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) over bar (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 = 15.122(1), c = 43.258(4) angstrom; (B) alpha = 15.125(1), c = 43.198(3) angstrom; (C) a = 15.036(2), c = 42.972(9) angstrom ; (D) a = 15.090(2), c = 43.050(9) angstrom; and (E) a = 15.1416(6), c = 43.123(2) angstrom. 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)(PO4)(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 Fe2+, Mn, and Mg. This formula was accepted by the CNMNC, and the four valid mineral species occurring in the fillowite group are fillowite (C = Mn), johnsomervilleite (C = Fe2+), chladniite (C = Mg), and galileiite (B and C = Fe2+). Stornesite-(Y) is discredited, since this mineral corresponds to Y-bearing chladniite.