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

Abstract The new mineral, gysinite-(La), with the ideal formula PbLa(CO 3 ) 2 (OH)⋅H 2 O, 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 μm in size, in interstices or enclosed in microcline, aegirine and nepheline. Associated minerals include nepheline, aegirine, microcline, natrolite, eudialyte, lamprophyllite, bastnäsite-(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 3½ to 4. The calculated density is 5.007 g/cm 3 . Optically, gysinite-(La) is biaxial (–), α= 1.832(2), β= 1.849(4), γ = 1.862(5) in white light and 2V meas = 81.6°. The empirical formula of gysinite-(La) is (La 0.93 Pb 0.61 Nd 0.23 Pr 0.14 Sr 0.04 Gd 0.02 Sm 0.01 Eu 0.01 Ca 0.01 ) Σ2 (CO 3 ) 2 (OH) 1.34 ⋅0.66H 2 O, which is calculated on the basis of general formula (REE x M 2+ 2– x )(CO 3 ) 2 (OH) x ⋅(2– x )H 2 O. The strongest eight lines of its powder X-ray diffraction pattern [ d , Å ( 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) Å, b = 8.5990(3) Å, c = 7.3901(4) Å, V = 321.90(2) Å 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.