Silica polymorphs: which crystalline form of silica is the most stable at room temperature under ambient pressure conditions?

Introduction / Context:
Silica (SiO2) exists in several polymorphs—each with its own stability range and thermal expansion behavior. Refractory engineers and ceramic technologists must know which phase is stable at use temperatures because phase changes can cause volume shifts and spalling.

Given Data / Assumptions:

• Ambient pressure, room temperature reference state.
• Common crystalline forms: quartz, tridymite, cristobalite.
• Amorphous varieties (e.g., opal) are not stable crystalline forms.

Concept / Approach:
Alpha-quartz is the thermodynamically stable crystalline form of silica at room temperature and atmospheric pressure. Tridymite and cristobalite are high-temperature polymorphs that can persist metastably on cooling but are not the equilibrium phase at room temperature. Their transformations involve significant expansion changes that must be managed in refractory formulations.

Step-by-Step Solution:
Identify equilibrium phase at 25 °C, 1 atm → alpha-quartz.Recognize tridymite/cristobalite are high-T forms with distinct expansion behaviors.Exclude noncrystalline opal as it is not a crystalline polymorph.Choose quartz as the correct stable form.

Verification / Alternative check:
Phase diagrams of SiO2 confirm quartz stability at low temperatures and transformations to tridymite/cristobalite at elevated temperatures, supporting the selection.

Why Other Options Are Wrong:
Cristobalite/Tridymite: high-temperature phases; metastable at room conditions.None of these: incorrect because alpha-quartz fits the criterion.Opal: amorphous, not a crystalline polymorph.

Common Pitfalls:
Confusing which phase is stable vs which has desirable thermal expansion for specific applications; stability and design choice are separate questions.

Final Answer:
Quartz (alpha-quartz)