Effects of sample absorption on Raman intensity for detection and quantification of minerals in two-component geological mixtures

Session: Advancing Mineralogy and Spectroscopy Across the Solar System in Honor of MSA Roebling Medalist M. Darby Dyar

Presenting Author:

Authors:

Abstract:

On Earth and other planets, field portable Raman instruments can characterize minerals in situ from an outcrop in a non-destructive and quick manner without any sample preparation. On Mars, the SuperCam and SHERLOC Raman instruments on the NASA Mars 2020 rover have identified both primary and secondary alteration minerals including olivine, pyroxene, perchlorate, phosphate, sulfate and carbonate (e.g., [1], [2]). However, a challenge with field instruments is that they are commonly detecting phases in mixtures because of the limits in the magnification that can be achieved. Consequently, the Raman scattered photons will originate from a larger area that could contain more than one phase. The effects of mixing can create challenges in detecting phases of interest, particularly when there are dark minerals in the field of view of the laser. Dark minerals in general are difficult to characterize with Raman spectroscopy due to its attenuation of the exciting laser as well as any Raman scattered photons. Motivated by the question of detection threshold of minerals on Mars with the SuperCam and SHERLOC Raman instruments, but broadly applicable to any Raman system, we present a model we have developed based on the theory of self-absorption in Raman spectroscopy. In this model, we parameterize the effects of sample absorption on Raman intensity for a two-component solid mixture and determine detection thresholds and phase quantification. Subsequently, we verified this model through lab measurements of three types of two-component mixtures: (1) light minerals + light minerals, (2) light minerals + dark minerals, (3) dark minerals + dark minerals. Raman intensity as a function of concentration is near linear for light+light mixtures but follows a power law for any mixtures containing dark minerals. We find that as little as 5 wt% of hydrated components in light mixtures can be detected while hydrated components in dark mixtures may require greater than 30 wt% to be detected. We will discuss how we can use these models to estimate the abundance of hydrated minerals that have been detected on Mars from orbital near-infrared spectroscopy such as sulfates and phyllosilicates.

[1] A. Corpolongo et al. (2023) JGR Planets, 128.

[2] E. Clavé et al. (2023) JGR Planets, 128.