• journal_title：American Mineralogist
• Contributor：Cristian Biagioni ; Elena Bonaccorsi ; Fernando Cámara ; Marcella Cadoni ; Marco E. Ciriotti ; Danilo Bersani ; Uwe Kolitsch
• Publisher：Mineralogical Society of America
• Date：2013-07-01
• Format：text/html
• Language：en
• Identifier：10.2138/am.2013.4366
• journal_abbrev：American Mineralogist
• issn：0003-004X
• volume：98
• issue：7
• firstpage：1322
• section：Articles

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) Å, V = 984.96(7) ų, Z = 2. Main diffraction lines of the X-ray powder diffraction pattern are [d in Å, (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 (Y_3.41Dy_0.16Er_0.15 Yb_0.09Gd_0.07Ca_0.05Pb_0.02Sm_0.01)_∑3.96Al_1.06(CO₃)_2.00(OH_10.35F_0.65)_∑11.00·6H₂O.

The crystal structure was solved by direct methods and refined on the basis of 840 observed reflections to R₁ = 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.