Crystal and molecular structure and spectroscopic behavior of isotypic synthetic analogs of the oxalate minerals stepanovite and zhemchuzhnikovite

Abstract

The crystal structure of synthetic stepanovite,Na[Mg(H2O)6][Fe(C2O4)3]·3H2O, and zhemchuzhnikovite,Na[Mg(H2O)6][Al0.55Fe0.45(C2O4)3]·3H2O, has beendetermined by single-crystal X-ray diffraction methods.The compounds are isotypic to each other and to the previouslyreported Na[Mg(H2O)6][M(C2O4)3]·3H2O (M:Cr, Al).They crystallize in the trigonal P3c1 space groupwith Z = 6 molecules per unit cell and (hexagonal axes)a = 17.0483(4), c = 12.4218(4) Å for the iron compound,and a = 16.8852(5), c = 12.5368(5) Å for the Al/Fe solidsolution. Comparison of our crystallographic results withprevious X-ray diffraction and chemical data of typestepanovite and zhemchuzhnikovite minerals providescompelling evidence that these natural materials possessthe same crystal and molecular structure as their syntheticcounterparts. It is shown that the originally reported unitcell for stepanovite represents a pronounced sub-cell andthat the correct unit cell and space group are based on weaksuperstructure reflections. The infrared and Raman spectraof both synthetic analogs were also recorded and are brieflydiscussed.