Crystal chemistry of zemannite-type structures: I. A re-examination of zemannite from Moctezuma, Mexico

The crystal structure of zemannite has been re-examined in order to address several features worthy of discussion. Analyses were performed on the type specimen and on material representative of Moctezuma. Type zemannite was found to refine in the space group P63 with a = 9.3877(5), c = 7.6272(4) Å and V = 582.12(7) Å3, whilst the unit-cell parameters in the same space group for the material representative of Moctezuma were a = 9.3921(13), c = 7.6230(15) Å and V = 582.3(2) Å3. The structural refinements undertaken were able to confirm for the first time the presence of a hydrogen bonding network in zemannite. An examination of the refinement of type zemannite in P63/m was also undertaken, showing that refining in the higher-symmetry space group does not show the ordering of framework octahedral metal cations. Zemannite-type minerals should therefore be refined in a non-centrosymmetric space group if possible, allowing the occupancies of the framework metal cations to refine. The chemical composition of zemannite was analysed by EMPA and by ICP-AES, showing conclusively that zemannite contains negligible Na, though the presence of Na should always be checked when analysing zemannite-type minerals. We also recommend that the formula of zemannite is revised to Mg0.5ZnFe3+(Te4+O3)3.(3 + n)H2O, where 0 ≤ n ≤ 1.5 from the current definition of Mg0.5ZnFe3+(Te4+O3)3.4.5H2O to better reflect the variable degree of hydration, since the type specimen is almost fully dehydrated and only contains three H2O molecules per formula unit.