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 degrees C. These experiments suggest that at subsolidus conditions the stable phase assemblages are hollandite-I+jadeite+stishovite (at pressures up to similar to 20 GPa), hollandite-II+jadeite+stishovite (at pressures between similar to 20 and similar to 24 GPa), and hollandite-II+calcium ferrite-type NaAlSiO4+stishovite (at pressures >similar to 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 similar to 22 GPa, and its fraction is positively correlated to temperature, attaining a value of similar to 51 mol% at 2200 degrees C. At supersolidus conditions the solidus and liquidus phases around 14 GPa are hollandite-I and jadeite, respectively; at similar to 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 similar to 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. (c) 2006 Elsevier B.V. All rights reserved.