Abstract

This is the first description of rare minerals found in paralavas from a recently discovered combustion metamorphic complex in the Choir–Nyalga basin, Central Mongolia. The identified minerals contain strongly variable concentrations of Si, Ti, Mg, Fe2+ and Fe3+ and most commonly have compositions intermediate in a series from kuratite Ca4Fe102+Ti2O4[Si8Al4O36] and rhönite Ca4(Mg,Fe2+)8Fe23+Ti2O4[Si6Al6O36] to low-Ti kuratite and unnamed Ti-free Fe2+-analogue of rhönite Ca4Fe82+Fe43+O4[Si8Al4O36]. The minerals crystallized in residual Si-Al-K and Si-Al-Ca-Fe immiscible melts after spinel, anorthite–bytownite, melilite, Al-clinopyroxene ± Mg-Fe olivine, together with Fe3+-bearing hercynite, Ca-rich fayalite, kirschsteinite, pyrrhotite ± native iron, wüstite, magnetite, celsian, hyalophane, Ba-orthoclase and fresnoite, but before nepheline ± kalsilite, and later sulfates, carbonates, an unidentified ‘X-mineral’ close to Al- and Fe-rich tobermorite and goethite. Micro-Raman spectroscopy of kuratite shows five bands near 133–155 (strong), 399–401, 545–566, 684–693 (strongest) and 828–839 cm−1.

The kuratite-bearing Nyalga paralavas have bulk compositions with MgO/(MgO+FeO+Fe2O3), mol.% ∼0.5 and a CIPW normative ratio of Ne/(Ne+Lc) = 0.23–0.76. Minerals of the rhönite–kuratite series formed during paralava crystallization at ∼1100°C. The diversity of the paralava mineral assemblages might result from local composition variations of Ca-rich silica-undersaturated melts derived from Fe-bearing carbonate-silicate sediments which were affected by nearby coal combustion sources at reducing conditions (IW-WM-QFM buffers) and at a nearly atmospheric pressure.

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