First high-pressure synthesis of rossmanitic tourmaline and evidence for the incorporation of Li at the X site
Autor: | Alexander Rocholl, Robert B. Trumbull, Andreas Ertl, Martin Kutzschbach, Bernd Wunder, Gerald Giester, Marija Krstulović |
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Přispěvatelé: | GFZ SIMS Publications, Deutsches GeoForschungsZentrum |
Rok vydání: | 2016 |
Předmět: |
Ionic radius
010504 meteorology & atmospheric sciences Tourmaline Crystal chemistry chemistry.chemical_element Mineralogy Institut für Umweltwissenschaften und Geographie 010502 geochemistry & geophysics 01 natural sciences Crystallography Spodumene chemistry Geochemistry and Petrology Formula unit ddc:550 General Materials Science Lithium Dumortierite Pegmatite 0105 earth and related environmental sciences |
Zdroj: | Physics and Chemistry of Minerals |
ISSN: | 1432-2021 0342-1791 |
DOI: | 10.1007/s00269-016-0863-0 |
Popis: | Lithium is an important component of some tourmalines, especially in chemically evolved granites and pegmatites. All attempts at synthesizing Li-rich tourmaline have so far been unsuccessful. Here we describe the first synthesis of rossmanitic tourmaline at 4 GPa and 700 °C in the system Li2O–Al2O3–SiO2–B2O3–H2O (LASBH) from seed-free solid starting materials consisting of a homogenous mixture of Li2O, γ-Al2O3, quartz and H3BO3. The solid run products after 12-day run duration comprise rossmanitic tourmaline (68 wt%), dumortierite (28 wt%) and traces of spodumene (3 wt%) and coesite (1 wt%). Tourmaline forms idiomorphic, large prismatic crystals (30 × 100 μm), which are inclusion free and chemically unzoned. The refined cell dimensions of the tourmaline are: a = 15.7396(9) A, c = 7.0575(5) A, V = 1514.1(2) A3. Conventionally, the Li+ ion is assumed to exclusively occupy the octahedral Y site in the tourmaline structure to a maximum of 2 Li per formula unit (pfu). However, the chemical composition of our synthetic tourmaline determined by electron microprobe and secondary ion mass spectroscopy results in the formula: X(☐0.67(11)Li0.33(11))Y(Al2.53(10)Li0.47(10))Z(Al6)T(Si5.42(15)B0.58(15))O 18 B (BO3) 3 V+W [(OH)2.40(3)O1.60(3)], wherein a significant amount of Li occupies the X site for charge balance requirements. Reliable assignment of the OH-stretching vibrations in a polarized single-crystal Raman spectrum such as a single-crystal XRD structure refinement, confirms the incorporation of Li at the X site [0.24(9) and 0.15(5) XLi pfu, respectively]. The SREF data show that the Li–O1 distances are shortened significantly in order to compensate for the smaller ionic radius of Li+ compared to Na+, K+ or Ca2+ at the X site, i.e., Li is closer to the Si6O18 ring and to a sevenfold coordination with oxygen. |
Databáze: | OpenAIRE |
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