gems, quartz, agate, chalcedony, mineralogy

This chapter reviews the geological occurrences, structures, and phase transitions of the low-pressure silica polymorphs—quartz, tridymite, and cristobalite. All these phases experience displacive transformations that involve structural contraction with decreased temperature, and research over the past three decades has sought out the mechanisms that control these transitions. The passage from β- to α-quartz is associated with an intermediate phase that is stable over a 1.3 °C temperature interval. X-ray diffraction and transmission electron microscopy have revealed that this phase consists of Dauphiné microtwins that are incommensurately modulated. Meteoritic and synthetic tridymite experience a series of structural alterations with decreasing temperature during which the symmetry changes from hexagonal (HP) to orthorhombic (OC, OS, and OP) to monoclinic (MC). Phase transition behavior in terrestrial tridymite (PO-n and MX-1) is more complex, probably due to a greater degree of structural disorder. The transformation from cubic β-cristobalite to tetragonal α-cristobalite is marked by a high spontaneous strain and a large hysteresis in the transition temperature. The three high-temperature polymorphs—β-quartz, HP-tridymite, and β-cristobalite—exhibit evidence for dynamical disorder, but the nature of the atomic oscillations in these phases remains an active area of investigation.