Mineralogy and crystal structure determination of Mg-fillowite

Accurate crystal structure determination of complex phosphate Mg-fillowite.—Na₂Ca(Mn₄Mg₂Fe)₇PO₄]₆ has been finished. Mg-fillowite occurs in muscovite-pegmatite in the Altay area, Xinjiang, China. It is of the trigonal lattice, with space group R-3, unit cell a=1.5143(3)nm, c=4.3191(2) nm, V=8.5736 nm³, Z = 18. The R-factor of the determination accuracy is R (I<2 σ(I)) = 0.0776. The cationic polyhedra consist of two kinds of structural units: compound column and screw column. In three-dimensional space, they are linked with each other through corner sharing or edge sharing to build the entire structure. There are 45 atoms in an asymmetric unit and 720 atoms in a unit cell. In terms of types and connection manners of the coordination polyhedra, the degree of complicity is rarely seen in all mineral crystal structures. Study of such a structure is significant for the classification of phosphate and the building rule of coordination polyhedra in a complex crystal structure.