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

Abstract

Haüyne mineral, belonging to sodalite group, characterizes two lithotypes of Mount Vulture area (Italy): phonolite of Toppo San Paolo and haüynophyre of Melfi. In the latter, haüyne phenocrysts can appear colourless‐white, grey‐to‐black or blue in colour. The blue colour of these crystals—sometimes in literature classified as lazurite—is known to be due to the presence of S3− polysulphides radical anion chromophores. Since it has been previously observed that heating these rocks up to 750 °C in oxidizing conditions leads haüyne crystals to acquire the blue colour, it was decided to investigate the reasons of this chromatic transformation. SEM‐EDS and Raman spot microanalyses were performed on single crystals of the pristine rocks; furthermore, detailed X‐ray and Raman maps were carried out on two white and two black haüyne crystals of haüynophyre before and after heating up to 750 °C. This allowed to investigate the role of very fine inclusions in the chromatic change: the sulphur‐, barium‐ and iron‐based species included in haüyne, principally ‘pyrrhotite‐like’ crystals and baryte, with the increasing of temperature and under control of oxygen and sulphur fugacity, give rise to the formation of new phases, chiefly haematite and new baryte crystals and of S3− groups.

Crystal Chemistry, Isomorphism, and Thermal Conversions of Extra-Framework Components in Sodalite-Group Minerals

“…Additionally, in the Raman spectrum of Sample 1, bands of SO 4 2− groups are observed at 987 cm −1 [symmetric stretching vibrations, A 1 (ν 1 ) mode], 616 cm −1 [bending vibrations, F 2 (ν 4 ) mode], and 442 cm −1 [bending vibrations, the E(ν 2 ) mode]. A series of distinct Raman bands (at 327, 650, and 682 cm −1 ) corresponding to S 4 molecules having cis-, gahcheand trans-conformations, respectively, as well as bands of carbonate groups (at 1953 and 1460 cm −1 ), H 2 S molecules (at 2580 cm −1 ), and hydronium cations H 3 O + (at 3005 and 3280 cm −1 ) (Table 2) are the specific feature of Sample 1 distinguishing it from all other haüyne samples studied in this work and elsewhere [1,2,48]. The presence of hydronium in Sample 1 is in agreement with the deficit of extra-framework cations determined by the electron microprobe analysis (Table 1).…”

“…Representative Raman spectra of cubic sodalite-group minerals are given in Figures 1-3. The assignment of bands in the Raman spectra of cubic sodalite-group minerals shown in Figures 1-3 (Table 2) was made in accordance with [3,4,[38][39][40][41][42][43][44][45][46][47][48][49][50]. Ideally, sodalite, bolotinaite and sapozhnikovite are the end-members of the solid-solution system with the general formula [Na7( ,Na)](Si6Al6O24){Cl,HS,[F0.5(H2O)2]}2.…”

“…The assignment of bands in the Raman spectra of cubic sodalite-group minerals shown in Figures 1-3 (Table 2) was made in accordance with [3,4,[38][39][40][41][42][43][44][45][46][47][48][49][50].…”

“…The S3 radical anion is a very strong chromophore [45] which causes the deep blue color of lazurite [5]. Blue color of haüyne is usually due to trace amounts of S3 − [3,4,44,48]. The main yellow chromophore in sodalite-group minerals is the S2 radical anion, which occurs in significant amounts in yellow varieties of bolotinaite (IMA 2021-088) earlier described as an "F-rich sodalite-group mineral" [4] as well as in above-described yellow products of heating of haüyne under reducing conditions.…”

A Possible Natural and Inexpensive Substitute for Lapis Lazuli in the Frederick II Era: The Finding of Haüyne in Blue Lead-Tin Glazed Pottery from Melfi Castle (Italy)

“…Lazurite [(Na,Ca) 8 Al 6 Si 6 O 24 (SO 4 ,S,Cl) 2 ] belongs to the mineral group of sodalite which mainly includes three other minerals: sodalite s.s. (Na 8 Al 6 Si 6 O 24 Cl 2 ), nosean (Na 8 Al 6 Si 6 O 24 SO 4 ), and haüyne [Na 6 Ca 2 Al 6 Si 6 O 24 (SO 4 ) 2 ]. In nature, lazurite, sodalite s.s. , and sometimes haüyne can be found with a blue color, due to the presence of groups [ 1 , 2 , 3 , 4 , 5 ]. The peculiarity of lazurite is not only related to its intense blue hue but also to the quite unchangeable characteristic of the color over time, essentially due to the idiochromatic character of the coloration.…”