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Rietveld refinement of a natural cobaltian mansfieldite from synchrotron data.

ABSTRACT: A structural refinement of a natural sample of a Co-bearing mansfieldite, AlAsO(4)·2H(2)O [aluminium orthoarsenate(V) dihydrate], has been performed based on synchrotron powder diffraction data, with 5% of the octa-hedral Al sites replaced by Co. Mansfieldite is the aluminium analogue and an isotype of the mineral scorodite (FeAsO(4)·2H(2)O), with which it forms a solid solution. The framework structure is based on AsO(4) tetra-hedra sharing their vertices with AlO(4)(H(2)O)(2) octa-hedra. Three of the four H atoms belonging to the two water mol-ecules in cis positions take part in O-H?O hydrogen bonding.

SUBMITTER: Zoppi M

PROVIDER: S-EPMC2968216 | biostudies-literature | 2009 Jan

REPOSITORIES: biostudies-literature

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Rietveld refinement of a natural cobaltian mansfieldite from synchrotron data.

Zoppi Matteo M Pratesi Giovanni G

Acta crystallographica. Section E, Structure reports online 20090110 Pt 2

A structural refinement of a natural sample of a Co-bearing mansfieldite, AlAsO(4)·2H(2)O [aluminium orthoarsenate(V) dihydrate], has been performed based on synchrotron powder diffraction data, with 5% of the octa-hedral Al sites replaced by Co. Mansfieldite is the aluminium analogue and an isotype of the mineral scorodite (FeAsO(4)·2H(2)O), with which it forms a solid solution. The framework structure is based on AsO(4) tetra-hedra sharing their vertices with AlO(4)(H(2)O)(2) octa-hedra. Three ...[more]

PMID: 21581741

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Project description:This study aims to solve the site-occupancy issues of any crystalline substance, showing that the structure refinement results should not be limited to a simple description of the crystal geometry but should go beyond that, providing refined site-scattering values consistent with the chemical data. A detailed description of the structure of the amphibole-supergroup minerals is very challenging owing to their complex chemical composition that renders the process of cation partition extremely difficult, particularly because of the occurrence of multivalent elements. Since amphiboles naturally occur under a fibrous morphology and have largely been used to produce asbestos, there is a growing demand for detailed and accurate structural data in order to study the relationships between structure, composition and toxicity. The present study proposes a recommended refinement procedure for both X-ray single-crystal structure refinement (SREF) and Rietveld analysis for tremolite, selected as a test case. The corresponding structural results are compared to estimate the ‘degree of confidence’ of the Rietveld refinement with regard to SREF. In particular, it is shown that the interpretation of the electron density of the tremolite structure by SREF is model dependent. By assuming that the site-scattering values from SREF should be as close as possible to those from electron microprobe analysis, as a crucial constraint for the correct description of the final crystal-chemical model, it is found that it is best satisfied by using partially ionized scattering curves (SCs) for O and Si, and neutral SCs (neutral oxygen curves or NOCs) for other atoms. This combination leads to the best fit to the diffraction data. Moreover, it is found that Rietveld refinement using NOCs produces the best structural results, in excellent agreement with SREF. It is worth noting that, due to the complexity of the diffraction pattern and the fairly large number of freely refinable parameters, refinements with different combinations of SCs produce results almost indistinguishable from a statistical point of view, albeit showing significant differences from a structural point of view.

Dataset Information

Rietveld refinement of a natural cobaltian mansfieldite from synchrotron data.

Publications

Rietveld refinement of a natural cobaltian mansfieldite from synchrotron data.

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