Raman spectroscopy of natural silica in Chicxulub impactite, Mexico

Abstract

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 α -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 Γ and the frequency ω of the symmetric SiOSi stretching vibrational band (A 1 mode) of the SiO 4 tetrahedra are the most amenable parameters for estimating the degree of crystallinity. In well-crystallized quartz, Γ =5 cm −1 and ω =464 cm −1 , while in highly disordered quartz this line shifts up to ω =455 cm −1 and broadens up to Γ =30 cm −1 . The Raman lineshapes appear to depend strongly on the degree of lattice disorder subsequent to impact events.