Phase relations in the system KAlSi3O8–NaAlSi3O8 at high pressure–high temperature conditions and their implication for the petrogenesis of lingunite

Multi-anvil experiments have been made with the composition (K0.2Na0.8)AlSi3O8 at 14–25 GPa and 1400–2400 °C. These experiments suggest that at subsolidus conditions the stable phase assemblages are hollandite-I + jadeite + stishovite (at pressures up to ∼20 GPa), hollandite-II + jadeite + stishovite (at pressures between ∼ 20 and ∼ 24 GPa), and hollandite-II + calcium ferrite-type NaAlSiO4 + stishovite (at pressures > ∼ 24 GPa), where hollandite-I and hollandite-II are distinguished by their chemical compositions and phase diagram topology. The maximum NaAlSi3O8 in hollandite occurs at ∼ 22 GPa, and its fraction is positively correlated to temperature, attaining a value of ∼ 51 mol% at 2200 °C. At supersolidus conditions the solidus and liquidus phases around 14 GPa are hollandite-I and jadeite, respectively; at ∼ 22 GPa, however, the solidus phase is jadeite while the liquidus phase is either stishovite or hollandite-II with a very low fraction of NaAlSi3O8. These experimental results therefore suggest that lingunite with the hollandite structure and ∼ 90 mol% NaAlSi3O8, found in some meteorites, is a disequilibrium product. It is similarly proposed that the phase with the structure of jadeite but the composition of plagioclase, found along with lingunite, is also a disequilibrium phase.