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The Structure of Charoite, (K,Sr,Ba,Mn)15А16(Ca,Na)

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The Structure of Charoite, (K,Sr,Ba,Mn)15А16(Ca,Na) Mineralogical Magazine, February 2010, Vol. 74(1), pp. 159–177 The structure of charoite, (K,Sr,Ba,Mn)15À16(Ca,Na)32[(Si70(O,OH)18 0)](OH,F)4.0˝nH2O, solved by conventional and automated electron diffraction 1,2, 3 2 2 3 4 I. ROZHDESTVENSKAYA *, E. MUGNAIOLI ,M.CZANK ,W.DEPMEIER ,U.KOLB ,A.REINHOLDT AND 4 T. WEIRICH 1 Department of Crystallography, Geological Faculty, Saint Petersburg State University, University emb. 7/9, St. Petersburg, 199034, Russia 2 Department of Crystallography, Institute of Geowissenschaften, Christian-Albrechts-University, Olshausenstrasse 40, D-24098, Kiel, Germany 3 Institute of Physical Chemistry, Johannes Gutenberg-University, Welderweg 11, D-55099, Mainz, Germany 4 Rheinisch-Westfaelische Technische Hochschule, Central Facility for Electron Microscopy, Aachen University, Ahornstrasse 55, D-52074, Aachen, Germany [Received 11 July 2009; Accepted 25 February 2010] ABSTRACT Charoite, ideally (K,Sr,Ba,Mn)15À16(Ca,Na)32[(Si70(O,OH)180)](OH,F)4.0·nH2O, a rare mineral from the Murun massif in Yakutiya, Russia, was studied using high-resolution transmission electron microscopy, selected-area electron diffraction, X-ray spectroscopy, precession electron diffraction and the newly developed technique of automated electron-diffraction tomography. The structure of charoite (a = ˚ ˚ ˚ ˚ 3 31.96(6) A, b = 19.64(4) A, c = 7.09(1) A, b = 90.0(1)º, V = 4450(24) A , space group P21/m)was solved ab initio by direct methods from 2878 unique observed reflections and refined to R1/wR2 = 0.17/0.21. The structure can be visualized as being composed of three different dreier silicate chains: a 10À 12À double dreier chain, [Si6O17] ; a tubular loop-branched dreier triple chain, [Si12O30] ; and a tubular 18À hybrid dreier quadruple chain, [Si17O43] . The silicate chains occur between ribbons of edge-sharing Ca and Na-octahedra. The chains of tetrahedra and the ribbons of octahedra extend parallel to the z + 2+ 2+ 2+ axis. K ,Ba ,Sr ,Mn and H2O molecules lie inside tubes and channels of the structure. On the basis of microprobe analyses and occupancy refinement of the cation sites, the crystal chemical formula of this charoite can be written as (Z =1):(K13.88Sr1.0Ba0.32Mn0.36)S15.56(Ca25.64Na6.36)S32 [(Si6O11(O,OH)6)2(Si12O18(O,OH)12)2(Si17O25(O,OH)18)2](OH,F)4.0·3.18H2O. KEYWORDS: charoite, crystal structure analysis, precession electron diffraction (PED), automated electron diffraction tomography (ADT). Introduction investigations of this mineral date back more than 50 years. Originally, it was described as the CHAROITE, a violet, valuable semi-precious stone mineral canasite, but more detailed studies is found uniquely in the alkaline intrusion of the revealed that it is a different mineral, and in Murun massif in Yakutiya, Sakha Republic, 1978, it was confirmed as a new mineral species Siberia, Russia (Vorob’ev, 2008). The first by Rogova et al. (1978). However, until recently, its chemical formula, unit-cell parameters, space group and structure have remained unclear. Charoite occurs in close association with other Ca-bearing alkaline minerals, such as frank- * E-mail: [email protected] amenite (K3Na3Ca5[Si12O30](OH)F3·H2O), cana- DOI: 10.1180/minmag.2010.074.1.159 site (K3Na3Ca5[Si12O30](OH,O)2.5F1.5), miserite # 2010 The Mineralogical Society I. ROZHDESTVENSKAYA ET AL. 3+ (K3 Ca10(Ca,M ) 2 [Si12O 30][Si2 O 7 ] 2 chain. According to the model, the silicate chains (O,F,OH)2 ·H2 O), tokkoite (K2 Ca4 are placed between bands formed by edge-sharing [Si7O18(OH)](F,OH)) and tinaksite (K2Ca2NaTi [(Ca,Na)O6] octahedra. The chains and bands [Si7O18(OH)]O) (Konev et al., 1996). The extend parallel to the z axis. Potassium ions and structures of all these minerals are characterized H2O molecules lie at the centres of the tubular by the occurrence of various types of dreier chains, and other K ions lie at the centres of eight- silicate chains. Close associations, intergrowths membered rings of SiO4 tetrahedra. and transformation of some minerals into others Despite the fact that the model of suggest their structural affinity and metasomatic (Rozhdestvenskaya et al., 2007, 2009a) displayed origin (Rozhdestvenskaya and Nikishova, 2002). important characteristics of the charoite structure, Although the structure of charoite was unknown, a definitive structure determination was still there was strong evidence that it belongs to the missing. For such astructure determinationthe group of alkaline Ca-silicates containing tubular collection of 3D-single crystal diffraction data is and/or other dreier chains (Frank-Kamenetskaya essential, but the acquisition of such data is and Rozhdestvenskaya, 2004). difficult owing to the very small sizes of charoite Despite many attempts to solve the charoite crystals. Furthermore, our recent studies have structure, or at least to construct a convincing shown that the charoite specimens not only structural model, the structure and stoichiometry contain other silicate minerals and an amorphous have remained enigmatic, and reported data were phase, but also individual asbestiform micro- partially inconsistent. The crystal chemical crystals of charoite having intergrowths of formula of charoite, inferred from X-ray powder different structural variants, one clearly being diffraction, was proposed as monoclinic, asecond one being metrically (K,Na)5(Ca,Ba,Sr)8[Si12O30][Si2O7][Si4O9] pseudo-orthorhombic, but still monoclinic and a (F,OH)2·nH2O; Z = 4; monoclinic, space group third one with adoubled a parameter Pm, P2orP2/m with a = 19.6, b = 32.02, c =7.25 (Rozhdestvenskaya et al., 2009b). Consequently, A˚ , b = 94.3º (Nikishova et al., 1985). we decided that for a reliable structure investiga- Chiragov and Shirinova (2004) proposed a tion, data from single-phase nanocrystals were model of the charoite structure on the basis of a required. miserite-like structure in which silicate-oxygen Nanoscale electron diffraction provides infor- chains are connected to double octahedral bands mation from single nanocrystals down to 5 nm in 8À and form [Ca8(Si12O30)2] structure blocks. diameter, as the electron beam can be focused These blocks are distributed about y = 0 and Ý down to this size, and the strong scattering of 8À and are connected to a [Si6O16] okenite band, or electrons allows a reasonable signal-to-noise ratio 6À a[Si 6O15] okenite net. even for nanovolumes. Conventional electron Based on the common features of alkaline Ca- diffraction often fails to give enough reflections silicate structures, X-ray powder diffraction, and required for areliablestructure solution. on high-resolution transmission electron micro- Moreover, it is well known that dynamic effects scopy (HRTEM), a structural model for charoite can result in spurious intensities when they are was proposed recently by Rozhdestvenskaya et al. collected from zones oriented along low-index (2007, 2009a). According to this model, charoite directions. In order to increase the number of is monoclinic (space group P21/m) with cell reflections collected from a nanocrystal, and at the parameters a = 32.296, b = 19.651, c = 7.16 A˚ , same time reduce both the electron dose on the b = 96.3º, V = 4517 A˚ 3, with ageneralformula sample and dynamic effects, a newly developed K6À7(Ca,Na)18[(Si6O17)(Si12O30)(Si18O45)] software module called ‘automated diffraction (OH,F)2·nH2O(Z = 2). The proposed model tomography’ (ADT) (Kolb et al., 2007, 2008) was contained three different one-dimensional infinite used. The ADT software allows a rich sampling of 10À silicate radicals: [Si6O17] kinked ribbons, reciprocal space in order to obtain unambiguous 12À 18À [Si12O30] tube radicals, and [Si18O45] tube cell parameters even for triclinic crystals, and radicals. Following Liebau’s (1985) classification gives a full 3D-visualization of the reciprocal terminology for silicates, we will henceforth use lattice. As a sample does not need to be oriented the appropriate notation for such silicate radicals: along low index zones, dynamic effects are 10À 12À [Si6O17] is adreier double chain,[Si 12O30] largely reduced. In order to reduce dynamic a tubular loop-branched dreier triple chain and effects further and to improve intensity integra- 18À [Si18O45] a tubular hybrid dreier quadruple tion, ADT can be coupled with ‘precession 160 THE STRUCTURE OF CHAROITE electron diffraction’ (PED) (Vincent and Midgley, electron microprobe using an accelerating 1994; Own, 2005; Avilov et al., 2007) whereby voltage of 15 kV and a probe current of 15 nA. almost complete quasi-kinematic intensities can For ADT investigations, the sample was be collected by electron diffraction from a ground, suspended in ethanol and sprayed onto a nanocrystal. Mugnaioli et al. (2009) showed that carbon-coated Cu grid using the sonifier described by this method it is possible to solve small by Mugnaioli et al. (2009). This method made it inorganic structures ab initio in one step, and also easy to disperse charoite nanocrystals so that there to detect the positions of such elements like was no overlap at high tilt angles. The data oxygen. collection was carried out using a Tecnai F30 In this paper we report our investigations of S-TWIN transmission electron microscope charoite, performed using a wide range of equipped with a field emission gun and operating techniques À HRTEM, selected-area electron at 300 kV. A FISCHIONE tomography holder diffraction (SAED), microprobe analysis, energy with a tilt range up to Ô70º was used. dispersive X-ray spectroscopy (EDX), PED and The ADT technique allows automatic collec- ADT, which have allowed the structure of tion of reflection data sets from a nanocrystal charoite to be determined for the first time. using electron diffraction. The ADT module alternates between scanning transmission electron microscopy (STEM) imaging for crystal tracking Experimental procedures in low-dose mode and nanoscale acquisitions Charoite occurs in different morphologies (Kolb et al., 2007, 2008). The module operates in (Evdokimov et al., 1995; Konev et al., 1996).
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