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

[1] 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 1011 m s1 at 600C to 9 109 m s1 at 850C, with an apparent activation enthalpy of 166 ± 91 kJ mol1. 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