The crystal structure of segelerite, Ca2Mg2Fe1.43+Al0.6(PO4)4(OH)2(H2O)8, from the Mount Deverell variscite deposit, Western Australia, has been refined using single-crystal X-ray data to wRobs = 0.048 for 2082 unique reflections and all H atoms were located during the refinement. Cell parameters are a = 14.7772(2) Å, b = 18.7079(2) Å, c = 7.2424(1) Å, space group Pbca. The H-bonding scheme is described and compared to that for the combinatorial polymorph, jahnsite. The crystal structures of both minerals comprise heteropolyhedral slabs of composition [XM1Fe23+(OH)2(PO4)4], that are linked together via corner-sharing of PO4 tetrahedra with isolated [M2(Op)2(H2O)4] octahedra. The structures differ in the mode of linkage of the M2 octahedra, which is via trans Op ligands in segelerite and via both trans and cis Op ligands in jahnsite. In segelerite, X = M1 = Ca, whereas in jahnsite-group minerals, X = Ca, Na, Mn2+ and M1 = Mn2+, Mg2+, Fe2+, Fe3+. X and M1 alternate along the 7 Å axis and it is proposed that different magnitudes of rotation of the Fe3+ octahedra about the 7 Å axis to accommodate the different coordination requirements of the X and M1 cations drives the symmetry changes in the two minerals so that a strong H-bonding network is maintained.

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