Crystal chemistry of the microporous tellurite minerals zemannite and kinichilite, Mg0.5[Me2+Fe3+(TeO3)3]-4.5H2O, (Me2+=Zn;Mn)
European Journal of Mineralogy Volume 7 Number 3 (1995), p. 509 - 524
published: May 19, 1995
manuscript accepted: Dec 19, 1994
manuscript received: Jul 25, 1994
ArtNo. ESP147050703005, Price: 29.00 €
Abstract Chemical and structural re-investigations of the microporous tellurite minerals zemannite and kinichilite revealed the previously undetermined arrangement of the non-framework atoms and led to the revision of their chemical formulas to Mgo.5[ZnFe3+(TeO3)3]4.5H2O (zemannite) and Mgo.5[(Mn2+,,Zn)Fe3+(TeO3)3]-4.5H2O (kinichilite). Crystal-structure investigations using single-crystal X-ray diffraction, electron microprobe and energydispersive X-ray analyses, as well as thermogravimetric and IR spectroscopic investigations were carried out on crystals from the type localities in Mexico (Moctezuma Mine) and Japan (Kawazu Mine), respectively. Only neglectably small amounts of Na2O have been found, although sodium was before assumed to be a significant part of the composition of both minerals. Instead of Na+ ions octahedrally coordinated [Mg(H2O)ô]2+ complexes and 'free' water molecules arranged in a hydrogen-bonded network were located within the tubular channels of the [(Zn,Mn)2+Fe3+(TeO3)3]~ framework. A reasonable [Mg(H2O)6(H2O)3]2+ ordering-scheme with the reduced symmetry 3 instead of 63/m was deduced for each single channel; this violates the P63/m framework symmetry. Mossbauer spectroscopy showed that the iron in zemannite and kinichilite is entirely octahedrally coordinated Fe3+. Therefore the presence of additional hydrogen for charge balance, as stated in the original formulas of zemannite and kinichilite, is unnecessary. Considerable Fe2O3 contents were determined for both zemannite and kinichilite and only the different contents of Zn and Mn allow them to be distinguished between the two species.