Schlüterite-(Y), ideally (Y,<i>REE</i>)₂Al(Si₂O₇)(OH)₂F, a new mineral species from the Stetind pegmatite, Tysfjord, Nordland, Norway: description and crystal structure

Abstract Schlüterite-(Y), ideally (Y, REE ) 2 Al(Si 2 O 7 )(OH) 2 F, 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/cm 3 . The indices of refraction are α = 1.755, β = 1.760, γ = 1.770, all ± 0.005, 2 Vobs = 71.8 (5)°, 2 Vcalc = 71°, non-pleochroic, optic orientation is X ˆ a = 83.1° (β obtuse), Y//b, Z ˆ c = 50.3° (β acute). Schlüterite-(Y) is monoclinic, space group P 2 1 / c , a 7.0722(2), b 5.6198(1), c 21.4390(4) Å, β 122.7756(3)°, V 716.43(5) Å 3 , 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, ( 14); 3.289, 51, (112); 2.728, 49, ( 16); 2.810, 37, (020); 3.013, 37, (( 16); 4.507, 36, (004). Chemical analysis by electron microprobe gave SiO 2 22.64, Al 2 O 3 9.45, Y 2 O 3 15.35, La 2 O 3 3.25, Ce 2 O 3 9.69, Pr 2 O 3 2.05, Nd 2 O 3 9.50, Sm 2 O 3 3.57, Gd 2 O 3 4.65, Dy 2 O 3 4.21, Er 2 O 3 2.31, Yb 2 O 3 1.86, F 2.71, H 2 O calc 3.78, O = F −1.14, sum 93.88 wt%. The H 2 O 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 0.73 Ce 0.32 Nd 0.30 Gd 0.14 Dy 0.12 La 0.11 Sm 0.11 Pr 0.07 Er 0.06 Yb 0.05 ) Σ=2.01 Al 0.99 Si 2.01 O 7 (OH) 2.24 F 0.76 . The crystal structure of schlüterite-(Y) was solved by direct methods and refined to an R 1 index of 1.8% based on 1422 unique observed reflections. In the structure of schlüterite-(Y), Al(OH) 4 O 2 octahedra share (OH)–(OH) edges to form [MΦ4] chains that are decorated by (Si 2 O 7 ) groups that bridge O vertices of neighbouring octahedra in a staggered fashion on either side of the chain. These [Al(OH) 2 (Si 2 O 7 )] chains extend parallel to b, and are linked into a continuous framework via bonds to interstitial [8] (Y, REE ) (= &lt;2.400 Å&gt;) and [9] (Y, REE ) (= &lt;2.548 Å&gt;) atoms.

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