Pseudo-isometric Silica Forms Explained

The question asks to identify the form of silica that exhibits pseudo-isometric crystal structure. Pseudo-isometry refers to a crystal system that appears geometrically similar to the isometric (cubic) system but is technically not cubic.

Silica Crystal Structures

Let's examine the crystal structures of the given silica forms:

• Quartz: Primarily exists in trigonal and hexagonal crystal systems at different temperatures. It is not pseudo-isometric.
• Tridymite: Exists in several polymorphs, typically monoclinic or orthorhombic depending on temperature and specific form. It is not pseudo-isometric.
• Coesite: A high-density form of silica, it crystallizes in the monoclinic system. It is not pseudo-isometric.
• Cristobalite: Exists in cubic form at high temperatures (above ~275°C) and transforms into tetragonal forms at lower temperatures. The high-temperature cubic form closely resembles the isometric system, leading to it being classified as pseudo-isometric.

Conclusion on Pseudo-isometry

Based on its structural characteristics, especially the high-temperature cubic polymorph, Cristobalite is considered pseudo-isometric among the given options.