Why do high-pressure phase transitions often produce denser mineral structures?

Under high pressure, materials undergo structural transformations to achieve denser, more closely packed atomic arrangements to minimize volume.1

For rock-forming minerals, such as oxides and silicates, these phase transitions often involve a structural rearrangement that increases the coordination number—the number of oxygen atoms surrounding a central metal or semimetal cation. For example, quartz transforms from a four-coordinated silicon structure to the denser stishovite form, which features six-coordinated silicon. Similarly, pyroxene transitions into the perovskite structure, increasing the coordination of its magnesium and silicon atoms.1

These transitions are driven by the system's response to extreme stress, where the material adopts a configuration with higher packing efficiency. While simple compression involves bond shortening or the bending of bond angles between atomic polyhedrons, phase transitions represent a fundamental shift in the material's geometry to a more compact state.12