A contribution to the crystal chemistry of the voltaite group: solid solutions, Mössbauer and infrared spectra, and anomalous anisotropy

Abstract

Voltaite is a mineral of fumaroles, solfatares, coal-fire gas vents, and acid-mine drainage systems. The nominal composition is K2Fe5 2+Fe3 3+Al(SO4)12·18H2O and the nominal symmetry is cubic, $$Fd\overline{3}c$$ . The tetragonal (I41/acd) superstructure of voltaite is known as the mineral pertlikite. In this study, we investigated 22 synthetic voltaite samples in which Fe2+ was partially or completely replaced by Mg, Zn, Mn, or Cd, by single-crystal and powder X-ray diffraction (both in-house and synchrotron). Two samples contained NH4 + instead of K+. The structure of voltaite is based on a framework defined by krohnkite-like heteropolyhedral chains which host both M3+ and M2+ in octahedral coordination. Unit cell dimensions of the end-members scale almost linearly with the size of M2+. In the Fe2+-Mg-Zn solid solutions, the Fe2+-Mg and Fe2+-Zn solutions are linear (ideal) in terms of their lattice-parameter variations. The Mg-Zn solid solution, however, is strongly non-ideal. A detailed analysis of the topology of the chains showed that this behavior originates in expansion and contraction of individual M2+-O bonds within the chains. In the Mg-Zn solid solution, some of the M2+-O bonds expand while none contract. In the other solid solutions, expansion of some M2+-O bonds is always compensated by contraction of the other ones. Parts of the nominally cubic crystals are optically anisotropic and their symmetry is found to be tetragonal by single crystal X-ray diffraction measurements. The coexistence of cubic and tetragonal sectors within a single crystal without any detectable difference in their chemical composition is difficult to explain in terms of growth of such composite crystals. Mossbauer and infrared spectra collected on our synthetic crystals conform with previously published data.