New data on welshite, e.g. Ca2Mg3.8Mn0.62+Fe0.12+Sb1.55+O2[Si2.8Be1.7Fe0.653+Al0.7As0.17O18], an aenigmatite-group mineral

Electron and ion microprobe data on two samples of welshite from the type l ocality of Langban, Sweden, gave analytical totals of 99.38-99.57 wt.% and BeO contents of 4.82-5.11 wt.%, corresponding to 1.692-1.773 Be/20 O. Mossb auer and optical spectra of one of these samples gave Fe-[iv](3+)/Sigma Fe = 0.91, Fe-[iv](2+)/Sigma Fe = 0.09, and no evidence of Mn3+. The resulting formula for this sample is Ca2Mg3.8Mn0.62+Fe0.12+Sb1.55+O2[Si2.8Be1.7Fe0.6 53+Al0.7As0.17O18], and that for the second sample, Ca2Mg3.8Mn0.12+Fe0.12+F 0.83+Sb1.25+O2[Si2.8Be1.8F0.653+Al0.25As0.25O18], is related by the substit ution involving tetrahedral and octahedral sites: 0.59([vi,iv])(Fe,Al)(3+) approximate to 0.42([vi])(Mg,Mn,Fe)(2+) + 0.21(Sb-[vi],As-[iv])(5+), i.e. 3 ([vi,iv]) M3+ = 2([vi])M(2+) + M-[vi,iv](5+). WelShite is distinctive among aenigmatite-group minerals in the high proportion of Fe 3+ in tetrahedral coordination and is unique in its Be content, substantially exceeding 1Be p er formula unit. Given the cation distributions in other minerals related t o aenigmatite, we think it is reasonable to assume that at least one tetrah edral site is >50% occupied by Be and that one octahedral site is >50% occu pied by Sb, so that welshite should be retained as a distinct species with its own name in the aenigmatite group.