Silica (SiO2) polymorphs vs. temperature: which crystalline form of silica is thermodynamically stable between 1470°C and its melting point near 1713°C?

Introduction:
Silica exhibits temperature-dependent polymorphism. Correctly identifying the stable phase at high temperatures is important for choosing silica bricks and predicting dimensional changes and thermal-shock behavior in furnaces.

Given Data / Assumptions:

• Temperature range: 1470°C up to the melting point (~1713°C).
• Common polymorphs: quartz (low/high), tridymite, cristobalite.
• We are concerned with equilibrium stability domains, not metastable persistence.

Concept / Approach:
Equilibrium phase relationships for SiO2 indicate that quartz is low-temperature, tridymite is intermediate (~870–1470°C), and cristobalite is the high-temperature phase stable from ~1470°C to melting. Bricks and linings must account for the volume changes associated with these transformations.

Step-by-Step Solution:
Identify the upper-temperature stable polymorph: cristobalite.Confirm boundaries: tridymite stabilizes below ~1470°C; quartz at still lower temperatures.Therefore, for 1470–1713°C, cristobalite is the correct phase.

Verification / Alternative check:
Phase diagrams and refractory textbooks list cristobalite as the stable phase approaching silica’s melting point.

Why Other Options Are Wrong:

• Tridymite: stable at intermediate temperatures below ~1470°C.
• Quartz: low-temperature region; transforms before reaching 1470°C.
• “None of these” and glassy silica: do not represent the equilibrium high-T crystalline silica.

Common Pitfalls:
Confusing metastable persistence (e.g., retained tridymite) with true equilibrium stability, and overlooking large volume changes during phase transitions.

Final Answer:
Cristobalite