• journal_title：Mineralogical Magazine
• Contributor：M. A. Cooper ; T. A. Husdal ; N. A. Ball ; Y. A. Abdu ; F. C. Hawthorne
• Publisher：Mineralogical Society of Great Britain and Ireland
• Date：2013-04-01
• Format：text/html
• Language：en
• Identifier：10.1180/minmag.2013.077.3.09
• journal_abbrev：Mineralogical Magazine
• issn：0026-461X
• volume：77
• issue：3
• firstpage：353
• section：Articles

Schlüterite-(Y), ideally (Y,REE)₂Al(Si₂O₇)(OH)₂F, is a new silicate mineral species from the Stetind pegmatite, Tysfjord, Nordland, Norway. It forms dense, fibrous, radiating aggregates (up to ~2 mm) diverging to individual needle-like crystals (up to ~1 mm long) in cavities. Crystals are acicular to bladed, flattened on {001} and elongated along [010], and the dominant form is {001}. Schlüterite-(Y) is transparent, pale pink with a white streak and a vitreous lustre, and does not fluoresce under short-wave ultraviolet light. Mohs hardness is 5½–6, and schlüterite-(Y) is brittle with an irregular fracture, and has no cleavage. The calculated density is 4.644 g/cm³. The indices of refraction are α = 1.755, β = 1.760, γ = 1.770, all ± 0.005, 2V_obs = 71.8 (5)°, 2V_calc = 71°, non-pleochroic, optic orientation is X ^ a = 83.1° (β obtuse), Y // b, Z ^ c = 50.3° (β acute). Schlüterite-(Y) is monoclinic, space group P2₁/c, a 7.0722(2), b 5.6198(1), c 21.4390(4) Å, β 122.7756(3)°, V 716.43(5) ų, Z = 4. The seven strongest lines in the X-ray powder-diffraction pattern are as follows: [d Å), I, (hkl)]: 4.769, 100, (012); 2.972, 55, (2İ14); 3.289, 51, (112); 2.728, 49, (2İ16); 2.810, 37, (020); 3.013, 37, ((1İ16); 4.507, 36, (004). Chemical analysis by electron microprobe gave SiO₂ 22.64, Al₂O₃ 9.45, Y₂O₃ 15.35, La₂O₃ 3.25, Ce₂O₃ 9.69, Pr₂O₃ 2.05, Nd₂O₃ 9.50, Sm₂O₃ 3.57, Gd₂O₃ 4.65, Dy₂O₃ 4.21, Er₂O₃ 2.31, Yb₂O₃ 1.86, F 2.71, H₂O_calc 3.78, O = F −1.14, sum 93.88 wt%. The H₂O content was determined by crystal-structure analysis. On the basis of 10 anions with (OH) + F = 3 a.p.f.u. (atoms per formula unit), the empirical formula is (Y_0.73Ce_0.32Nd_0.30Gd_0.14Dy_0.12La_0.11Sm_0.11Pr_0.07Er_0.06Yb_0.05)_Σ=2.01Al_0.99Si_2.01O₇(OH)_2.24F_0.76. The crystal structure of schlüterite-(Y) was solved by direct methods and refined to an R₁ index of 1.8% based on 1422 unique observed reflections. In the structure of schlüterite-(Y), Al(OH)₄O₂ octahedra share (OH)–(OH) edges to form [MΦ₄] chains that are decorated by (Si₂O₇) groups that bridge O vertices of neighbouring octahedra in a staggered fashion on either side of the chain. These [Al(OH)₂(Si₂O₇)] chains extend parallel to b, and are linked into a continuous framework via bonds to interstitial ^[8](Y,REE) (= <2.400 Å>) and ^[9](Y,REE) (= <2.548 Å>) atoms.