The influence biofilm medium chemistry on calcium oxalate biomineralization

It was experimentally shown that the chemical composition of biofilms on the surface of carbonate and other Ca-bearing rocks significantly affect the ratio between crystallizing calcium oxalates (whewellite Ca(C₂O₄)·H₂O and weddellite Ca(C₂O₄)·(2.5-x)H₂O) and their morphology.

It was established that metastable weddellite crystallize in the range of stable whewellite, provided that calcium concentration in the initial solution is higher than the concentration of oxalate ions, or with the addition of small amounts of organic acids (citric malic, succinic, and fumaric). Without whewellite, weddellite formed in more alkaline conditions (at pH>5.7). In the presence of citric acid, weddellite formed almost solely (as small dipyramidal crystals). With the addition of malic, succinic, or fumaric acids to the medium (separately or together), the amount of weddellite crystalls decreased, but the size of its crystals increased. The simultaneous presence of a number of chemical impurities typical of biofilms (K⁺, Mg²⁺, Sr²⁺, Cl^-, SO₄^2-, CO₃^2-, PO₄^3-) in the crystallization medium in addition to citrate ions, also lead to the formation of practically monophasic weddellite, represented by very small, poorly faceted, isometric crystals. The addition of these components individually or in pairs lead to a decrease in the amount of crystallizing weddellite relative to whewellite (especially in the presence of phosphate, carbonate, or strontium ions) with a simultaneous increase in the size of weddellite crystals.

The added micro components actively interact with crystallizing oxalates, either replacing Ca²⁺ ions or adsorbing on the faces of growing crystals. The presence of two series of solid solutions, isomorphous (Ca,Sr)[C₂O₄]·(2.5–x)H₂O) (sp.gr. I4/m) and isodimorphous Ca[C₂O₄]·H₂O(sp.gr. P₂₁/c)−Sr[C₂O₄]·H₂O(sp.gr. P ̅1) was experimentally proven for the first time. Additionally the presence in the medium of strontium (together with phosphate), potassium (only in significant amounts), as well as carbonate, magnesium and sulfate ions add tetragonal prism facets to the morphology of dipyramidal weddellite crystals, which can be explained by their selective adsorption on the edges of the latter.