Spectral characterization of lab-made Nakhlitic rock powders: effects of crystal/glass ratio and acquisition geometry.

Silicates are the main constituent of volcanic terrains on terrestrial planets in the Solar system. On Earth, we know that volcanic terrains are constituted by lava flows and fragmented pyroclasts whose texture presents both glassy and crystalline phases. Understanding the influence of glass/crystal ratio on the spectral response of volcanic rocks is therefore focal to interpret remotely sensed spectra that are commonly used to interpret the geology of terrestrial planets. Thus, we performed spectral characterization of lab-made mafic volcanic products which were  synthesized in the PVRG labs with the aim to present different degrees of crystallinity.

Samples were synthesized  by mixing and melting oxides to resemble the composition of a Nakhlite meteorite. First we produced a homogeneous silicate glass (Nglass) which was then used to prepare three more samples by melting at 1500°C and then cooling down slowly (52-56 °C per hour) towards subliquidus temperatures of ca. 1200°C (N12), 1100°C (N11), and 1000°C (N10), respectively. Each sample stayed for 48 hours at the target temperature and was finally quenched in air.

SEM and XRPD analyses and Rietveld method quantitative phase analyses were performed to assess type and degree of crystallinity, showing how N11 and N10 present a similar mineralogical assemblage with ca. 30% of glass and crystal species including augite, magnetite, cristobalite and other minor phases similar to the mineralogical composition of a natural Nakhlite. Sample N12 presents less diverse mineralogy (augite and magnetite) and ca. 70% of glass. 

Bi-directional reflectance spectra was collected at room temperature in the 1-4.2 µm range considering a set of 3 incidence angles (i = 0°; 30°; 60°) and emergence (e) angles between -70° and 70° using the custom-made bidirectional reflectance spectro-goniometers SHINE at the Cold Surface Spectroscopy facility (CSS) of the IPAG laboratory in the frame of the Trans-National Access program, project number 21-EPN-FT1-025, of Europlanet 2024.

Spectral analyses show how, in the Visible and Near-Infrared, increasing crystallinity causes slopes of spectra to gradually shifts from positive for glasses towards flat-negative for crystalline material. The spectral features of the single mineral phases are barely distinguishable for N10 and N11, where spectra are flattened probably because of the presence of magnetite, whereas the spectral signature of Fe in augite is distinguishable for N12 located at ~0.9 and ~1.15 µm.
Changing observation geometry, reflectance values and spectral slope show important variations while the bands position remains unchanged. We observe important dependence of band and slope in correspondence of low phase (< 30°) angle and high phase angle (> 100°).

To identify distinctive features we used principal component analysis (PCA) obtaining four clusters in the PC space which are relatable to the four samples. K-means clustering was used to verify our clustering obtaining a very low level of misclassification, especially regarding Nglass.

These results provide further information on the spectral response of synthesized rock samples, especially for what concerns glass-bearing materials, that can be used for modeling of spectral information coming from volcanic rocky bodies in the Solar system.