New data on welshite, e.g. Ca_2Mg_(3.8)Mn_(0.6)~(2+)Sb_(1.5)~(5+)O_2(Si(2.8)Be_(1.7)Fe_(0.65)~(3+)Al_(0.7)As_(0.17)O_(18)), an aenigmatite-group mineral

摘要

Electron and ion microprobe data on two samples of welshite from the type locality of Langban, Sweden, gave analytical totals of 99.38-99.57 wt. percent and BeO contents of 4.82-5.11 wt. percent, corresponding to 1.692-1.773 Be/20 O. Mossbauer and optical spectra of one of these samples gave ~(iv)Fe~(3+)/#SIGMA# Fe = 0.91, ~(iv)Fe~(2+)/#SIGMA# Fe = 0.09, and no evidence of Mn~(3+). The resulting formula for this sample is Ca_2Mg_(3.8)Mn_(0.6)~(2+)Sb_(1.5)~(5+)O_2(Si_(2.8)Be_(1.7)Fe_(0.65)~(3+)Al_(0.5)As_(0.25)O_(18)), is related by the substitution involving tetrahedral and octahedral sites: 0.59~(vi, vi)(Fe,Al)~(3+) approx = 0.42~(vi)(Mg,Mn,Fe)~(2+) + 0.21(~(vi)Sb, ~(iv)As)~(5+), i.e. 3~(vi,iv)M~(3+) = 2~(vi)M~(2+) + ~(vi,iv)M~(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 1 Be per formula unit. Given the cation distributions in other minerals related to aenigmatite, we think it is reasonable to assume that at least one tetrahedral site is > 50 percent occupied by Be and that one octahedral site is > 50 percent occupied by Sb, so that welshite should be retained as a distinct species with its own name in the aenigmatite group.