Reaction rims are aggregates of one or more minerals that overgrow and partially replace a relic, unstable mineral, as a result of a chemical reaction within a rock. The replacement reaction typically occurs along grain boundaries, but also fractures and cleavage surfaces. Reaction rims can consist of a single mineral (monomineralic) or be aggregates of two or more minerals (polymineralic). A corona (or coronitic texture/structure) is a type of reaction rim characterized by the concentric (closed) arrangement of one or more minerals that encircle a relic phases. The term ‘corona’ comes, indeed, from the Latin term for ‘crown’. A corona can consist of a single mineral (in this case it is also known as moat) or several minerals, which are arranged in worm-like fine-grained aggregates (symplectitic corona). In general, any intergrowth characterized by vermicular structure (i.e. small, irregular ‘pipes’ of a mineral finely intergrown with another) is known as a symplectite.

A zoning is a special type of reaction rim that occurs in minerals that are solid solutions, i.e. can incorporate different elements and show chemical variability (e.g. feldspars, amphiboles, garnet…). Zoning patterns can be highlighted by variations in the angle of extinction or color of a mineral at the scale of a thin section. Some special techniques (like element X-ray mapping) can highlight non obvious zoning patterns. There are two types of zoning in rocks: growth zoning, when a mineral rim equilibrate to different pressure-temperature conditions or melt composition during mineral growth, and reaction zoning, when the zoned rim of a mineral forms due to reactions with other phases or fluid/melt along the rim.

Reaction rims are very important because they mark reactions ‘frozen’ in the structure of a rock. Reaction rims can be produced by solid-state metamorphic reactions, for example between two solids and/or with fluids and partial melt or, in igneous rocks, by reaction with solid crystals and magma. In metamorphic rocks, they may be triggered by changes in pressure-temperature conditions, but also due to the interaction with external fluids (i.e. metasomatism). In igneous rocks they may be a result of crystals that are not in equilibrium with the surrounding melt and react with it. They can also form due to late magmatic (solid-state) fluid-rock interaction and hydrothermal alteration. Reaction rims tend to be more common in high-grade metamorphic rocks (e.g. granulite, eclogite) and ultramafic rocks, likely because the limited availability of aqueous fluids prevents reactions to run to completion.