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

Abstract

ABSTRACTThe crystal structures of carlfrancisite, ideally Mn32+(Mn2+,Mg,Fe3+,Al)42(As3+O3)2(As5+O4)4[(Si,As5+)O4]8(OH)42, hexagonal (rhombohedral),R3̄mwithZ= 6, and unit-cell parameters:a= 8.2238(2),c= 205.113(6) Å andV= 12013.5(4) Å3, from the Kombat mine, Otavi Valley, Namibia, and mcgovernite, ideally Zn3(Mn2+,Mg,Fe3+,Al)42(As3+O3)2(As5+O4)4[(Si,As5+)O4]8(OH)42, hexagonal (rhombohedral),R3̄mwithZ= 6, and unit-cell parameters:a= 8.2061(3),c= 204.118(8) Å andV= 11903.8(6) Å3, from Sterling Hill, New Jersey, have been solved by direct methods and refined toR1values 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: As2O512.89, As2O33.33, P2O50.50, V2O50.74, SiO28.96, Al2O30.78, FeO 0.22, MnO 53.25, MgO 9.37, H2O(calc) 8.42, sum 98.50 wt.%; mcgovernite: As2O513.06, As2O33.71, SiO29.34, Al2O30.20, 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.There are 18 crystallographically distinct cation sites in both carlfrancisite and mcgovernite. There are two [4]-coordinatedAssites fully occupied by As5+, and fourTsites occupied by Si and As5+in solid solution. TheAs(3) site has triangular pyramidal coordination with <As(3)–O> distances of 1.808 and 1.817 Å, typical of As3+. TheZ(1) site, occupied by Mn2+in carlfrancisite and Zn in mcgovernite, has tetrahedral coordination, and theZ(2) site is only partly occupied by Mg and Mn2+in both structures. There are nineMsites, all of which are octahedrally coordinated and contain dominantly Mn2+and Mg, with minor Al and Fe. There are three cation sites with significant vacancies:As(3),Z(2),M(3), and there are complicated patterns of short-range order involving the cations and vacancies at these sites.The carlfrancisite–mcgovernite structure contains 84 layers of approximately close-packed polyhedra along one translation onc. The anions in the structure are arranged in approximately close-packed layers orthogonal to thecaxis. Fourteen layers stack along thecaxis in the sequence |**hhchch•hchchh| (* denotes a layer of anions displaced from close packed). There are eight distinct layers of cation-centred polyhedra that repeat via a centre of symmetry at the origin, and via theR-centering to give 84 layers per unit-cell.