Very often, temperature contours are omitted. This is not a problem if the system is simple, but if there are maxima or minima on the liquidus surface, the diagram becomes completely useless without contours.
Steven Dutch, Natural and Applied Sciences, University
of Wisconsin - Green Bay
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Mixtures of three distinct materials often melt and solidify in a ternary eutectic relationship. A good and very important geological example is the system quartz-anorthite-K-feldspar. If you understand binary eutectics well, ternary eutectics are very straightforward. Since ternary systems are plotted on triangle diagrams, a little review of triangle diagrams is in order first.
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Consider the three binary systems A-B, B-C, and C-A. We can arrange the three diagrams in three dimensions as shown here. As long as we only consider two components, we can work with the binary diagrams as usual. |
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The space inside the triangular prism represents systems with all three components present. There will be a field where A crystallizes first, a field where B crystallizes first, and one where C crystallizes first. |
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In reality, there is a liquidus surface covering the triangle and as the system cools, it will slide down the surface. |
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This diagram shows the relationship between temperature in
the binary systems and temperature on the triangle diagram.
Very often, temperature contours are omitted. This is not a problem if the system is simple, but if there are maxima or minima on the liquidus surface, the diagram becomes completely useless without contours. |
In the diagrams below, the overall system composition is a hollow white square and the melt and solid compositions are shown in red and blue, respectively. The history of the melt composition is shown in magenta and the solid in green. Blue is used to show geometric relationships between the solid, system, and melt compositions.
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The first thing that will happen is that one component will begin to crystallize. In this case it is B. As we remove B from the melt, the melt composition will migrate straight away from the B corner as shown. |
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Eventually the melt will hit a field boundary and then a
second component will begin to form. In this case it is C. The composition
of the melt will migrate away from both B and C in the general direction
of A. The path of the melt is shown in magenta.
Since C is now forming along with B, the solid composition migrates in the direction of C. The melt, system, and solid compositions always lie on a straight line as shown. |
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Finally the melt reaches the ternary eutectic and all three
components begin to form. The eutectic is usually a temperature minimum so
the melt does not move once it reaches the eutectic.
Since all three components are now forming, the solid composition moves into the interior of the triangle. The melt, system, and solid compositions always lie on a straight line as shown. Since the melt stays at the eutectic, the solid migrates toward the system composition as shown in green. Once it reaches the system composition, the entire system is solidified. |
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Given the rules above, we can divide the triangle into six fields with crystallization orders as shown. Note that any order is possible. |
One important ternary system is quartz-K feldspar-plagioclase. According to Bowen's reaction series, we expect to find plagioclase forming first in igneous rocks, then potassium feldspar, and last of all quartz. Since a large fraction of igneous rocks lie in the plagioclase rich corner of the diagram, we find that order in many cases and it's a useful rule of thumb. However, in silica rich magmas, quartz can crystallize first, and the three minerals can form in any order depending on the composition of the magma.
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Created November 22, 1999, Last Update 30 June 2003
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