Cation distribution in Mg, Mn-bearing babingtonite from Arvigo, Val Calanca, Grisons, Switzerland

The crystal structures of three babingtonites, space group P (1) over bar , of simplified Ca2Fe2+Fe3+[Si5O14OH] composition were refined from X-ray sin gle-crystal data. Two samples originated from low-temperature Alpine fissur es at Arvigo (Switzerland) and one from a skarn at Westfield (Mass.). Both Arvigo samples were enriched in Mg (0.21-0.28 p.f.u.) and Mn (0.21-0.26 p.f .u.) yielding the following cell dimensions, Arvigo 1: a = 7.468(1.), b = 1 2.161(4),c = 6.675(2) A, alpha = 86.09(3), beta = 93.94(2), gamma = 112.21( 2)degrees, and Arvigo 2: a = 7.480(1), b = 12.159(1), c = 6.678(2) Angstrom , alpha = 86.10(2), beta = 93.92(1), gamma = 112.08(2)degrees. Site populat ion refinements for the Arvigo, babingtonite crystal (Arvigo 2) that displa yed epitaxially]hedenbergite whiskers yielded significant Mg on the Fe2 sit e, commonly occupied by ferric iron and minor Al. For stoichiometry reasons significant Fe3+ must be assumed on Fe1, commonly occupied by ferrous iron , Mn2+, and Mg. Only a very low degree of Fe2+/Fe3+ disorder could be deriv ed for the skarn babingtonite from Westfield confirming published Mossbauer data.