ternary - 1 GLY 421: Igneous & Metamorphic Petrology...

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1 El-Shazly, A. K., 2004 Ternary Phase Diagrams General principles: Construction of ternary phase diagrams: Ternary phase diagrams are constructed by projecting the liquidus surfaces of a three dimensional diagram (a triangular prism in which T is plotted on the vertical axis against the compositions of three components on the base of the prism) onto the compositional ternary diagram (Fig. 1). On such diagrams or projections, these temperatures are shown as dashed contours . The intersection of two liquidus surfaces is also projected on the same compositional ternary and is known as a boundary curve . The areas between boundary curves where one liquid coexists with a solid are known as " primary fields " (Fig. 2). Because T is still a variable on these ternary projections, we can still apply the condensed phase rule to determine the variance of a system. Accordingly, any point in a primary field (at T below the liquidus for this composition) will have two degrees of freedom. Boundary curves on the other hand will be univariant (as long as the system remains isobaric, and as long as two solid phases coexist with a liquid along these curves). We shall later see that there are several types of boundary curves, depending on the type of system. From the above discussion, it is quite clear that ternary systems are a bit more complicated than binary ones. The primary fields on these projections have to be labelled, and the diagram has to have T contours to allow us to determine the variance of the system and to understand the sequence of equilibrium crystallization. However, many ternary diagrams do not show T contours. These can still be "read" and interpreted using several simple rules. Definition: Alkemade line: An alkemade line is a straight line joining the compositions of two phases that share a boundary curve. On Fig. 3, the sides XY and XZ of the ternary diagram XYZ are alkemade lines, so are the lines Y-YZ, Z-YZ and X-YZ. Some rules: 1- A melt of a composition 30% X, 20% Y and 50% Z undergoing equilibrium crystallization will end up crystallizing 30% X, 20% Y and 50% Z! 2- A melt with a bulk composition lying in a "primary field" will first crystallize the solid phase of this primary field, and the liquid will then change its composition along a line drawn through this bulk composition and joining the composition of this solid (represented by a point on an Alkemade line if the solid phase is an intermediate compound e.g. YZ on Fig. 3, or one of the apices of the ternary diagram) and the boundary curve (Fig. 4). 3- The percentage of crystals formed at any point can be determined using the lever rule. For example, on the ternary diagram sphene - anorthite - wollastonite (Fig. 5), at a temperature represented by point "B", the % of crystals formed from a melt of composition "A" will be: % crystals = 100[AB/BC] (Fig. 5).
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2 El-Shazly, A. K., 2004 4- After considerable cooling, the liquid will eventually have a composition lying on a
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This note was uploaded on 02/28/2012 for the course GLY 421 taught by Professor Staff during the Fall '11 term at Marshall.

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ternary - 1 GLY 421: Igneous & Metamorphic Petrology...

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