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Oxy-Dravite from Wołowa Góra Mountain, Karkonosze Massif, SW Poland: Crystallochemical and Structural Studies

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Oxy-Dravite from Wołowa Góra Mountain, Karkonosze Massif, SW Poland: Crystallochemical and Structural Studies Mineralogical Magazine, August 2018, Vol. 82(4), pp. 913–928 Oxy-dravite from Wołowa Góra Mountain, Karkonosze massif, SW Poland: Crystallochemical and structural studies 1,* 2,3 1 1 1 4 ADAM PIECZKA ,ANDREAS ERTL ,MATEUSZ P. S ĘK ,DIANA TWARDAK ,SYLWIA ZELEK ,ELIGIUSZ SZEŁĘG AND 3 GERALD GIESTER 1 AGH University of Science and Technology, Department of Mineralogy, Petrography and Geochemistry, al. Mickiewicza 30, 30-059 Kraków, Poland 2 Mineralogisch-Petrographische Abt., Naturhistorisches Museum, Burgring 7, 1010 Wien, Austria 3 Institut für Mineralogie und Kristallographie, Geozentrum, Universität Wien, Althanstrasse 14, A-1090 Wien, Austria 4 University of Silesia, Faculty of Earth Sciences, Department of Geochemistry, Mineralogy and Petrography, 41- 200 Sosnowiec, Będzińska 60, Poland [Received 4 July 2017; Accepted 5 September 2017; Associate Editor: Mark Welch] ABSTRACT Yellowish dravitic tourmaline (dominated by the oxy-dravite component) associated with secondary fluor-dravite/fluor-schorl and dravite/schorl tourmalines was found in a quartz vein cropping out in the eastern part of the Karkonosze Mountains range, SW Poland. The crystal structure of this tourmalinewasrefinedtoanR1 value of 1.85% based on single-crystal data, and the chemical composition was determined by electron-microprobe analysis. The tourmaline, a representative of □ 2þ the alkali-tourmaline group, has the structural formula: (Na0.75Ca0.12 0.13)Σ1(Mg1.93Al0.95Ti0.06Fe0:04 V0.01)Σ3(Al5.38Mg0.62)Σ6B3Si6O27(OH)3(O0.46OH0.33F0.21)Σ1, and is characterized by an extremely high Mg/(Mg + Fe) ratio of 0.97–0.99, the WO2– content that reaches 0.59 apfu resulting in a local predominance of the oxy-dravitic component and Mg–Al disorder on the octahedral Yand Z sites of the order of 0.64 apfu. This disordering results in an increasing <Z–O> distance with ∼1.925 Å, and unit- cell parameters a = 15.916(1) Å and c = 7.180(1) Å. The tourmaline formed during Variscan prograde metamorphism under the influence of a released (H2O,B,F)-bearing fluid. The fluid mobilized the most soluble components of partly altered silicic volcaniclastic material of theLateCambriantoEarly Ordovician bimodal volcanism to become the protolith for adjacent quartzo-feldspathic schists and amphibolites, and propagated them into the surrounding granitic gneisses of the Kowary unit in the eastern metamorphic cover of the Karkonosze granite. K EYWORDS: dravitic tourmaline, oxy-dravite, fluor-dravite, dravite, chemical composition, crystal structure, order-disorder. Introduction corresponding to the generalized chemical formula XY3Z6(T6O18)(BO3)3V3W (Henry et al., MINERALS of the tourmaline supergroup are 2011), where X, Y, Z, T, B, V (=O3) and W (=O1) common accessory phases occurring in numerous denote structural sites occupied as follows. igneous, metamorphic and sedimentary rocks. They are complex borosilicates with compositions X ¼ Naþ; Kþ; Ca2þ; Pb2þ; A ðÞvacancy ; Y ¼ Fe2þ; Mg2þ; Mn2þ; Al3þ; Liþ; Fe3þ; *E-mail: [email protected] 2þ; 3þ; 3þ; 4þ; 2þ; 2þ; ...; https://doi.org/10.1180/minmag.2017.081.069 Zn Cr V Ti Cu Ni Copyright © Mineralogical Society of Great Britain and Ireland 2018 Downloaded from https://pubs.geoscienceworld.org/minmag/article-pdf/82/4/913/4580044/s0026461x18000129a.pdf by University of Arizona user on 04 April 2019 ADAM PIECZKA ET AL. þ þ þ þ þ a valid mineral of the tourmaline supergroup Z ¼ Al3 ; Mg2 ; Fe3 ; Cr3 ; V3 ; :::; (Bosi and Skogby, 2012, 2013). Occasionally it T ¼ Si4þ; Al3þ; B3þ; occurs in rocks of Cr- and V-rich environments. Such tourmaline species can be enriched in V and ¼ 3þ; B B Cr, and sometimes also in Fe3+. These elements can V ¼ OHÀ; O2À; and even be the dominant occupants at the Y site and/or Z site, e.g. in oxy-vanadium-dravite (Bosi et al., W ¼ OHÀ; FÀ; O2À: 2013a), originally described as ‘vanadium-dravite’ by Reznitsky et al.(2001), oxy-chromium-dravite, vanadio-oxy-dravite, vanadio-oxy-chromium-dravite, The extremely complex chemical composition chromo-alumino-povondraite (Baksheev et al., results in numerous colour varieties, sometimes 2011; Bosi et al., 2012; 2013b; 2014a,b; 2017; showing gem-quality features, although the most Reznitskij et al. 2014) and bosiite (Ertl et al., 2016). common Fe-bearing crystals are usually dark- The Wołowa Mountain tourmaline, however, features brown to black. a relatively simple chemistry with negligible V In Poland, occurrences of tourmaline-supergroup and Cr, and probably absent Fe3+, which reflects minerals are generally limited to metamorphic and the chemistry of the environment in which the igneous rocks of the Sudetes region and crystalline tourmaline grew. rocks of the Tatra Mountains (Pieczka, 1996; Gawedą et al., 2002). Tourmalines in those areas are mainly very dark green to black Fe-bearing Occurrence species, mainly schorl. The Góry Sowie Block is the only geological unit in Poland known for The Karkonosze granite massif is the largest occurrences of multicoloured Li-bearing tourma- Variscan pluton in the Western Sudetes at the lines, where they may be found in LCT-type northeastern margin of the Bohemian Massif, pegmatites, e.g. in the vicinity of Gilów (Pieczka hosted by the Izera Metamorphic Complex on the et al., 2004) and Piława Górna (Pieczka et al., northwest, and the South Karkonosze Metamorphic 2010). Additionally, a dark blue dravite was Unit, the East Karkonosze Metamorphic Unit and described from a small lens of a LCT-type the Kaczawa Metamorphic Complex on the north- pegmatite hosted by serpentinite of the Szklary east (Fig. 1). The East Karkonosze Metamorphic massif (Pieczka, 2007), considered to be an Unit comprises the Kowary–Czarnów Unit divided external representative of the Góry Sowie pegma- into the Kowary gneiss series and the Czarnów titic melts intruded into ultrabasic rocks of the schist series and the Leszczyniec Unit (Mazuret al., Sudetic ophiolite before ∼380 Ma (Pieczka et al., 2007). The Kowary gneiss series, which contacts 2015). the granite, is composed of gneisses, mica-schists, In addition to the Góry Sowie area, Wołowa amphibolites, phyllites and marbles (ibid.). The Mountain in the eastern part of Karkonosze region of Wołowa Góra Mountain located ∼3km Mountains is the only locality in the Polish part SW from Kowary, marks the NE termination of the of the Sudetes with occurrences of schorlitic to Karkonosze montane range and is the SW part of dravitic tourmalines, varying in colour from black the East Karkonosze Metamorphic Unit in Poland. through olive-green to yellow (Lis et al., 1965). The region is composed mainly of leucocratic Pieczka (1996) gives brief details of Fe-bearing augen gneisses associated with granitic gneisses tourmalines from the locality; in the present paper and subordinate layered gneisses of the Kowary we describe the yellowish dravitic tourmaline in gneiss series, surrounded on the south and west by term of its crystal chemistry and structure. Oxy- an assemblage of chlorite-mica schists, quartzo- dravite tourmaline, the end-member species usually feldspathic schists and amphibolites, formed dominant in the dravitic tourmaline studied, was from differentiated igneous and volcanogenic– mentioned in the tourmaline classification proposal sedimentary ∼500 Ma protoliths (Berg, 1923; Lis of Hawthorne and Henry (1999). Subsequently the et al., 1965;Różanski,́ 1995; Oberc-Dziedzic et al., term was used in the classification of Mg- and Al- 2010). A pass between Wołowa Mountain and bearing tourmalines with the O2–-dominant W site, Czoło Mountain in the main Karkonosze range is and recently it was approved by the International the well-known locality of a milky quartz vein with Mineralogical Association Commission on New brannerite, associated with tourmalines showing Minerals, Nomenclature and Classification as colours varying from black, found in the host 914 Downloaded from https://pubs.geoscienceworld.org/minmag/article-pdf/82/4/913/4580044/s0026461x18000129a.pdf by University of Arizona user on 04 April 2019 OXY-DRAVITE FROM POLAND: CRYSTALLOCHEMICAL AND STRUCTURAL STUDIES FIG.1.(a) Geological sketch of the East Karkonosze Metamorphic Unit (after Pieczka et al., 2009). (b) Detailed geological map of the Budniki–Wołowa Mountain region (after Różanski,́ 1995; fide Mochnacka et al., 2008) shown as a black dotted frame in (a). The occurrence of the quartz vein with dravitic tourmaline is marked by a grey star. granitic gneisses, through olive-green to yellow part. Brannerite, gersdorffite, pale-coloured micas, in the quartz vein itself (Lis et al., 1965). The an apatite-group mineral, pyrite, pharmacosiderite surrounding granitic gneisses are medium- to and Fe oxides were recognized as the associated coarsely-crystalline, light-pinkish rocks composed minerals (Lis et al., 1965). of microcline, microcline perthite, quartz and pale- coloured mica, with strong albitization, which almost completely altered the primary K-feldspars, Methods leading commonly to the formation of chessboard- Electron microprobe analysis (EMPA) twinned albite. The contact between the host granitic gneisses and the quartz vein is some Electron microprobe analyses were performed at metres long and 5–50 cm thick emphasized by the the Inter-Institute Analytical Complex for Minerals presence of altered dark micas and dark green and Synthetic Substances at the University of tourmaline. The tourmaline occurs as radial Warsaw on a longitudinal section of an aggregate aggregates, a few centimetres thick, composed of composed of yellowish tourmaline crystals, using a elongated, prismatic crystals (Lis et al., 1965; Cameca SX 100 electron microprobe operating in Pieczka, 1996), which become pale green inside the wavelength-dispersive mode under the following vein, reaching a yellowish coloration in its axial conditions: accelerating voltage of 15 kV, beam 915 Downloaded from https://pubs.geoscienceworld.org/minmag/article-pdf/82/4/913/4580044/s0026461x18000129a.pdf by University of Arizona user on 04 April 2019 ADAM PIECZKA ET AL. current of 20 nA, beam diameter of 2 µm, peak anisotropic displacement parameters for all non- count-time of 20 s and background time of 10 s.
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