Phase Equilibria

The stability of silica polymorphs under varying pressure is a crucial concept in geology, particularly within the study of mineral phase transitions. Silica, or silicon dioxide (SiO₂), exists in several polymorphic forms, each stable under particular temperatures and pressures. These polymorphs include Quartz, Tridymite, Coesite, and Stishovite.

Let's elucidate the order of these polymorphs with respect to stability under increasing pressure:

1. Quartz: It is the most common form of silica at the Earth's surface pressures and relatively low temperatures. However, its stability decreases as pressure increases.
2. Coesite: This denser form of silica becomes stable at higher pressures compared to Quartz. It is typically found in ultra-high-pressure metamorphic rocks.
3. Stishovite: At even higher pressures, Stishovite becomes the stable form of SiO₂. It has a rutile-type structure and is particularly noted for its high density, making it stable at extremely high pressures, such as those found deep within the Earth.

Thus, the stability order under increasing pressure is: Quartz – Coesite – Stishovite.

Tridymite is another polymorphic form of silica that is stable at high temperatures but low pressures, and it does not contribute as significantly to the stability under increasing pressure.

Among the given options, the correct orders that reflect the stability sequence with increasing pressure are:

• Quartz – Coesite – Stishovite
• Tridymite – Coesite – Stishovite

Initial melt: 1000 moles total = 900 An + 100 Di → bulk composition = 90% An (point P).

On cooling from P to Q, only anorthite crystallizes (field: Anorthite + Melt).

At Q, the melt composition is 50% An (from the diagram).

Let L = moles of melt remaining at Q.

Anorthite mass balance:

So, anorthite crystallized:

The answer is 800 moles.