High-temperature transformation of silica: conversion of a silica mineral to cristobalite at elevated temperatures is accompanied by which change(s) in bulk properties?

Introduction / Context:Silica-based refractories undergo polymorphic changes on heating. The formation of cristobalite at high temperature alters the crystal framework and therefore the bulk physical properties. Designers must anticipate these changes to manage dimensional stability and thermal-shock behavior.

Given Data / Assumptions:

• Cristobalite is the high-temperature SiO2 polymorph stable above ~1470°C.
• We compare average room-temperature densities of the phases.
• Bulk volume is inversely related to density for a fixed mass.

Concept / Approach:Among common SiO2 phases, quartz has higher density (~2.65) than cristobalite (~2.27–2.33). Thus, transformation to cristobalite is associated with a reduction in specific gravity (density). For a given mass, a lower density implies a larger volume, not a reduction in volume. Therefore, only the density decreases; the statement about volume reduction does not hold for the same mass of solid.

Step-by-Step Solution:Recall densities: quartz > cristobalite.Infer property change: density decreases on forming cristobalite.Relate density and volume at fixed mass: lower density → larger volume.Hence, “reduction in specific gravity” alone is correct.

Verification / Alternative check:Materials handbooks list cristobalite with lower density than quartz or tridymite. Dimensional change studies in silica bricks highlight the impact of phase composition on bulk volume and thermal cycling response.

Why Other Options Are Wrong:Reduction in volume: opposite to the density trend at fixed mass.Both (a) & (b): contradicts the volume–density relationship.Neither / Increase in specific gravity: inconsistent with measured densities.

Common Pitfalls:Confusing transient thermal expansion with permanent phase-related density changes.Ignoring kinetic barriers that control the extent of transformation in service.

Final Answer:Reduction in specific gravity (density)