Plagioclase: The System An-Ab

The feldspar of common basalts is by no means pure anorthite, but instead an intermediate plagioclase near An50. This means that we could much improve the Di-An model of basalts if we could study a model of Di-An50, or in other words part of the ternary system Di-An-Ab. It would be folly to do this, however, without first examining the nature of all the binary combinations, An-Ab and Di-Ab as well as Di-An, already discussed.