Structure Refinement of Monoclinic Astrophyllite

Abstract

The crystal structure of monoclinic magnesium astrophyllite has been redetermined. The samples were from Khibina alkaline massifs, Russia. K2NaNa(Fe,Mn)4Mg2Ti2[Si4O12]2(OH)4(OH,F)2, Mr = 1202.31, monoclinic, A2, a = 10.370 (3), b = 23.129 (5), c = 5.322 (1) Å, β = 99.55 (2)°, V = 1258.8 (5) Å3, Z = 2, Dx = 3.173 Mg m−3, Mo Kα, λ = 0.71069 Å, μ = 3.74 mm−1, F(000) = 1179, T = room temperature, R = 0.057 for 1720 reflections. The determination indicated that the coordination number of Ti is five. In tetrahedra of astrophyllite the open-branched Si—O chain connected with Ti—O semi-octahedra by corner-sharing formed a continuous sheet. Na atoms have two types of coordination. Their coordination numbers are eight and six. The Na atoms with coordination number eight are situated between layers. Other types of Na atoms are situated within the layers and form Na—O octahedra with partial occupancies.

Crystal-structure refinement of a Zn-rich kupletskite from Mont Saint-Hilaire, Quebec, with contributions to the geochemistry of zinc in peralkaline environments

“…The AGM structure can be considered as two composite sheets stacked along [001] in a 2:1 ratio. (Liebau, 1985) which are in turn cross-linked by corner-sharing Df 6 octahedra [f = unspecified anion], or DO 5 polyhedra as in magnesium astrophyllite (Shi et al, 1998), where D = Nb,Ti,Zr. The resultant Si:D ratio is 4:1.…”

High-temperature behaviour of astrophyllite, K2NaFe7 2+Ti2(Si4O12)2O2(OH)4F: a combined X-ray diffraction and Mössbauer spectroscopic study

“…Minerals of the astrophyllite group are found mainly in alkaline intrusions (Piilonen et al 2003a). To date, the astrophyllite supergroup consists of three mineral groups and includes 12 minerals (Sokolova et al 2017a): astrophyllite (Weibye 1848; Woodrow 1967;Piilonen et al 2003a, b), niobophyllite (Nickel et al 1964;Cámara et al 2010), zircophyllite (Kapustin 1972(Kapustin , 1973Sokolova and Hawthorne 2016), tarbagataite (Stepanov et al 2012), nalivkinite (Agakhanov et al 2008(Agakhanov et al , 2016Uvarova et al 2008), bulgakite (Agakhanov et al 2016), kupletskite (Semenov 1956;Piilonen et al 2001;Piilonen et al 2006), niobokupletskite (Piilonen et al 2000), kupletskite-(Cs) (Yefimov et al 1971;Bayliss 2007;Cámara et al 2010), devitoite (Kampf et al 2010), sveinbergeite (Khomyakov et al 2011), and lobanovite (former "magnesioastrophyllite") (Shi et al 1998;Sokolova and Cámara 2008;Sokolova et al 2017b).…”

High-temperature Fe oxidation coupled with redistribution of framework cations in lobanovite, K2Na(Fe2+4Mg2Na)Ti2(Si4O12)2O2(OH)4– the first titanosilicate case

“…1) consists of heteropolyhedral (H) and octahedral (O) layers stacked . In lobanovite, adjacent HOH blocks are connected through interlayer alkali and alkaline earth cations: K, Na, Ca, Sr and Ba (Peng & Ma, 1963;Shi et al, 1998;Sokolova & Cá mara, 2008). The general formula of astrophyllite-supergroup minerals is A 2p B r C 7 D 2 (T 4 O 12 ) 2 X O D2 X O A4 X P Dn , where C and D are cations of the O and H sheets, C = [6] (Fe 2+ , Mn, Fe 3+ , Na, Mg or Zn) at the M(1-4) sites; D = [6,5] (Ti, Nb, Zr, Fe 3+ ); T = Si, minor Al; A 2p B r is the composition of the intermediate block where p = 1, 2; r = 1, 2; A = K, Cs, Li, Ba, H 2 O, &; B = Na, Ca, Ba, H 2 O, &; X = X O D2 X O A4 X P Dn = O, OH, F and H 2 O; n = 0, 1, 2 (Sokolova et al, 2017a).…”