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.
Highlights
Erythrite and köttigite belong to the arsenate series of vivianite-group minerals with the general formula of M3 (AsO4 )2 ·8 H2 O, where M represents divalent cations of Co, Cu, Fe, Mg, Ni, and Zn
We report on the chemical composition, structure refinement, and results of Raman spectroscopy of an extended erythrite–köttigite [(Co1–x Znx )3 (AsO4 )2 ·8H2 O] solid solution from a polymetallic ore deposit in Miedzianka, Sudetes Mts, SW Poland
8 of 24 the was inferred from the spatial relationships between observed minerals: erythrite → Zn-rich erythrite → Co-rich köttigite → köttigite (Figures 4 and 5)
Summary
Erythrite and köttigite belong to the arsenate series of vivianite-group minerals with the general formula of M3 (AsO4 )2 ·8 H2 O, where M represents divalent cations of Co (erythrite), Cu (babánekite), Fe (parasymplesite), Mg (hörnesite), Ni (annabergite), and Zn (köttigite). Members of the arsenate series are isostructural (space group C2/m), as revealed by the structure refinement of annabergite [1] and its Mg-rich variety cabrerite [2] as well as babánekite [3], erythrite [1,4,5], köttigite [6], köttigite–parasymplesite intermediate [7], and parasymplesite [8] The latter was originally referred to as symplesite [8], which is at present the name for a triclinic dimorph of parasymplesite. We report on the chemical composition, structure refinement, and results of Raman spectroscopy of an extended erythrite–köttigite [(Co1–x Znx ) (AsO4 )2 ·8H2 O] solid solution from a polymetallic ore deposit in Miedzianka, Sudetes Mts, SW Poland. Miedzianka is unique in this respect, as two other extended solid solutions (77.5 mol% cornwallite–pseudomalachite and 75 mol% kipushite–philipsburgite) have been observed among base-metal arsenates and phosphates in samples from this locality [18]
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