Redefinition of the formula for aldermanite, [Mg(H 2 O) 6 ][Na(H 2 O) 2 Al 3 (PO 4) 2 (OH,F) 6 ]H 2 O, and its crystal structure

Aldermanite from Tom's quarry in the Kapunda-Angaston area of the Mount Lofty Ranges, South Australia has been characterised by electron microprobe analyses and single-crystal structure analysis. The empirical formula is Na0.72K0.13Ca0.06Mg1.15Al2.92(PO4)2.05[(OH)2.92F2.96]Σ5.888.91H2O, based on 23 anions. Analysis of a specimen from the type locality, the nearby Klemm's quarry, Moculta, gave a similar formula, Na0.59K0.06Ca0.36Mg0.92Al3.16(PO4)1.97[(OH)4.08F2.70]Σ6.788.36H2O. Na and F were not analysed in the original description of the mineral. The ideal end-member formula is [Mg(H2O)6][Na(H2O)2Al3(PO4)2(OH)6]H2O, compared to the original formula of Mg5Al12(PO4)8(OH)22nH2O with n ≈ 32. Aldermanite is monoclinic, P21/c with a = 13.524(3), b = 9.958(2), c = 7.013(1) Å and β = 97.40(3)°. The crystal structure of aldermanite is built from sawtooth layers of cis- and trans-corner-connected, Al-centred octahedra, decorated with corner-connected PO4 tetrahedra to give (100) layers of composition Al3(PO4)2(OH,F)6. Interlayer Mg(H2O)6 octahedra and H2O molecules hold the layers together through H bonding. The corner-connected octahedra form 6-membered rings that are centred by 8-coordinated Na and have a topology identical to a 3-octahedra-wide {110} slice of the pyrochlore structure. This pyrochlore element contains intersecting kagomé nets of Al atoms, parallel to (111) and (11) of cubic pyrochlore. Minerals of the walentaite group, as well as zirconolite-3O polytypes have the same type of intersecting kagomé nets of small cations.