Atomic Dynamics and Structural Transformations in Chalcedony as a Model Cryptocrystalline Multiphase System at Non-Ambient Conditions

Abstract

The behaviour of chalcedony, a natural polycrystalline system consisting of cryptocrystalline (i.e., submicron-sized) quartz and moganite, has been investigated via in situ Raman spectroscopy under non-ambient conditions, complemented by hybrid Hartree–Fock/density-functional-theory (HF-DFT) simulations of moganite phonon modes at high pressure. The combined experimental and computational results clearly indicate that the peak neat 503 cm⁻¹ arises exclusively from the moganite SiO₄-ring mode rather than from OH librations of silanol groups. At high pressure and room temperature, the quartz fraction in chalcedony becomes metastable against coesite at 2.4 GPa and develops structural defects because of the anisotropic elastic strain arising from the interaction between cryptocrystalline moganite and quartz under the applied hydrostatic compressible stress. This process can trigger amorphization of quartz at pressures lower than those commonly observed in a quartz single crystal. At high temperature and ambient pressure, both quartz and moganite Raman peaks measured in chalcedony are excellent markers of both moganite and quartz α-β transformations. Further, we show that upon heating–cooling cycles of chalcedony, a part of the moganite fraction transforms into quartz, if the temperature of α-β quartz phase transition is crossed, confirming the overall metastability of the moganite structure with respect to that of α quartz. Moreover, the α-β moganite transition affects the rate of phonon softening on quartz on the approach to the phase-transition temperature. Our results demonstrate that the mutual impact of quartz and moganites is achieved via an admixture of phonons belonging to different phases but having the same symmetry and type of atomic vibrations, emphasizing the key role of Raman spectroscopy in studying structural transformations in multiphase systems.