Lusernaite-(Y), Y4Al(CO3)2(OH,F)11{middle dot}6H2O, a new mineral species from Luserna Valley, Piedmont, Italy: Description and crystal structure

Abstract

The new mineral species lusernaite-(Y), ideally Y4Al(CO3)2(OH,F)11·6H2O, has been discovered in small fractures of the “Luserna Stone,” a leucocratic orthogneiss belonging to the Dora-Maira massif, Western Alps, Italy. It occurs as colorless, thin platelets, with white streak and mica-like pearly luster, elongated along [100] and flattened on {010}, arranged in radiating aggregates. Lusernaite-(Y) is associated with aeschynite-(Y), albite, “chlorite,” hematite, pyrite, quartz, and titanite. Lusernaite-(Y) has a perfect cleavage on {010} and a less marked one probably on {100}. Its calculated density is 2.810 g/cm3. In plane-polarized light, it is transparent, with parallel extinction and positive elongation. Lusernaite-(Y) is biaxial positive; its optical orientation is a = Z , b = X , c = Y . Owing to the crystal morphology, only two refractive indices could be measured, corresponding to β = 1.566(2) and γ = 1.577(2). Lusernaite-(Y) is orthorhombic, space group Pmna , with a = 7.8412(3), b = 11.0313(5), c = 11.3870(4) A, V = 984.96(7) A3, Z = 2. Main diffraction lines of the X-ray powder diffraction pattern are [ d in A, ( I ), ( hkl )]: 11.02 (100) (010), 7.90 (49) (011), 5.66 (25) (002), 5.06 (24) (012), 4.258 (33) (112), 3.195 (27) (220), 3.095 (21) (212). Raman spectroscopy confirmed the presence of CO3 groups (sharp peak at 1096 cm−1); due to the very strong luminescence, the bands of the OH and H2O groups could not be seen. Chemical analyses by electron microprobe gave (wt%) Al2O3 6.11, Y2O3 43.52, La2O3 0.02, Ce2O3 0.04, Nd2O3 0.03, Sm2O3 0.16, Gd2O3 1.39, Dy2O3 3.46, Er2O3 3.15, Yb2O3 2.09, CaO 0.33, PbO 0.37, H2O 22.76, CO2 9.95, F 1.40, O≡F −0.59, sum 94.19; H2O and CO2 were determined from structure refinement. The empirical formula by assuming the presence of 2 (CO3)2− groups, 11 (OH,F)− anions, and 6 H2O groups, in agreement with micro-Raman and structural results, is (Y3.41Dy0.16Er0.15 Yb0.09Gd0.07Ca0.05Pb0.02Sm0.01)∑3.96Al1.06(CO3)2.00(OH10.35F0.65)∑11.00·6H2O. The crystal structure was solved by direct methods and refined on the basis of 840 observed reflections to R 1 = 6.8%. In the structure of lusernaite-(Y), yttrium and REE cations occupy two distinct sites, Y1 and Y2, both in eightfold coordination. The structure is built by layers parallel to (010), formed by chains of edge-sharing Y-centered polyhedra (Y1), which run along [100], and are connected along c through Al-centered octahedra. These chains are decorated on one side by corner-sharing chains of Y-centered polyhedra (Y2), and on the other side by CO3 groups. Along [001] the decorated chains alternate their polarity. Lusernaite-(Y), named after the type locality, the Luserna Valley, shows a new kind of structure among the natural carbonates of REE. Its origin is related to the circulation of hydrothermal solutions during the late-stage Alpine tectono-metamorphic events.