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

Hauyne mineral, belonging to sodalite group, characterizes two lithotypes of Mount Vulture area (Italy): phonolite of Toppo San Paolo and hauynophyre of Melfi. In the latter, hauyne 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 degrees C in oxidizing conditions leads hauyne 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 hauyne crystals of hauynophyre before and after heating up to 750 degrees C. This allowed to investigate the role of very fine inclusions in the chromatic change: the sulphur-, barium- and iron-based species included in hauyne, 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.