CRYSTAL-CHEMISTRY OF THE TSUMCORITE-GROUP MINERALS - NEW DATA ON FERRILOTHARMEYERITE, TSUMCORITE, THOMETZEKITE, MOUNANAITE, HELMUTWINKLERITE, AND A REDEFINITION OF GARTRELLITE

The general formula for the tsumcorite-group minerals is Me(1)Me(2)(2) (XO4)(2)(OH,H2O)(2); Me(1) = 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 th e minerals ferrilotharmeyerite, tsumcorite, thometzekite (sulfatian), mounanaite, helmutwinklerite, gartrellite and zincian gartrellite were investigated regarding optical properties, morphology, chemical compo sition, infrared and Mossbauer spectroscopy, and crystal structure. Cr ystal symmetry is either (i) triclinic in the case of an ordered occup ation of two Me(2) sites, (ii) triclinic due to ordering of the hydrog en bonds in the case of species with 2 water molecules per formula uni t or (iii) monoclinic in;the other cases. Cell parameters were refined from X-ray powder data; powder patterns are given for thometzekite (s ulfatian), mounanaite, helmutwinklerite, gartrellite and zincian gartr ellite. Mossbauer measurements confirmed the iron in ferrilotharmeyeri te, tsumcorite, gartrellite and zincian gartrellite to be completely t rivalent. Electron microprobe analyses showed that at least partial so lid solution is common for the Me(2) and less frequent for the X and M e(1) site. In addition to homovalent substitution [e.g. Zn2+ <----> Cu 2+ or (AsO4)(3-) <----> (PO4)(3-)] there are 3 heterovalent exchange m echanisms with a coupled substitution involving OH/H2O groups: (i) on the Me(1) 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 s ymmetry, 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 miner als isotypy is proved. Infrared absorption spectra indicate two indepe ndent and strong hydrogen bonds. In particular, there is no evidence f or protonated arsenate groups. The chemical formulas of ferrilotharmey erite 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.