Effect of excess aluminium on phase relations in the system Qz-Ab-Or: experimental investigation at 2 kbar and reduced H2O-activity
Holtz, Francois; Johannes, Wilhelm; Pichavant, Michel
European Journal of Mineralogy Volume 4 Number 1 (1992), p. 137 - 152
published: Feb 13, 1992
manuscript accepted: Jul 16, 1991
manuscript received: Oct 25, 1990
ArtNo. ESP147050401011, Price: 29.00 €
Abstract 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 2 kbar, 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. 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-H2O-CO2) and the peraluminous (Qz-Ab-Or-Al2O3-H2O-CO2) systems shows that 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 Qz35Ab36Or29 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). 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.