Raman spectroscopy of natural silica in Chicxulub impactite, Mexico

A series of natural silica impactite samples from Chicxulub (Mexico) was investigated by Raman microprobe (RMP) analysis. The data yield evidence for high-pressure shock metamorphism in the rock. The impactite contains three polymorphs of silica: the original alpha-quartz, and two high-pressure varieties - coesite and disordered quartz representing various degrees of crystallinity. We found systematic changes in frequencies and half-widths of the Raman bands, caused by increasing irregularities of bond-lengths and bond-angles and a general breaking-up of the structure as a result of impact events. Therefore, RMP is an adequate tool for measuring the crystallinity of disordered quartz. The half-width F and the frequency a) of the symmetric Si-O-Si stretching vibrational band (A, mode) of the SiO4 tetrahedra are the most amenable parameters for estimating the degree of crystallinity. In well-crystallized quartz, Gamma = 5 cm(-1) and omega = 464 cm(-1), while in highly disordered quartz this line shifts up to omega = 455 cm(-1) and broadens up to Gamma = 30 cm(-1). The Raman lineshapes appear to depend strongly on the degree of lattice disorder subsequent to impact events.