Basic ferric phosphates: a crystallochemical principle.
A polyatomic complex of iron-oxygen octahedral face-sharing triplets corner-linked to four other octahedra occurs in the atomic arrangements of the basic iron phosphates dufrenite, rockbridgeite, beraunite, and laubmannite. This complex is further knit together by the phosphate tetrahedra. The family of basic ferric phosphates arises from the variety of ways in which the complexes can be linked along a third "variable" crystallographic axis.
Moore
P B
PB