Abstract

The crystal structure of hagendorfite (Na,Ca)MnFe2(PO4)3 from type locality has been determined and it was found to be isostructural with alluaudite. It accepts space group symmetry C2/c both at room temperature and at 100 K. At room temperature the lattice parameters are a = 11.9721(9) Å, b = 12.5988(8) Å, c = 6.5029(5) Å, β = 114.841(8)° with Z = 4. For reasons of comparison the structure of an alluaudite sensu strictu (s.s.) from Buranga (Rwanda) was re-determined. The specific arrangement of M(1) and M(2) octahedral sites and of P(1) and P(2) tetrahedral sites gives rise to two different channels aligned along the crystallographic c-axis, containing the A(1) and A(2)’ sites. In both compounds the A(1) site is fully occupied and shows a mixed occupation of Na+, Ca2+ and Mn2+ (hagendorfite) and Ca2+ and Na+ (alluaudite s.s.). The A(2)’ is fully occupied by Na+ in hagendorfite and partly filled by 0.14 Na+ atoms per formula unit in Buranga alluaudite. The structural topology of hagendorfite is described in detail and structural parameters are compared to alluaudite s.s. and to other, mostly synthetic compounds also crystallizing in the alluaudite structure type. 57Fe Mössbauer spectroscopy shows that iron is exclusively in the trivalent state in alluaudite s.s. while in hagendorfite about 2/3 of the total iron are in the divalent state. The ferrous iron resonance absorption contribution appears to be broad and can only be refined with four different Fe2+ subcomponents, which are all ascribed to the M(2) site. The four different spectroscopic signals presumably arise from different next nearest neighbour occupations of adjacent M(2) and A(1) sites.

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