Stability region of K0.2Na0.8AlSi3O8 hollandite at 22 GPa and 2273 K

Lingunite (hollandite with Na contents of 80-85 mol%) discovered in the shock veins of strongly shocked meteorites is an important signature of shock metamorphism. To seek the stability region of lingunite, phase relations in the system KAlSi3O8-NaAlSi3O8 have been investigated by multi-anvil experiments at pressures of 20-23 GPa and temperatures of 1873 and 2273 K. Phase assemblages of hollandite + jadeite + stishovite, hollandite + calcium ferrite-type NaAlSiO4 + stishovite and hollandite single phase have been recovered, depending on the pressure-temperature conditions and the compositions of starting materials. Both pressure and temperature have large effects on the solubility of Na in hollandite, and hollandite with 79 mol% Na, similar to the natural lingunite in terms of Na content, has been firstly synthesized at 22 GPa and 2273 K. The stability region of K0.2Na0.8AlSi3O8 hollandite is comparable to the typical pressure-temperature conditions of the shock veins of strongly shocked meteorites (20-25 GPa and 2273-2500 K).