A Structure Hierarchy for the Aluminofluoride Minerals

The structure hierarchy hypothesis states that structures may be ordered hierarchically according to the polymerization of coordination polyhedra of higher bond-valence, and such hierarchies are useful in understanding the general compositional, structural, and paragenetic variations within the structural group of interest. Here we develop a structure hierarchy for the aluminofluoride minerals based on the polymerization of the dominant (AlΦ6) octahedra and their linkage with other strongly bonded complex anionic groups. The minerals are divided first into the following categories: (1) simple aluminofluorides and (2) compound aluminofluorides containing other oxyanions. The minerals are then ordered according to the polymerization of the constituent polyhedra into a coherent structural hierarchy. The chemical composition and crystal-chemical details of the ions of the interstitial complex are a collective function of the Lewis acidity of the interstitial cations; the presence of interstitial anions, both simple [F–, (OH)–] and complex [(SO4)2–]; self-polymerization of the (AlF6)3– octahedra; and polymerization with both Mg(F,OH)6 octahedra and other complex anions: (SO4)2–, (PO4)3–, (CO3)2–.