Evrim Yazgin
Cosmos science journalist
New research suggests that electricity generated by earthquakes might be the key to the formation of giant gold nuggets.
The study by geologists at Melbourne’s Monash University is published in Nature Geoscience today.
Humans have prized gold for millennia. It’s shine, hue and relative rarity have turned it into a valuable commodity which has formed the basis of economies and a sign of wealth.
Beyond jewellery and a store of value gold is also a critical component of computers and communications equipment.
According to the US Geological Survey, 244,000 tons of gold have been discovered to date – about 57,000 of this is still in underground reserves.
The total amount of gold found would fit into a cube with 23-metre sides.
It’s no wonder, then, that humans have spent so much time trying to figure out how to produce gold. Alchemists for centuries believed that they could turn lead – atomic number 82 – into the precious element gold – atomic number 79.
Scientists more recently have tried to explain how gold is produced in nature. Some have even been able to produce gold nanoparticles in the lab.
“The standard explanation is that gold precipitates from hot, water-rich fluids as they flow through cracks in the Earth’s crust,” says Monash researcher Chris Voisey, lead author of the new study. “As these fluids cool or undergo chemical changes, gold separates out and becomes trapped in quartz veins.”
“While this theory is widely accepted, it doesn’t fully explain the formation of large gold nuggets, especially considering that the concentration of gold in these fluids is extremely low.”
The researchers sought to understand better how large gold nuggets form through testing a new theory: piezoelectricity.
The piezoelectric effect was first discovered in 1880 by French brothers and physicists Jacques and Pierre Curie – the latter being the husband of physicist Marie Curie. Piezoelectricity occurs when certain solid materials can convert mechanical energy into electrical energy, or vice-versa.
Some crystals, like quartz, can generate a piezoelectric voltage when a mechanical stress is applied. This effect is commonly found in quartz clocks and BBQ lighters.
Voisey’s team wondered if the mechanical stresses produced during earthquakes could see piezoelectricity produced in quartz, leading to the electrical and chemical changes required to make large gold nuggets.
To test their idea, the team tried to replicate the conditions quartz might experience during an earthquake.
The quartz crystals were submerged in a gold-rich fluid and stress applied using a motor. The quartz samples were then studied under a microscope.
“The results were stunning,” says co-author Andy Tomkins. “The stressed quartz not only electrochemically deposited gold onto its surface, but it also formed and accumulated gold nanoparticles. Remarkably, the gold had a tendency to deposit on existing gold grains rather than forming new ones.”
This is because quartz is an electrical insulator while gold is a conductor. The result is that gold deposited on the quartz due to piezoelectric voltages become a pole of attraction for other dissolved gold particles.
“Our discovery provides a plausible explanation for the formation of large gold nuggets in quartz veins,” Voisey says. “In essence, the quartz acts like a natural battery, with gold as the electrode, slowly accumulating more gold with each seismic event.”
