A Raman study of chalcogen species in sodalite-group minerals from the volcanic rocks of Latium (Italy)

Sodalite-group minerals are silicates belonging to the zeolite group hosting in their cages a variety of cations and anions. Four mineral species of the sodalite-group are known in nature, the main difference being connected to the identity of the chemical species in the cages, particularly the sulfur molecular arrangements. Notably, the sodalite-group minerals show different colours; lazurite, in particular, is the main constituent of the well-known lapis lazuli, a material used throughout the human history as a precious stone or as a blue pigment for artworks. In this paper, we exploit Raman and Fourier-transform infrared (FTIR) spectroscopy to highlight an existing connection between the chalcogen species entrapped within the samples originated from the volcanic region of Latium (Italy) and their final colour. We show that Raman spectroscopy is a very valuable tool in characterizing the extraframework content in these minerals. Further, our analysis demonstrates how the blue hue of the sodalite species is related to the S-3(-)/SO42- relative content. We finally show how most specimen are also rich in CO2, a feature addressed by combination with FTIR spectroscopy. These findings have a particular interest not only in geology, but also in Cultural Heritage studies.