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

Bradley R. Hacker; David C. Rubie; Stephen H. Kirby; Steven R. Bohlen

doi:10.1029/2004jb003302

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

Bradley R. Hacker·David C. Rubie·Stephen H. Kirby·Steven R. Bohlen

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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 &sim;3 × 10 −11 m s −1 at 600°C to &sim;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.

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Journal of Geophysical Research B: Solid Earth Article

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Bradley R. Hacker, David C. Rubie, Stephen H. Kirby, Steven R. Bohlen. 2005-03-19. The calcite → aragonite transformation in low-Mg marble: Equilibrium relations, transformations mechanisms, and rates. https://doi.org/10.1029/2004jb003302

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