Deriving and Balancing Metamorphic Reactions
Dexter Perkins (University of North Dakota)
Given a list of minerals, how do we determine all the possible reactions that can occur
between them? There are many ways to approach this question and the answer can
sometimes be quite elusive.
One Way To Proceed
•Make a list of all the phases and their formulas.
•Identify the chemical system and the number of components
•Use the phase rule to determine how many phases are in a normal univariant
•Make a list of all possible reactions, identifying them by the phases they DO NOT
•Balance the reactions
Example Involving 5 Phases and 3 Components
Consider the phases:
•Wo: Wollastonite CaSiO
•Ky: Kyanite Al
•An: Anorthite CaAl
•Gr: Grossular Ca
•Qz: Quartz SiO
The chemical system is CaO-Al
; it contains 3 components. The phase rule tells
us that a univariant reaction will include 4 phases (unless it is degenerate). So, each
reaction will be missing one phase.
Possible reactions are (absent phase in parentheses):
1. (Wo) involves Ky, An, Gr, Qz
2. (Ky) involves Wo, An, Gr, Qz
3. (An) involves Wo, Ky, Gr, Qz
4. (Gr) involves Wo, Ky, An, Qz
5. (Qz) involves Wo, Ky, An, Gr
Consider the first reaction (Wo). It involves kyanite, anorthite, grossular and quartz.
For fairly simple systems involving few phases, we can usually
identify and balance reactions by inspection.
For this example, note that only two of the phases (An, Gr) contain Ca, so they must be
on opposite sides of the reaction. Also, Gr contains 3 Ca, compared to 1 in An.
Therefore we can start with: