Among the crystalline polymorphs of silica (SiO2) used in refractories—quartz, cristobalite, and tridymite—which form exhibits the highest specific gravity (i.e., the greatest density)?

Introduction / Context:
Silica (SiO2) occurs in multiple crystalline forms—quartz, tridymite, and cristobalite—that transform with temperature. In refractory engineering, the density (or specific gravity) of these polymorphs matters because it influences thermal expansion behavior, thermal shock response, and the strength-to-weight balance of bricks and shapes. The question asks which crystalline form possesses the greatest specific gravity, a property closely linked to the packing of the crystal lattice.

Given Data / Assumptions:

• We compare the specific gravity of crystalline SiO2 polymorphs typically encountered in refractory practice: quartz, tridymite, and cristobalite.
• Amorphous silica (fused silica) is not a crystalline polymorph but is sometimes referenced for contrast.
• Room-temperature values are intended unless otherwise noted.

Concept / Approach:
Polymorphs differ solely in how SiO4 tetrahedra are arranged, not in chemical composition. Denser packing yields a higher specific gravity. Quartz is the most densely packed of the common silica polymorphs at ambient conditions. Typical specific gravities are approximately: quartz ≈ 2.65, cristobalite ≈ 2.27–2.33, and tridymite ≈ 2.26–2.28. These differences stem from open-framework features in cristobalite and tridymite relative to quartz, which lead to lower bulk density for those phases.

Step-by-Step Solution:
List typical specific gravities: quartz (~2.65), cristobalite (~2.3), tridymite (~2.27).Compare the values: quartz > cristobalite ≈ tridymite.Therefore, the polymorph with the highest specific gravity is quartz.

Verification / Alternative check:
Materials handbooks and phase-diagram texts consistently list quartz as the densest of the common silica polymorphs at room temperature, corroborating the ranking presented above.

Why Other Options Are Wrong:
Cristobalite: less dense due to more open crystal structure at ambient conditions.Tridymite: similarly less dense than quartz.All have the same specific gravity: incorrect; polymorphs differ in density.Amorphous silica: included as a distractor; not a crystalline polymorph and typically shows lower apparent density.

Common Pitfalls:
Assuming identical density because the chemical formula is the same.Confusing high-temperature stability with room-temperature density.

Final Answer:
Quartz