Abstract
The interplay between stress and chemical processes is a fundamental aspect of how rocks evolve, relevant for understanding fracturing due to metamorphic volume change, deformation by pressure solution and diffusion creep, and the effects of stress on mineral reactions in crust and mantle. There is no agreed microscale theory for how stress and chemistry interact, so here I review support from eight different types of the experiment for a relationship between stress and chemistry which is specific to individual interfaces: (chemical potential) = (Helmholtz free energy) + (normal stress at interface) × (molar volume). The experiments encompass temperatures from -100 to 1300 degrees C and pressures from 1 bar to 1.8 GPa. The equation applies to boundaries with fluid and to incoherent solid–solid boundaries. It is broadly in accord with experiments that describe the behaviours of free and stressed crystal faces next to solutions, that document flow laws for pressure solution and diffusion creep, that address polymorphic transformations under stress, and that investigate volume changes in solid-state reactions. The accord is not in all cases quantitative, but the equation is still used to assist the explanation. An implication is that the chemical potential varies depending on the interface, so there is no unique driving force for reaction in stressed systems. Instead, the overall evolution will be determined by combinations of reaction pathways and kinetic factors. The equation described here should be a foundation for grain-scale models, which are a prerequisite for predicting larger scale Earth behaviour when stress and chemical processes interact. It is relevant for all depths in the Earth from the uppermost crust (pressure solution in basin compaction, creep on faults), reactive fluid flow systems (serpentinisation), the deeper crust (orogenic metamorphism), the upper mantle (diffusion creep), the transition zone (phase changes in stressed subducting slabs) to the lower mantle and core mantle boundary (diffusion creep).
Highlights
Pressure influences all chemical reactions, including those that occur in the Earth spanning simple transformations such as diamond to graphite to complex ones including many phases. This implies that stress, a more general state in which forces per unit area are different in different directions, must influence reactions
As discussed for a hydrostatic system A can be written as a difference between chemical potential of reactants and products—according to Eq 1 in a stressed system that will be dependent on the interfaces involved
I cannot see how pressure solution and diffusion creep (4) could be explained using thermodynamics based on mean stress, I cannot see how such an approach could explain oriented microstructures (6), and for olivine, ice, and quartz it is not in accord with conditions for polymorphic transformation (7)
Summary
Pressure influences all chemical reactions, including those that occur in the Earth spanning simple transformations such as diamond to graphite to complex ones including many phases. Argue that there is no Gibbs free energy in a stressed system, and local chemical potentials vary from place to place, governed by different interface orientations and local normal stresses Both approaches reduce to hydrostatic thermodynamics when stress is isotropic, but their predictions are quite different, so clarification is required. For a single component solid the chemical potential is equal to the Gibbs free energy per mole as follows:. As discussed for a hydrostatic system A can be written as a difference between chemical potential of reactants and products—according to Eq 1 in a stressed system that will be dependent on the interfaces involved.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
