Sapphirine-bearing assemblages in the system MgO-Al (sub 2) O (sub 3) -SiO (sub 2) ; a continuing ambiguity
Sapphirine-bearing assemblages in the system MgO-Al (sub 2) O (sub 3) -SiO (sub 2) ; a continuing ambiguity (in From field observation to experimental petrology and back; a special issue to honour Werner Schreyer, Walter Maresch (editor), Edward Grew (editor) and Friedrich Seifert (editor))
European Journal of Mineralogy (October 2008) 20 (5): 721-734
Based on experimental data for reactions involving sapphirine in the system MgO-Al (sub 2) O (sub 3) -SiO (sub 2) (MAS), thermodynamic properties of the sapphirine solid solution end-members have been optimized for both ideal and non-ideal models, and internally consistent P-T grids are proposed. According to the calculated P-T relationships in MAS, the sapphirine + quartz assemblage, which is widely recognized as indicative of ultrahigh-temperature metamorphism, can be stable down to 840 degrees C and 0.67 GPa, i.e., at temperatures up to 150 degrees C lower than estimated by Kelsey et al. (2004). The sapphirine + kyanite assemblage has been found stable at temperatures below 860 degrees C and 1.13 GPa, whereas the sapphirine + forsterite assemblage may be stable below 800 degrees C only under specific conditions of a very low activity of water. Calculations of P-T relationships involving sapphirine using the THERMOCALC dataset revealed its inconsistency with both experimental and natural assemblages.
