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Compositional phase diagrams
Uses of compositional phase diagrams:
(i) They show the relationship between the bulk composition of a rock and its mineral
assemblage.
(ii) They help illustrate the stable mineral assemblage at a specified P and T
(iii) They help "predict" metamorphic reactions that may have taken place (by
comparing mineral assemblages of rocks of the same bulk composition but from
different metamorphic zones within the area of interest).
(iv) They are useful for monitoring compositional variations in phases as a function of
changing P and T (for minerals with solid solutions).
For twodimensional representation (i.e. on a paper), the diagram must have at most 3
components. The compositions of the phases in a metamorphic rock are then plotted on
this diagram. Those phases believed to coexist in equilibrium are then joined by
tie
lines
. According to the mineralogical phase rule, the maximum number of phases
occurring in equilibrium in an arbitrarily selected rock will most likely be less than or
equal to the number of components of that system. Therefore, for a three component
system and a common rock with a variance of 2, this "maximum number of phases in
equilibrium" will be 3. On the other hand, if the phase rule is strictly followed, it would
predict that on all ternary diagrams (three component diagrams) representing
equilibrium mineral assemblages, tie lines should connect only three phases if P and T
lie in a
divariant field
, four phases (one pair of crossed tielines) if P and T lie on a
univariant line
, and five phases (five crossing tie lines) if P and T are fixed at an
invariant point
.
Types of Compositional Ternary Diagrams
A ternary diagram is ideally suited for a threecomponent system. However, rocks are
complex chemical systems, containing 10 to 13 components. Nevertheless, ternary
diagrams can be constructed for any rock by making some assumptions. Clearly, the
usefulness of the ternary diagram will depend on the validity of these assumptions!! It is
therefore necessary to first identify the "compositional group" to which the rock in
question belongs, make your "assumptions" and select the "type" of compositional
ternary to be used.
Commonly used ternary diagrams include:
1 ACF diagram: Suitable for mafic rocks and calcsilicates.
2 AFM diagram: Useful for pelitic rocks.
3 CAS (CaO  Al
2
O
3
 SiO
2
) diagram: Useful for marly rocks (calcareous mudstones).
4 MCS (MgO  CaO  SiO
2
) diagram: Useful for ultramafic rocks.
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Construction of Compositional Phase diagrams:
Although the selection of a compositional ternary diagram depends on the rock type
studied, the assumptions made for the same "type" of ternary diagram may differ from
one rock type to another, even if the "same" ternary system is used. We must therefore
understand the basis for making these assumptions in order to construct a compositional
ternary diagram.
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 Fall '11
 Staff
 ACF Diagram, projection phase

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