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X-ray–driven reaction front dynamics at calcite-water interfaces

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Science  18 Sep 2015:
Vol. 349, Issue 6254, pp. 1330-1334
DOI: 10.1126/science.aab3272

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Driving dissolution with x-rays

Carbonate minerals are important for Earth's carbon cycle. They precipitate directly from solution into diverse materials, depending on their physical or biological source. Whether carbonate minerals grow or dissolve is controlled by the thermodynamic drivers of the mineral/water interface. To control and observe the reactions, Lanaait et al. developed a synchrotron x-ray technique that images calcium carbonate surfaces in water and selectively tunes the solution saturation state (see the Perspective by Wolthers). The x-ray beam drives fast-moving reaction fronts far from equilibrium that are more limited by solution-ion transport than by surface processes.

Science, this issue p. 1330; see also p. 1288

Abstract

The interface between minerals and aqueous solutions hosts globally important biogeochemical processes such as the growth and dissolution of carbonate minerals. Understanding such processes requires spatially and temporally resolved observations and experimental controls that precisely manipulate the interfacial thermodynamic state. Using the intense radiation fields of a focused synchrotron x-ray beam, we drove dissolution at the calcite/water interface and simultaneously probed the dynamics of the propagating reaction fronts using surface x-ray microscopy. Evolving surface structures were controlled by the time-dependent solution composition, as characterized by a kinetic reaction model. At extreme disequilibria, we observed the onset of reaction front instabilities with velocities of > 30 nanometers per second. These instabilities serve as a signature of transport-limited dissolution of calcite under extreme disequilibrium.

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