| Summary: | Y-rich hainite occurs in nepheline syenite pegmatite of the Sakharjok massif (Kola Peninsula, Russia). It forms euhedral prismatic crystals up to 2 mm in length as well as rims around an unidentified mineral phase (silicate of Ca, Y, Zr and Ti). The mineral is triclinic, space group P-1, a 9.6054(10), b 5.6928(6), c 7.3344(7) Å, α 89.903(2), β 101.082(2), γ 100.830(2)°, V 386.32(7) Å3, Z=1. The calculated density is 3.39 g/cm3. Chemical composition of Sakharjok hainite is different from the previously published data by much higher Y and Nb contents up to 0.72 and 0.20 atoms per formula unit, respectively, by the two- to five-fold depletion in the LREEs and by the strong enrichment of the HREEs. From the single-crystal X-ray diffraction data, there is a significant amount of Y in the M1 site associated with the absence of Zr in it. Nb and Zr are concentrated in the M5 site substituting Ti. Combination of single-crystal X-ray diffraction data and electron microprobe data give the empirical formula (Ca1.04Y0.63REE0.24Mn0.02)Σ1.93(Na0.92Ca0.77)Σ1.69Ca2.00 (Na0.65Ca0.10)Σ0.75(Ti 0 .60Zr0.21Nb0.15Fe0 .03)Σ0.99 ((Si4.00Al0.02)Σ4.02O14) (F2.61O1.39)Σ4.00.
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