1. X-ray–driven reaction front dynamics at calcite-water interfaces
by Nouamane Laanait, Erika B. R. Callagon, Zhan Zhang, Neil C. Sturchio, Sang Soo Lee, and Paul Fenter
Science
Volume 349(6254):
September 18, 2015
Published by AAAS

2. Fig. 1 Imaging the calcite/water interface while simultaneously driving it far from equilibrium.
Imaging the calcite/water interface while simultaneously driving it far from equilibrium. (A) Optical configuration of the x-ray reflection interface microscope. (Inset) XRIM image of steps on a pristine calcite surface. (B) (Top) Photoelectron (e–ph) generated by absorption of an incident x-ray photon in the calcite crystal. The photoelectron propagates to the mineral/water interface and disrupts the local equilibrium through the formation of short-lived radicals and hydrated electrons (e–aq). (Bottom) Elastically reflected x-rays image the calcite surface topography, with an angle of incidence α and wave vector kin. The imaging was performed at a scattering condition, Q = 2 |kin| sin(α) = 2.1 Å−1. (C) Images of etch pits formed in response to solution undersaturation. All XRIM images were flat-field–corrected and scaled to correct for distortions due to the viewing angle of the lens with respect to the crystal surface (10). Scale bars, 2 μm.
Nouamane Laanait et al. Science 2015;349:
Published by AAAS

3. Fig. 2 Calcite dissolution driven by a pulsed x-ray beam.
Calcite dissolution driven by a pulsed x-ray beam. (A) X-ray images acquired during the 3-s irradiation pulse (t, experimental time; τ, total time under irradiation). Sites 1, 2, and 4, dislocation etch pit; site 3, homogeneously nucleated pit; site 5, pit annihilation by surface retreat. arb., arbitrary. Scale bars, 3 μm. (B) Time-dependent area measurements of sites 3 and 4. (Inset) Final shape of site 4. (C) The homogeneously nucleated pit of site 3 assumes the shape of a conical frustum at the indicated time. (D) Velocity field analysis of the reaction fronts that drive the lateral expansion of the pit in site 3. The arrows indicate the front direction, whereas their length and color give the magnitude of the front velocity in nanometers per second. The times when the reaction front vector fields were extracted are indicated in the figures.
Nouamane Laanait et al. Science 2015;349:
Published by AAAS

4. Fig. 3 The predicted evolution of the composition and saturation state of a calcite equilibrated solution due to irradiation.
The predicted evolution of the composition and saturation state of a calcite equilibrated solution due to irradiation. (A) Evolution of pH for continuous (dashed blue lines) and pulsed (red line) irradiation [and similarly in (D) and (E)]. (Inset) Irradiation sequences begin at t = 0 s. Dashed blue line, continuous exposure; solid red line, pulsed irradiation. (B) Time-dependent concentrations of solution species in response to a single x-ray square-wave pulse (off at t = 3 s). (C) Oscillations of solution concentrations irradiated by square-wave pulses. (D) Variation of the calcite solution saturation index under irradiation. (E) Calcite dissolution (instantaneous) rates predicted by the model (10). (F) Temporal evolution of calcite saturation index, Ω, as a function of absorbed radiation dose. XRIM subjects the system to a dose of 4 × 105 Gy/s under constant illumination (dashed curve) and an average of 105 Gy/s under pulsed illumination, whereas a typical in situ transmission electron microscope (TEM) subjects the system to ~108 Gy/s (9). At continuous doses higher than 104 Gy/s, the system reaches a steady state but not equilibrium.
Nouamane Laanait et al. Science 2015;349:
Published by AAAS

5. Fig. 4 Calcite reaction front instabilities under constant irradiation.
Calcite reaction front instabilities under constant irradiation. (A) Time sequence showing the homogeneous nucleation of pits (sites 1 and 2). At later times, the appearance of a pit (site 3), whose reaction front undergoes a distortion that resembles a “wormhole” or fingering instability, is shown. Scale bars, 3 μm. (B) Reaction front velocity field analysis of site 2 at different times during its evolution, showing a clear asymmetry in velocity distribution without a distinct preferential direction. (C) Area measurements of the dissolution modes in (A).
Nouamane Laanait et al. Science 2015;349:
Published by AAAS