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

Olivine group minerals were formed in melilite-nepheline paralavas in two combustion metamorphic (CM) complexes as result of thermal alterations of sedimentary rocks caused by multistage current and quaternary wild coal fires. Products of decomposition of the olivine solid solution into Ca-bearing fayalite and kirschsteinite have been found in paralavas of the Khamaryn–Khural–Khiid CM complex. And, besides these two minerals, forterite–fayalite, monticellite–kirschsteinite and simplectite intergrowths of kirschsteinite with nepheline were detected in paralavas of the Nyalga PM complex. Olivines are differing by their textural features, index value Mg# = Mg/(Mg + Fe) and their crystallization temperature as a consequence of local variations of formation conditions and composition of silica-undersaturated Ca-rich mafic initial melts of melilite–nepheline paralavas. Olivines of the monticellite–kirschsteinite series containing up to 86 mol. % of kirschsteinite end-member occur only in paralavas containing xenoliths of thermally modified carbonate–silicate sedimentary rocks. Crystallization of these olivines together with gehlenitic melilite took place in the process of reactionary interaction between mafic paralava melts and xenoliths of marly limestones with large variations in contents of silicate (clay–feldspar) admixture.