Volume changes of silica polymorphs: which statement about transformations is correct?

Introduction:
Silica’s polymorphic transformations induce volume changes that can crack linings if not accommodated. Correctly quantifying these changes is important for choosing silica or alumino-silicate bricks in zones that cycle through transformation temperatures.

Given Data / Assumptions:

• Common SiO2 polymorphs: quartz, tridymite, cristobalite.
• Transformations occur at high temperatures under equilibrium conditions.
• Approximate volume changes are adequate for engineering judgment.

Concept / Approach:
Quartz to tridymite transformation is associated with a significant specific-volume increase on heating (often cited around 12–16% depending on reference and form). Changes between tridymite and cristobalite are comparatively smaller and not described as “very large expansion.”

Step-by-Step Solution:
Evaluate statement (a): accepted as approximately a 16% volume increase → TRUE.Evaluate statement (b): tridymite ↔ cristobalite change is not “very large”; hence FALSE.Therefore, only (a) is correct.

Verification / Alternative check:
Thermal expansion data for silica bricks and phase-change tables corroborate a large volume jump when passing from quartz to higher-temperature forms, especially near transition ranges.

Why Other Options Are Wrong:

• (b) exaggerates the tridymite→cristobalite expansion.
• (c) and (d) contradict the mixed truth of the two statements.
• (e) is opposite in direction; quartz→cristobalite results in higher specific volume, not a decrease.

Common Pitfalls:
Assuming all silica phase transitions have similar magnitude; the quartz transition effects are notably more problematic for lining design.

Final Answer:
Quartz → Tridymite conversion involves about a 16% volume increase