Si-rich bergslagite from a granitic pegmatite at Tennvatn, north Norway

Abstract

Si-rich bergslagite from an amazonite-cleavelandite pegmatite at Tennvatn in Sørfold, Nordland, north Norway, has the empirical composition Ca1Ca1.00 Ca2Ca0.068(Be0.94Si0.06)(As0.774Si0.226)O4[(OH)0.97 O0.03], based on a single-crystal structure refinement (MoKα radiation, CCD area detector, 293 K, space group P21/c, a = 4.878(1), b = 7.810(2), c = 10.130(2) Å, β = 90.09(3)[ddot], V = 385.92(15) Å3, Z = 4, R1(F) = 4.48%) and electron-microprobe analyses of Ca, As and Si. The structure refinement revealed the presence of a previously unknown, partially occupied and octahedrally coordinated Ca2 site that provides charge-balance compensation. The sample has the highest content of Si ever reported in bergslagite, and can be compared to (As0.87Si0.08) found in bergslagite from the type locality (Långban, Sweden). The results also indicate that the previously postulated OH substitution scheme in type bergslagite is erroneous. Substantial substitution of Si for As in arsenates and arsenosilicates seems to be relatively rare; some examples are discussed.