CRYSTAL CHEMISTRY OF THE WYLLIEITE GROUP OF PHOSPHATE MINERALS

Three samples of minerals belonging to the wyllieite mineral group were structurally investigated: wyllieite from the Buranga pegmatite, Rwanda (A); wyllieite from the Malpensata pegmatite, Italy (B); and qingheiite from the Santa Ana pegmatite, Argentina (C). Their crystal structures were refined, based on single-crystal X-ray diffraction data, to R1 ¼ 2.72% (A), 3.53% (B), and 2.46% (C). Unit-cell parameters are: a 11.954(2), b 12.439(2), c 6.406(1) A? , b 114.54(1)8 (A); a 11.983(1), b 12.423(1), c 6.381(1) A? , b¼114.54(1)8 (B); a 11.878(3), b 12.448(2), c 6.438(2) A? , b 114.49(1)8 (C). The structure consists of kinked chains of edge-sharing octahedra stacked parallel to {101}. These chains are formed by a succession of M(2a)-M(2b) octahedral pairs, linked by slightly larger M(1) octahedra. Equivalent chains are connected in the b direction by the P(1), P(2a), and P(2b) phosphate tetrahedra to form sheets oriented perpendicular to [010]. These interconnected sheets produce channels parallel to c, channels that contain the large X cations. The X(1a) site is coordinated by anions in a distorted octahedral coordination, whereas the X(1b) site coordination can be described as a very distorted cube. The morphology of the X(2) coordination corresponds to a very distorted gable disphenoid with a [7 þ 1] coordination, similar to the coordination of the A(2)' site in the alluaudite structure. The structural features of these phosphate minerals are compared to those of other wyllieite-type phosphates: ferrorosemaryite, rosemaryite, and qingheiite-(Fe2þ). These new structural data indicate that Al is predominant at the M(2a) site in the investigated samples, with Fe2þ, Fe3þ, or Mg at the M(2b) site. Variations of unit-cell parameters, of bond distances, and of distortion coefficients among members of the wyllieite group are discussed in detail.