The stability of clinochlore +quartz at low pressure

The univariant dehydration reaction which defines the maximum thermal stability of the assemblage clinochlore + quartz, has been bracketed, with reversed experiments at Р_H2O = P_total. A smooth curve drawn between the brackets passes through the coordinates 0.5 kbar, 482°; 1 kbar, 500°; 2 kbar, 527°; 3 kbar, 551°; 4 kbar, 574°C. The position of the curve is probably within ± 7°C of the stated temperature at each pressure. Synthetic clinochlore, talc, cordierite, and natural quartz mixed in the appropriate proportions with an excess of distilled deionized water were used as starting materials for the bracketing experiments. Reversibility was established by determining the relative growth or diminution (as judged by examining relative intensities of X-ray reflections) of the high with respect to the low temperature assemblage. The established curve lies 55°C below the synthesis point at P_H2O = 2 kbar established by Fawcett and Yoder for Mg-chlorite + quartz and 50°C below the reversed bracket at Р_H2O = 1 kbar established by Velde for clinochlore. Phase relations in the water-saturated, silica-rich portion of the system K₂O–MgO–Al₂O₃–SiO₂–H₂O are more complex than had been suspected, due to the intersection of the phase boundary for the reaction above with the phase boundary for the reaction muscovite + clinochlore + quartz = cordierite + phlogopite + H₂O.