In a binary solid-solution phase diagram (e.g., Albite-Anorthite plagioclase system), a tie-line connecting the liquidus and solidus curves at a given temperature links the coexisting equilibrium melt composition and solid (crystal) composition.
At a chosen temperature, the horizontal tie-line intersects the liquidus at point a, giving the composition of the coexisting melt, and intersects the solidus at...
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