AFM study of the epitaxial growth of brushite (CaHPO4{middle dot}2H2O) on gypsum cleavage surfaces

Abstract

The epitaxial overgrowth of brushite (CaHPO 4 ·2H 2 O) by the interaction of phosphate-bearing, slightly acidic, aqueous solutions with gypsum (CaSO 4 ·2H 2 O) was investigated in situ using atomic force microscopy (AFM). Brushite growth nuclei were not observed to form on the {010} gypsum cleavage surface, but instead formed in areas of high dissolution, laterally attached to gypsum [101] step edges. During the brushite overgrowth the structural relationships between brushite (Aa) and gypsum (A2/a) result in several phenomena, including the development of induced twofold twining, habit polarity, and topographic effects due to coalescence of like-oriented crystals. The observed brushite growth is markedly anisotropic, with the growth rate along the main periodic bond chains (PBCs) in the brushite structure increasing in the order [101] > [101] > [010], leading to tabular forms elongated on [101]. Such a growth habit may result from the stabilization of the polar [101] direction of brushite due to changes in hydration of calcium ions induced by the presence of sulfate in solution, which is consistent with the stabilization of the gypsum [101] steps during dissolution in the presence of HPO 2-4ions. The coupling between growth and dissolution was found to result in growth rate fluctuations controlled by the changes in the solution composition.

Mechanism of water extraction from gypsum rock by desert colonizing microorganisms

“…S3) confirmed that the microbial cells assembled primarily on the {011} planes of gypsum. It is likely that these specific planes have a rougher surface (23), which enables stronger adhesion, but may also facilitate accelerated water access (i.e., via enhanced dissolution kinetics) (24,25). Cyanobacteria-like cells were identified from morphological features in the microbial colonies ( Fig.…”

Importance of Mass Transport and Spatially Heterogeneous Flux Processes for in Situ Atomic Force Microscopy Measurements of Crystal Growth and Dissolution Kinetics

“…This 3D model investigates the influence of convection and the impact of the AFM tip/holder and general cell geometry on mass transport in a square ROI for a setup in which the bottom wall of the AFM fluid is formed by the crystal surface under investigation, as in a number of studies. 12,15,18,19,31,35,44,45 The surface concentration at the ROI is simulated at different flow rates and for different bulk concentrations. Dissolution of an ionic crystal is a stoichiometric reaction to satisfy the assumption of an electro-neutral solution and to maintain a constant charge at the crystal surface; as such, for illustrative purposes we reduce the mass transport model to the simulation of a single species.…”

The Link between Brushite and Gypsum: Miscibility, Dehydration, and Crystallochemical Behavior in the CaHPO4·2H2O–CaSO4·2H2O System

“…2H 2 O was considered in depth by Heijnen and Hartman 18 , who presented a comparative study of their crystal morphologies using an A-setting for the three unit cells. More recent papers [19][20][21][22] illustrates how this structural similarity favors the development of oriented intergrowths between these three minerals.…”