Ilyukhinite (H₃O,Na)₁₄Ca₆Mn₂Zr₃Si₂₆O₇₂(OH)₂ ∙ 3H₂O, a New Mineral of the Eudialyte Group

A new eudialyte-group mineral, ilyukhinite, ideally (H₃O,Na)₁₄Ca₆Mn₂Zr₃Si₂₆O₇₂(OH)₂ · 3H₂O, has been found in peralkaline pegmatite at Mt. Kukisvumchorr, Khibiny alkaline pluton, Kola Peninsula, Russia. It occurs as brownish orange, with vitreous luster anhedral grains up to 1 mm across in hydrothermally altered peralkaline rock, in association with aegirine, murmanite, albite, microcline, rhabdophane-(Ce), fluorite, sphalerite and molybdenite. The Mohs hardness is 5; cleavage is not observed. D_meas 2.67(2), D_calc 2.703 g/cm³. Ilyukhinite is optically uniaxial (–): ω = 1.585(2), ε = 1.584(2). The IR spectrum is given. The average chemical composition of ilyukhinite (wt %; electron microprobe, ranges given in parentheses; H₂O determined by gas chromatography) is as follows: 3.07 (3.63–4.43) Na₂О, 0.32 (0.28–0.52) K₂O, 10.63 (10.26–10.90) CaO, 3.06 (2.74–3.22) MnO, 1.15 (0.93–1.37) FeO, 0.79 (0.51–0.89) La2O3, 1.21 (0.97–1.44) Ce₂O₃, 0.41 (0.30–0.56) Nd₂O₃, 0.90 (0.77–1.12) TiO₂, 10.94 (10.15–11.21) ZrO₂, 1.40 (0.76–1.68) Nb₂O₅, 51.24 (49.98–52.28) SiO₂, 1.14 (0.89–1.37) SO₃, 0.27 (0.19—0.38) Cl, 10.9(5 )H₂O,–0.06–O = C1, total is 98.27. The empirical formula is H_36.04(Na_3.82K_0.20)(Ca_5.65Ce_0.22La_0.14Nd_0.07)(Mn_1.285Fe_0.48)(Zr_2.645Ti_0.34)Nb_0.31Si_25.41S_0.42Cl_0.23O_86.82. The crystal structure has been solved (R = 0.046). Ilyukhinite is trigonal, R3m; a = 14.1695(6) Å, b = 31.026(1) Å, V = 5394.7(7) ų, Z = 3. The strongest XRD reflections [d, Å (I, %) (hkl)] are 11.44 (82) (101), 7.09 (70) (110), 6.02 (44) (021), 4.371 (89) 205), 3.805 (47) (303, 033), 3.376 (41) (131), 2.985 (100) (315, 128), 2.852 (92) (404). Ilyukhinite was named in memory of Vladimir V. Ilyukhin (1934–1982), an outstanding Soviet crystallographer. The type specimen of ilyukhinite has been deposited in the collection of the Natural History Museum, University of Oslo, Norway.