz crystal. In effect, the charge can drive redox reactions (reduction/oxidation) in nearby liquids, neutralizing dissolved ions and causing them to come out of solution.
This may be self-reinforcing: as small metal deposits form on the quartz surface, they facilitate electron exchange with the liquid in its immediate vicinity, leading to the deposition of even more metal in the same location. This also reduces the concentration of the metal in the nearby solution, facilitating the diffusion of additional metal ions to that location, so the liquid itself doesn’t have to keep circulating in the same place.
Finally, this concept also requires a source of strain to produce the piezoelectric effect in the first place, but remember that this is all happening in active fault zones, so there is no shortage of strain.
And the evidence
Figuring out whether this kind of thing happens in active fault zones is incredibly difficult for a variety of reasons, but it’s relatively easy to soak some quartz in a gold-containing solution and see what happens, so that’s the latter route the Australians took.
The gold was provided in the form of a solution of gold chloride ions or a suspension of gold nanoparticles. The quartz crystals were either pure quartz or taken from gold-rich areas that already contained small deposits of gold. The crystals themselves were subjected to strain at frequencies equivalent to those produced by small earthquakes, and the experiment was run for one hour.
Whether from dissolved gold or suspended gold nanoparticles, one hour was enough to form small gold deposits on the pure quartz crystals. In the case of naturally formed quartz, the gold was deposited in existing locations where gold metal was present, rather than forming additional deposits.
The researchers note that much of the quartz in the deposits is disordered, rather than single crystalline. Disordered material has many small randomly oriented crystals, and the piezoelectric effect of any one of these crystals is usually countered by its neighbors. Gold will therefore preferentially form on the single crystals, which may explain why it is found in large clumps in these deposits.
So this is a pretty plausible hypothesis: it explains something puzzling, relies on established processes, and has some experimental support. Since activity on the active faults is likely to remain slow and hard to access, the next step would be to get long-term information on the deposition rates by this process and to physically compare these deposits with those found in natural environments.
Nature Geoscience, 2024. DOI: 10.1038/s41561-024-01514-1 (About DOIs)
