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ternary_ACF - 1 Compositional phase diagrams Uses of...

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1 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 two-dimensional 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 tie-lines) 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 three-component 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 calc-silicates. 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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2 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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