Materials Science

[Recusant]

While the previous item shows an intriguing possibility for materials science, this one is just cool, in my opinion.

"Radical Theory Suggests Earthquakes Spark Gold Nuggets Into Existence" | Science Alert

New findings by scientists in Australia could challenge what we thought we knew about the way gold nuggets bloom in vast reefs beneath our feet.

Under pressures of hundreds of megapascals (tens of thousands of pounds per square inch) and boiling hot temperatures, water squeezed up from the depths of Earth's crust carries dissolved gases, metals, and minerals to the surface with every quake and shudder of a seismic event.

As any good prospector knows, buried seams of crystalized silicon dioxide – better known as quartz – are fertile ground for gold mining, with both materials precipitating out of solution under strikingly similar conditions.

Though the basic mechanisms behind the precious ore's formation have been understood for some time, certain details have never quite added up, and new research from scientists at Australia's Monash University, the CSIRO, and the Australian Nuclear Science and Technology Organisation challenge the conventional views on how gold forms.

[. . .]

Silicon dioxide is an incredibly unique material. Where other crystals are relatively symmetrical, quartz forms with a bias that produces a voltage when stressed – a phenomenon known as the piezoelectric effect.

With every tremor of Earth's crust, seams of quartz will crackle with static currents as voltages emerge and electrons rebalance.

This charge jump is unlikely to move very far given quartz is an insulating material. Gold, on the other hand, is a great conductor of electricity, raising the possibility that electrochemical reactions within quartz seams might serve as a catalyst, drawing sufficient gold from solution in concentrated spots through repeated cycles of tiny shakes.

[. . .]

What was simulated in the lab using concentrated solutions and extensive periods of shaking would of course take far longer in the real world with dilute solutions and occasional tremors.

On geological timescales, however, the process could be relatively rapid. Without the added zap of stressed quartz, it's difficult to even explain how gold might accumulate in such rich deposits in the first place.

[Continues . . .]

The paper is behind a paywall.

Abstract:

Gold nuggets occur predominantly in quartz veins, and the current paradigm posits that gold precipitates from dilute (<1 mg kg⁻¹ gold), hot, water ± carbon dioxide-rich fluids owing to changes in temperature, pressure and/or fluid chemistry.

However, the widespread occurrence of large gold nuggets is at odds with the dilute nature of these fluids and the chemical inertness of quartz. Quartz is the only abundant piezoelectric mineral on Earth, and the cyclical nature of earthquake activity that drives orogenic gold deposit formation means that quartz crystals in veins will experience thousands of episodes of deviatoric stress.

Here we use quartz deformation experiments and piezoelectric modelling to investigate whether piezoelectric discharge from quartz can explain the ubiquitous gold–quartz association and the formation of gold nuggets. We find that stress on quartz crystals can generate enough voltage to electrochemically deposit aqueous gold from solution as well as accumulate gold nanoparticles.

Nucleation of gold via piezo-driven reactions is rate-limiting because quartz is an insulator; however, since gold is a conductor, our results show that existing gold grains are the focus of ongoing growth. We suggest this mechanism can help explain the creation of large nuggets and the commonly observed highly interconnected gold networks within quartz vein fractures.