High spatial resolution Raman mapping of complex mineral assemblages: Application on phosphate mineral sequences in pegmatites

Abstract

Phosphates are a group of complex minerals widespread on Earth that can crystallize in several environments. Changes in environmental conditions during crystallization are reflected in the formation of a sequence of phosphate minerals, termed paragenesis. Accordingly, analyses of textural and chemical properties of complex mineral assemblages are essential to reconstruct the history of rocks and solid materials. Raman mapping can detect those mineralogical changes in complex assemblages with high spatial resolution, successfully determining the paragenetic sequence. Specifically, Raman spectroscopy is more advantageous than conventional methods because it requires little sample preparation, it is nondestructive, and it is capable of identifying hydrated minerals and phases composed of light elements (e.g., H and Li). In this study, the parageneses of two phosphatic assemblages from the Buranga pegmatite, a P–Li–Nb–Ta‐rich magmatic rock, are reported in detail. The first case shows the replacement of primary F‐rich montebrasite [LiAl(PO4)(OH,F)] by secondary F‐poor montebrasite [LiAl(PO4)OH]. Mapping of the OH‐stretching peak position and width identifies the mineral phases, illustrates their spatial relations, and allows to estimate the F content distribution. The second case investigates a sequence of phosphate minerals in a complex assemblage (trolleite + rosemaryite ➔ lazulite–scorzalite ➔ samuelsonite ➔ wardite). The application of multivariate analysis methods to characterize complex assemblages is assessed, and the classical least squares method is identified as the best suited for this setting. An analysis of Raman images shows that Buranga samples experienced a progressive hydration process during cooling, followed by fluid‐assisted mineral reactions with remobilization of elements, such as Ca and Na.

A confocal-controlled Raman-LIBS hybrid microscope with high stability and spatial resolution

“…Although all these methodologies can handle the different irradiances required by Raman and LIBS measurements, they lack high-precision autofocus capability and are therefore prone to low spatial resolution and poor long-term stability. These shortcomings reduce the applicability of these methods to geological samples with a complex topography 20,21 or to unknown extraterrestrial minerals. 22,23 The spatial resolution of a Raman-LIBS hybrid system is mainly limited by the size of the ablation crater formed during the LIBS excitation process.…”

Benefits of using multiple Raman laser wavelengths for characterizing defects in a UO2 matrix

“…Additionally, Raman spectra collected over an area with an automated XY movement stage can be compiled into maps with profiles such as peak height or chemical identification of different regions using reference spectra. [ 35–37 ] Depending on the stage movement resolution, microscope objective, and laser wavelength being used, maps with submicrometer resolution are possible. For instance, Raman mapping has been used with UO 2 to measure many grains and compare T 2g intensities with crystallographic orientations measured by electron backscatter diffraction (EBSD), in order to investigate correlations between Raman signatures and grain orientations.…”