Epidote in the context of Dynamic metamorphism

Metamorphism is the transformation of existing rock (the protolith) to rock with a different mineral composition or texture. Metamorphism takes place at temperatures in excess of 150 °C (300 °F), and often also at elevated pressure or in the presence of chemically active fluids, but the rock remains mostly solid during the transformation. Metamorphism is distinct from weathering or diagenesis, which are changes that take place at or just beneath Earth's surface.

Various forms of metamorphism exist, including regional, contact, hydrothermal, shock, and dynamic metamorphism. These differ in the characteristic temperatures, pressures, and rate at which they take place and in the extent to which reactive fluids are involved. Metamorphism occurring at increasing pressure and temperature conditions is known as prograde metamorphism, while decreasing temperature and pressure characterize retrograde metamorphism.

Breccia (/ˈbrɛ(t)ʃiə/ BRETCH-ee-ə, BRESH-; Italian: [ˈbrettʃa]; Italian for 'breach') is a clastic rock composed of large angular broken fragments of minerals or rocks cemented together by a fine-grained matrix.

The word has its origins in the Italian language, in which it means 'rubble'. A breccia may have a variety of different origins, as indicated by the named types including sedimentary breccia, fault or tectonic breccia, igneous breccia, impact breccia, and hydrothermal breccia.

A metamorphic reaction is a chemical reaction that takes place during the geological process of metamorphism wherein one assemblage of minerals is transformed into a second assemblage which is stable under the new temperature/pressure conditions resulting in the final stable state of the observed metamorphic rock.

Examples include the production of talc under varied metamorphic conditions:

Greenschists are metamorphic rocks that formed under the lowest temperatures and pressures usually produced by regional metamorphism, typically 300–450 °C (570–840 °F) and 2–10 kilobars (29,000–145,000 psi). Greenschists commonly have an abundance of green minerals such as chlorite, serpentine, and epidote, and platy minerals such as muscovite and platy serpentine. The platiness gives the rock schistosity (a tendency to split into layers). Other common minerals include quartz, orthoclase, talc, carbonate minerals and amphibole (actinolite).

Greenschist is a general field petrologic term for metamorphic or altered mafic volcanic rock. In Europe, the term prasinite is sometimes used. A greenstone is sometimes a greenschist but can also be rock types without any schistosity, especially metabasalt (spilite). However, basalts may remain quite black if primary pyroxene does not revert to chlorite or actinolite. To qualify for the name, a rock must also exhibit schistosity or some foliation or layering. The rock is derived from basalt, gabbro or similar rocks containing sodium-rich plagioclase feldspar, chlorite, epidote and quartz.

Wollastonite is a calcium inosilicate mineral (CaSiO₃) that may contain small amounts of iron, magnesium, and manganese substituting for calcium. It is usually white. It forms when impure limestone or dolomite is subjected to high temperature and pressure, which sometimes occurs in the presence of silica-bearing fluids as in skarns or in contact with metamorphic rocks. Associated minerals include garnets, vesuvianite, diopside, tremolite, epidote, plagioclase feldspar, pyroxene and calcite. It is named after the English chemist and mineralogist William Hyde Wollaston (1766–1828).

Despite its chemical similarity to the compositional spectrum of the pyroxene group of minerals—where magnesium (Mg) and iron (Fe) substitution for calcium ends with diopside and hedenbergite respectively—it is structurally very different, with a third SiO4−4 tetrahedron in the linked chain (as opposed to two in the pyroxenes).