Yuchuanite-(Y), Y₂(CO₃)₃?H₃O, a new hydrous yttrium carbonate mineral from the Yushui Cu deposit, South China

A new mineral species, yuchuanite-(Y), ideally Y-2(CO3)(3)center dot H2O, has been discovered and characterized in the Yushui Cu deposit in South China. The mineral occurs in bedded/massive ore and is associated with bornite, chalcopyrite, galena, sphalerite, bastn & auml;site-(Y), xenotime-(Y), anhydrite, and quartz. Individual crystals range in size from 30 to 300 mu m. No twinning is observed. The mineral is colorless and transparent with a vitreous luster. The calculated density is 3.62 g/cm(3). An electron microprobe analysis yields the empirical formula (based on 10 O apfu), (Y1.61Yb0.11Er0.11Dy0.08Ho0.03 Gd0.02Tm0.02)(Sigma 1.99)(CO3)(3)center dot H2O. Yuchuanite-(Y) is triclinic, with space group P(1) over bar (#2), Z = 6, and unit-cell parameters a = 6.2134(3) & Aring;, b = 8.9697(3) & Aring;, c = 19.9045(7) & Aring;, alpha = 91.062(3)degrees, beta = 90.398(3)degrees, gamma = 91.832(3)degrees, and V = 1108.54(8) & Aring;(3). The structure is constructed from (110) sheets of eight-coordinated Y polyhedra and C trigonal planar groups. All Y polyhedra are linked by shared edges. The Y atoms occupy six independent crystallographic sites of two different coordination types: [YO7(H2O)] and [YO8]. The chemical composition of yuchuanite-(Y) is similar to tengerite-(Y), Y-2(CO3)(3)center dot 2-3H(2)O, but is distinct in the crystal structure, such as crystal system, space group, and unit cell, from that of tengerite-(Y). The Y polyhedra of tengerite-(Y) are nine-coordinated, while those of yuchuanite-(Y) are eight-coordinated. Moreover, their structures could be both described as sheet structures built up from Y polyhedra and CO3 trigonal planar groups but link together in significantly different ways. Thus, yuchuanite-(Y) is not a polytype of tengerite-(Y) but is an independent mineral species.