Crystal chemistry of zemannite-type structures; I, A re-examination of zemannite from Moctezuma, Mexico
Crystal chemistry of zemannite-type structures; I, A re-examination of zemannite from Moctezuma, Mexico
European Journal of Mineralogy (October 2018) 31 (3): 519-527
- alkali metals
- cell dimensions
- chemical composition
- crystal chemistry
- crystal structure
- dehydration
- electron probe data
- formula
- hydration
- metals
- Mexico
- Moctezuma Mexico
- order-disorder
- refinement
- sodium
- Sonora Mexico
- space groups
- spectra
- substitution
- synthetic materials
- tellurates
- tellurites
- type specimens
- X-ray diffraction data
- hydrogen bonding
- zemannite
- atomic emission spectra
- Moctezuma Mine
- ICP atomic emission spectra
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) Aa and V = 582.12(7) Aa (super 3) , 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) Aa and V = 582.3(2) Aa (super 3) . 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 Mg (sub 0.5) ZnFe (super 3+) (Te (super 4+) O (sub 3) ) (sub 3) .(3 + n)H (sub 2) O, where 0 < or = n < or = 1.5 from the current definition of Mg (sub 0.5) ZnFe (super 3+) (Te (super 4+) O (sub 3) ) (sub 3) .4.5H (sub 2) O to better reflect the variable degree of hydration, since the type specimen is fully dehydrated and only contains three H (sub 2) O molecules pfu.