RAMAN AND X-RAY ANALYSES ON CHALCEDONY AND CHRYSOPRASE by Shu-Cheng Yu, Heng-Ying Su and Jiann-Shing Lee, Department of Earth Sciences, National Cheng-Kung University, Tainan, Taiwan

An X-ray diffraction study and a Raman spectroscopic analysis at elevated pressure conditions have been carried out on two cryptocrystalline silica minerals, chalcedony and chrysoprase. The chalcedony samples are of fibrous variety and the chrysoprase specimens are granular in microstructure. X-ray diffraction measurements indicate that the crystallinity index of the chalcedony is inferior to that of the chrysoprase. The variation in crystallinity between these two samples is also consistent with the Raman spectroscopic analysis. At ambient conditions, a principal Raman peak of the chrysoprase has a wavenumber of 466.1cm-1 instead of 464.3cm-1 for a crystalline quartz specimen. With increasing pressure, this Raman peak shifts toward a higher wavenumber region, and its FWHM value increases. At 15 GPa, this 466.cm-1 peak has moved to 565.0cm-1. The peak has significantly broadened in peak width, indicating the deterioration of the crystallinity of the sample.

No obvious structural phase transformations were observed within the studied pressure range for both cryptocrystalline silica samples. The pressure effect on amorphization of cryptocrystalline SiO2 is more profound for chalcedony than for chrysoprase.