The mechanism of porosity formation during solvent-mediated phase transformations

Abstract

Solvent-mediated solid-solid phase transformations often result in the formation of a porous medium, which may be stable on long time scales or undergo ripening and consolidation. We have studied replacement processes in the KBr-KCl-H 2 O system using both in situ and ex situ experiments. The replacement of a KBr crystal by a K(Br,Cl) solid solution in the presence of an aqueous solution is facilitated by the generation of a surprisingly stable, highly anisotropic and connected pore structure that pervades the product phase. This pore structure ensures efficient solute transport from the bulk solution to the reacting KBr and K(Br,Cl) surfaces. The compositional profile of the K(Br,Cl) solid solution exhibits striking discontinuities across disc-like cavities in the product phase. Similar transformation mechanisms are probably important in controlling phase-transformation processes and rates in a variety of natural and man-made systems.

The legacy of crystal-plastic deformation in olivine: high-diffusivity pathways during serpentinization

“…When a fluid phase is present, mass transport can take place via advection and the diffusion of elements through an interconnected three-dimensional fluid network. Mineral re-equilibration in this environment is dominated by dissolution-reprecipitation, with rates much faster than diffusion even on a laboratory time-scale (e.g., Martin and Fyfe 1970;Putnis 2009;King et al 2010;Raufaste et al 2011). Upon dissolution, elements that are released into the fluid create an elemental reservoir, from which some elements are removed during the precipitation of new phases and some remain in solution.…”

Sculpting of Rocks by Reactive Fluids

“…This reaction and the corresponding solid solution-aqueous solution phase diagram has been described in detail by Putnis and Metzger (2004) and Putnis et al (2005). More recently, the formation of the porous reaction product was studied by in situ and ex situ experiments by Raufaste et al (2011). Raufaste et al (2011) addressed two main problems.…”

“…In both examples, the replacement is pervasive and thus the systems somehow manage to produce porosity during the reaction. The mechanism of porosity production during replacement in the KBr-KCl system is known from the recent experiments of Raufaste et al (2011), while the mechanism of pervasive porosity generation during charnockitisation is unknown. Is it possible to understand patterns similar to what is seen in h without invoking small scale processes?…”

“…More recently, the formation of the porous reaction product was studied by in situ and ex situ experiments by Raufaste et al (2011). Raufaste et al (2011) addressed two main problems. First, what mechanism generates the observed porosity in the product phase, and second, how is structural information transmitted from the parent to the product phase?…”

“…Propagation of the cylinders into the original crystal takes place by dissolution at the roof of the saucer shaped pores (denoted by wave patterns) coupled with precipitation of a more Cl-rich phase at the floor (dotted pattern). Mass transfer with the bulk external fluid takes place through the central channels (modified from Raufaste et al, 2011).…”