Element transfer mechanisms during mineral re-equilibration in a fluid

Structural or chemical transformattons in minerals are driven by changes in the physical and/or chemical environment and in thermodynamic terms are the result of transforming the system to a new equilibrium. In many cases, the chemical environment is changed by the introductton of an aqueous ffuid whose compositton is not in equilibrium with the solid mineral phase. In the Earth’s crust such ffuids may be saline (Na-rich) so that for example when a potassium-rich mineral such as orthoclase KAlSi3O8 comes into contact with a saline solutton the thermodynamics dictates that there should be exchange of K for Na in the solid mineral resulttng in the formatton of albite NaAlSi3O8 , a process known as albittsatton, with the potassium removed to the ffuid phase. Such an ion exchange process can be readily reproduced experimentally in a hydrothermal solutton and is found to be pseudomorphic i.e. the mineral retains its original morphology. This could suggest that the transformatton might be achieved by a solid state interdiffusion mechanism with K diffusing out and Na diffusing into the solid driven by the chemical potenttal gradient between the solid and ffuid phase. However, at relattvely low hydrothermal condittons (<~500 0C), the kinettcs of solid-state diffusion (SSD) is far too slow to account for the fast albittsatton observed experimentally. This led researchers to propose that a pseudomorphic replacement could also be achieved by a coupled dissolutton-reprecipitatton (CDR) mechanism whereby the dissolutton of a surface layer of the parent mineral results in an interfacial ffuid that is supersaturated with respect to the product phase (in this case albite) which may then precipitate on the surface of the dissolving phase. For the whole mineral grain to be replaced by albite requires that the CDR mechanism generates porosity so that the ffuid can migrate through the parent, dissolving and simultaneously reprecipitattng at a moving interface through the grain. See Putnis and Putnis, 2007,2022; Putnis, 2009; Ruiz-Agudo et al., 2014; Altree-Williams et al., 2015 for comprehensive reviews of the CDR mechanism.