The crystal structure of stenonite, Sr 2 [AlF 5 CO 3 ], a 5.450(2), b 8.704(2), c 13.150(3) Aa, beta 98.72(2) degrees , V 616.6(1) Aa 3 , Z = 4, space group P2 1 /n has been solved by direct methods and refined to a residual R = 6.0% for 1219 observed (3sigma ) reflections. The AlOe 6 octahedron (Oe: unspecified anion) and CO 3 triangle link by corner-sharing to form a finite [AlCO 3 F 5 ] (super 4-) cluster. These clusters are linked into a three dimensional structure - and - coordinated Sr cations. A structural classification of the aluminofluoride minerals is set up on the premise that crystal structures may be classified according to the polymerization of co-ordination polyhedra with higher bond-valences. The minerals are divided first into two categories: (1) simple aluminofluorides and (2) compound aluminofluorides containing another strongly bonded complex anionic group. They are further subdivided according to the polymerization of the AlOe 6 octahedra, in (1), or of these octahedra with other complex anionic groups, in (2). The degree of polymerization within the complex aluminofluorides is shown to be a function of the Lewis basicity of the constituent cation polyhedra.--Modified journal abstract.