A tribute to Centennial Bragg Nobel Prize: crystal structure of synthetic Stepanovite and Zhemchuzhnikovite minerals

O. E. PIRO; G. E. ECHEVERRÍA; E. J. BARAN

La Plata

Congreso; XI Reunión de la Asociación Argentina de Cristalografía (X AACr); 2015

AACr

The characterization of minerals at atomic resolution dates back one hundredth years ago to the dawn of structural X-ray diffraction by the pioneering work of W. L. Bragg on alkaline halides [1]. However, full crystallographic characterization of minerals by structural diffraction methods is frequently hampered by several drawbacks, including unavailability of natural samples, the usual lack of purity and other disorders of these materials, and the difficulty in finding natural single crystals suitable for detailed structural work. We show here in the cases of Stepanovite, NaMg[Fe(C2O4)3].9H2O, and Zhemchuzhnikovite, NaMg[AlxFe1-x(C2O4)3].9H2O, minerals that synthetic inorganic chemistry followed by single crystal X-ray diffraction employing modern data collection and advanced space group determination and crystal structure solution [2] procedures have the potential to circumvent these shortcomings, providing a wealth of new structural information on these minerals which has remained hidden to the scientist?s eye since their discovery more than fifty years ago in vein lets of coal deposits located at the estuary of the Lena River in Siberia, Russia.Synthetic Stepanovite and Zhemchuzhnikovite minerals are isomorphic to each other and to the previously reported synthetic NaMg[M(C2O4)3].9H2O (M: Cr, Al) isomorphic pair [3]. They crystallize in the trigonal P3c1 space group with Z=6 molecules per unit cell. The metal (M) and sodium ions are at crystal general positions, in octahedral environments coordinated to three oxalate molecules in a propeller-like conformation. These metal ions are in a honeycomb-like layered arrangement of alternated metal and sodium atoms linked through sharing oxalate ligands. There are three different octahedral [Mg(H2O)6]2+ ions at C3 lattice sites filling the honeycomb holes and hence giving rise to an electrically neutral layered structure which can be described as Na[Mg(H2O)6][Fe(C2O4)3]. Neighboring layers are bridged by the remaining three water molecules through relative strong H-bond interactions and therefore the whole crystal can be better described as Na[Mg(H2O)6][Fe(C2O4)3].3H2O.Comparison of our crystallographic results with early X-ray diffraction and chemical data of Stepanovite and Zhemchuzhnikovite minerals [4] provides compelling evidence that these natural materials basically possess the same crystal and molecular structure as their synthetic counterparts.