METAMORPHIC REACTIONS

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 METAMORPHIC REACTIONS

  • It is an expression of how the minerals got to their final state

  • It is a chemical reaction that produces, or changes, an assemblage of metamorphic mineral and fluid phases in a rock.

POLYMORPHIC TRANSFORMATIONS

  • Chemical composition remains same

  • Mineralogy will change with change in temperature and pressure

  • Isochemical reaction

  • Depends upon tempearature and pressure but not on partial pressure of fluid

         Ex: Polymorphism of SiO2, Aluminosilicates, Graphite-Diamond, Calcite-Aragonite






EXSOLUTION REACTIONS

  • Occur in solid-solution mineral series when a solvus is encountered

  • The process involves the unmixing of the solid solution within the composition range covered by the solvus.                                   

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SOLID-SOLID NET TRANSFER REACTIONS

  • This reaction occurs only in solid state

  • Differ from polymorphic transformations: involve solids of differing composition

  • In this reaction, two or more solid mineral combine and form one or more solid mineral phase essentially in solid state

  • This reaction depends on only temperature and pressure

          Ex: NaAlSi2O6 + SiO2 = NaAlSi3O8



DEVOLATIZATION REACTIONS

  • It either consumes or releases volatiles such as H2O or CO2

  • It depends on tempearature, pressure along with partial fluid pressure

  • It is a prograde reaction

TYPES OF DEVOLATIZATION REACTIONS

1) Dehydration reaction :

  • Prograde reaction that releases H2O

  • Muscovite + Quartz = Orthoclase + Silimanite + Water

          Ex: KAl2 Si3AlO10(OH)2 + SiO2 = KAlSi3O8 + Al2 SiO5 + H2O



2) Decarbonation reaction :

  • Prograde reaction that releases CO2

  • Calcite+Quartz = Wollastonite + CO2

          Ex: CaCO3 + SiO2 = CaSiO3 + CO2


3) Combined dehydration and decarbonation reaction

  • The reaction in which both CO2 and H2O relaeses

  • Tremolite + Calcite + Quartz = Diopside + 3 CO2 + H2O

    Ex: Ca2Mg5Si8O221OH22 + 3 CaCO3 + 2 SiO2 = 5 CaMgSi2O6 + 3 CO2 + H2O

ION-EXCHANGE REACTIONS

  • The exchange of ions between two solid minerals occur

  • Used as geothermometer (large 𐤃s and small 𐤃V and 𐤃H)

  • Fe-Mg exchange between Opx and Cpx:

    Ex: MgSiO3 + CaFeSi2O6 = FeSiO3 + CaMgSi2O6


TIE-LINE FLIP REACTIONS

  • Involve two phases becoming stable together that were previously unstable together, and vice versa.

  • As with terminal reactions, they may be solid-solid reactions or may involve a fluid phase.

    Ex: chlorite + garnet = biotite + staurolite


TERMINAL REACTIONS

  • Involve the creation of a new phase from two or three other phases, or (in the other direction) the decomposition of one phase into two or three others.

  • May be solid-solid reactions or they may involve a fluid phase

    Ex: chloritoid = staurolite + garnet + chlorite

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