Hexagonal ternary apatite

Hughes et al. (1990) refined the atomic arrangement of a hexagonal ternary apatite with the column anion composition of (F0.39CI0.33OH0.28), and much of the following is taken contextually or verbatim from their description of the phase. 

In a study of ternary apatites sensu stricto, Hughes et al. (1990) revealed a monoclinic variant of ternary apatite that illustrates the unaddressed complexities of apatite crystal chemistry. In the P2 lb variety, the positions of the non-column atoms are similar to those in P6j,/m ternary apatite, but the reduction in symmetry results from ordering in the anion columns, similar to the cause of monoclinicity in the chlorapatite and hydroxylapatite end-members. The positions of the column anions in hexagonal and monoclinic ternary apatite are depicted in Figure 5. The petrologic implications of monoclinic ternary apatite remain unexplored, and it is unknown how common the phase is. 

Ternary rare earth arsenates having hexagonal apatite structure have been prepared by Escobar and Baran (1982a). As with phosphates and vanadates their composition can vary widely. 

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