Polymorphs of silica and temperature stability: Which crystalline form of silica is the stable phase below about 870 °C under standard pressure?

Introduction / Context:
Silica (SiO2) has several polymorphs that are stable over different temperature ranges. Knowing which phase is stable at a given temperature is important for kiln design, refractory selection, and thermal shock management.

Given Data / Assumptions:

• Atmospheric pressure conditions.
• Temperature threshold of interest: about 870 °C.

Concept / Approach:
At room temperature up to about 870 °C, the stable polymorph is quartz (with an alpha-to-beta transition near 573 °C). At higher temperatures, tridymite becomes stable, followed by cristobalite at still higher temperatures. Thus, for temperatures below roughly 870 °C, quartz is the stable form.

Step-by-Step Solution:
Recall stability sequence with rising temperature: quartz → tridymite → cristobalite.Compare the given temperature range (below 870 °C) to the sequence.Select quartz as the correct answer.

Verification / Alternative check:
Ceramic materials references show polymorph stability fields; quartz dominates at lower temperatures under standard pressure.

Why Other Options Are Wrong:
Tridymite and cristobalite are high-temperature forms stable above quartz’s range. “None of these” is incorrect because quartz fits the condition.

Common Pitfalls:

• Confusing the alpha–beta transition of quartz (≈573 °C) with a change to a different polymorph.
• Assuming cristobalite forms readily at low temperature; it requires higher temperatures or catalysts/nuclei.

Final Answer:
Quartz