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The calcite ??? aragonite transformation in low-Mg marble: Equilibrium relations, transformations mechanisms, and rates

Journal of Geophysical Research B: Solid Earth

By:
, , ,
DOI: 10.1029/2004JB003302

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Abstract

Experimental transformation of a rather pure natural calcite marble to aragonite marble did not proceed via the expected straightforward polymorphic replacement. Instead, the small amount of Mg in the starting material (0.36 wt %) was excluded from the growing aragonite and diffused preferentially into the remaining calcite grains, producing Mg-rich calcite rods that persisted as relicts. Nucleation of aragonite occurred exclusively on grain boundaries, with aragonite [001] oriented subparallel to calcite [0001]. The aragonite crystals preferentially consumed the calcite crystal on which they nucleated, and the reaction fronts developed preferentially along the {010} and {110} planes of aragonite. Each aragonite neoblast that grew was nearly free of Mg (typically <0.1 wt %). The excess Mg was taken up by the calcite grains in between, stabilizing them and causing a few volume percent rodlike relicts of Mg-enriched calcite (up to 10 wt % MgO) to be left behind by the advancing reaction front. The aragonite growth rates are approximately linear and range from ???3 ?? 10-11 m s-1 at 600??C to ???9 ?? 10-9 m s-1 at 850??C, with an apparent activation enthalpy of 166 ?? 91 kJ mol-1. This reaction mechanism and the resultant texture are akin to cellular precipitation reactions in metals. Similar transformation textures have been reported from high-Mg marbles in Japan and China that disproportionated to low-Mg calcite and dolomite. Copyright 2005 by the American Geophysical Union.

Additional Publication Details

Publication type:
Article
Publication Subtype:
Journal Article
Title:
The calcite ??? aragonite transformation in low-Mg marble: Equilibrium relations, transformations mechanisms, and rates
Series title:
Journal of Geophysical Research B: Solid Earth
DOI:
10.1029/2004JB003302
Volume
110
Issue:
3
Year Published:
2005
Language:
English
Larger Work Type:
Article
Larger Work Subtype:
Journal Article
First page:
1
Last page:
16
Number of Pages:
16