Question

Choose the CORRECT statement regarding crystallization of a single feldspar of composition Or$_{50}$Ab$_{50}$ in the Albite–Orthoclase system.

• A. The mineral can form in hypersolvus but not in subsolvus feldspar system.
• B. The mineral can form in subsolvus but not in hypersolvus feldspar system.
• C. The mineral can crystallize in both hypersolvus and subsolvus feldspar system.
• D. The mineral can crystallize neither in hypersolvus nor in subsolvus feldspar system.

Hint:

Think "hyper = one feldspar (solid solution), sub = two feldspars (split across the solvus)." Intermediate Ab–Or compositions are single-phase only above the solvus.

Answer 1

The Correct Option is A

Step 1: Recall hypersolvus vs.\ subsolvus.
In the alkali-feldspar (Ab–Or) system, a hypersolvus granite crystallizes one alkali-feldspar solid solution at high $T$ (above the solvus). A subsolvus granite crystallizes two feldspars: a K-rich alkali feldspar and an Ab-rich plagioclase; compositions lie on opposite sides of the solvus.

Step 2: Apply to Or$_{50$Ab$_{50}$.}
An intermediate composition like Or$_{50}$Ab$_{50}$ can exist as a single alkali-feldspar crystal only above the solvus (hypersolvus conditions). On cooling it exsolves to perthite. Under subsolvus conditions, the melt partitions into two coexisting feldspars rather than a single intermediate-composition feldspar; hence Or$_{50}$Ab$_{50}$ does not crystallize as one phase.

Final Answer:
\[ \boxed{\text{(A) Hypersolvus only; not subsolvus.}} \]