Editing Metamorphism (section)

===Recrystallization===
[[File:Basalt-hand-sample.tif|thumb|upright=1.35|Basalt hand sample showing fine texture]]
[[File:Amphibolite (Archean, 3.1-3.2 Ga; Norris South roadcut, Madison County, Montana, USA) 1 (45574881922).jpg|thumb|upright=1.35|Amphibolite formed by metamorphism of basalt showing coarse texture]]
The change in the grain size and orientation in the rock during the process of metamorphism is called [[Recrystallization (geology)|recrystallization]]. For instance, the small [[calcite]] crystals in the sedimentary rocks [[limestone]] and [[chalk]] change into larger crystals in the metamorphic rock [[marble]].{{sfn|Yardley|1989|pp=127, 154}} In metamorphosed [[sandstone]], recrystallization of the original [[quartz]] sand grains results in very compact [[quartzite]], also known as metaquartzite, in which the often larger quartz crystals are interlocked.{{sfn|Jackson|1997|loc="metaquartzite"}} Both high temperatures and pressures contribute to recrystallization. High temperatures allow the [[atom]]s and [[ion]]s in solid crystals to migrate, thus reorganizing the crystals, while high pressures cause solution of the crystals within the rock at their points of contact (''[[pressure solution]]'') and redeposition in pore space.{{sfn|Yardley|1989|pp=154–158}}

During recrystallization, the identity of the mineral does not change, only its texture. Recrystallization generally begins when temperatures reach above half the melting point of the mineral on the [[Kelvin]] scale.{{sfn|Gillen|1982|p=31}}

Pressure solution begins during diagenesis (the process of lithification of sediments into sedimentary rock) but is completed during early stages of metamorphism. For a sandstone protolith, the dividing line between diagenesis and metamorphism can be placed at the point where strained quartz grains begin to be replaced by new, unstrained, small quartz grains, producing a ''mortar texture'' that can be identified in [[thin section]]s under a polarizing microscope. With increasing grade of metamorphism, further recrystallization produces ''foam texture'', characterized by polygonal grains meeting at triple junctions, and then ''porphyroblastic texture'', characterized by coarse, irregular grains, including some larger grains ([[porphyroblasts]].){{sfn|Howard|2005}}

[[File:Mylonite Strona.jpg|thumb|upright=1.35|A mylonite (through a [[petrographic microscope]])]]
Metamorphic rocks are typically more coarsely crystalline than the protolith from which they formed. Atoms in the interior of a crystal are surrounded by a stable arrangement of neighboring atoms. This is partially missing at the surface of the crystal, producing a ''[[surface energy]]'' that makes the surface thermodynamically unstable. Recrystallization to coarser crystals reduces the surface area and so minimizes the surface energy.{{sfn|Yardley|1989|pp=148–158}}

Although grain coarsening is a common result of metamorphism, rock that is intensely deformed may eliminate [[strain energy]] by recrystallizing as a fine-grained rock called ''[[mylonite]]''. Certain kinds of rock, such as those rich in quartz, [[carbonate mineral]]s, or olivine, are particularly prone to form mylonites, while feldspar and garnet are resistant to mylonitization.{{sfn|Yardley|1989|p=158}}
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