Experimental study of the stability of sudoite and magnesiocarpholite and calculation of a new petrogenetic grid for the system FeO–MgO–Al₂O₃–SiO₂–H₂O

ABSTRACT

The solid‐solid reaction magnesiocarpholite = sudoite + quartz has been bracketed between 350 and 500°C, 6.3 and 7.8 kbar. Because it is impossible to synthesize end‐member sudoite, all experiments were carried out using natural minerals as starting materials. Although mineral compositions were very close to those of the end‐members, the effect of the fluorine content in carpholite was significant. Particularly in those experiments where sudoite grows at the expense of carpholite, electron microprobe analysis of the run products shows that a more stable F‐rich carpholite crystallizes too, and consumes the fluorine released in solution by the breakdown of the original carpholite.

Our experimental results are combined, through a thermodynamic analysis, with a previous data set and with previous experimental data concerning the relative stability of chlorite, talc and magnesiocarpholite with excess of quartz and water as a function of P–T and AlAl(SiMg)_‐1 substitutions in phyllosilicates. This allows us to constrain the feasible thermodynamic parameters (H°_f, sud; S° sud) and (H°_f,car; S°_car) for the Mg end‐members. Using the partition coefficients calculated from natural parageneses, we have computed a petrogenetic grid for the system FeO–MgO–Al₂O₃–SiO₂–H₂O. It demonstrates that parageneses involving sudoite and carpholite can be used as indicators of P–T conditions, up to 600° C, 8 kbar for sudoite, and at higher pressure for carpholite.