Long-range and short-range cation order in the crystal structures of carlfrancisite and mcgovernite
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Bibliographic record
Abstract
ABSTRACT The crystal structures of carlfrancisite, ideally Mn 3 2+ (Mn 2+ ,Mg,Fe 3+ ,Al) 42 (As 3+ O 3 ) 2 (As 5+ O 4 ) 4 [(Si,As 5+ )O 4 ] 8 (OH) 42 , hexagonal (rhombohedral), R 3̄ m with Z = 6, and unit-cell parameters: a = 8.2238(2), c = 205.113(6) Å and V = 12013.5(4) Å 3 , from the Kombat mine, Otavi Valley, Namibia, and mcgovernite, ideally Zn 3 (Mn 2+ ,Mg,Fe 3+ ,Al) 42 (As 3+ O 3 ) 2 (As 5+ O 4 ) 4 [(Si,As 5+ )O 4 ] 8 (OH) 42 , hexagonal (rhombohedral), R 3̄ m with Z = 6, and unit-cell parameters: a = 8.2061(3), c = 204.118(8) Å and V = 11903.8(6) Å 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: As 2 O 5 12.89, As 2 O 3 3.33, P 2 O 5 0.50, V 2 O 5 0.74, SiO 2 8.96, Al 2 O 3 0.78, FeO 0.22, MnO 53.25, MgO 9.37, H 2 O(calc) 8.42, sum 98.50 wt.%; mcgovernite: As 2 O 5 13.06, As 2 O 3 3.71, SiO 2 9.34, Al 2 O 3 0.20, FeO 1.38, MnO 44.58, ZnO 8.81, MgO 8.89, H 2 O(calc) 8.24, sum 98.21 wt.%. The H 2 O 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]-coordinated As sites fully occupied by As 5+ , and four T sites occupied by Si and As 5+ in solid solution. The As (3) site has triangular pyramidal coordination with < As (3)–O> distances of 1.808 and 1.817 Å, typical of As 3+ . The Z (1) site, occupied by Mn 2+ in carlfrancisite and Zn in mcgovernite, has tetrahedral coordination, and the Z (2) site is only partly occupied by Mg and Mn 2+ in both structures. There are nine M sites, all of which are octahedrally coordinated and contain dominantly Mn 2+ 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 on c . The anions in the structure are arranged in approximately close-packed layers orthogonal to the c axis. Fourteen layers stack along the c axis 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 the R -centering to give 84 layers per unit-cell.
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| Category | Codex | Gemma |
|---|---|---|
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