CRYSTAL CHEMISTRY AND NOMENCLATURE OF FILLOWITE-TYPE PHOSPHATES

The crystal chemistries of five samples of minerals belonging to the fillowite group were structurally investigated: (A) fillowite from the Buranga pegmatite, Rwanda; (B) fillowite from the Kabira pegmatite, Uganda; (C) johnsomervilleite from Loch Quoich, Scotland; (D) johnsomervilleite from the Malpensata pegmatite, Italy; and (E) chladniite from the Sapucaia pegmatite, Minas Gerais, Brazil. Their crystal structures were refined in space group R 3 (No. 148), using single-crystal X-ray diffraction data, to R 1 values of (A) 3.79%, (B) 3.52%, (C) 4.14%, (D) 4.04%, and (E) 5.59%. Unit-cell parameters are: (A) a ¼ 15.122(1), c ¼ 43.258(4) Å; (B) a ¼ 15.125(1), c ¼ 43.198(3) Å; (C) a ¼ 15.036(2), c ¼ 42.972(9) Å; (D) a ¼ 15.090(2), c ¼ 43.050(9) Å; and (E) a ¼ 15.1416(6), c ¼ 43.123(2) Å. The asymmetric unit contains 15 cation sites with coordinations ranging from V to IX, as well as six P sites. The complex structure can be split into three types of chains running parallel to the c axis. These chains are composed of edge-and face-sharing polyhedra. Detailed cation distributions were determined for all five samples, and their comparison allowed us to establish the general formula A 3 BC 11 (PO 4) 9 for fillowite-type phosphates, where A represents the group of sites mainly occupied by Na, B the Ca sites, and C the sites containing the divalent cations Fe 2þ , Mn, and Mg. This formula was accepted by the CNMNC, and the four valid mineral species occurring in the fillowite group are fillowite (C ¼ Mn), johnsomervilleite (C ¼ Fe 2þ), chladniite (C ¼ Mg), and galileiite (B and C ¼ Fe 2þ). Stornesite-(Y) is discredited, since this mineral corresponds to Y-bearing chladniite.