David C. Rubie
Stephen H. Kirby
Steven R. Bohlen
Bradley R. Hacker
2005
<p><span>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</span><sup>−11</sup><span><span class="Apple-converted-space"> </span>m s</span><sup>−1</sup><span><span class="Apple-converted-space"> </span>at 600°C to ∼9 × 10</span><sup>−9</sup><span><span class="Apple-converted-space"> </span>m s</span><sup>−1</sup><span><span class="Apple-converted-space"> </span>at 850°C, with an apparent activation enthalpy of 166 ± 91 kJ mol</span><sup>−1</sup><span>. 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.</span></p>
application/pdf
10.1029/2004JB003302
en
AGU Publications
The calcite → aragonite transformation in low-Mg marble: Equilibrium relations, transformations mechanisms, and rates
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