Micro-Raman Determination of the Composition of Ugrandite Garnets

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Micro-Raman Spectroscopy and Luminescence Studies (2009) 3037.pdf MICRO-RAMAN DETERMINATION OF THE COMPOSITION OF UGRANDITE GARNETS. Danilo Bersani1, Sergio Andò2, Pietro Vignola3, Iari-Gabriel Marino1 and Pier Paolo Lottici1, 1Dipartimento di Fisi- ca, Università di Parma, Viale G.P. Usberti 7a, 43100 Parma (Italy), 2Dipartimento di Scienze Geologiche e Geotec- nologie, Università di Milano-Bicocca, Piazzale della Scienza 4, 20126 Milano (Italy), 3Consiglio Nazionale delle Ricerche (CNR), Istituto per la dinamica dei processi ambientali (IDPA), Via Mario Bianco 9, 20131 Milano (Italy). A fast method to obtain the molar composition of A simple software routine, optimized for the garnets starting from their Raman spectra was recently ugrandite family, was prepared in order to calculate the proposed [1]. The method, called MIRAGEM – MIcro- composition of garnets in terms of molar ratio of the RAman Garnets Evaluation Method – is based on the pure end-members (andradite, grossular and uvarovite) assumption that in a solid-solution garnet the Raman starting from the frequencies of their main Raman wavenumbers are linear combinations of end-member bands. The role played by water (hydrogarnets) is dis- wavenumbers, weighted by their molar fraction. cussed. The results are compared with the composi- In this work, the application of the MIRAGEM me- tions obtained by means of Wavelength Dispersive thod to garnets belonging to the ugrandite family is System (WDS) and Energy Dispersive System (EDS) discussed in detail. The Raman spectra of garnets in electron microprobe measurements, showing very the andradite-grossular and grossular-uvarovite series good agreement, as visible in the triangular plots re- are shown in Figs 1 and 2. ported in Fig. 3. Fig. 3: graphical comparison between the composi- tions of ugrandite garnets obtained by means of WDS- EMPA and MIRAGEM. Some applications in high-resolution provenance studies of modern sands and ancient sandstones are Fig. 1: Raman spectra of garnets in the andradite- shown. This non-destructive and friendly technique grossular series. can thus be routinely used in petrographic and heavy- mineral analyses to rapidly determine the distribution of garnet types and the size-density correlation in dif- ferent sediment samples. [1] D. Bersani, S. Andò, P. Vignola, G. Moltifiori, I.-G. Marino, P. P. Lottici and V. Diella, (2009) Spec- trochim. Acta A: Mol. Biomol. Spectrosc., do- i:10.1016/j.saa.2008.11.033 Fig. 2: Raman spectra of garnets in the grossular- uvarovite series. .

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