METASTABLE PHASE-RELATIONS IN THE CHLORITE-H2O SYSTEM

The dehydroxylation reactions of Mg-rich chlorite-IIb (clinochlore) an d Fe,Mg-rich chlorite-IIb (i.e., Mg-rich chamosite) were determined by using high-pressure differential thermal analysis (HP-DTA) to pressur es of about 770 bars and temperatures of about 900-degrees-C. These re actions may be described in the metastable binary system chlorite-H2O. All experiments were reducing, with the Fe remaining in the Fe2+ stat e for all Fe-containing phases. For conditions involving open capsules and P(H2O) < P(tot) 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 (clinochl ore at about 690-degrees-C, chamosite at near 670-degrees-C). For cond itions involving closed capsules and P(H2O) = P(tot), the reaction inv olving clinochlore occurs at the same P-T conditions as for open-capsu le experiments. However, chamosite does not show an apparent reaction, although it is interpreted that such a reaction occurs over a large t emperature range and thus the reaction is not detectable on the DTA th ermogram. At higher pressures (e.g., 500 bars), a back reaction occurs when chlorite is formed from dehydroxylated chlorite during the quenc h in the presence of H2O. This back reaction complicates the interpret ation of the phases present in both open- and closed-capsule experimen ts where H2O vapor is present, either as H2O added initially or as H2O generated during the reaction. From a metastable invariant point at a bout 855-degrees-C and 50 bars (clinochlore) and about 776-degrees-C a nd 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 chl orite structure with an oxide-like interlayer, A is a postulated anhyd rous metastable phase, and V and L refer to vapor and a liquidlike pha se, respectively. Reaction 1 is located at 817 (clinochlore) and 740-d egrees-C (chamosite) at 1 bar. Reaction 3 terminates in a singular poi nt at slightly higher and unknown temperatures and pressures above the invariant point. Two other reactions, MC + V = L and MC = L, originat e at the singular point. At a given pressure, the Fe-rich MC phase the rmally decomposes at a much lower temperature than the Mg-rich MC phas e, perhaps suggesting that the oxide interlayer in the Mg-rich MC phas e is different from the Fe-rich MC phase.