Publication: Nanoscale dissolution and growth on anhydrite cleavage faces
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Publication Date
2009
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Elsevier Science Ltd
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Abstract
In situ atomic force microscopy (AFM) was used to study the molecular-scale reactivity of anhydrite (1 0 0), (0 1 0), and
(0 0 1) faces exposed to water and CaSO4 aqueous solutions at room temperature. In pure water, dissolution occurs by step
retreat and etch pit nucleation and growth. Both the kinetics of the step retreat and the shape of the etch pits are surface-specific
and crystallographically controlled. In CaSO4 aqueous solutions with concentrations ranging from 0.030 mol/l to
0.075 mol/l, the growth kinetics on anhydrite (1 0 0) and (0 1 0) faces was studied. Growth is also strongly controlled by crystallographic
constraints and occurs exclusively from pre-existing step edges by highly anisotropic spreading of monolayers
(3.5A ˚ in height). The AFM observations demonstrate that monolayer growth can occur on anhydrite (0 1 0) and (1 0 0)
faces even from slightly supersaturated solutions. In addition, the comparison of the step kinetics on anhydrite faces shows
that the mechanisms of step dissolution and growth are essentially the same, with the direction of migration of crystal building
units being reversed at the anhydrite saturation point. Moreover, the analysis of both high resolution AFM images and lateral
force microscopy (LFM) images confirms that the newly-formed monolayers are anhydrite growing in structural continuity
with the original (1 0 0) and (0 1 0) surfaces. However, the formation of the first monolayers is metastable and two-dimensional
nucleation and further multilayer growth of anhydrite are strongly inhibited even at high supersaturations.