Crystal-Growth Behavior in Ca-Mg Carbonate Bacterial Spherulites

Spherulites composed of aragonite, magnesian calcite, and calcian-magnesian (-manganoan) kutnahorite-type carbonates were precipitated by two halophilic bacterial strains in porous solid as well as liquid media at high salinity. Although Mg and Ca are geochemically similar elements, Ca is preferentially incorporated into aragonite structures in liquid media whereas Mg remains in the solution and/or precipitates to form struvite crystals. In solid media, crystal growth features clearly correlate with reticular parameters and the Mg content of the Ca-Mg and Ca-Mg(Mn) carbonates. The increased salinity in these media leads to the incorporation of Mg (and Mn) into the carbonate structure under growth conditions farther and farther from equilibrium. Although calcite is the stable phase in the Earth surface environments, carbonates denser than pure calcite, like aragonite and Mg-rich calcite, are kinetically favored in the studied bacterial precipitates.