Abiological formation of formic acid on rocks in nature

The formation of formic acid by ants is well known since the discovery by S. Fisher in 1670. In general, it has been believed that formic acid is not formed abiologically in nature. To evaluate the possibility of abiological formation of formic acid on silicate rock, a photochemical reduction of CO2 using silicate rock powders (amphibolite, gneiss, granite, granodiorite, quartzdiorite, shale and zeolite) suspended in water was investigated under sunlight at ambient temperature and pressure.It was found from the study that 4.6×10−6 g formic acid/m2 is formed daily on wetting-silicate rocks exposed to sunlight in nature. The range of the expected global production rate is between 2.5 and 3000 kt/a or more. Therefore, formic acid formed on wet-rocks might play an important role in the weathering of rocks.