Original paper

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

Nèmeth, Pèter Khomyakov; Ferraris, Giovanni; Menshikov, Yuriy P.

Abstract

A new mineral species, nechelyustovite, (Ba0.75Sr0.25K0.17Ce0.02Ca0.01〉0.80)Σ2.00{(Na2.20Ti0.94Mn0.62Ca0.20Fe0.04)Σ4.00[(Ti1.33Nb0.67)Σ2.00O2Si4O14](O1.30H2O0.70)Σ2.00}·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; Dmeas = 3.32-3.42(2), Dcalc = 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.

Keywords

new mineral speciesnechelyustovitebykovaiteheterophyllosilicateskhibiny massifkola peninsulatem study