Gysinite-(La), PbLa(CO3)(2)(OH)center dot H2O, a new rare earth mineral of the ancylite group from the Saima alkaline complex, Liaoning Province, China

The new mineral, gysinite-(La), with the ideal formula PbLa(CO3)(2)(OH).H2O, has been discovered in lujavrite from the Saima alkaline complex, Liaoning Province, China. It commonly occurs as subhedral to anhedral, granular and platy crystals of 5 to 50 mu m in size, in interstices or enclosed in microcline, aegirine and nepheline. Associated minerals include nepheline, aegirine, microcline, natrolite, eudialyte, lamprophyllite, bastnasite-(Ce), parasite-(Ce), ancylite-(La), ancylite-(Ce), bobtraillite, britholite-(Ce), thorite, calcite and galena. The crystallisation of gysinite-(La) may be related to the post-magmatic carbonation event. Gysinite-(La) crystals are generally transparent, colourless, or pale yellow, with a vitreous lustre and white streak. It is brittle with an uneven fracture, and the estimated Mohs hardness is 31/2 to 4. The calculated density is 5.007 g/cm(3). Optically, gysinite-(La) is biaxial (-), alpha= 1.832(2), beta= 1.849(4), gamma = 1.862(5) in white light and 2V(meas) = 81.6 degrees. The empirical formula of gysinite-(La) is (La0.93Pb0.61Nd0.23Pr0.14Sr0.04Gd0.02Sm0.01Eu0.01Ca0.01)(Sigma 2)(CO3)(2)(OH)(1.34).0.66H(2)O, which is calculated on the basis of general formula (REExM2-x2+)(CO3)(2)(OH)(x).(2-x)H2O. The strongest eight lines of its powder X-ray diffraction pattern [d, angstrom (I, %) (hkl)] are: 5.596 (21) (011), 4.349 (100) (110), 3.732 (68) (111), 2.984 (61) (121), 2.667 (21) (031), 2.363 (48) (131), 2.090 (29) (221) and 2.028 (21) (212). Gysinite-(La) is orthorhombic, in the space group Pmcn, and unit-cell parameters refined from single-crystal X-ray diffraction data are: a = 5.0655(2) angstrom, b = 8.5990(3) angstrom, c = 7.3901(4) angstrom, V = 321.90(2) angstrom(3) and Z = 2. It is a new member of the ancylite group and isostructural with gysinite-(Nd), but with La and Pb dominant in the metal cation sites in the structure.