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American Mineralogist

Journal of Earth and Planetary Materials

Ed. by Baker, Don / Xu, Hongwu / Swainson, Ian


IMPACT FACTOR 2017: 2.645

CiteScore 2018: 2.55

SCImago Journal Rank (SJR) 2018: 1.355
Source Normalized Impact per Paper (SNIP) 2018: 1.103

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1945-3027
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Volume 100, Issue 1

Issues

Synthetic and natural ammonium-bearing tourmaline

Bernd Wunder / Eleanor Berryman
  • GeoForschungsZentrum Potsdam, 14473 Potsdam, Germany
  • Fachgebiet Mineralogie-Petrologie, Technische Universität Berlin, 13355 Berlin, Germany
  • Other articles by this author:
  • De Gruyter OnlineGoogle Scholar
/ Birgit Plessen / Dieter Rhede / Monika Koch-Müller / Wilhelm Heinrich
Published Online: 2015-01-10 | DOI: https://doi.org/10.2138/am-2015-5055

Abstract

Due to the similar ionic radius of K+ and NH4+, K-silicates can incorporate a significant amount of NH4. As tourmaline is able to accommodate K in its crystal structure at high and ultrahigh pressure, we test if this also holds true for NH4.

Piston-cylinder experiments in the system (NH4)2O-MgO-SiO2-Al2O3-B2O3-H2O at 4.0 GPa, 700 °C, with B2O3 and NH4OH in excess produce an assemblage of tourmaline, phengite, and coesite. The tourmaline crystals are up to 10 × 40 μm in size. EMP analyses indicate that the tourmalines contain 0.22 (±0.03) wt% (NH4)2O and are solid solutions mainly along the magnesio-foitite and “NH4-dravite” join with the average structural formula X[(NH4)0.08(1)o0.92(1)]Y[Mg2.28(8)Al0.72(8)]Z[Al5.93(6)Si0.07(6)]T[Si6.00(5)O18](BO3)3(OH)4.

NH4 incorporation is confirmed by characteristic <N-H> stretching and bending modes in the IR-spectra of single crystals on synthetic tourmaline. Further evidence is the increased unit-cell parameters of the tourmaline [a = 15.9214(9) Å, c = 7.1423(5) Å, V = 1567.9(2) Å3] relative to pure magnesio-foitite.

Incorporation of NH4 in natural tourmaline was tested in a tourmaline-bearing mica schists from a high-P/low-T (>1.2 GPa/550 °C) metasedimentary unit of the Erzgebirge, Germany, rich in NH4. The NH4-concentrations in the three main NH4-bearing phases are: biotite (~1400 ppm) > phengite (~700 ppm) > tourmaline (~500 ppm). This indicates that tourmaline can act as important carrier of nitrogen between the crust and the deep Earth, which has important implications for a better understanding of the large-scale light element cycle.

Keywords: Tourmaline; high-pressure synthesis; ammonium; Erzgebirge mica-schists; nitrogen cycle

About the article

Received: 2014-05-08

Accepted: 2014-08-06

Published Online: 2015-01-10

Published in Print: 2015-01-01


Citation Information: American Mineralogist, Volume 100, Issue 1, Pages 250–256, ISSN (Online) 1945-3027, ISSN (Print) 0003-004X, DOI: https://doi.org/10.2138/am-2015-5055.

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© 2015 by Walter de Gruyter Berlin/Boston.

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