The <i>M</i> ₂ (PO ₄ ) ₂ Φ ₈ cyclic tetramer – a flexible structure-building unit – Part 1: Primary pegmatite phosphate minerals

Abstract. The minerals of the triphylite, alluaudite, zwieselite, and graftonite groups are amongst the most abundant of the primary phosphate minerals in granitic pegmatites. An analysis of the crystal structures of the four groups from the perspective of the connectivity between the metal atom polyhedra and the PO4 tetrahedra for the individual metal atom sublattices has resulted in the identification of structural motifs that are common to the four primary mineral groups. All four structure types contain 2-polyhedra-wide ribbons formed from corner linking of M2(PO4)2Φ8 cyclic tetramers, oriented along [001]. The ribbons in the four structure types all have the same topology but correspond to different geometrical isomers due to different orientations of the tetrahedra and different pairs of polyhedral edges involved in the ribbon formation. In the alluaudite, zwieselite, and graftonite group minerals, the polymerised cyclic tetramer (PCT) ribbons are connected along [010] by octahedral edge sharing, giving the same topology of (100) layers, with correspondingly similar b and c unit-cell parameters for the structures of the three mineral groups. The same PCT ribbons are also present in arrojadite–dickinsonite group primary phosphates. The persistent presence of the same type of structural unit across several different mineral groups is related to the high flexility of the cyclic tetramers to adjust to different crystal chemistries by rotation and buckling about the polyhedral corner linkages.