Polymorphic transformations of silica: Identify the correct sequence of stable forms of SiO2 as temperature increases.

Introduction / Context:
Silica (SiO2) has several polymorphs—quartz, tridymite, and cristobalite—each stable over different temperature ranges. These phase changes are crucial in refractory processing, as they influence thermal expansion, microcracking, and dimensional stability of silica bricks and castables.

Given Data / Assumptions:

• Temperature ranges are approximate because kinetics and impurities can shift transformation temperatures.
• We are discussing equilibrium stability fields under typical ceramic firing conditions.
• Statements concern stability ranges rather than instantaneous kinetics.

Concept / Approach:
As temperature rises, silica shifts from quartz to tridymite to cristobalite. Standard ceramic references place the quartz–tridymite change near 870°C and the tridymite–cristobalite transition around 1470°C. Correct identification of these phases helps explain thermal shock behavior and firing schedules for silica refractories.

Step-by-Step Solution:

Verification / Alternative check:
Phase diagrams and high-temperature XRD studies corroborate the sequence and ranges used in refractory texts and standards.

Why Other Options Are Wrong:

• Any single statement alone is incomplete; the correct answer acknowledges the full sequence.

Common Pitfalls:
Confusing cristobalite formation temperature or assuming irreversible conversion; on cooling, metastable retention can occur, impacting thermal expansion cycles.

Final Answer:
All of the statements (a), (b) and (c) are correct.