CSIR NET June 2016 Q79: Petrology & Mineralogy
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 point b, giving the composition of the plagioclase crystals in equilibrium with that melt. This melt-crystal tie-line is fundamental to reading equilibrium crystallization/melting behaviour from binary phase diagrams. Option 2 is correct.
1) Incorrect — the diagram shows a continuous solid-solution series (plagioclase), not coexisting pure end-member phases. 2) Correct — a and b represent, respectively, the melt composition (on the liquidus) and the coexisting plagioclase composition (on the solidus). 3) Incorrect — both points do not lie on the solidus; a is on the liquidus. 4) Incorrect — both points do not lie on the liquidus; b is on the solidus.