Long-range and short-range cation order in the crystal structures of carlfrancisite and mcgovernite

摘要

The crystal structures of carlfrancisite, ideally Mn-3(2+)(Mn2+ ,Mg,Fe3+ ,Al)(42)(As3+O3)(2)(As5+O4)(4)[(Si,As5+) O-4](8)(OH)(42),hexagonal (rhombohedral), R (3) over barm with Z=6, and unit-cell parameters: a = 8.2238(2), c = 205.113(6) and V = 12013.5(4) (3), from the Kombat mine, Otavi Valley, Namibia, and mcgovernite. ideally Zn-3(Mn2+,Mg,Fe3+,Al)(42)(As3+O3)(2)(As5+O4)(4)[(Si,As5+) O-4]8(OH)(42), hexagonal (rhombohedral), R (3) over barm with Z = 6, and unit-cell parameters: a = 8.2061(3), c = 204.118(8) and V = 11903.8(6) (3), from Sterling Hill, New Jersey, have been solved by direct methods and refined to R-1 values of 3.37 and 5.02% for 3837 and 3772 unique observed reflections, respectively; they are isostructural. Chemical analysis by electron microprobe and crystal-structure refinement gave the following compositions: carlfrancisite: As2O5 12.89, As2O3 3.33, P2O5 0.50, V2O5 0.74, SiO2 8.96, Al2O3 0.78, FeO 0.22, MnO 53.25, MgO 9.37, H2O (calc) 8.42, sum 98.50 wt.%; mcgovernite: As2O5 13.06, As2O3 3.71, SiO2 9.34, Al2O3 0.20, FeO 1.38, MnO 44.58, ZnO 8.81, MgO 8.89, H2O(calc) 8.24, sum 98.21 wt.%. The H2O contents and the valence states of As were determined by crystal-structure analysis.