Before humans built solar cells to turn light into electricity and maybe even before plants stored it as chemical energy, rocks were doing the same thing. The first examples of natural but non-biological conversion of light to electricity have been found, and it seems it is actually common, taking place on surface coatings of rocks found in Chinese deserts. The process may even play an important part in making the Earth habitable.
Many desert rocks have thin coatings of iron and manganese. While examining samples from the Gobi desert under bright light, Professor Anhuai Lu of Peking University found electric currents on the surface, but only in the vicinity of deposits of these metals. The weaker the light, the less current, demonstrating the coatings are turning the photons into moving electrons. The coatings are also quite stable, so generation probably lasts all day.
However, don't imagine you could charge your phone off rocks if trapped in the desert with a flat battery. The currents are tiny, Lu reports in Proceedings of the National Academy of Sciences, even when the rocks are exposed to light several times more powerful than the Sun.
Less than 1/10,000th of the photons in the sunlight are converted to moving electrons by the manganese-rich coatings, and those heavy in iron alone do even worse. That compares with commercial solar cells, which get close to 100 percent at the right wavelengths.
On the other hand, if these varnishes cover much of the 35 million square kilometers (13 million square miles) of deserts worldwide, this could make cumulative effects substantial. Humans may have no way to harness this resource, but photoelectrotrophic bacteria are a different matter. Other researchers have stimulated the growth of certain bacteria using minerals that convert sunlight to electricity.
There’s always a hope discoveries like this could take the search for better solar panels down new and better paths. That may be a vain hope given the low efficiency, but the findings may have other uses. Some of the bacteria Lu thinks are feeding off these currents draw carbon dioxide from the air and turn it to acetate, although we don't yet know if this represents a major carbon sink.
Lu found that manganese concentrations in these veneers can be 100 times those in the body of the rock, but only on the surfaces exposed to sunlight. Experiments showed manganese-rich coatings grow quickly under strong visible or ultraviolet light, but slowly, if at all, in shade. There could, therefore, be a cycle where sunlight stimulates manganese-rich varnishes, which then increase energy production. These manganese-rich coatings have sufficient chemical potential to convert water to oxygen, although again we don’t know how globally significant this is.