This document summarizes chemical equilibrium and metamorphic reactions in metamorphic rocks, including retrograde metamorphism. It defines metamorphic reactions, univariant and divariant reactions, and discusses low-grade and high-grade metamorphism as well as prograde and retrograde metamorphism. Retrograde metamorphism occurs during uplift and cooling as the minerals adjust to lower temperatures, though the reactions may not fully reverse due to efficient fluid removal during prograde metamorphism.

1. Name : Darji Darshan. R
Geology paper no: GEL 409
Roll No: 03
Supervision: Dr.Rahul SIR
M. G. Science institute
( Geology department)
Navrangpura, Ahmedabad, Gujarat 380009
darshandaiya234@gmail.com
Chemical equilibrium in metamorphic rocks
( equilibrium reaction )
Retrograde metamorphism

2. Content
★ Introduction
★ Metamorphic Reaction
★ Types of Metamorphic Reaction
★ Grade of Metamorphic Reaction
★ Retrograde Metamorphism
★ References

3. INTRODUCTION
• The equilibrium model for metamorphism is founded
on the metamorphic facies principle, the repeated
association of the same mineral assemblages in rocks
of different bulk composition that have been
metamorphosed together.

4. Metamorphic Reaction
A metamorphic reaction is a chemical reaction that
takes place during the geological process of
metamorphism where in on assemblage of
minerals is transformed into a second assemblage
which is stable under the new
temperature/pressure conditions resulting in the
final stable state of the observed metamorphic
rock.

5. Conti….
Examples
would include the production of talc under varied
metamorphic conditions:
serpentine + carbon dioxide—> talc + magnesite + water
chlorite + quartz—> kyanite + talc + water.

6. Types Of Metamorphic Reaction
Mainly have a two types Of metamorphic reaction
1) Univariant reaction
2) diavariant reaction

7. Univariant Reactions
A univariant reaction is one that plots as a line or
curve on a pressure-temperature diagram.
If all phases in the reaction are present in the rock,
then we know that the rock must have been
metamorphosed at some pressure and
temperature along the reaction boundary Consider
for example the simple Al2SiO5 system with excess
SiO2 and H2O.

8. Conti…
In low grade metamorphic in this system, the
reaction:
Al2Si4O10(OH)2 <=> Al2SiO5 + 3SiO2 + H2O
Pyrophyllite Ky or Andal Qtz fluid

9. defines a reaction boundary on a P-T diagram. This boundary can be
determined experimentally or can be calculated using thermodynamic
properties of the phases involved. If we find a rock that contains
pyrophyllite, quartz, and an Al2SiO5 mineral, then we know that
metamorphism took place somewhere along the trajectory of the reaction
boundary.

10. Divariant Reactions
In the cases discussed above, the univariant reactions that were
considered involved reaching a point in pressure temperature
space where a reaction occurred resulting in a sudden change in
mineral assemblage.
These reactions can be considered discontinuous reactions
because they occur along specific pressure temperature curves.
Because many minerals are solid solutions, it is also possible to
have discontinuous reactions that result in a gradual change in
composition of the minerals, but not necessarily the formation of
new minerals.

11. Conti….
These reactions are also considered divariant reactions
because they occur over a wide range of pressure and
temperature conditions.
Consider the hypothetical case of rocks that contain minerals
like chlorite and garnet, which are both Mg-Fe solid
solutions.
The reaction that occurs with increasing temperature (at
constant pressure) is:
Chlorite +Qtz=>Garnet + Mg-richer Chlorite + H2O

12. ● This reaction begins at a temperature of T1 where an initial Mg-
poor garnet is produced. As temperature increases, say to T2,
both the garnet and the chlorite become more Mg-rich. The
reaction continues over a range of temperature until eventually
the temperature reaches T3 at which point the much more Mg-
rich chlorite disappears leaving garnet with Mg/(Mg/Fe) ratio the
same as that in the initial chlorite.

13. Conti….
● We say that this reaction is a continuous reaction because there
is no change in mineral assemblage between T1 and T3, but
there is a reaction occurring and its effect is to change the
compositions of the solid solution minerals. Note the similarity
of this idea to the melting behavior of Fe-Mg solid solution

14. Grade of Metamorphism
Metamorphic grade is a general term for describing
the relative temperature and pressure conditions
under which metamorphic rocks form.
https://egyankosh.ac.in/

15. Continue…
• Low-grade metamorphism
• High-grade metamorphism
• Prograde (= progressive) metamorphism
• Retrograde (= retrogressive) metamorphism

16. Low Grade Metamorphism
Low-grade metamorphism takes place at temperatures
between about 200 to 320oC, and relatively low pressure.
Low grade metamorphic rocks are generally characterized by
an abundance of hydrous minerals.
High-grade metamorphism
takes place at temperatures greater than 320oC and
relatively high pressure. As grade of metamorphism
increases, hydrous minerals become less hydrous, by losing
H2O, and non-hydrous minerals become more common

17. Prograde metamorphism
involves the change of mineral assemblages
(paragenesis) with increasing temperature and (usually)
pressure conditions. These are solid state dehydration
reactions, and involve the loss of volatiles such as water
or carbon dioxide.
Retrograde metamorphism
(diaphthoresis, retrogressive metamorphism) is the
mineralogical adjustment of relatively high-grade
metamorphic rocks to temperatures lower than those of
their initial metamorphism

18. Retrograde metamorphism
In general, the changes in mineral assemblage and mineral composition
that occur during burial and heating are referred to as prograde
metamorphism, whereas those that occur during uplift and cooling of a
rock represent retrograde metamorphism.
If thermodynamic equilibrium were always maintained, one might expect
all the reactions that occur during prograde metamorphism to be reversed
during subsequent uplift of the rocks and reexposure at Earth’s surface; in
this case, metamorphic rocks would never be seen in outcrop.
However, two factors mitigate against complete retrogression of
metamorphic rocks during their return to Earth’s surface.

19. Continue…
First is the efficient removal of the water and carbon dioxide released
during prograde devolatilization reactions by upward migration of the fluid
along grain boundaries and through fractures.
Because almost all the water released during heating by reactions—such as
when chlorite (Fe9Al6Si5O20(OH)16) reacts with quartz (4SiO2) to yield
garnet (3Fe3Al2Si3O12) and water (8H2O)—is removed from the site of
reaction, the reaction cannot be reversed during cooling unless water is
subsequently added to the rock.
Thus, garnet can be preserved at Earth’s surface even though it is
thermodynamically unstable at such low temperatures and pressures.

20. Continue…..
garnets are often rimmed by small amounts of chlorite and quartz,
indicating that limited quantities of water were available for the reverse of
the reaction given above to proceed during cooling.
Retrograde features such as these reaction rims can be mapped to yield
information on pathways of fluid migration through the rocks during uplift
and cooling.
In other rocks, such as high-temperature gneisses, mineral compositions
often reflect temperatures too low to be in equilibrium with the preserved
mineral assemblage.
In these samples, it is clear that certain exchange reactions operated in a
retrograde sense even when the net-transfer reactions were frozen in
during prograde metamorphism.

21. https://opengeology.org/

22. References
★ https://www2.tulane.edu/
★ https://www.britannica.com/
★ WWW.WIKIPEDIA.COM
★ https://egyankosh.ac.in