Anomalous behavior at the I2/a to Imab phase transition in SiO₂-moganite: An analysis using hard-mode Raman spectroscopy

The silica polymorph moganite is commonly intergrown with quartz in microcrystalline silica varieties that are less than ∼100 Ma in age. Synchrotron X-ray diffraction suggests that a displacive phase transition occurs when moganite is heated above ∼570 K, with an increase in symmetry from I2/a to Imab. In the present study, we employed hard-mode Raman spectroscopy to confirm the existence of the α-β moganite transformation and to offer complementary insight into the transition mechanism. Our analysis of the displacement of the 501 Δcm^-1 symmetric stretching-bending vibration (B_3g mode) with changing temperature strongly supports the existence of a monoclinic-to-orthorhombic phase transition between 570 and 590 K. Between 593 and 723 K, however, the mode remained fixed at 496 Δcm^-1. This behavior was repeated on cooling, but with a hysteresis of over 100 K. We offer three hypotheses that may explain this observation: (1) the intergrowth of nanoscale quartz lamellae within moganite may exert a strain that inhibits the transition; (2) the transition may exhibit a martensitic character marked by the co-existence of α- and β-moganite over a finite temperature interval; and (3) the α- and β-moganite transition may occur via an intermediate phase.