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Experimental growth of diopside + merwinite reaction rims; the effect of water on microstructure development

Bastian Joachim, Emmanuel Gardes, Boza Velickov, Rainer Abart and Wilhelm Heinrich
Experimental growth of diopside + merwinite reaction rims; the effect of water on microstructure development
American Mineralogist (January 2012) 97 (1): 220-230

Abstract

The growth rate and internal organization of bimineralic diopside (CaMgSi (sub 2) O (sub 6) )-merwinite (Ca (sub 3) MgSi (sub 2) O (sub 8) ) reaction rims produced by a solid-state reaction between monticellite (CaMgSiO (sub 4) ) and wollastonite (CaSiO (sub 3) ) single crystals was determined at 900 degrees 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 mu m 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 D (sub bulk,MgO) (super Di+Mw) = 10 (super -16.3+ or -0.2) m (super 2) /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 MwDiMw 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 SiO (sub 2) . 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 MwDiMw layers. Traces of water have a major influence on relative component mobilities, internal rim organization, and the microstructural development of reaction zones.


ISSN: 0003-004X
EISSN: 1945-3027
Coden: AMMIAY
Serial Title: American Mineralogist
Serial Volume: 97
Serial Issue: 1
Title: Experimental growth of diopside + merwinite reaction rims; the effect of water on microstructure development
Affiliation: Free University Berlin, Institute of Geological Sciences, Berlin, Germany
Pages: 220-230
Published: 201201
Text Language: English
Publisher: Mineralogical Society of America, Washington, DC, United States
References: 37
Accession Number: 2012-030321
Categories: General mineralogy
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 2 tables
Secondary Affiliation: German Research Centre for Geosciences Potsdam, DEU, GermanyUniversite de Caen Basse-Normandie, FRA, FranceLeibniz-Institut fuer Kristallzuechtung, DEU, GermanyUniversity of Vienna, AUT, Austria
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, copyright, Mineralogical Society of America. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201216
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