Silica polymorph stability for refractory design: among quartz, tridymite, and cristobalite, which SiO2 phase is thermodynamically stable up to approximately 1470°C under typical kiln conditions?

Introduction / Context:
Silica (SiO2) exhibits multiple polymorphs with distinct thermal expansion and transformation behaviors, which strongly influence the performance of silica-bearing refractories. Understanding the stability fields is essential for predicting dimensional change and spalling tendencies.

Given Data / Assumptions:

• Equilibrium or near-equilibrium transformations in a kiln.
• Key transitions: alpha- to beta-quartz near 573°C; tridymite stabilizes at higher temperatures; cristobalite stabilizes above ~1470°C.
• Pressure effects are negligible in typical furnace service.

Concept / Approach:
From room temperature to ~870°C, quartz dominates (alpha then beta). From roughly 870°C to about 1470°C, tridymite is the stable polymorph. Above ~1470°C, cristobalite becomes stable until close to the melting region. These fields explain expansion anomalies and why silica bricks are carefully heat-treated.

Step-by-Step Solution:
Map temperature ranges to polymorphs.Identify the target upper limit 1470°C.Select tridymite as the stable phase up to that temperature.

Verification / Alternative check:
Silica phase diagrams in refractory references consistently present the sequence: quartz → tridymite → cristobalite with transitions near 870°C and 1470°C, respectively.

Why Other Options Are Wrong:
Quartz: stable only up to ~870°C.Cristobalite: stable above ~1470°C, not below.Opal / None: contradict established phase equilibria.

Common Pitfalls:
Confusing kinetic sluggishness with equilibrium stability.Overlooking the sharp expansion at the quartz transition (~573°C) during heat-up.

Final Answer:
Tridymite