Smectite synthesis at low temperature and neutral pH in the presence of succinic acid

In some modern natural environments, authigenic clays occur in close spatial association with microbial cells and biofilms. These clays may be the result of a microbially influenced mineralization process. To test this hypothesis, we conducted laboratory experiments with organic molecules that are commonly produced by microbes and may promote mineral nucleation within biofilms. Solutions used for the experiments contained different organic acids (i.e., oxalic acid, succinic acid, citric acid, and EDTA), sodium silicate, and different cations (i.e., Al, Mg, Ca, K). The solutions, with a pH adjusted to 7, were incubated at 25 °C for 2.5 or 6 months. Precipitates showing a crystalline habit formed exclusively in the presence of succinic acid. In the other experiments, amorphous silica was the only precipitated phase. Transmission electron microscopy (TEM) observations including analytical electron microscopy (AEM) analyses and selected area electron diffraction (SAED) patterns indicate that the minerals formed in the presence of succinic acid are smectites with variable Al and Mg octahedral occupancy. Mg-rich smectites show a composition broadly similar to stevensite. Succinic acid is commonly produced by aerobic microbes as intermediate product of the tricarboxylic acid cycle. Thus, this biological degradation process might promote the formation of authigenic smectites in a large variety of natural environments. Our results may lead to a revised interpretation of some smectites present in ancient sedimentary rocks, which may have formed through a microbially influenced mineralization process similar to that simulated with our experiments.