The calcite→aragonite transformation in low-Mg marble:: Equilibrium relations, transformation mechanisms, and rates -: art. no. B03205

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 similar to 3 x 10(-11) m s(-1) at 600 degrees C to similar to 9 x 10(-9) m s(-1) at 850 degrees 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.