Journal of Geosciences, 62 (2017), 25–36 DOI: 10.3190/jgeosci.229 Original paper Magnesiovesuvianite, Ca19Mg(Al,Mg)12Si18O69(OH)9, a new vesuvianite-group mineral Taras L. PANIKOROVSKII1*, Vladimir V. SHILOVSKIKH2, Evgenia Yu. AVDONTSEVA1, Andrey A. ZOLOTAREV1, Vladimir Yu. KARPENKO3, Anton S. MAZUR4, Victor N. YAKOVENCHUK5, Ayya V. BAZAI5, Sergey V. KRIVOVICHEV1, Igor V. PEKOV6 1 Department of Crystallography, St. Petersburg State University, University Emb. 7/9, 199034, St. Petersburg, Russia; [email protected] 2 Department of Colloidal Chemistry, Saint-Petersburg State University, University Av. 26, 198504, St. Petersburg, Russia 3 Fersman Mineralogical Museum, Russian Academy of Sciences, Leninskiy Av. 18/2, 115162, Moscow, Russia 4 Center for Magnetic Resonance, St. Petersburg State University, University Av. 26, 198504, St. Petersburg, Russia 5 Nanomaterials Research Centre, Kola Science Centre of the Russian Academy of Sciences, Fersman Str. 14, Apatity, 184200, Murmansk Region, Russia 6 Faculty of Geology, Moscow State University, Vorobievy Gory, 119991 Moscow, Russia * Corresponding author Magnesiovesuvianite (IMA 2015-104), ideally Ca19Mg(Al,Mg)12Si18O69(OH)9, a new vesuvianite-group member, was found in an old museum specimen labelled as vesuvianite from the Tuydo combe, near Lojane, Republic of Macedonia. It occurs as radiating aggregates up to 2 cm across consisting of acicular tetragonal crystals (up to 7 mm long and 5–40 μm thick) with distinct fibre-optic effect. Associated minerals are calcite, grossular–andradite and clinochlore. Single crystals of magnesiovesuvianite are transparent, light pink with a silky lustre. Dominant crystal forms are {100}, {110}, {101} and {001}. The Mohs’ hardness is 6. The measured and calculated densities are 3.30(3) and 3.35 g/cm3, respectively. Mineral is optically uniaxial, negative, ω = 1.725(2), ε = 1.721(2). The chemical composition (wt. % electron-microprobe data) is: SiO2 36.73, Al2O3 20.21, CaO 36.50, MgO 1.80, MnO 0.18, FeO 0.04, Na2O 0.01, H2O 3.10, total 98.37. The empirical formula based on 19 (Ca + Na) apfu is: (Ca18.99Na0.01)Σ19.00(Mg0.60Al0.40)Σ1.00(Al11.05Mg0.70Mn0.07Fe0.02)Σ11.84Si17.84O68.72(OH)9. Absorption bands in the IR spectrum are: 393, 412, 443, 492, 576, 606, 713, 802, 906, 968, 1024, 3200, 3450, 3630 cm–1. The eight strongest lines of the powder X-ray diffraction pattern are (I–d(Å)–hkl): 23–10.96–110, 22–3.46–240, 33–3.038–510, 100–2.740–432, 21–2.583–522, 94–2.365–620, 19–2.192–710, 25–1.6165–672. Magnesiovesuvianite is tetragonal, space group P4/n, unit-cell parameters refined from the powder data are a 15.5026(3), c 11.7858(5), 3 V 2832.4(2) Å , Z = 2. The crystal structure has been refined toR 1 = 0.027 for 3266 unique observed reflections with |Fo| – ≥ 4σF. The structure refinements provided scattering factors of theY 1A,B sites close to 12 e that supports predominant occupancy of these sites by the Mg2+ cations, in perfect agreement with the 27Al MAS NMR data. Magnesiovesuvianite is a member of the vesuvianite group with Mg2+ as a dominant cation at the Y1 site. The name magnesiovesuvianite is given to highlight the species-defining role of Mg. Keywords: magnesiovesuvianite, new mineral, light conductor, cation ordering, Lojane, Republic of Macedonia Received: 16 August, 2016; accepted: 17 February, 2017; handling editor: J. Sejkora The online version of this article (doi: 10.3190/jgeosci.229) contains supplementary electronic material. 1. Introduction active elements such as 222Rn, 220Rn or La (Li et al. 2009; Malczewski and Dziurowicz 2015; Tang et al. 2015). Due to significant chemical variations and the dependence At present the vesuvianite group consists of six of the symmetry (space group) upon crystallization temper- minerals: vesuvianite itself Ca19(Al,Mg,Mn,Fe)13Si18 ature (Gnos and Armbruster 2006), minerals of the vesuvi- O68(OH,F,Cl,O)10 (Werner 1795), wiluite Ca19(Al,Mg, anite group seem to indicate conditions of formation and, Fe,Ti)13(B,Al,‪)5Si18O68(O,OH)11 (Groat et al. 1998), thus, are of practical interest to geology and geochemistry. fluorvesuvianite Ca19(Al,Mg,Mn,Fe)13Si18O68(F,OH)10 3+ Perfect acicular vesuvianite crystals and whiskers show (Britvin et al. 2003), manganvesuvianite Ca19Mn 3+ 3+ 2+ pronounced light-conducting properties (Galuskin et al. (Al,Mn ,Fe )10(Mg,Mn )2Si18O69(OH)9 (Armbruster 2+ 2007). Owing to their structural and chemical complexity, et al. 2002), cyprine Ca19Cu (Al,Mg)12Si18O69(OH)9 vesuvianite-group minerals are also considered as potential (Panikorovskii et al. in print a) and alumovesuvianite components of silicate materials for the immobilization of Ca19Al(Al,Mg)12Si18O69(OH)9 (Panikorovskii et al. in dangerous waste products incorporating toxic and radio- print b). Taking into account the modular structure, www.jgeosci.org T. Panikorovskii, V. Shilovskikh, E. Avdontseva, A. Zolotarev, V. Karpenko, A. Mazur, V. Yakovenchuk, A. Bazai, S. Krivovichev, I. Pekov acb Fig. 1a – A general view of the vesuvianite crystal structure projected along the c axis (grossular module highlighted by white circle). b – The grossular module in the vesuvianite structure. c – Strings of the Y1–X4–X4–Y1 positions in the vesuvianite structure. hydrogarnet-type substitutions, incorporation of so- positions (Groat et al. 1992). Single crystal X-ray and dium and boron, and different cation coordinations, EXAFS investigations by Valley et al. (1985) and Ohkawa the general formula of vesuvianite-group minerals can et al. (1992) demonstrated that Mg predominates in the be written as X16X12X4Y1Y24Y38T0–5[(ZO4)10–x(H4O4)x] five-coordinated Y1 site in vesuvianites from Georgetown, (Z2O7)4(OH,O)10–11, where x < 3, X are seven- to nine- California, USA, and Chichibu, Saitama Prefecture, Japan. 2+ coordinated sites (Ca, Na, K, Fe , REE), X4 has a square In this paper, we report data for magnesiovesuvianite, antiprism coordination (Ca, Na), Y1 has a square pyra- ideally Ca19Mg(Al,Mg)12Si18O69(OH)9, a new vesuvianite- midal coordination (Fe3+, Mg, Al, Fe2+, Cu2+), Y2 and group mineral with a predominance of Mg at the Y1A,B Y3 have octahedral coordination (Al, Mg, Zn, Fe2+, Fe3+, sites. Both the new mineral and its name have been 2+ 3+ Mn , Mn , Ti, Cr, Zn), T0–5 (B, Fe or Al) is the additional approved by the IMA Commission on New Minerals, site with triangular and tetrahedral coordination, whereas Nomenclature and Classification (IMA 2015-104). ZO4 (Si, H4O4) and Z2O7 are ortho- and diortho- groups, respectively (Warren and Modell 1931; Coda et al. 1970; Dyrek et al. 1992; Groat et al. 1992; Armbruster and 2. Occurrence Gnos 2000b; Galuskin et al. 2003; Aksenov et al. 2016, Panikorovskii et al. 2016a, b, c). Magnesiovesuvianite was found in an old museum speci- The crystal structure of vesuvianite-group minerals has men from the Tuydo combe, near Lojane, Republic of a modular character (Fig. 1a) and may be considered as Macedonia, deposited in the systematic collection of the containing one-dimensional grossular modules running Fersman Mineralogical Museum of the Russian Academy along the fourfold axis (Allen and Burnham 1992). The of Sciences, Moscow, Russia, under catalogue no. 59783 modules are linked by the Si2O7 groups (Fig. 1b); thereby (acquired in 1957). This specimen is considered as the it forms framework with channels occupied by the Y1A,B holotype of magnesiovesuvianite. (Y1) and X4A,B (X4) sites (Fig. 1c) in the Y1A–X4A– Associated minerals are calcite, garnet of the gros- X4B–Y1B sequence with the distances smaller than 1.3, sular–andradite series and clinochlore. The new mineral 1.3 and 2.5 Å between the Y1A–X4A, Y1A–X4A and occurs in cavities of a garnet-bearing rodingite and defi- X4A–X4B positions, respectively. Various occupancies of nitely has a hydrothermal origin. Y1A,B and X4A,B positions lead to the different ordering schemes and, as a result, to different space groups (Giu- seppetti and Mazzi 1983; Fitzgerald et al. 1986b; Pavese 3. General appearance, physical properties et al. 1998; Armbruster and Gnos 2000a, b). and optical data The majority of vesuvianite samples contain significant amounts of Mg (usually in the range of 0.8–6.7 wt. %), Magnesiovesuvianite forms radiating aggregates up to which mostly occupies the octahedrally coordinated Y3 2 cm across consisting of acicular tetragonal crystals 26 Magnesiovesuvianite, a new mineral 4. Chemical composition 4.1. Electron microprobe data Chemical analyses of magnesiovesuvianite (Tab. 1) were obtained by Cameca MS-46 electron microprobe (20 kV, 20–30 nA, beam diameter 10 µm) and by a HITACHI S-3400N scanning electron microscope equipped with the INCA Wave 500 system (WDS mode; 20 kV; 10 nA: beam diameter 5 µm). Contents of other elements with the atomic numbers higher than eight were below the detection limits. The H2O content was analysed by thermogravimetric analysis (TGA). Carbon dioxide and boron were not analysed because of the absence of bands corresponding to the C–O and B–O Fig. 2 Cluster of pink magnesiovesuvianite crystals with fibre-optic vibrations in the IR spectrum. The empirical formula effect. of magnesiovesuvianite, calculated on the basis of 19 (up to 7 mm long and 5–40 μm thick) with a distinct (Ca + Na) apfu and taking into account the solid-state fibre-optic effect (Fig. 2). This effect is used as a means magic-angle spinning nuclear magnetic resonance of transmitting light between the two ends of the fibre, (MAS NMR) data, is: (Ca18.99Na0.01)Σ19.00(Mg0.60Al0.40)Σ1.00 e.g. in computer networking. Crystals are elongated along (Al11.05Mg0.70Mn0.07Fe0.02)Σ11.84Si17.84O68.72(OH)9. The ideal- [001]; the dominant forms are {100} and/or {110}, both ized formula is Ca19Mg(Al,Mg)12Si18O69(OH)9, which usually with striation along [001]; crystals are terminated ideally requires MgO 2.82, CaO 37.13, Al2O3 19.54, SiO2 by the {111} and {101} or {001} faces (Fig.