Crystal Chemistry of the Copper Oxalate Biomineral Moolooite: The First Single-Crystal X-ray Diffraction Studies and Thermal Behavior

Abstract

Moolooite, Cu(C2O4)·nH2O, is a typical biomineral which forms due to Cu-bearing minerals coming into contact with oxalic acid sources such as bird guano deposits or lichens, and no single crystals of moolooite of either natural or synthetic origin have been found yet. This paper reports, for the first time, on the preparation of single crystals of a synthetic analog of the copper-oxalate biomineral moolooite, and on the refinement of its crystal structure from the single-crystal X-ray diffraction (SCXRD) data. Along with the structural model, the SCXRD experiment showed the significant contribution of diffuse scattering to the overall diffraction data, which comes from the nanostructural disorder caused by stacking faults of Cu oxalate chains as they lengthen. This type of disorder should result in the chains breaking, at which point the H2O molecules may be arranged. The amount of water in the studied samples did not exceed 0.15 H2O molecules per formula unit. Apparently, the mechanism of incorporation of H2O molecules governs the absence of good-quality single crystals in nature and a lack of them in synthetic experiments: the more H2O content in the structure, the stronger the disorder will be. A description of the crystal structure indicates that the ideal structure of the Cu oxalate biomineral moolooite should not contain H2O molecules and should be described by the Cu(C2O4) formula. However, it was shown that natural and synthetic moolooite crystals contain a significant portion of “structural” water, which cannot be ignored. Considering the substantially variable amount of water, which can be incorporated into the crystal structure, the formula Cu(C2O4)·nH2O for moolooite is justified.