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

Abstract

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 S3−/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.

Blue coloured haüyne from Mt. Vulture (Italy) volcanic rocks: SEM‐EDS and Raman investigation of natural and heated crystals

“…The spectra collected in these areas show a very sharp peak centred at 988 cm À1 , together with the other typical signals of baryte (458, 618, 646 and 1145 cm À1 [24] ; Figure 7a). The broad band around 990 cm À1 , generally associated to the sulphate group intrinsically characteristic of the haüyne mineral structure, [2][3][4][5]14,23] can actually include and hide the principal signal of barium sulphate inclusions. The mapping carried out after heating shows a homogeneous distribution of the intensities (Figure 6c), and the sulphate signals are visible at 445 and 988 cm À1 (Figure 7d).…”

“…As concerns Raman studies on haüyne, only few data on sodalite‐group minerals are present in the literature. [ 3–5,9–15 ] The aim of such studies has largely been the understanding of the reason for their different colouring, which has been debated upon for a long time. [ 3–5,9–15 ] The first investigations [ 9 ] succeeded in measuring and classifying the vibrational modes which resulted in Raman signals and managed to correlate experimental measurements with calculations based on group theory.…”

“…Haüyne was focused upon by Ballirano [ 4 ] by combining Raman data with different analytical techniques, underlining the key role played by (poly‐sulphur radical) in modifying the cation arrangement in M sites, ultimately affecting the sample colouration. Della Ventura et al, [ 3 ] on the other hand, examined the content ratio of to , relating it to the gradation of blue in lazurites.…”

“…Raman investigation of cationic and anionic groups in the extra-frameworks has been a pivotal topic for sodalite group minerals, and many authors [3,4,[13][14][15] correlated the entrapped species with different colourations.…”

GeoRaman

“…They were able to demonstrate that crichtonite group minerals can be readily distinguished from other Ti-, Fe-, Cr-and Zr-containing oxides (e.g., ilmenite or those of spinel-and magnetoplumbite groups). Insights in the knowledge of blue sodalite group minerals are provided by Della Ventura et al [10] combining Raman and μ-Fourier-transform infrared (FTIR) spectroscopies to study calchogen species in the haüynelazurite series from the comagmatic volcanic region of latium. Examining the polarized Raman scattering on a single crystal of hematite and observing the spectral differences between basal and prism facets, Marshall et al [11] assign the previously unidentified mode at 245 cm −1 and confirm that at 294 cm −1 , additionally highlighting the polarization behaviour of the 1,320 cm −1 mode.…”