MS17-P04 Structural evolution of hydrated calcium oxalatesCalcium oxalates are represented in nature by three hydrated forms: whewellite (CaC2O4·H2O; COM), weddellite (CaC2O4·(2+x)H2O; COD) and caoxite (CaC2O4·3H2O; COT). Calcium oxalates are also found among the pathogenic mineral precipitates in human bone marrow, myocardium, joints, lungs, liver, thyroid gland, intestinal mucosa, eyes, and urinary system. Oxalates span therefore several fields (medicine, biology, mineralogy, materials science, etc.), which is reflected in a large number of publications. Nevertheless, many questions remain unresolved. For instance, the role of water in the formation of calcium oxalate crystal structures as well as the mechanisms of phase transition is still unclear.
Thermal stability, structural evolution pathways and phase transition mechanisms of the calcium oxalates COM, COD and COT have been analyzed using single crystal and powder X-ray diffraction techniques at non ambient conditions. The reduction of H2O content in the structures increases dimensionality from dimers and chains to the layered structural units and from rarefied to denser sheets within the compounds whose structures are based on the 2D units. While studying the phase transitions pathways within the calcium oxalate family, two crystalline compounds have been structurally characterized for the first time (α-CaC2O4 and CaC2O4·H2O), among which the novel COM modification has been obtained for the first time as well. 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 towards 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. Finally, we’d like to emphasize the particular importance of the structural and chemical evolution studies of calcium oxalates, as their phase transitions occur at temperatures typical of the human body and can therefore have a significant effect on health.
This work was supported by President of Russian Federation grant for leading scientific schools (no. NSh-3079.2018.5), Russian Foundation for Basic Research (no. 16-05-00986) and by the People Program (Marie Curie Actions) of the European Union's Seventh Framework Program FP7/2007-2013/ under REA grant agreement IRSES-GA-2013-610547-TAMER. The XRD measurements have been performed at the X-ray Diffraction Centre of St. Petersburg State University.References:
Keywords: calcium oxalate, crystal strusture, phase transition