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.

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

• 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