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

O. E. PIRO; G. E. ECHEVERRÍA; A. C. GONZÁLEZ-BARÓ ; E. J. BARAN

PHYSICS AND CHEMISTRY OF MINERALS

SPRINGER

Lugar: Berlin; Año: 2016 vol. 43 p. 287 - 300

0342-1791

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, have been determined by single-crystal X-ray diffraction methods. The compounds are isotypic to each other and to the previously reported Na[Mg(H2O)6][M(C2O4)3]×3H2O (M: Cr, Al). They crystallize in the trigonal P3c1 space group with 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 solid solution. Comparison of our crystallographic results with previous X-ray diffraction and chemical data of type stepanovite and zhemchuzhnikovite minerals provides compelling evidence that these natural materials possess the same crystal and molecular structure as their synthetic counterparts. It is shown that the originally reported unit cell for stepanovite represents a pronounced sub-cell, and that the correct unit cell and space group are based on weak superstructure reflections. The infrared and Raman spectra of both synthetic analogues were also recorded and are briefly discussed.