Plumbogaidonnayite, PbZrSi₃O₉?2H₂O, a new Pb-member of the gaidonnayite group from the Saima alkaline complex, Liaoning Province, China

Plumbogaidonnayite, ideally PbZrSi3O9 center dot 2H(2)O, is a new gaidonnayite-group mineral discovered as a secondary product derived from the alteration of eudialyte from the Saima alkaline complex, China. It occurs as aggregates (up to 1 mm) composed of subhedral to anhedral or platy crystals (individually 5-50 mu m), associated closely with microcline, natrolite, aegirine, gaidonnayite, georgechaoite, zircon, bobtraillite and britholite-(Ce) in eudialyte pseudomorphs. The crystals are transparent, colourless or light brown with a vitreous lustre. Plumbogaidonnayite is brittle with conchoidal fracture, and it has a Mohs hardness of similar to 5 and a calculated density of 3.264 g/cm(3). It is optically biaxial (+) with alpha = 1.61(3), beta = 1.63(3) and gamma = 1.66(4). The calculated 2V is 80 degrees, with the optical orientations X, Y and Z parallel to the crystallographic a, b and c axes, respectively. The empirical formula is (Pb0.70Ca0.17Ba0.01K0.11Na0.01Y0.01)(Sigma 1.01)(Zr1.00Hf0.01Ti0.01)(Sigma 1.02)Si3.01O9 & sdot;2H(2)O calculated on the basis of nine oxygen atoms per formula unit and assuming the occurrence of two H2O groups. Plumbogaidonnayite is orthorhombic, P2(1)nb, a = 11.7690(4) angstrom, b = 12.9867(3) angstrom, c = 6.66165(16) angstrom, V = 1018.17(5) angstrom(3) and Z = 4. The nine strongest lines of its powder XRD pattern [d in angstrom (I, %) (hkl)] are: 6.489 (36) (020), 5.803 (100) (101), 4.661 (27) (021), 4.336 (29) (121), 3.640 (30) (221), 3.114 (79) (112), 2.947 (27) (400), 2.622 (27) (241) and 2.493 (27) (312). Plumbogaidonnayite has a similar spiral chain framework structure with gaidonnayite and georgechaoite, which is composed of SiO4 tetrahedra and ZrO6 octahedra, but with disordered extra-framework sites (cations and H2O groups) characterised by the substitution of 2Na(+) (K+)-> Pb2+ (Ca2+) + square (vacancy). The discovery of plumbogaidonnayite adds a new perspective on the cation ordering and heterovalent substitution mechanism in gaidonnayite-group minerals.