Rock-forming feldspathoids of the sodalite–sapozhnikovite series from the Lovozero alkaline complex (Kola peninsula, Russia): isomorphism, thermal and radiation-induced transformations and genetic mineralogy

Sulfur-enriched sodalite-group feldspathoids from the Lovozero alkaline complex (Kola peninsula, Russia) and products of their laboratory, anthropogene, and natural thermal and radiation-induced transformations were studied using EMPA, single-crystal XRD, and Raman, IR, ESR and optical spectroscopy. Sodalite Na8[Al6Si6O24]Cl2 and sapozhnikovite Na8[Al6Si6O24](HS)2 form a continuous isomorphous series [with the Cl:HS ratio variation (in mol.%) from Cl100(HS)0 to Cl12(HS)88] in highly agpaitic feldspathoid syenites and their pegmatites. In Lovozero, hydrosulfide anion HS— turned out the major form of sulfidic sulfur occurrence in minerals of this group including sodalite-hackmanite. It is found that sapozhnikovite and HS-rich sodalite are important rock-forming minerals of some Lovozero rocks; a new rock, poikilitic nepheline-sapozhnikovite syenite was discovered. Sapozhnikovite and intermediate members of the sodalite–sapozhnikovite series are the sensitive geochemical indicator, an oxymeter which indicates reducing conditions of mineral formation. Under heating, HS− anion in sodalite-sapozhnikovite series minerals destroys and sulfur forms polysulfide groups: radical anion S2●− (500–600 °C) and further radical anion S3●− (700 °C and above). The S3●− groups also appear in the result of radiation-induced transformation of these minerals. Under natural radioactive irradiation at the contact with Th-enriched steenstrupine, an intermediate member of the sodalite-sapozhnikovite series transformed to an earlier unknown in nature S3●−-rich variety of sodalite with the simplified formula Na8[Al6Si6O24][Cl,(S3)].