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

Abstract Plumbogaidonnayite, ideally PbZrSi 3 O 9 ⋅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 μm), 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 ~5 and a calculated density of 3.264 g/cm 3 . It is optically biaxial (+) with α = 1.61(3), β = 1.63(3) and γ = 1.66(4). The calculated 2V is 80°, with the optical orientations X , Y and Z parallel to the crystallographic a , b and c axes, respectively. The empirical formula is (Pb 0.70 Ca 0.17 Ba 0.01 K 0.11 Na 0.01 Y 0.01 ) Σ1.01 (Zr 1.00 Hf 0.01 Ti 0.01 ) Σ1.02 Si 3.01 O 9 ⋅2H 2 O calculated on the basis of nine oxygen atoms per formula unit and assuming the occurrence of two H 2 O groups. Plumbogaidonnayite is orthorhombic, P 2 1 nb , a = 11.7690(4) Å, b = 12.9867(3) Å, c = 6.66165(16) Å, V = 1018.17(5) Å 3 and Z = 4. The nine strongest lines of its powder XRD pattern [ d in Å ( 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 SiO 4 tetrahedra and ZrO 6 octahedra, but with disordered extra-framework sites (cations and H 2 O groups) characterised by the substitution of 2Na + (K + )→Pb 2+ (Ca 2+ ) + □ (vacancy). The discovery of plumbogaidonnayite adds a new perspective on the cation ordering and heterovalent substitution mechanism in gaidonnayite-group minerals.