This does not really help us understand why we have different igneous rocks, but it does seem to show that there is some order in nature. To more closely examine this order let's look only at the plagioclase feldspars. Because plagioclase occurs in most igneous rocks. So if we can understand how and why feldspars form we may have some understanding about how different rocks form. Plot of temperature vs. composition.
Upper line is liquidus .
We can begin by examining the crystallization path of a liquid of composition X 0 . It cools to temperature X 1 and at that point the first crystals begin to form. As temperature continues to fall liquid composition shifts along liquidus to X 2 and solid crystals shift in composition along the solidus to C 2 . This is an example of equilibrium crystallization. Now let's look at what happens when we remove some of the crystals from the liquid as they form rather than allowing them to remain in contact with the liquid and change composition as they did in the example above.