The Earth recycles water and other elements in a vast system that involves the oceans, minerals, magma, and the atmosphere. We studied the part of the system that involves minerals, specifically, lawsonite and epidote because they contain both water and iron. Iron in these minerals is usually assumed to be Fe³⁺, but we discovered an unexpected amount of Fe²⁺. Reactions involving different states of Fe and water in minerals affect many processes related to element cycling in the Earth.