Crystal chemistry of type paulkerrite and establishment of the paulkerrite group nomenclature

A single-crystal structure determination and refinement has been conducted for the type specimen of paulkerrite. The structure analysis showed that the mineral has monoclinic symmetry, space group P21/c, not orthorhombic, Pbca, as originally reported. The unit-cell parameters are aCombining double low line10.569(2), bCombining double low line20.590(4), cCombining double low line12.413(2)Å, and βCombining double low line90.33(3)°. The results from the structure refinement were combined with electron microprobe analyses to establish the empirical structural formula A1[(H2O)0.98K0.02]ς1.00 A2K1.00 M1(Mg1.02Mn0.982+)ς2.00 M2(Fe1.203+Ti0.544+Al0.24Mg0.02)ς2.00 M3(Ti0.744+ Fe0.263+)ς1.00 (PO4)4.02X[O1.21F0.47(OH)0.32]ς2.00(H2O)10.3.95H2O, which leads to the end-member formula (H2O)KMg2Fe2Ti(PO4)4(OF)(H2O)10.4H2O. A proposal for a paulkerrite group, comprising orthorhombic members benyacarite, mantiennéite, pleysteinite, and hochleitnerite and monoclinic members paulkerrite and rewitzerite, has been approved by the International Mineralogical Association's Commission for New Minerals, Nomenclature and Classification. The general formulae are A2M12M22M3(PO4)4X2(H2O)10.4H2O and A1A2M12M22M3(PO4)4X2(H2O)10.4H2O for orthorhombic and monoclinic species, respectively, where ACombining double low lineK, H2O, (Combining double low linevacancy); M1Combining double low lineMn2+, Mg, Fe2+, Zn (rarely Fe3+); M2 and M3Combining double low lineFe3+, Al, Ti4+ (and very rarely Mg); XCombining double low lineO, OH, F. In monoclinic species, K and H2O show an ordering at the A1 and A2 sites, whereas O, (OH), and F show a disordering over the two non-equivalent X1 and X2 sites, which were hence merged as X2 in the general formula. In both monoclinic and orthorhombic species, a high degree of mixing of Fe3+, Al, and Ti occurs at the M2 and M3 sites of paulkerrite group members, making it difficult to get unambiguous end-member formulae from the structural determination of the constituents at individual sites. To deal with this problem an approach has been used that involves merging the compositions at the M2 and M3 sites and applying the site-total-charge method. The merged-site approach allows end-member formulae to be obtained directly from the chemical analysis without the need to conduct crystal-structure refinements to obtain the individual site species. Copyright: