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Several important concepts emerge from an examination of melting behavior in two‐component systems such as diopside–anorthite:

1 The composition of first melts in such systems is the same – is invariant – for a wide range of system compositions

2 Melt compositions depend on the proportion of melting so that increasing degrees of partial melting cause liquid compositions to change

3 Changes in liquid composition depend on the composition of the crystals being incorporated into the melt.

Invariant melting

38albite–orthoclase phase diagram

orthoclase38albite38

ssss1 Albite–orthoclase phase diagram at atmospheric pressure.

The result is a solid composed of sodic plagioclase, with a potassic orthoclase component in solid solution. For systems with >40% Or, initial crystals are relatively enriched in the potassium feldspar (orthoclase) component. As such crystals continue to separate on cooling, they react continuously with the melt so that crystal composition changes down the solidus as the remaining liquid changes composition down the liquidus, both toward decreasing Or content until all the melt is used up. The result is a rock composed of a feldspar solid solution. For systems with >40% Or, these crystals may be thought of as potassic orthoclase crystals with an albite component in solid solution. All solid solutions between the two end members are stable at high temperatures (and low pressures) after they begin to cool below the solidus temperature.