Crystal chemistry of the tsumcorite-group minerals. New data on ferrilotharmeyerite, tsumcorite, thometzekite, mounanaite, helmutwinklerite, and a redefinition of gartrellite
Krause, Werner; Belendorff, Klaus; Bernhardt, Heinz-Jürgen; Mccammon, Catherine; Effenberger, Herta; Mikenda, Werner
European Journal of Mineralogy Volume 10 Number 2 (1998), p. 179 - 206
published: Mar 31, 1998
manuscript accepted: Nov 6, 1997
manuscript received: Dec 9, 1996
ArtNo. ESP147051002001, Price: 29.00 €
Abstract The general formula for the tsumcorite-group minerals is Me(l)Me(2)2(XO4)2(OH,H2O)2; Me(l) = Pb, Ca, Na, and partly Bi; Me(2) = Fe, Mn, Cu, Zn, Co, Ni, and partly Al; X = P, As, V, S. Out of this group the minerals ferrilotharmeyerite, tsumcorite, thometzekite (sulfatian), mounanaite, helmutwinklerite, gartrellite and zincian gartrellite were investigated regarding optical properties, morphology, chemical composition, infrared and Mossbauer spectroscopy, and crystal structure. Crystal symmetry is either (i) triclinic in the case of an ordered occupation of two Me(2) sites, (ii) triclinic due to ordering of the hydrogen bonds in the case of species with 2 water molecules per formula unit or (iii) monoclinic in the other cases. Cell parameters were refined from X-ray powder data; powder patterns are given for thometzekite (sulfatian), mounanaite, helmutwinklerite, gartrellite and zincian gartrellite. Mossbauer measurements confirmed the iron in ferrilotharmeyerite, tsumcorite, gartrellite and zincian gartrellite to be completely trivalent. Electron microprobe analyses showed that at least partial solid solution is common for the Me(2) and less frequent for the X and Me(l) site. In addition to homovalent substitutions [e.g. Zn2+ ↔ Cu2+ or (ASO4)3- ↔ (PO0)3-] there are 3 heterovalent exchange mechanisms with a coupled substitution involving OH/H2O groups: (i) on the Me(l) site, e.g. [(Ca)2+(H2O)]2+ ↔ [(Bi)3+(OH)-]2+, (ii) on the Me(2) site, e.g. [(Zn,Cu,Co,Ni)2+(H2O)]2+ ↔ [(Fe,Al,Mn)3+(OH)]2+, and (iii) on the X site, e.g. [(AsO4)3-(H2O)]3- ↔ [(SO4)2-(OH)-]3-. Crystals of ferrilotharmeyerite, tsumcorite, thometzekite (sulfatian), and mounanaite have monoclinic symmetry, space group C2/m. The crystal structures were investigated by single-crystal X-ray diffraction. The structural data for tsumcorite given by Tillmanns & Gebert (1973) were confirmed, for the other minerals isotypy is proved. Infrared absorption spectra indicate two independent and strong hydrogen bonds. In particular, there is no evidence for protonated arsenate groups. The chemical formulas of ferrilotharmeyerite and lotharmeyerite (for analogy reasons) have to be revised on a 10 oxygen basis to Ca(Fe3+,Zn)2(AsO4)2(OH,H2O)2, and Ca(Mn3+,Zn)2 (AsO4)2(OH,H2O)2, respectively. The triclinic members of the tsumcorite group are gartrellite, zincian gartrellite, phosphogartrellite, helmutwinklerite, and probably (sulfate-free) thometzekite; the space group is P1¯, with a pronounced monoclinic C-centered pseudoctW. A redefinition based on type material was performed for gartrellite; it has the ideal formula PbCuFe3+(AsO4)2(OH)(H2O), but it frequently contains substantial amounts of Zn (at least up to 12.6 wt.% ZnO), whereas the ratio Cu:Fe is close to 1:1. The triclinic distortion is caused by an ordered arrangement of FeO6 octahedra and tetragonal bi-pyramidal Cu[4+2]O6 polyhedra, as shown by Rietveld refinement of powder data. The weak superstructure reflections of helmutwinklerite observed by single-crystal X-ray investigations are obviously caused by an ordering of the hydrogen bonds.
ferrilotharmeyerite • tsumcorite • thometzekite (sulfatian) • mounanaite • helmutwinklerite • gartrellite • zincian gartrellite • redefinition • crystal chemistry • Mossbauer study • infrared spectra • crystal structure.