Publication: Nanoscopic Characteristics of Anhydrite (100) Growth
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2012
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American Chemical Society
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Abstract
The growth of anhydrite (100) surface in contact with supersaturated aqueous solutions (βanh = 1 -3.6) under low hydrothermal conditions (T = 60 -120 ºC) has been studied by use of a hydrothermal atomic force microscope (HAFM). Our observations show that growth on this surface occurs by lateral spreading ofmonomolecular layers (3.5 Å in height) and is highly anisotropic, with [001] and [001] alternating as fast and slow directions in successive monolayers. This anisotropic growth is evidence of strong structural control, which becomes less intense as temperature and/or supersaturation increases. The growth anisotropy affects the development of spirals, determining the combination of fast-moving and slow-moving steps to form bilayer steps around the emergence point of screw dislocations and leading to nonconstant spread rates. As a result, the overall efficiency of spiral growth mechanism is highly dependent on the interaction between slow-moving bilayers and fast-moving monolayers originating from different dislocations. Formation of two-dimensional nuclei occurs only at Tg>º80 ºCand βanhg>2, two-dimensional nucleation density always being very low (< 1nucleus/μm2) under the conditions explored. These facts, together with the slow kinetics of anhydrite growth in comparison to the much faster kinetics of gypsum growth, might explain the frequentmetastable formation of gypsum crystals under temperatures corresponding to the stability field of anhydrite.