Original paper
Segelerite from the Mount Deverell variscite deposit, Western Australia. Hydrogen bonding and relationship to jahnsite
Grey, Ian E.; Mumme, W. Gus; Downes, Peter J.; Grguric, Benjamin A.; Gable, Robert W.
European Journal of Mineralogy Volume 31 Number 3 (2019), p. 465 - 471
17 references
published: Jul 9, 2019
published online: Jan 10, 2019
manuscript accepted: Dec 18, 2018
manuscript revision received: Dec 6, 2018
manuscript received: Nov 14, 2018
Abstract
The crystal structure of segelerite, Ca2Mg2 Fe 1.4 3 + 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 wR obs = 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 [XM1 Fe 2 3 + (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.
Keywords
segelerite • jahnsite • H-bonding • crystal structure • single-crystal refinement • combinatorial polymorphs