Olivine-Group Minerals from Melilite–Nepheline Paralavas of Combustion Metamorphic Complexes of Mongolia

This paper reports the olivine-group minerals from melilite–nepheline paralavas of two combustion metamorphic (CM) complexes of Mongolia. The paralavas formed by thermal alterations of sedimentary rocks due to multistage modern and ancient (since the Quaternary) wild coal fires. The decomposition products of the olivine solid solution into Ca-bearing fayalite and kirschsteinite have been found in paralavas of the Khamaryn–Khural–Khiid CM complex. Apart from these minerals, members of isomorphic series forsterite–fayalite, and monticellite–kirschsteinite, and simplectite-like intergrowths of kirschsteinite with nepheline were identified in the paralavas of the Nyalga CM complex. The studied minerals differ in structural features, Mg# value, and crystallization temperature as a consequence of local variations in formation conditions and composition of silica-undersaturated Ca-rich mafic melts initial for melilite–nepheline paralavas. The olivines of the monticellite–kirschsteinite series reaching 86 mol % of kirschsteinite end-member occur only in paralavas bearing xenoliths of thermally modified carbonate-silicate sedimentary rocks. The crystallization of these olivines together with gehlenitic melilite took place through interaction between mafic paralava melts and xenoliths of marly limestones with large variations in content of silicate (clay–feldspar) admixture.