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

Voltaite is a mineral of fumaroles, solfatares, coal-fire gas vents, and acid-mine drainage systems. The nominal composition is K 2 Fe 5 2+ Fe 3 3+ Al(SO 4 ) 12 ·18H 2 O and the nominal symmetry is cubic, . The tetragonal ( I 4 1 / acd ) superstructure of voltaite is known as the mineral pertlikite. In this study, we investigated 22 synthetic voltaite samples in which Fe 2+ 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 NH 4 + instead of K + . The structure of voltaite is based on a framework defined by kröhnkite-like heteropolyhedral chains which host both M 3+ and M 2+ in octahedral coordination. Unit cell dimensions of the end-members scale almost linearly with the size of M 2+ . In the Fe 2+ -Mg-Zn solid solutions, the Fe 2+ -Mg and Fe 2+ -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 M 2+ -O bonds within the chains. In the Mg-Zn solid solution, some of the M 2+ -O bonds expand while none contract. In the other solid solutions, expansion of some M 2+ -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. Mössbauer and infrared spectra collected on our synthetic crystals conform with previously published data.