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

Schluterite-(Y), ideally (Y,REE)(2)Al(Si2O7)(OH)(2)F, is a new silicate mineral species from the Stetind pegmatite, Tysfjord, Nordland, Norway. It forms dense, fibrous, radiating aggregates (up to similar to 2 mm) diverging to individual needle-like crystals (up to similar to 1 mm long) in cavities. Crystals are acicular to bladed, flattened on {001} and elongated along [010], and the dominant form is {001}. Schluterite-(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 51/2-6, and schluterite-(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 alpha = 1.755, beta = 1.760, gamma = 1.770, all +/- 0.005, 2V(obs) = 71.8 (5)degrees, 2V(calc) = 71 degrees, non-pleochroic, optic orientation is X (boolean AND) a = 83.1 degrees (beta obtuse), Y // b, Z (boolean AND) c = 50.3 degrees (beta acute). Schluterite-(Y) is monoclinic, space group P2(1)/c, a 7.0722(2), b 5.6198(1), c 21.4390(4) angstrom, beta 122.7756(3)degrees, V 716.43(5) angstrom(3), Z = 4. The seven strongest lines in the X-ray powder-diffraction pattern are as follows: [d (angstrom), I, (hkl)]: 4.769, 100, (012); 2.972, 55, ((2) over bar 14); 3.289, 51, (112); 2.728, 49, ((2) over bar 16); 2.810, 37, (020); 3.013, 37, (((1) over bar 16); 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, H2Ocalc 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 (Yr(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))(Sigma=2.01)Al0.99Si2.01O7(OH)(2.24)F-0.76. The crystal structure of schluterite-(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 schluterite-(Y), Al(OH)(4)O-2 octahedra share (OH)-(OH) edges to form [M Phi(4)] 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 angstrom>) and ([9])(Y,REE) (= <2.548 angstrom>) atoms.