Quantitative analysis of binary mineral mixtures using Raman microspectroscopy: Calibration curves for silica and calcium carbonate minerals and application to an opaline silica nodule of volcanic origin

We have developed a method for the quantitative analysis of binary mixtures of minerals using a common Raman microspectrometer having backscattering optical configuration (180° illumination) and a rotating sample stage. Using the averaged Raman spectra of mineral mixtures, the calibration curves of four mixtures of calcium carbonate and silica minerals-calcite and quartz, aragonite and calcite, quartz and cristobalite, and coesite and quartz-are constructed. The calibration curves express the correlation between the weight fraction and the relative intensities of the Raman bands intrinsic to the binary mixture. This technique can be used to map the phase distributions in one or two dimensions across an analytical sample surface and conduct quantitative analyses of samples containing inclusions with dimensions on the order of a few microns. As an example of the quantitative analysis using Raman microspectroscopy, the distribution of quartz and cristobalite in a silica nodule in volcanic rock from Akaze, Ishikawa prefecture is investigated. It is confirmed by X-ray diffraction, optical observation, and Raman mapping analysis that microquartz and opal-C are horizontally layered in the silica nodule. The line profile of the Raman spectra across the two regions reveals a sequential change in the weight ratio from quartz to cristobalite.