Editing Metamorphic rock (section)

=== Mineralogical changes ===
[[File:Basalt-hand-sample.tif|thumb|upright=1.5|Basalt hand-sized sample showing fine-grained texture]]
[[File:Amphibolite (Archean, 3.1-3.2 Ga; Norris South roadcut, Madison County, Montana, USA) 1 (45574881922).jpg|thumb|upright=1.5|[[Amphibolite]] formed by metamorphism of basalt]]
<!-- Detailed discussion of mineralogical and textural changes should go in [[Metamorphism]]. The discussion here should be general and illustrative only. -->
[[Metasomatism]] can change the bulk composition of a rock. Hot fluids circulating through pore space in the rock can dissolve existing minerals and precipitate new minerals. Dissolved substances are transported out of the rock by the fluids while new substances are brought in by fresh fluids. This can obviously change the mineral makeup of the rock.{{sfn|Yardley|1989|page=5}}

However, changes in the mineral composition can take place even when the bulk composition of the rock does not change. This is possible because all minerals are stable only within certain limits of temperature, pressure, and chemical environment. For example, at atmospheric pressure, the mineral [[kyanite]] transforms to [[andalusite]] at a temperature of about {{cvt|190|C||}}. Andalusite, in turn, transforms to [[sillimanite]] when the temperature reaches about {{cvt|800|C||}}. All three have the identical composition, {{chem2|Al2SiO5}}. Likewise, [[forsterite]] is stable over a broad range of pressure and temperature in [[marble]], but is converted to [[pyroxene]] at elevated pressure and temperature in more silicate-rich rock containing [[plagioclase]], with which the forsterite reacts chemically.{{sfn|Yardley|1989|pages=32-33, 110, 130-131}}

Many complex high-temperature reactions may take place between minerals without them melting, and each mineral assemblage produced indicates the temperatures and pressures at the time of metamorphism. These reactions are possible because of rapid diffusion of atoms at elevated temperature. Pore fluid between mineral grains can be an important medium through which atoms are exchanged.{{sfn|Yardley|1989|page=5}}