Structural ordering in calcioferrite-group minerals. Refinement of the crystal structure of fanfaniite from the Hühnerkobel pegmatite, Bavaria

Abstract Fanfaniite, Ca 4 Mn 2+ Al 4 (PO 4 ) 6 (OH) 4 ·12H 2 O, from the Hühnerkobel pegmatite mine, Bavaria, has been characterised by chemical analyses and synchrotron single-crystal diffraction. The average crystal structure was refined in space group C 2/ c (cell parameters a = 10.055(2), b = 24.132(5), c = 6.2590(10) Å, β = 91.35(3)°) to compare with reported monoclinic structures of other calcioferrite-group minerals with general formula Ca 4 AB 4 (PO 4 ) 6 (OH) 4 ·12H 2 O, A = Mn 2+ , Fe 2+ , Mg, B = Al, Fe 3+ . The average structure contains disordered half-occupied A sites and associated coordinated water molecules. The diffraction data for fanfaniite contains weak reflections that violate the c -glide condition, as also reported for montgomeryite, and in addition contains extremely weak, diffuse reflections requiring a doubling of a , as reported for kingsmountite. Structure refinements were conducted for the noncentrosymmetric C 2 model used for montgomeryite and for the P $\bar 1$ model used for kingsmountite. The fanfaniite diffraction data is better explained by the triclinic model with doubled a cell parameter, although the extent of ordering of the A -site cations is considerably lower (56%) than reported for kingsmountite (85%). If the C 2 model contributes, it can only be at the scale of the unit cell.