Effect of excess aluminium on phase relations in the system Qz-Ab- Or: experimental investigation at 2kbar and reduced H₂O-activity

Liquidus phase relationships have been investigated in the peraluminous system Qz (SiO₂) - Ab (NaAlSi₃O₈) - Or (KAlSi₃O₈) - Al₂O₃ - H₂O - CO₂. The experiments have been performed at 2kbar, 750 to 850°C, and at H₂O-undersaturated conditions (H₂O-activity close to 0.5). An aluminosilicate phase (mullite) coexists with all melts of peraluminous compositions. The comparison of the subaluminous and the peraluminous systems shows that all liquidus temperatures are lower in the peraluminous system. The position of the quartz-feldspar cotectic is slightly shifted towards more Qz-rich compositions in the peraluminous system. The projected composition of the thermal minimum is Qz₃₅Ab₃₆ Or₂₉ in the subaluminous system and Qz₃₈Ab₃₄Or₂₈ in the peraluminous system. Peraluminous melts coexisting with mullite contain 1.2 to 2.4% normative corundum at the investigated temperatures (750 to 850°C). Different temperatures have not been observed for the beginning of melting of crystal mixtures composed of quartz + alkali feldspar and quartz + alkali feldspar + sillimanite. This may be due to kinetic problems and is discussed. -from Authors