Segelerite from the Mount Deverell variscite deposit, Western Australia; hydrogen bonding and relationship to jahnsite

The crystal structure of segelerite, Ca (sub 2) Mg (sub 2) Fe (sub 1.4) (super 3+) Al (sub 0.6) (PO (sub 4) ) (sub 4) (OH) (sub 2) (H (sub 2) O) (sub 8) , from the Mount Deverell variscite deposit, Western Australia, has been refined using single-crystal X-ray data to wR (sub obs) =0.048 for 2082 unique reflections and all H atoms were located during the refinement. Cell parameters are a=14.7772(2) Aa, b=18.7079(2) Aa, c=7.2424(1) Aa, 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 [XM1Fe (sub 2) (super 3+) (OH) (sub 2) (PO (sub 4) ) (sub 4) ], that are linked together via corner-sharing of PO (sub 4) tetrahedra with isolated [M2(O (sub p) ) (sub 2) (H (sub 2) O) (sub 4) ] octahedra. The structures differ in the mode of linkage of the M2 octahedra, which is via trans O (sub p) ligands in segelerite and via both trans and cis O (sub p) ligands in jahnsite. In segelerite, X=M1=Ca, whereas in jahnsite-group minerals, X=Ca, Na, Mn (super 2+) and M1=Mn (super 2+) , Mg (super 2+) , Fe (super 2+) , Fe (super 3+) . X and M1 alternate along the 7 Aa axis and it is proposed that different magnitudes of rotation of the Fe (super 3+) octahedra about the 7 Aa 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.