Nechelyustovite, a new heterophyllosilicate mineral, and new data on bykovaite: a comparative TEM study

A new mineral species, nechelyustovite, (Ba_0.75Sr_0.25K_0.17Ce_0.02Ca_0.01□_0.80)_∑2.00{(Na_2.20Ti_0.94Mn_0.62Ca_0.20Fe_0.04) _∑4.00 [(Ti_1.33Nb_0.67) _∑2.00O₂Si₄O₁₄](O_1.30H₂O_0.70) _∑2.00}·4.325H₂O (electron microprobe), was collected from a hydrothermally altered pegmatite body emplaced in the nepheline syenites near their contact with ijolite–urtites in the south-western part of the Khibiny alkaline massif, Kola Peninsula, Russia, in the underground Kirovskii Mine at Mount Kukisvumchorr. It forms rosettes scattered in a natrolite matrix which are up to 1–5 cm in diameter and composed of extremely fine (0.01–0.1 mm) bounded flakes and lamellae. Nechelyustovite is associated with natrolite, belovite-(La), belovite-(Ce), gaidonnayite, nenadkevichite, epididymite, fluorapophyllite, sphalerite and submicrometric barytolamprophyllite. It is creamy with greyish, bluish or yellowish shades; streak is white, lustre vitreous, pearly or silvery; translucent, transparent in fine flakes formed by [010] elongated and (001) flattened lamellae; H = 3 (Mohs); {001} perfect and {100} medium cleavages; brittle; fracture uneven; D_meas = 3.32–3.42(2), D_calc = 3.22 g/cm³. Biaxial (+); at 589 nm α = 1.700(3), β= 1.710(3), γ= 1.734(3); 2V(calc) 66°; X ~ c, Y ~ a, Z ~ b. Nechelyustovite, simplified as (Ba,Sr,K,□)₂{(Na,Ti,Mn)₄[(Ti,Nb)₂O₂Si₄O₁₄](O,H₂O,F)₂}·4.5H₂O, is a new heterophyllosilicate member of the mero-plesiotype bafertisite series and a transmission electron microscopy (TEM) study shows that it occurs as two polytypes intergrown at submicrometric scale: polytype 1M, P2/m, a = 5.37, b = 7.00, c = 24.05 Å, β= 91.1°, Z = 2; polytype 2M, A2/m, a = 5.38, b = 7.04, c = 48.10 Å, β= 91.1°, Z = 4. The spacing (Å) and intensities of the most intense X-ray powder diffraction peaks are 24.06 (100), 7.05 (13), 5.95 (36), 3.95 (25), 2.828 (42), 2.712 (19) and 2.155 (13). A TEM study of bykovaite, simplified as (Ba,Na,K,□)₂{(Na,Ti,Mn)₄[(Ti,Nb)₂O₂Si₄O₁₄](H₂O,F, OH)₂}·3.5H₂O, shows that also this heterophyllosilicate occurs as two polytypes intergrown at submicrometric scale: polytype 1M, P112/m, a = 5.552, b = 7.179, c = 25.47 Å, γ= 91.1°, Z = 2; polytype 2M, I112/m, a = 5.552, b = 7.179, c = 50.94 Å, γ= 91.1°, Z = 4. Hypotheses on the crystal structure of the two minerals are discussed.