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Reactions between rocks and fluid at low temperature - known as retrograde metamorphism, alteration, or weathering - often lead to solid hydration, carbonation and/or oxidation. Such reactions increase the solid mass and decrease the solid density, leading to an increase in solid volume. In turn, these volume changes can affect fluid flow and reactivity. In negative feedback systems, reaction products fill porosity and armor reactive surfaces, reducing permeability and reaction rates. In positive feedback systems, increasing solid volume causes local stresses that lead to fracture, maintaining or enhancing permeability and reactive surface area. Such processes are exemplified by alteration and weathering of mantle peridotite, where it is exposed at the surface. Reactions are fast because the mantle is far from equilibrium with the atmosphere and the oceans, and volume changes can be very large. Reaction-driven cracking is probably important in a variety of tectonic settings, where it facilitates hydration of peridotite, weakening the rheology of the shallow mantle, within and above subduction zones and along oceanic fracture zones. We are also studying this process with the idea of designing engineered systems that emulate natural feedbacks. Such knowledge could be applied to achieve rapid and inexpensive mineral carbonation for carbon capture and storage, improve yields from geothermal power plants and from “in situ solution mining”, facilitate extraction of oil and gas from low permeability reservoirs, and prevent cracking and corrosion of caprocks and well cement in a variety of settings.