The calcite → aragonite transformation in low-Mg marble: Equilibrium relations, transformations mechanisms, and rates
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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.
| 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 |
| Publisher | AGU Publications |
| Description | 16 p. |
| First page | 1 |
| Last page | 16 |
| Online Only (Y/N) | N |
| Additional Online Files (Y/N) | N |
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