Identify the correct chemical formula of the refractory mineral “spinel,” frequently used in magnesia–spinel and alumina–spinel brick systems.

Introduction / Context:
Spinel is a key phase in many modern refractories. It appears as an in-situ reaction product (e.g., MgO + Al2O3 → MgAl2O4) or as a pre-formed aggregate added to formulations. Spinel improves thermal shock resistance, restrains slag penetration, and can mitigate thermal expansion mismatch in magnesia- and alumina-based refractories.

Given Data / Assumptions:

• “Spinel” here refers to magnesium aluminate spinel, not generic spinel-structure compounds.
• We consider stoichiometric composition for the ideal phase.
• Hydrated or sulfate forms are distractors.

Concept / Approach:
Magnesium aluminate spinel has the formula MgAl2O4. It crystallizes in the cubic system and belongs to the broader spinel structural family AB2O4. In refractory bricks, spinel’s presence can reduce thermal stress and improve corrosion resistance against many slags, especially when distributed as fine, well-bonded grains.

Step-by-Step Solution:
Recall spinel family general formula: AB2O4.Assign A = Mg and B = Al for magnesium aluminate → MgAl2O4.Exclude hydrates and sulfates; these are not the refractory spinel phase.

Verification / Alternative check:
Phase diagrams of the MgO–Al2O3 system show spinel as a stable intermediate compound with the fixed stoichiometry MgAl2O4, corroborating the formula.

Why Other Options Are Wrong:
MgAl2O3: incorrect stoichiometry.MgSO4: magnesium sulfate, not a refractory ceramic used as spinel.MgAl2O3·2H2O: a hydrate-type distractor, not the high-temperature spinel phase.Al2O3·SiO2: mullite precursor/related composition, not spinel.

Common Pitfalls:
Confusing magnesium aluminate spinel with alumino-silicate phases like mullite.Assuming any AB2O4 compound is “spinel” without specifying A and B cations.

Final Answer:
MgAl2O4