Crystal chemistry and Raman spectroscopy of the johnbaumite–hydroxylellestadite mineral series and associated As- and B-bearing minerals from a pyrometamorphic xenolith of the Upper Chegem Caldera, North Caucasus, Kabardino-Balkaria, Russia

Abstract A calcium-silicate xenolith (no. 11) from the ignimbrite of the Upper Chegem Caldera in Kabardino-Balkaria, Russia, has revealed a diverse mineral assemblage with As- and B-bearing phases from the apatite supergroup such as the svabite and johnbaumite–hydroxylellestadite series, in addition to cahnite and datolite. Three distinct zones of variable arsenic content have been investigated. Notably, the outermost altered zone adjacent to the ignimbrite hosts the highest concentration of arsenic and arsenate minerals. A detailed structural analysis using Raman spectroscopy was carried out to investigate the distribution of boron and arsenic in tetrahedral coordination. This has provided the basis for describing a solid-solution system between hydroxylellestadite, svabite and johnbaumite and can be used as a novel technique for identifying apatite-supergroup minerals. One aim of the analysis was to elucidate the origin of various elements and content levels, particularly in relation to the distance from the xenolith–ignimbrite contact. The presence of boron and arsenic, probably derived from ignimbrites, highlights the important role of volcanic rocks as potential contributors of these elements in mineral formation processes.