Metamorphic reactions Flashcards
How does mienrals respond to change of metamorphic conditions?
They go towrds equilibrium!
- minerals change
- metamorphic reactions happens
What are the most important metamorphic reactions?
- Polymorphic transformations
- Solid-solid nettransfer reaction
- Devolatilization reactions
- Continuous reactions
- Ion exchange reactions
What is polymorphic reactions?
changes of solid minerals of same composition (polymorphs) only because of P or T changes
What is the catch with polymorphic reactions?
there is little potential energy for the reaction
- often end of as metastable (non-equillibrium assemblages)
ex. diamond brought to surface, doesn’t change to grafite
What is solid-solid nettransfer reactions?
Solids of different compositions.
Change in models amounts of the phase
(minerals react to create new minerals?)
if limited they are discontinuous
- line, and then new mineral (:)) by increase of T or P
What is Devolatilization reactions?
Reactions release or consume volatiles
- depends on P, T and Partial pressure of the volatile
- very common
How does the devolitalization reaction affect the isograd?
i can make it move towrds lower temp. bc it is sensitive to the voltiles partial pressure
What types of devolatilization reactions are there?
- Dehydration reaction (frees water)
- Decarbonation reaction (frees CO2)
- Combination (frees both)
- Prograde reaction (either consumes water and frees CO2 or the opposite)
What is continuos reactions?
continually changing composition of phases bc. of the reaction
- Solid solution systems
- range of P and T
ex. breakdown of chlorite to get garnet,
Difference continiuous and discontiniuous reactions?
one changes apruptly to another mineral while the other is gradual change of the chemistry/composition of the phase
What is ion exchange reactions?
Reciprocal (gensidig) exchange of components between 2(+) minerals.
ex. Fe-Mg exchange between ortho- and clinopyroxene, or garnet and biotite
How is ion exchange different from continiuous reactions in the way it affect the cation ratios?
Ion exchange doesn’t change the modal composition of the phases - continiuous reactions does
What is the ideal form of Ion exchange reactions?
A(Fe) + B(Mg) = A(Mg) + B(Fe)
How can ploymorphic reactions be seen in compatibily diagrams?
Tie-lines does not change but a component (mineral) does
Reaction: Ky -> Si
What reactions can show crossing tie-lines in a compatibility diagram?
-Solid-solid nettransfer
-devolatilization
-continiuous
Reaction: Ms + Qtz -> Or + Sil (+ H2O)
