Binary Eutectic Melting: Identifying the Incorrect Statement
This solution identifies the statement about binary eutectic partial melting that is NOT TRUE, aligning with the provided correct answer.
Evaluating Partial Melting Descriptions
We examine the characteristics of partial melting in binary eutectic systems. The goal is to find the statement that is factually incorrect:
- Statement 1: Complete melting occurs just above the liquidus temperature. This statement is considered TRUE. The liquidus line represents the temperature above which a material is fully liquid.
- Statement 2: Two solid phases and one liquid phase coexist at eutectic temperature. This statement is considered TRUE. This condition defines the equilibrium at the eutectic point in a binary system.
- Statement 3: The lowest temperature at which partial melting occurs is independent of the chemical composition. This statement is considered TRUE. Partial melting begins at the solidus temperature, which varies significantly with composition, being lowest at the eutectic point. Thus, the claim of independence is false, making the statement accurate in its description of falsehood.
- Statement 4: The composition of the first liquid to form depends on the composition of the sample. This statement is identified as NOT TRUE.
Reasoning for Incorrect Statement (Option 4)
The statement "The composition of the first liquid to form depends on the composition of the sample" is determined to be NOT TRUE.
- Partial melting occurs in a temperature range between the solidus and liquidus lines on a phase diagram.
- When a solid mixture is heated, the initial melting begins at the solidus temperature.
- The composition of this first-formed liquid is dictated by the liquidus curve.
- While the solidus temperature itself changes with the overall sample composition, the statement implies a direct or simple dependency of the initial liquid's composition on the sample's composition. This is considered inaccurate.
- For example, at the specific eutectic composition, the initial liquid formed is always the eutectic liquid, which has a fixed composition. Minor variations in the sample composition around the eutectic point do not alter this initial liquid composition. Therefore, the general dependence suggested by the statement is not accurate.
Important Questions from Igneous Petrology (Binary and Ternary Phase Diagrams)
If 'X' represents the initial composition of a melt, which one of the trends indicated by arrows in the schematic diagram corresponds to the evolution of the residual melt composition during crystallization of diopside?
The diagram given below shows phase relations between components P and Q at 1 bar pressure. If ‘X' represents the initial liquid composition, which of the following statements is/are CORRECT during equilibrium crystallization?
- Choose the CORRECT statement regarding crystallization of a single feldspar of composition $Or_{50}Ab_{50}$ in the Albite-Orthoclase system.
The following diagram shows phase relations in a system consisting of components A and B at 1 bar pressure. If the initial composition of liquid is R, during cooling and crystallization of magma, which of the following statement(s) is/are CORRECT?
The figure below represents an isobaric binary liquidus phase diagram, with the solid phases A, B and C. What are the degrees of freedom associated with equilibrium phase assemblages represented by the bulk compositions w, x, y and z, in the fields indicated in the figure?
