Yakovenchukite-(Y), K3NaCaY2[Si12O30](H2O)4, is a new REE silicate found in a thin (3–4 cm) sodalite-aegirine-microcline veinlet cutting ijolite-urtite at Mt. Kukisvumchorr, Khibiny alkaline massif, Kola Peninsula, Russia. The mineral occurs as small prismatic crystals in intimate association with microcline, aegirine, calcite, catapleiite, donnayite-(Y), fl uorapophyllite, fluorite, galena, lead, litharge, molybdenite, natrolite–gonnardite, pyrochlore, rinkite, strontianite, and vuorijarvite-K. Yakovenchukite-(Y) is creamy to colorless, with vitreous luster. The streak is white. The mineral is transparent, non-fluorescent. The Mohs hardness is about 5. The mineral is brittle. Cleavage is perfect on {100}, distinct on {010}, fracture is stepped. Densities are 2.83 g/cm3 (measured by sink/fl oat in heavy liquids) and 2.72 g/cm3 (calculated). Yakovenchukite-(Y) is biaxial (+): nα = 1.520(5), nβ = 1.525(5), nγ = 1.538(5) (589 nm), 2V(meas.) = 60 ± 5°, 2V(calc.) = 61.86°. The optical orientation is Y = c, X = a, Z = b, pleochroism is not observed. Chemical analysis by electron microprobe (wt%): Na2O 4.32, K2O 10.73, CaO 3.42, Y2O3 15.49, Ce2O3 0.10, Dy2O3 0.68, Er2O3 0.88, Tm2O3 0.18, Yb2O3 1.53, ThO2 0.62, SiO2 57.55, H2O (by the Penfield method) 4.70, total 100.20. The empirical formula (based on Si = 12 apfu) is (K2.85 Na0.15)Σ3.00 Na1.00 (Ca0.71 Na0.60)Σ1.31 (Y1.72 Yb0.10 Er0.06 Dy0.05 Th0.03 Ce0.01 Tm0.01 Ca0.05)Σ2.03 [Si12 O30.02]·3.27H2O. According to single-crystal X-ray study yakovenchukite-(Y) is orthorhombic, Pcca, a = 14.972(8), b = 14.137(7), c = 14.594(8) Å, V = 3089(3) Å3, Z = 4. The strongest lines of the X-ray powder diffraction pattern are [ dbs(Å) (Iobs) (hkl)]: 7.00 (40) (020), 6.57 (60) (102), 4.20 (50) (222), 3.337 (100) (331), 3.248 (90) (024), 3.101 (40) (142), 3.014 (80) (422), 2.608 (40) (404). The crystal structure of yakovenchukite-(Y) belongs to a new structure type of minerals and inorganic compounds. It is based on microporous octahedral-tetrahedral framework of SiO4-tetrahedra and YO6-octahedra. Silicate tetrahedra share corners to form unprecedented [Si12O30] sheets consisting of 4-, 6-, and 14-membered rings. The sheets are parallel to (001) and are linked into 3D framework through YO6 octahedra. Ca2+, K+, and Na+ cations are located within the framework cavities. The octahedral-tetrahedral framework possess channels extended along the a axis. The channel dimensions are 4.9 × 6.2 Å, which means the free crystallographic diameter of 2.2 × 3.5 Å, that allows classifying yakovenchukite-(Y) as a microporous material. Yakovenchukite-(Y) is the latest low-temperature hydrothermal mineral, formed by alteration of earlier REE-rich minerals (pyrochlore, rinkite, etc.). The mineral is named in honor of Victor N. Yakovenchuk, a mineralogist at the Geological Institute of the Kola Science Centre of the Russian Academy of Sciences, for his outstanding contribution to the mineralogy of alkaline and alkaline-ultrabasic massifs.

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