Identify the correct chemical formula for mullite, the key crystal phase that imparts strength to many alumino-silicate refractories.

Introduction / Context:
Mullite is the principal high-temperature phase in many alumino-silicate refractories. Its composition controls properties such as creep resistance, thermal shock behavior, and corrosion resistance in service.

Given Data / Assumptions:

• Mullite is an alumina-rich phase within the Al2O3–SiO2 system.
• We seek the stoichiometric formula most commonly cited for mullite.

Concept / Approach:
The widely accepted composition for mullite is 3Al2O3 · 2SiO2. Actual mullites can show slight deviations, but the idealized formula remains the standard reference for exam and design contexts.

Step-by-Step Solution:
Eliminate silica-rich ratios like Al2O3 · 2SiO2 or 2Al2O3 · 3SiO2.Select the alumina-rich stoichiometry: 3Al2O3 · 2SiO2.

Verification / Alternative check:
Materials science texts consistently list mullite close to the 3:2 alumina-to-silica molar ratio.

Why Other Options Are Wrong:
They represent different alumino-silicate ratios, not the canonical mullite composition.

Common Pitfalls:
Confusing mullite with metakaolin or sillimanite-derived phases; mullite is distinctly alumina-rich.

Final Answer:
3Al2O3 · 2SiO2