Crystal chemistry of zemannite-type structures: II. Synthetic sodium zemannite

New zemannite-type phases, from Na 1.25 Fe 0.75 3 + Zn 1.25 ( Te O 3 ) 3 ∙ 3 H 2 O to Na 0.55 Fe 1.44 3 + Zn 0.56 ( Te O 3 ) 3 ∙ 3 H 2 O have been synthesised hydrothermally under basic conditions from a mixture of tellurium dioxide, zinc oxide, and either iron (III) oxide or iron (III) nitrate nonahydrate. Analysis by electron microprobe and inductively coupled plasma atomic emission spectrometry showed considerable variation in the Fe(III) and Zn contents of the framework. Single crystals of the compound produced from the iron (III) oxide syntheses were analysed, showing lower unit-cell parameters than natural zemannite, with a = 9.2620(9) and c = 7.6148(7) Å. The structure differs from type zemannite by the presence of Na and a water molecule which bridges the channel and framework by bonding via the oxygen atom. The unit-cell parameters of the compounds produced from iron (III) nitrate nonahydrate syntheses were refined from powder data and shown to increase with increasing pH. The results of this study show the potential for solid solution to exist in natural zemannite-type minerals and gives a stable boundary for the formation of pure zemannite-type compounds at basic pH levels, between pH 11.5 and pH 14.