Phase Composition and Microstructure of UHPHT Vein Glass from Giant Diamondiferous Kara Impact Crater by Raman Mapping

Abstract

2D and 3D Raman mapping allows to observe “in situ” distribution of phases and relationship of minerals in rock samples, different mineral aggregates, mineral inclusions within crystals and other quantitative spectroscopic studies of mineral substances and materials. The method is realized in accumulating and processing of big data array of Raman spectra (RS) from selected areas of a studied sample with subsequent visualization in XY plane at 2D mapping or in the form of 3D experimental models with XYZ coordinates in a case of 3D Raman mapping. During the mapping different analytical data can be visualized as maps such as spatial distribution of mineral phases, degrees of their crystallinity, orientation of crystallites and their size, residual tension in minerals and inclusions and many other parameters. As 3D Raman mapping demands higher requirements to quality of samples, takes significantly long time expenditure and big data for processing, 2D Raman mapping looks to be the most effective for different mineralogical applications. In this work we demonstrate advantages of 2D Raman mapping on the example of studying of the impact ultrahigh-pressure high-temperature (UHPHT) vein glasses with melt-crystallized ultrahigh-pressure silica phase – coesite, for the first time found within diamondiferous impactites of the giant Kara meteorite crater. The mapping was produced with processing of typical spectra ranges of recognized mineral phases met in analyzed areas. As a result of the conducted studies it has been established what according to spectroscopic data coesite hangs to central parts of UHPHT silica glass located in general matrix of aluminosilicate glass. The sites of silicate glass are characterized by fully amorphous state that is obviously shown in its typical ranges. The areas with a high luminescence correspond to inclusions of smectite micro-drops in UHPHT silica glass isolations that has been confirmed with the accompanying study with use of electron probe microanalysis (EPMA), energy dispersive spectroscopy (EDS) and electron diffraction (ED). The used 2D Raman mapping to the impact vein glasses of the Kara meteorite crater allowed to reveal distribution of mineral phases in UHPHT glass, showing distribution of amorphous and crystalline phases that is undoubted advantage of the Raman spectroscopy. In full text of the paper technical features of Raman 2D mapping are shown that can be useful for similar mineral objects and materials with complex phase structure and different degree of ordering.