| The alluaudite-group is composed of four subgroups: the alluaudite-, wyllieite-, bobfergusonite- and "elephantite"-subgroups. A significant issue in the crystal chemistry of the alluaudite-group minerals is the choice of cell orientation; the alluaudite-group can be presented in C2/c or l2/a. The l2/a space group has been selected due to the smaller beta angle, which is conformable with the space group P21/c for the wyllieite and "elephantite" structure, and P21/n for the bobfergusonite structure.;The proposed general formulae for the alluaudite-group is: A B C D2 (PO4)3 and the wyllieite-subgroup is: A(1) A(2) B2 C2 D(1)2 D(2)2 (PO4)6; both general formulae are based on a simple lettering scheme for each of the crystallographic sites. A nomenclature scheme is presented for both the alluaudite- and wyllieite-subgroups based on occupants at the C- and D-sites.;The Cross Lake pegmatite field occurs over an area of about 200 km 2 at the western margin of the Sachigo subprovince in central Manitoba. (Abstract shortened by UMI.)... |
