Crystal Chemistry of an Erythrite-Köttigite Solid Solution (Co3–xZnx) (AsO4)2·8H2O

Abstract

A wide compositional range, covering about 90% of an expected erythrite-köttigite substitutional solid solution with extreme compositions of (Co2.84Mg0.14Zn0.02) (AsO4)2·8H2O and (Zn2.74Co0.27) (AsO4)2·8H2O, was revealed in a suite of samples from a polymetallic ore deposit in Miedzianka, SW Poland. Members of the solid solution series were examined by means of Electron Probe Microanalysis (EPMA), Scanning Electron Microscopy (SEM)/Energy-Dispersive Spectrometer (EDS), X-ray single-crystal and powder diffraction, and Raman spectroscopy. Metal cations were randomly distributed between two special octahedral sites in the erythrite–köttigite structure. In response to Co ↔ Zn substitutions, small but significant changes in bond distances (particularly in [AsO4] tetrahedra), rotation, and distortion of co-ordination polyhedra were observed. Two sub-series of dominant cationic substitutions (Co-Mg-Ni and Co-Fe-Zn) were noted within the arsenate series of vivianite-group minerals linked by erythrite. The paragenetic sequence erythrite → Zn-rich erythrite → Co-rich köttigite → köttigite reflects the evolution of the solution’s pH towards increased acidity and a relative increase in the concentration of Zn ions following precipitation of erythrite.

Thermodynamics and crystal structures of krautite, Mn[AsO3(OH)]⋅H2O, koritnigite, Zn[AsO3(OH)]⋅H2O and cobaltkoritnigite, Co[AsO3(OH)]⋅H2O

“…They can accommodate many divalent metals in various structural types. The vivianite-group arsenates, for example, have the general formula M 3 (AsO 4 ) 2 ⋅8H 2 O, where M = Ni, Co, Zn, Mg, Mn, Fe and Cu (Sejkora et al , 2014; Plášil et al , 2017; Ciesielczuk et al , 2018; Siuda and Macioch, 2018; Dumańska-Słowik et al , 2018). Natural members of this group are mostly Ni or Co rich.…”