Metamorphism - Lecture 6 Metamorphic Reactions

I. Metamorphic reactions and composition diagrams

In addition to showing graphically what the equilibrium mineralogy should be given a bulk composition, a major reason to use composition diagrams is to show what reactions have occurred between grades.


Two different pelitic units with slightly different bulk composition in Barrovian metamorphic sequence.

  Chlorite zone Biotite zone
UNIT A kspar>musc>chl ksp>musc>biot
UNIT B chl>musc>ksp chl>musc>biot

Both rock types have the same minerals in the chlorite zone, but the abundances are different

Question: What is the biotite forming reaction in the area?



II. Types of metamorphic reactions

A. Solid - solid reactions

DS/DV remains constant for all solid-solid reactions, thus P/T slope remains the same. So reaction is very dependent upon pressure


Types of solid-solid reactions

1. Polymorphic transformations

2. Coupled reactions (= net transfer reaction)

3. Exsolution or solvus reactions


B. Dehydration reactions

With increasing metamorphism, minerals get more dehydrated. Shape of curves can be gotten from Clapeyron equation (dP/dT = DS/DV), but you'll be spared that this quarter. Dehydration reactions are always curved in P-T space.



Progressive dehydration or "drying out" of rock:

Mud loose clay sized particles

(pore water expelled during burial & compaction)

10-30 vol.% porosity filled with H2O



clay minerals (~14% H2O in mineral)

ex) kaolinite Al2Si2O5(OH)4

5-10 wt.% H2O found in mineral
Slate/Schist mica minerals

ex) muscovite KAl2Si3O10(OH)2

4 wt.% H2O
High grade gneiss


biotite + amphibole

ex) amph ~2 wt.% H2O, but <10% of rock

<0.5 wt.% H2O



III. Reading metamorphic reactions off composition diagrams

A. Appearance/Disappearance of a phase

Example: Dehydration of talc

Talc -> enstatite + quartz + fluid

Mg3Si4O10(OH)2 -> 3MgSiO3 + SiO2 + H2O

B. Tieline flip

Example: dehydration of muscovite

Muscovite + quartz -> kspar + sillimanite + H2O