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Raman spectroscopy is a powerful technique in the context of planetary exploration because it provides information on mineral identification, chemistry, and abundance. For Raman spectrometers with large spot sizes, multiple mineral phases can be investigated by collecting a single Raman spectrum. There is a lack of methodology for quantifying mineral species in mixtures due to the independent signal strengths of different materials in Raman spectra. Two techniques are presented in this work for quantifying common rock-forming minerals: partial least-squares multivariate analysis and a novel approach called Raman cross-section proxies (numerical metrics associated with specific Raman features). This paper targets 20 mineral species relevant to the mineralogy of the planet Mars. Mineral end-member samples and 187 binary mineral-mineral mixtures (mixture of two distinct minerals) are used for multivariate modeling. Eighteen diamond-mineral mixtures are used to derive Raman cross-section proxies. Mineral abundance proportions are predicted for the binary mineral-mineral mixtures with known mineralogical content to evaluate the efficacy of the two quantitative methods. Technique performance is mineral dependent. The root mean square error for unseen predictions (RMSE-P) using Raman crosssectionproxies ranges from ±3.2–17.0 vol%. For the multivariate models, the cross-validated root mean square error (RMSE-CV) ranges from ±8.8 to 26.2 vol%. Although these error estimates are not directly comparable, they provide the most accurate error estimate currently available. Different scenarios may favor the use of one or the other of the two quantitative methods. This work provides fundamental groundwork that can be applied to common rock-forming minerals on terrestrial planets, including Mars. Quantification of mineral abundances aids in answering critical geologic questions related to ancient primary and altered rocks as well as planetary processes and conditions. [ABSTRACT FROM AUTHOR]

Parent body thermal metamorphism is an important process that alters the structure of organic matter in the parent asteroid of meteorites. Increasing and progressing thermal metamorphism results in carbonization and graphitization of carbonaceous matter in the parent body. Such modifications in the carbon structures can be studied by Raman microspectroscopy, thanks to its high sensitivity to structure and bonding within carbonaceous molecules. We have characterized polyaromatic carbonaceous matter in a total of 24 Antarctic CV3 and CO3 chondrites using micro-Raman imaging spectroscopy in an effort to better understand parent body thermal metamorphism and assess its effects on the carbon structures. Raman spectral parameters of the first-order carbon peaks (D and G) were extracted from at least 200 spectra for each meteorite and were compared to deduce relationships that yield information regarding the thermal metamorphism conditions. We also show, for the first time, spectral trends and relations of the second-order carbon peaks (2D and D+G) within the 2500–3200 cm–1 with thermal metamorphic history. The second-order peaks appear to contain information that is lacking in the first-order peaks. Based on the second-order carbon peak parameters, we tentatively classify four CV3 chondrites into subtypes, and reclassify another. Peak metamorphic temperatures of the investigated meteorites have been estimated based on the width of the D band as well as the calculated Raman spectral curvature. Estimated temperatures appear to correlate well with the assigned petrologic types. We have calculated higher peak metamorphic temperatures for the CV3 chondrites than for the considered CO3 chondrites and further showed that the peak metamorphic temperatures of CV3oxA chondrites are higher than those of CV3oxB, indicating possibly different metamorphic conditions for the two oxidized subtypes. We observe that there is a relatively larger temperature increase going from CO3.2 to CO3.4 (150 °C increase) compared to CO3.4–CO3.6 (20 °C), which may indicate that the graphitization and structural ordering of carbon reach a critical temperature regime around petrologic type CO3.3. [ABSTRACT FROM AUTHOR]

Pigeonite (P21/c clinopyroxene) crystallizes in various terrestrial and extraterrestrial rocks. However, because it breaks down ("inverts") in slowly cooled rocks, bulk natural samples of pigeonite from coarse-grained rocks are not available. We have synthesized eight samples of pigeonite with compositions of Wo8 and Wo10 [where Wo (mol%) = 100Ca/(Ca+Mg+Fe2+)] and X ranging from 20 to 60 [where X = 100Fe2+/(Mg+Fe2+)]. These samples are suitable for spectroscopic and other studies that require bulk samples. Because of relatively fine grain size (mainly 5-50 mm) and slight grain-to-grain variation in composition, they are generally not suitable for studies requiring individual crystals. We will make samples available for appropriate investigations, especially if the techniques used are non-destructive and the samples can be returned after use. [ABSTRACT FROM AUTHOR]

Tissintite is a shock-induced, Ca-rich mineral, isostructural to jadeite, observed in several meteorite samples such as the martian shergottite Tissint. It may form within a “Goldilocks Zone,” indicating a potential to provide strict constraints on peak pressure and temperature conditions experienced during impact. Here we present the first laboratory synthesis of tissintite, which was synthesized using a large volume multi-anvil apparatus at conditions ranging from 6–8.5 GPa and 1000–1350 °C. For these experiments, we utilized a novel heating protocol in which we reached impact-relevant temperatures within 1 s and in doing so approximated the temperature-time conditions in a post-shock melt. We have established that heating for impact-relevant timescales is not sufficient to completely transform crystalline labradorite to tissintite at these pressures. Our findings suggest that tissintite forms from amorphous plagioclase during decompression. [ABSTRACT FROM AUTHOR]

The hydroxy sulfate jarosite [(K,Na,H3O)Fe3(SO4)2(OH)6] has both been discovered on Mars, and is associated with areas of highly acidic runoff on Earth. Because jarosite is extremely sensitive to formation conditions, it is an important target mineral for remote sensing applications. Yet at visible and near infrared (VNIR) wavelengths, where many spacecraft spectrometers collect data, the spectral abundance of a mineral in a mixture is not linearly correlated with the surface abundance of that mineral. Radiative transfer modeling can be used to extract quantitative abundance estimates if the optical constants (the real and imaginary indices of refraction, n and k) for all minerals in the mixture are known. Unfortunately, optical constants for a wide variety of minerals, including sulfates like jarosite, are not available. This is due, in part, to the inherent difficulty in obtaining such data for minerals that tend to crystallize naturally as fine-grained (~10 μm) powders, like many sulfates including jarosite. However, the optical constants of powders can be obtained by inverting the equation of radiative transfer and using it to model laboratory spectra. In this paper, we provide robust n and k data for synthetic potassium, hydronium, and sodium jarosite in the VNIR. We also explicitly describe the calculation procedures (including access to our Matlab code) so that others may obtain optical constants of additional minerals. Expansion of the optical constants library in the VNIR will facilitate the extraction of quantitative mineral abundances, leading to more in-depth evaluations of remote sensing target locations. [ABSTRACT FROM AUTHOR]

We have determined the mid-IR optical constants of one alkali feldspar and four pyroxene compositions in the range of 250-4000 cm-1. Measured reflectance spectra of oriented single crystals were iteratively fit to modeled spectra derived from classical dispersion analysis. We present the real and imaginary indices of refraction (n and k) along with the oscillator parameters with which they were modeled. While materials of orthorhombic symmetry and higher are well covered by the current literature, optical constants have been derived for only a handful of geologically relevant monoclinic materials, including gypsum and orthoclase. Two input parameters that go into radiative transfer models, the scattering phase function and the single scattering albedo, are functions of a material's optical constants. Pyroxene is a common rock-forming mineral group in terrestrial bodies as well as meteorites and is also detected in cosmic dust. Hence, having a set of pyroxene optical constants will provide additional details about the composition of Solar System bodies and circumstellar materials. We follow the method of Mayerhöfer et al. (2010), which is based on the Berreman 4 × 4 matrix formulation. This approach provides a consistent way to calculate the reflectance coefficients in lowsymmetry cases. Additionally, while many models assume normal incidence to simplify the dispersion relations, this more general model applies to reflectance spectra collected at non-normal incidence. [ABSTRACT FROM AUTHOR]