Chemistry of mullite refractories: Mullite, an important phase in alumino-silicate refractories, has which of the following idealized chemical formulas?

Introduction / Context:
Mullite is the principal crystalline phase that imparts strength and thermal stability to many alumino-silicate refractories and porcelain bodies. Identifying its stoichiometry is a foundational materials science fact useful in refractory selection and kiln operation.

Given Data / Assumptions:

• Mullite is an alumino-silicate phase.
• We seek the idealized stoichiometric formula.

Concept / Approach:
The idealized composition of mullite is commonly written as 3Al2O3·2SiO2 (approximately 71.8 wt% Al2O3). Natural and industrial mullites may deviate slightly, but this formula captures the key alumina-rich nature that differentiates it from simple alumina or silica.

Step-by-Step Solution:
List options and eliminate non-alumino-silicate compounds (e.g., ZrSO4, ThO2).Recognize mullite as an alumina-rich alumino-silicate phase.Select 3Al2O3·2SiO2 as the correct formula.

Verification / Alternative check:
Phase diagrams of the Al2O3–SiO2 system show the mullite field near the 3:2 alumina to silica molar ratio.

Why Other Options Are Wrong:
Al2O3 is corundum (pure alumina), not mullite. ZrSO4 and ThO2 are unrelated compounds.

Common Pitfalls:

• Confusing mullite with corundum or sillimanite group minerals.
• Assuming mullite equals “alumina” without silica; mullite is a mixed oxide phase.

Final Answer:
3Al2O3·2SiO2