Crystal chemistry of organic minerals – salts of organic acids: the synthetic approach

The term ‘organic minerals’ means naturally occurring crystalline organic compounds including metal salts of formic, acetic, citric, mellitic, methanesulfonic and oxalic acids. As for the rest of the (inorganic) minerals, the primary tool to disclose their crystal and molecular structure and therefore to understand their mutual relationship with each other and with synthetic analogues and also their physicochemical properties is X-ray diffraction crystallography ever since the dawn of this methodology in 1913. The structure of several synthetic organic minerals was solved well before the discovery of their natural counterpart. On the other hand, complete crystal structure determination of early discovered organic minerals had to await the advent of combined synthetic and advanced X-ray diffraction methods to fully unveil their crystal structures. We review here the crystal chemistry of organic minerals and show the importance of structural studies on their synthetic analogues. This will be highlighted by case studies on the recently reported synthetic novgorodovaite, Ca2(C2O4)Cl2·2H2O, and its heptahydrate analogue, Ca2(C2O4)Cl2·7H2O, and the isotypic to each other stepanovite, NaMg[Fe(C2O4)3]·9H2O, and zhemchuzhnikovite, NaMg[AlxFe1−x(C2O4)3]·9H2O.