The formation mechanism of sedimentary dolomite has remained a persistent mystery in Earth sciences. While microbial extracellular polymeric substances (EPS), particularly their polysaccharide components, are recognized to facilitate protodolomite formation by overcoming kinetic barriers associated with Mg²⁺ hydration, the underlying mechanisms remain debated. Prevailing hypotheses attribute this catalytic effect to negatively charged functional groups in polysaccharides, suggesting they promote Mg²⁺ dehydration via chelation. However, direct experimental evidence supporting this mechanism has been lacking. To test this hypothesis, we examined the influence of polysaccharide surface charge on the Ca-Mg carbonate crystallization at ambient conditions, comparing negatively-charged xanthan gum with neutrally-charged guar gum. Contrary to conventional expectations, our results demonstrated that protodolomite formation occurred at room temperature irrespective of polysaccharide charge. Instead, solution chemistry emerged as the primary factor controlling Mg incorporation into carbonates. Furthermore, polysaccharides promoted the formation of amorphous calcium-magnesium carbonate (ACMC), which served as a metastable precursor in a non-classical nucleation pathway. The stabilization of ACMC by polysaccharide suppressed dissolution-reprecipitation processes and enabled the transformation into protodolomite with negligible Mg loss. These findings suggest that a wider range of microbial or organic materials could have contributed to low-temperature dolomite precipitation in natural settings. This expands our understanding of microbial influences on carbonate mineralogy and implies that organic matter-rich environments may have served as significant but previously overlooked locations for dolomite formation throughout Earth's history.

Dolomite problem,Microbial EPS,Polysaccharides,Amorphous calcium-magnesium carbonate (ACMC),Non-classical crystallization