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American Mineralogist

Journal of Earth and Planetary Materials

Ed. by Baker, Don / Xu, Hongwu


IMPACT FACTOR 2018: 2.631

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1945-3027
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Volume 97, Issue 1

Issues

Experimental growth of diopside + merwinite reaction rims: The effect of water on microstructure development

Bastian Joachim
  • Corresponding author
  • Institute of Geological Sciences, Free University Berlin, Malteserstrasse 74-100, 12249 Berlin, Germany
  • German Research Centre for Geosciences (GFZ), Section 3.3 Chemistry and Physics of Earth Materials, Telegrafenberg, 14473 Potsdam, Germany
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  • Other articles by this author:
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/ Emmanuel Gardés
  • German Research Centre for Geosciences (GFZ), Section 3.3 Chemistry and Physics of Earth Materials, Telegrafenberg, 14473 Potsdam, Germany
  • CEA-CNRS-ENSICAEN-Université de Caen Basse-Normandie, Bd H. Becquerel, BP 5133, 14070 Caen Cadex 5, France
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/ Boza Velickov / Rainer Abart
  • Department for Lithospheric Research, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria
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/ Wilhelm Heinrich
  • German Research Centre for Geosciences (GFZ), Section 3.3 Chemistry and Physics of Earth Materials, Telegrafenberg, 14473 Potsdam, Germany
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  • De Gruyter OnlineGoogle Scholar
Published Online: 2015-04-02 | DOI: https://doi.org/10.2138/am.2011.3819

Abstract

The growth rate and internal organization of bimineralic diopside (CaMgSi2O6)-merwinite (Ca3MgSi2O8) reaction rims produced by a solid-state reaction between monticellite (CaMgSiO4) and wollastonite (CaSiO3) single crystals was determined at 900 °C, 1.2 GPa, and run durations from 5 to 65 h using conventional piston-cylinder equipment. Overall reaction rim thickness ranges from 3.8 to 20.9 μm and increases with the square root of time, indicating that rim growth is diffusion controlled. Symmetrical makeup of the internal microstructure implies that rims grow from the original interface toward both reactants at identical rates, indicating that diffusion of MgO across the rim controls overall growth, with an effective bulk diffusion coefficient of DDi Mwbu+lk,MgO = 10−16.3 ± 0.2 m2/s. At the initial stages, a “lamellar type” microstructure of alternating palisade-shaped diopside and merwinite grains elongated normal to the reaction front is generated, which gradually transforms into a “segregated multilayer type” microstructure with almost perfectly monomineralic Mw|Di|Mw layers oriented parallel to the reaction fronts at long run durations. This is due to changes in relative component mobilities. Whereas the “lamellar” microstructure develops when MgO is substantially more mobile than the other components, formation of the “segregated multilayer” microstructure requires additional mobility of at least one of the other components, CaO or SiO2. We assume that a significant change in relative component mobilities is caused by continuous entrance of minute amounts of water from the piston-cylinder solid pressure medium through the capsule walls, as revealed by the presence of OH-defects in a reactant after the runs, and supported by water-containing powder experiments that only produce monomineralic Mw|Di|Mw layers. Traces of water have a major influence on relative component mobilities, internal rim organization, and the microstructural development of reaction zones.

Keywords : Reaction rims; reaction kinetics; solid-state diffusion; lamellar microstructure; IR spectroscopy; segregation; water

About the article

Received: 2011-02-10

Accepted: 2011-09-01

Published Online: 2015-04-02

Published in Print: 2012-01-01


Citation Information: American Mineralogist, Volume 97, Issue 1, Pages 220–230, ISSN (Online) 1945-3027, ISSN (Print) 0003-004X, DOI: https://doi.org/10.2138/am.2011.3819.

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© 2015 by Walter de Gruyter Berlin/Boston.Get Permission

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