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Crystal structure of Fe (sub 2) (AsO (sub 4) )(HAsO (sub 4) )(OH)(H (sub 2) O) (sub 3) , a dehydration product of kankite

Juraj Majzlan, Lukas Palatinus and Jakub Plasil
Crystal structure of Fe (sub 2) (AsO (sub 4) )(HAsO (sub 4) )(OH)(H (sub 2) O) (sub 3) , a dehydration product of kankite
European Journal of Mineralogy (September 2015) 28 (1): 63-70

Abstract

We report the crystal structure of a dehydration product of the mineral kankite (FeAsO (sub 4) .3.5H (sub 2) O). The structure was solved and refined by precession electron diffraction tomography. Initially, we believed that we solved the structure of kankite; this mineral, however, decomposes in vacuum to Fe (sub 2) (AsO (sub 4) )(HAsO (sub 4) )(OH)(H (sub 2) O) (sub 3) (=FeAsO (sub 4) .2H (sub 2) O). The crystal structure was solved in the space group Cc and the model was refined by the full-matrix least-squares method by Jana2006. The model converged to R (sub (obs)) = 12.02 %, wR (sub (obs)) = 12.49 % (with GOF = 6.39) for 1139 observed reflections with [I (sub obs) > 3sigma (I)]. The structure of the dehydration product of kankite consists of corrugated heteropolyhedral sheets. Pairs of Fephi (sub 6) octahedra, flanked by five adjacent arsenate tetrahedra, could be seen as the building units of the corrugated sheets. Variable-temperature powder X-ray diffraction showed that kankite dehydrates to FeAsO (sub 4) .2H (sub 2) O at 55-56 degrees C. Based on the similar topology of FeAsO (sub 4) .2H (sub 2) O and the mineral lausenite [Fe (sub 2) (SO (sub 4) ) (sub 3) .5H (sub 2) O], and the structural relationship between lausenite and kornelite [Fe (sub 2) (SO (sub 4) ) (sub 3) .7.5H (sub 2) O], we conjecture that the structure of kankite could be also built by corrugated sheets. One polyhedral linkage between an Fephi (sub 6) octahedron and an Asphi (sub 4) tetrahedron in the sheet of the dehydrated kankite needs to be broken to allow for stretching of the sheets, the introduction of an additional H (sub 2) O molecule into the sheet and perhaps also additional H (sub 2) O molecules in between the sheets.


ISSN: 0935-1221
EISSN: 1617-4011
Serial Title: European Journal of Mineralogy
Serial Volume: 28
Serial Issue: 1
Title: Crystal structure of Fe (sub 2) (AsO (sub 4) )(HAsO (sub 4) )(OH)(H (sub 2) O) (sub 3) , a dehydration product of kankite
Affiliation: Friedrich-Schiller University, Institute of Geosciences, Jena, Germany
Pages: 63-70
Published: 20150930
Text Language: English
Publisher: Schweizerbart'sche Verlagsbuchhandlung (Naegele u. Obermiller), Stuttgart, Germany
References: 21
Accession Number: 2015-107323
Categories: Mineralogy of non-silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 4 tables
N49°58'42" - N49°58'42", E15°16'03" - E15°16'03"
Secondary Affiliation: Academy of Sciences of the Czech Republic, Institute of Physics, CZE, Czech Republic
Country of Publication: Germany
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, copyright, Schweizerbart'sche Verlagsbuchhandlung. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201546
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