OSCILLATORY ZONING IN ERYTHRITE, IDEALLY CO3(ASO4)2•8H2O

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) ų. The empirical formula for erythrite, obtained with electron-probe micro-analysis (EPMA), is [Co_2.78Zn_0.11Ni_0.07Fe_0.04]_Ʃ3.00(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. 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.