Chapter 5.4, Problem 64P

Figure 5.32 shows the phase diagram of plagioclase feldspar, which can be considered a mixture of albite $\left({\text{NaAlSi}}_{\text{3}}{\text{O}}_{\text{8}}\right)$ and anorthite $\left({\text{CaAl}}_2{\text{Si}}_2{\text{O}}_{\text{8}}\right)$.

1. Suppose you discover a rock in which each plagioclase crystal varies in composition from center to edge, with the centers of the largest crystals composed of 70% anorthite and the outermost parts of all crystal made of essentially pure albite. Explain in some details how this variation might arise. What was the composition of the liquid magma from which the rock formed?
2. Suppose you discover another rock body in which the crystals near the top are albite–rich while the crystals near the bottom are anorthite–rich. Explain how this variationmight arise.

Figure 5.32. The phase diagram of plagioclase feldspar (at atmospheric pressure). From N. L. Bowen, “The Melting Phenomena of the Plagioclase Feldspars,” American Journal of Science 35, 577–599 (1913).