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H-bonding scheme in allactite: a combined single-crystal X-ray and neutron diffraction, optical absorption spectroscopy, FTIR and EPMA-WDS study

Published online by Cambridge University Press:  02 January 2018

G. Diego Gatta*
Affiliation:
Dipartimento di Scienze della Terra, Università degli Studi di Milano, Via Botticelli 23, I-20133 Milano, Italy CNR – Istituto di Cristallografia, Sede di Bari, Via G. Amendola 122/o, I-70126 Bari, Italy
Ferdinando Bosi
Affiliation:
Dipartimento di Scienze della Terra, Sapienza Università di Roma, Piazzale Aldo Moro 5, I-00185 Roma, Italy CNR – Istituto di Geoscienze e Georisorse, UOS Roma, P.le Aldo Moro 5, 00185 Roma, Italy
Maria Teresa Fernandez Diaz
Affiliation:
Institut Laue-Langevin, BP 156, F-38042 Grenoble Cedex 9, France
Ulf Hålenius
Affiliation:
Department of Geosciences, Swedish Museum of Natural History, Box 50007, SE-10405 Stockholm, Sweden

Absatract

The crystal chemistry of allactite from Långban, Värmland (Sweden) was investigated by single-crystal X-ray and neutron diffraction, optical absorption spectroscopy, Fourier-transform infra-red spectroscopy (FTIR) and electron microprobe analysis by wavelength-dispersive spectroscopy (EPMA-WDS). The optical spectra indicate the presence of Mn in valence state 2+ only. Assuming 16 O atoms per formula unit, arsenic as As5+ and the (OH) content calculated by charge balance, the resulting formula based on the EPMA-WDS data is (Mn2+6.73Ca0.13Mg0.12Zn0.02)∑7.00(As5+)2.00O16H8, very close to the ideal composition Mn7(AsO4)2(OH)8. In the unpolarized FTIR spectrum of allactite, fundamental (OH)-stretching bands are observed at 3236, 3288, 3387, 3446, 3484, 3562 and 3570 cm–1, suggesting that a number of OH environments, with different hydrogen bond strengths, occur in the structure. The neutron structure refinement shows that four independent H sites occur in allactite with full site occupancy, all as members of hydroxyl groups. The complex hydrogen-bonding scheme in the allactite structure is now well defined, with at least nine hydrogen bonds energetically favourable with mono-, bi- and trifurcated configurations.

Type
Research Article
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2016

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