The interaction of crystalline and amorphous phases in the determination of VNIR and MIR spectral signatures: insights from the PVRG rock spectra database

Magmatism and volcanism are key processes that shaped the surface of rocky planets in the Solar System, and understanding how magmatism occurred on these planets is essential to reconstruct their geological histories.  It is very likely that volcanic terrains on planetary surfaces are covered by products that have a porphyric, aphanitic or hyalocrystalline texture, in which crystalline phases represent only a fraction of the rocks that are mainly composed by amorphous materials such as glasses.

Remote investigation of planetary surfaces combines geomorphological analysis with spectral data. Regarding the latter, the interpretation of spectral information from planetary terrains needs to be based on the comparison with reference material.

Up to now, libraries and  repositories are mostly report information about so-called planetary analogues consisting of crystalline materials rather than amorphous products. This happens because of the lack of spectral features of amorphous products. However, a the presence of amorphous material together with crystalline phases deeply influence the spectral response of the latter [1].

For this reason, the Petro-Volcanology Research Group at the University of Perugia started to build up a spectral database of volcanic rocks, both natural and synthetic, to understand how and how much are crystalline and amorphous products influencing each other in determining the spectral response of a rock/terrain. In this work we report the spectral characterization of various products within two ranges: Visible and Near Infrared (VNIR) and Mid-Infrared (MIR).

The accounted products consist of:

- Synthetic silicate glasses with wide range of chemical composition [2,3]

- Synthetic planetary analogues consisting of rocky materials containing both amorphous and crystalline products [4]

- Natural volcanic rocks

The overall analysis of the spectral response shows that, within the MIR, it is relatively easy to link the wavelength position of well-known spectral features (Christiansen feature, Reststrahlen Bands, Transparency features) to the silica content of the product itself, no matter which crystalline phases are present within the rock. This finding is of pivotal importance since the determination of silica content is fundamental for a characterization of volcanic products.

On the other hand, investigations and analyses on VNIR spectra show that the interpretation within such is more complicated. Indeed, a small presence of Iron-bearing crystals can deeply influence the VNIR spectrum. However, some constrains can be build using empirical parameters and machine-learning approaches.

All the spectra produced and published by our research group is or will be made available to everyone, open-source, within the platform hosted by ASI-SSDC (www.ssdc.asi.it/rockspectra/), making it possible for scientist from all-over the world to integrate such data in other researches or to directly compare planetary spectra to laboratory data [4].

[1]  Horgan, B. H., Cloutis, E. A., Mann, P., & Bell III, J. F. (2014). Near-infrared spectra of ferrous mineral mixtures and methods for their identification in planetary surface spectra. Icarus, 234, 132-154.
[2] Pisello, A., Ferrari, M., De Angelis, S., Vetere, F. P., Porreca, M., Stefani, S., & Perugini, D. (2022). Reflectance of silicate glasses in the mid-infrared region (MIR): Implications for planetary research. Icarus, 388, 115222.
[3] Pisello, A., De Angelis, S., Ferrari, M., Porreca, M., Vetere, F. P., Behrens, H., ... & Perugini, D. (2022). Visible and near-InfraRed (VNIR) reflectance of silicate glasses: Characterization of a featureless spectrum and implications for planetary geology. Icarus, 374, 114801.
[4] Pisello, A., Zinzi, A., Bisolfati, M., Porreca, M., & Perugini, D. (2022). A new spectral database for silicate glasses: a fundamental resource to interpret characteristics of volcanic terrains on planetary bodies (No. EPSC2022-539). Copernicus Meetings.