Two common minerals in igneous rocks are Albite (NaAlSi3O8), and Anorthite (CaAl2Si2O8). Both of these minerals are plagioclase feldspars. When a plagioclase feldspar crystallizes, it forms as a solid state solution between Albite and Anorthite. The composition of the liquid magma (melt) will change as precipitation occurs, as will the composition of the solid precipitate. Anorthite has a higher melting point than Albite. It will therefore crystallize first as the melt cools. The liquidus on the graph is the line above which everything is liquid. The solidus is the line below which everything is solid. Consider a melt with a composition of 50 weight% Albite. This is represented by the dashed line on the graph. As the melt cools, the status of the melt will move down the dashed line until it reaches point A. This is the point where it intersects the liquidus. The composition at point B represents the composition of the initial precipitate. Point A represents the composition of the liquid at the moment of precipitation. As the melt cools further, the composition of the liquid and solid will move in lock step until the solid arrives at the dashed line, which represents the bulk composition of the available material. The Lever rule can be used to determine the ratio of liquid to solid in the melt at any given point as it cools. Consider the melt with liquid composition C and solid composition D. The ratio of liquid to solid is equivalent to the ratio of y to x.
Img Credit: http://www.tulane.edu/~sanelson/images/phdifig3.gif
Posted by Matt Matteri
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