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Minerals 2017, 7(8), 136; https://doi.org/10.3390/min7080136

Growth Oscillatory Zoning in Erythrite, Ideally Co3(AsO4)2·8H2O: Structural Variations in Vivianite-Group Minerals

Department of Geoscience, University of Calgary, Calgary, AB T2N 1N4, Canada
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Received: 11 July 2017 / Revised: 28 July 2017 / Accepted: 30 July 2017 / Published: 2 August 2017
(This article belongs to the Special Issue Fundamentals and Frontiers in Mineralogy)
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Abstract

The crystal structure of an oscillatory zoned erythrite sample from Aghbar mine, Bou Azzer, Morocco, was refined using synchrotron high-resolution powder X-ray diffraction (HRPXRD) data, Rietveld refinement, space group C2/m, and Z = 2. The crystal contains two sets of oscillatory zones that appear to have developed during epitaxial growth. The unit-cell parameters obtained are a = 10.24799(3) Å, b = 13.42490(7) Å, c = 4.755885(8) Å, β = 105.1116(3)°, and V = 631.680(4) Å3. The empirical formula for erythrite, obtained with electron-probe micro-analysis (EPMA), is [Co2.78Zn0.11Ni0.07Fe0.04]∑3.00(AsO4)2·8H2O. Erythrite belongs to the vivianite-type structure that contains M1O2(H2O)4 octahedra and M22O6(H2O)4 octahedral dimers that are linked by TO4 (T5+ = As or P) tetrahedra to form complex layers parallel to the (010) plane. These layers are connected by hydrogen bonds. The average <M1–O>[6] = 2.122(1) Å and average <M2–O>[6] = 2.088(1) Å. With space group C2/m, there are two solid solutions: M3(AsO4)2·8H2O and M3(PO4)2·8H2O where M2+ = Mg, Fe, Co, Ni, or Zn. In these As- and P-series, using data from this study and from the literature, we find that their structural parameters evolve linearly with V and in a nearly parallel manner despite of the large difference in size between P5+ (0.170 Å) and As5+ (0.355 Å) cations. Average <T–O>[4], <M1–O>[6], and <M2–O>[6] distances increase linearly with V. The average <As–O> distance is affected by M atoms, whereas the average <P–O> distance is unaffected because it contains shorter and stronger P–O bonds. Although As- and P-series occur naturally, there is no structural reason why similar V-series vivianite-group minerals do not occur naturally or cannot be synthesized. View Full-Text
Keywords: erythrite; vivianite-group minerals; structure; chemistry; oscillatory zones; synchrotron high-resolution powder X-ray diffraction (HRPXRD); Rietveld refinement erythrite; vivianite-group minerals; structure; chemistry; oscillatory zones; synchrotron high-resolution powder X-ray diffraction (HRPXRD); Rietveld refinement
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Antao, S.M.; Dhaliwal, I. Growth Oscillatory Zoning in Erythrite, Ideally Co3(AsO4)2·8H2O: Structural Variations in Vivianite-Group Minerals. Minerals 2017, 7, 136.

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