OCCURRENCE AND DISTRIBUTION OF MOGANITE IN AGATE CHALCEDONY - A COMBINED MICRO-RAMAN, RIETVELD, AND CATHODOLUMINESCENCE STUDY/

Agate/chalcedony samples of different origin were investigated by perf orming Raman, X-ray diffraction (using Rietveld refinement), and catho doluminescence measurements. These analyses were performed to measure the content and spatial distribution of the silica polymorph moganite, which is considered to represent periodic Brazil-law twinning of a-qu artz at the unit-cell scale in agate/chalcedonies. Homogeneous standar d samples including the nearly a-quartz free moganite type material fr om Gran Canaria were analysed in order to compare results of the X-ray diffractometry and Raman spectroscopy techniques and to provide a cal ibration curve for the Raman results. However, due to the different le ngth scales analysed by the two techniques, the ''moganite content'' i n microcrystalline SiO2 samples measured by Raman spectroscopy (short- range order) was found to be considerably higher than the ''moganite c ontent'' measured by X-ray diffractometry (long-range order). The diff erence is explained by the presence of moganite nanocrystals, nano-ran ge moganite lamellae, and single Brazil-law twin-planes that are detec ted by vibrational spectroscopy but that are not large enough (in the sense of coherently scattering lattice domains) to be detected by X-ra y diffractometry. High resolution Raman analysis provides a measure of the moganite content and its spatial variation in microcrystalline si lica samples with a lateral resolution in the Gun-range. Variations in the moganite-to-quartz ratio are revealed by varying intensity ratios of the main symmetric stretching-bending vibrations (A(1) modes) of c c-quartz (465cm(-1)) and moganite (502cm(-1)), respectively. Traces of Raman microprobe analyses perpendicular to the rhythmic zoning of aga tes revealed that the moganite-to-quartz ratio is often not uniform bu t shows a cyclic pattern that correlates with the observed cathodolumi nescence pattern (colour and intensity). Data obtained from an agate s ample from a fluorite deposit near Okorusu, Namibia and from a volcani c agate from Los Indios, Cuba were selected for detailed presentation. Variations of cathodoluminescence and Raman data between single bands in agates suggest alternating formation of fine-grained, highly defec tive chalcedony intergrown with moganite, and coarse-grained low-defec t quartz. Multiple zones indicate dynamic internal growth during a sel f-organizational crystallization process from silica-rich fluids.