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

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) angstrom, b = 13.42490(7) angstrom, c = 4.755885(8) angstrom, beta = 105.1116(3)degrees, and V = 631.680(4) angstrom(3). The empirical formula for erythrite, obtained with electron-probe micro-analysis (EPMA), is [Co2.78Zn0.11Ni0.07Fe0.04]Sigma 3.00(AsO4)(2) center dot 8H(2)O. Erythrite belongs to the vivianite-type structure that contains M1O(2)(H2O)(4) octahedra and M2(2)O(6)(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) angstrom and average <M2-O>[6] = 2.088(1) angstrom. With space group C2/m, there are two solid solutions: M-3(AsO4)(2) center dot 8H(2)O and M-3(PO4)(2) center dot 8H(2)O 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 angstrom) and As5+ (0.355 angstrom) 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.