Polezhaevaite-(Ce) NaSrCeF 6 is a new member of the gagarinite mineral group [hexagonal, P6 3 /m, a = 6.207(7) Γ
, c = 3.801(9) Γ
, V = 126.8(2) Γ
3 , Z = 1]. It is found in a natrolitized microcline-aegirinesodalite lens within apatite-rich urtite at Mt. Koashva (Khibiny massif, Kola Peninsula, Russia) in association with Π°egirine, albite, arfvedsonite, astrophyllite, burbankite, catapleiite, chlorbartonite, djerfisherite, elpasolite, fluorapatite, fluorite, galena, hydroxylapatite, ilmenite, lamprophyllite, lorenzenite, leucophanite, microcline, natrolite, nepheline, orickite, pectolite, pyrochlore, sodalite, sphalerite, strontiofluorite, tainiolite, titanite, vinogradovite, and villiaumite. Polezhaevaite-(Ce) occurs as parallel and sheaf-like aggregates of extremely thin fibers (up to 1 mm long and <1 Β΅m thick), which fill leaching voids within burbankite crystals in natrolite and mantles around partially dissolved burbankite crystals in intimate association with strontiofluorite. Polezhaevaite-(Ce) is translucent (transparent in separate fibers), snowy-white, with a silky luster (in aggregates) and a white streak. Cleavage is not observed; fracture is splintery (in aggregates). The Mohs hardness of individual crystals could not be determined and approaches 3 in aggregates. In transmitted light, the mineral is colorless, uniaxial positive: Ξ΅ = 1.497( 5), Ο = 1.490(5) (for Ξ» = 589 nm). D calc = 4.646 g/cm 3 , D meas = >4.2 g/cm -3 . The mean chemical composition determined by electron microprobe is (wt%): Na 5.27, Ca 3.08, Sr 29.72, Ba 0.48, La 11.76, Ce 14.12, Pr 0.49, Nd 3.09, F 31.95, total 99.96. Empirical formula calculated on the basis of F = 6 apfu is: (Na 0.82 Ca 0.18 ) Ξ£=1.00 (Sr 1.21 Ce 0.36 La 0.30 Ca 0.09 Nd 0.08 Ba 0.01 ) Ξ£=2.06 F 6 (charge imbalance is +0.05). Its simplified formula is NaSrCeF 6 . The strongest X-ray powder-diffraction lines [d in Γ
, (I), (hkl)] are: 5.416(40)(100), 3.120(100)(101, 110), 2.198(70) (201), 1.796(90) (121, 211, 300, 102), 1.554(30)(220), 1.173(70) (321, 231, 140, 410,132, 312, 113). The mineral is named in honor of Lyudmila Ivanovna Polezhaeva (b. 1935), a Russian expert in electron microprobe analysis of minerals for her contribution to the mineralogy of alkaline rocks.