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Crystal structure and Mössbauer spectroscopy of the synthetic amphibole potassic-ferri-ferrorichterite at 298 K and low temperatures (80–110 K)

Institute of Crystallography, Rheinisch-Westfälische-Technische Hochschule Aachen, Jägerstraβe 17–19, D-52056 Aachen, Germany

^* mailing address: Neuhofen 224/1, A-4910 Ried im Innkreis, Austria e-mail: redhammer{at}xtal.rwth-aachen.de

The crystal structure of a synthetic potassic-ferri-ferrorichterite, ideally K(Ca_0.5Na_1.5)Fe₅Si₈O₂₂(OH)₂ was refined from single-crystal X-ray imaging plate diffraction data at 298 K and 110 K. The title compound is monoclinic, S.G. C2/m, Z = 2, with a = 10.145(1) Å, b = 18.184(1) Å, c = 5.296(1) Å, β = 104.42(1)° at 298 K (sample Ri1). The structure is consistent with the general amphibole structure type. There is no crystallographic phase transition down to 110 K. The two nonequivalent T sites and the octahedral M1 and M3 sites behave very rigidly upon cooling. Bond lengths for the distorted M sites slightly decrease as temperature is lowered. The tetrahedral chain kinking increases by 0.9° between 298 K and 110 K. Similar behavior was found for a second sample investigated by single crystal X-ray diffraction. Mössbauer spectra, collected at 298 K, yield rather high ferric iron concentrations up to 2.0 a.p.f.u for the three samples investigated. Fe³⁺ prefers the M2 site. Three components were detected within the ferrous quadrupole splitting distribution (QSD) of the C-type sites and where assigned to the M1, M3 and M2 sites respectively. This assignment is consistent with polyhedral distortion and gives correct site occupancies. The magnitude of the quadrupole splitting of Fe²⁺ on M1 and M3 sites shows a dependency on the ferric iron content (mean cation radius of the M2 site) of the sample.

Key-words: crystal structure, Mössbauer spectroscopy, synthesis, amphibole, richterite.

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