The dehydroxylation reactions of Mg-rich chlorite-IIb (clinochlore) and Fe,Mg-rich chlorite-IIb (i.e., Mg-rich chamosite) were determined by using high-pressure differential thermal analysis (HP-DTA) to pressures of about 770 bars and temperatures of about 900 °C. These reactions may be described in the metastable binary system chlorite-H2O. All experiments were reducing, with the Fe remaining in the Fe2+ state for all Fe-containing phases. For conditions involving open capsules and PH2O < Ptot, the chlorite phase undergoes the dehydroxylation of the interlayer to form a modified chlorite structure (MC) + vapor (V). Above about 100 bars, this reaction is nearly isothermal (clinochlore at about 690 °C, chamosite at near 670 °C). For conditions involving closed capsules and PH2O = Ptot, the reaction involving clinochlore occurs at the same P-T conditions as for open-capsule experiments. However, chamosite does not show an apparent reaction, although it is interpreted that such a reaction occurs over a large temperature range and thus the reaction is not detectable on the DTA thermogram. At higher pressures (e.g., 500 bars), a back reaction occurs when chlorite is formed from dehydroxylated chlorite during the quench in the presence of H2O. This back reaction complicates the interpretation of the phases present in both open- and closed-capsule experiments where H2O vapor is present, either as H2O added initially or as H2O generated during the reaction.

From a metastable invariant point at about 855 °C and 50 bars (clinochlore) and about 776 °C and 30 bars (chamosite) four reactions emanate: (1) MC = A + V, (2) A + V = L, (3) MC = L + V, and (4) MC + A = L, where MC is a modified chlorite structure with an oxide-like interlayer, A is a postulated anhydrous metastable phase, and V and L refer to vapor and a liquidlike phase, respectively. Reaction 1 is located at 817 (clinochlore) and 740 °C (chamosite) at 1 bar. Reaction 3 terminates in a singular point at slightly higher and unknown temperatures and pressures above the invariant point. Two other reactions, MC + V = L and MC = L, originate at the singular point. At a given pressure, the Fe-rich MC phase thermally decomposes at a much lower temperature than the Mg-rich MC phase, perhaps suggesting that the oxide interlayer in the Mg-rich MC phase is different from the Fe-rich MC phase.

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