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

Abstract

Liquidus phase relationships have been investigated in the peraluminous system Qz (SiO 2 ) - Ab (NaAlSi 3 O 8 ) - Or (KAlSi 3 O 8 ) - Al 2 O 3 - H 2 O - CO 2 . The experiments have been performed at 2 kbar, 750 to 850°C and at H 2 O-undersaturated conditions (H 2 O-activity close to 0.5). An aluminosilicate phase (mullite) coexists with all melts of peraluminous compositions. Liquidus phase relationships determined in this system correspond therefore to liquidus temperatures of quartz or alkali feldspar in presence of mullite. The comparison of the subaluminous (Qz-Ab-Or-H 2 O-CO 2 ) and the peraluminous (Qz-Ab-Or-Al 2 O 3 -H 2 O-CO 2 ) systems shows hat all liquidus temperatures are lower in the peraluminous system. The minimum liquidus temperature is 20±5°C lower in the peraluminous system. The position of the quartz-feldspar cotectic is slightly shifted towards more Qz-rich compositions (3 ± 1 wt% normative Qz content) in the peraluminous system. The projected composition of the thermal minimum is Qz 35 Ab 36 Or 29 in the subaluminous system and Qz 38 Ab 34 Or 28 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). In contrast to the results obtained in liquidus experiments, 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