Thermal stability,
structural evolution pathways, and phase transition
mechanisms of the calcium oxalates whewellite (CaC2O4·H2O), weddellite (CaC2O4·(2+x)H2O), and caoxite
(CaC2O4·3H2O) have been
analyzed using single crystal and powder X-ray diffraction (XRD).
During single crystal XRD heating experiments, α-CaC2O4 and the novel calcium oxalate monohydrate have been
obtained and structurally characterized for the first time. The highest
thermal expansion of these compounds is observed along the direction
of the hydrogen bonds, whereas the lowest expansion and even contraction
of the structures occur due to the displacement of neighbor layered
complexes toward each other and to an orthogonalization of the monoclinic
angles. Within the calcium oxalate family, whewellite should be regarded
as the most stable crystalline phase at ambient conditions. Weddellite
and caoxite transform to whewellite during dehydration-driven phase
transition promoted by time and/or heating.
The effect of bacteria that present in the human urine (Escherichia coli, Pseudomonas aeruginosa, Klebsiella pneumoniae, and Staphylococcus aureus) was studied under the conditions of biomimetic synthesis. It was shown that the addition of bacteria significantly affects both the phase composition of the synthesized material and the position of crystallization boundaries of the resulting phosphate phases, which can shift toward more acidic (struvite, apatite) or toward more alkaline (brushite) conditions. Under conditions of oxalate mineralization, bacteria accelerate the nucleation of calcium oxalates by almost two times and also increase the amount of oxalate precipitates along with phosphates and stabilize the calcium oxalate dihydrate (weddellite). The multidirectional changes in the pH values of the solutions, which are the result of the interaction of all system components and the crystallization process, were analyzed. The obtained results are the scientific basis for understanding the mechanisms of bacterial involvement in stone formation within the human body and the creation of biotechnological methods that inhibit this process.
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