Metamorphic facies

A metamorphic facies is a set of mineral assemblages characteristic of a particular range of pressure and temperature. It helps geologists infer the conditions under which a rock changed.

Metamorphism: The solid-state transformation of rocks through changes in temperature, pressure, or fluid conditions. Metamorphic facies classify the conditions experienced during this transformation.
Metamorphic reaction: A chemical reaction among minerals during metamorphism, producing a new mineral assemblage. Reactions create or remove the minerals used to recognize facies.
Greenschist facies: A low- to medium-temperature metamorphic facies commonly characterized by chlorite, actinolite, and epidote. Its characteristic green minerals make it a familiar example of facies-based classification.
Eskola's metamorphic facies: The facies classification proposed by Finnish geologist Pentti Eskola to relate mineral assemblages to metamorphic conditions. Eskola established the framework from which modern facies classification developed.
Mineral assemblage: A group of minerals occurring together in a rock. Facies are identified by characteristic mineral assemblages rather than by a single mineral.
Mineral stability: The range of physical and chemical conditions under which a mineral can persist. Facies boundaries reflect changes in which minerals are stable.
Amphibolite facies: A medium- to high-temperature metamorphic facies commonly characterized by amphibole and plagioclase. Its assemblages illustrate how increasing temperature changes facies-defining minerals.
Pentti Eskola: A Finnish geologist who developed the metamorphic facies concept in the early twentieth century. His work made mineral assemblages a systematic basis for classifying metamorphic conditions.
Pressure–temperature path: The sequence of pressure and temperature conditions a rock experiences through time. A rock’s path determines which facies it enters and how its minerals evolve.
Metamorphic phase diagram: A diagram showing stable mineral assemblages across specified pressure, temperature, and composition conditions. Phase diagrams map the conditions associated with facies-defining assemblages.

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