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

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Marco E. Ciriotti
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Long-range and short-range cation order in the crystal structures of carlfrancisite and mcgovernite

Messaggio da Marco E. Ciriotti » ven 18 mag, 2018 18:05

Prossima pubblicazione.

Referenza:
▪ Hawthorne, F.C. (2018): Long-range and short-range cation order in the crystal structures of carlfrancisite and mcgovernite. Mineralogical Magazine, 82, (in press).
Marco E. Ciriotti

«Things are interesting only in so far as they relate themselves to other things»

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Marco E. Ciriotti
Messaggi: 24905
Iscritto il: ven 25 giu, 2004 11:31
Località: via San Pietro, 55 I-10073 Devesi/Cirié TO - Italy
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Re: Long-range and short-range cation order in the crystal structures of carlfrancisite and mcgovernite

Messaggio da Marco E. Ciriotti » ven 09 nov, 2018 20:43

Pubblicazione effettuata.

Referenza:
▪ Hawthorne, F.C. (2018): Long-range and short-range cation order in the crystal structures of carlfrancisite and mcgovernite. Mineralogical Magazine, 82, 1101-1118.

Abstract:
The 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), R-3̄m with Z = 6, and unit-cell parameters: a = 8.2238(2), c = 205.113(6) Å and V = 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), R-3̄m with Z = 6, and unit-cell parameters: a = 8.2061(3), c = 204.118(8) Å and V = 11903.8(6) Å3, from Sterling Hill, New Jersey, have been solved by direct methods and refined to R1 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.
There are 18 crystallographically distinct cation sites in both carlfrancisite and mcgovernite. There are two [4]-coordinated As sites fully occupied by As5+, and four T sites occupied by Si and As5+ 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 As3+. The Z(1) site, occupied by Mn2+ in carlfrancisite and Zn in mcgovernite, has tetrahedral coordination, and the Z(2) site is only partly occupied by Mg and Mn2+ in both structures. There are nine M sites, 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 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.
Marco E. Ciriotti

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