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The albite-epidote-hornfels facies is a facies at low pressure and relatively low temperatures. It is named for the two minerals albite and epidote, though they are also stable in other facies. Hornfels is a rock formed by contact metamorphism, a process that characteristically involves high temperatures but low pressures/depths. This facies is ...
Zeolite facies describes the mineral assemblage resulting from the pressure and temperature conditions of low-grade metamorphism.. The zeolite facies is generally considered to be transitional between diagenetic processes which turn sediments into sedimentary rocks, and prehnite-pumpellyite facies, which is a hallmark of subseafloor alteration of the oceanic crust around mid-ocean ridge ...
The transformation converts the minerals in the protolith into forms that are more stable (closer to chemical equilibrium) under the conditions of pressure and temperature at which metamorphism takes place. [5] [6] Metamorphism is generally regarded to begin at temperatures of 100 to 200 °C (212 to 392 °F).
Illite crystallinity is a technique used to classify low-grade metamorphic activity in pelitic rocks. [1] Determining the " illite crystallinity index " allows geologists to designate what metamorphic facies and metamorphic zone the rock was formed in and to infer what temperature the rock was formed.
It is an important rock-forming mineral in low- to medium-grade metamorphic rock formed by metamorphism of mafic or pelitic rock. [9] It is also common in igneous rocks, usually as a secondary mineral, formed by alteration of mafic minerals such as biotite , hornblende , pyroxene , or garnet . [ 10 ]
The greenstone belts are surrounded by high-grade gneiss terrains showing highly deformed low-pressure, high-temperature (over 500 °C (932 °F)) metamorphism to the amphibolite or granulite facies. These form most of the exposed rock in Archean cratons.
Figure 1. Petrogenetic grid for metapelites (several authors). [1] [2] Metamorphic facies included are: BS = Blueschist facies, EC = Eclogite facies, PP = Prehnite-Pumpellyite facies, GS = Granulite facies, EA = Epidote-Amphibolite facies, AM = Amphibolite facies, GRA = Granulite facies, UHT = Ultra-High Temperature facies, HAE = Hornfels-Albite-Epidote facies, Hbl = Hornblende-Hornfels facies ...
The degree of deformation and metamorphism along with rock type determines the kind of cleavage feature that develops. Generally, these structures are formed in fine grained rocks composed of minerals affected by pressure solution. [1] Cleavage is a type of rock foliation, a fabric element that describes the way planar features develop in a ...