A STRUCTURAL HIERARCHY FOR PHOSPHATE MINERALS

Large-cation polyhedra occur in group (1) and group (2) phosphates, as cations such as Al and Ca often occur together in minerals. In principle, octahedra should not occur (by definition) in group (3) phosphates however, in some cases, the structural affinity of the mineral indicates it to be a large-cation phosphate mineral (e.g., whidocWte, 

As noted above, (PO4) tetrahedra can polymerize with other (PO4) groups, plus tetrahedrally coordinated Be, Zn, B, Al and Si. However, in minerals, only the following polymerizations are known (P04)-(P04), (P04)-(Bect)4), (P04)-(Zn04) and (P04)-(A104). 

Although there seems no obvious reason why (PO4) should not polymerize with (B( )4) or (Si t)4) groups, this has not been observed, although there are several minerals containing both (PO4) and (B( )4) or (Si( )4) groups in which the different polyhedra do not polymerize. 

---