Abstract

The dehydroxylation of brucite was determined in the presence of NaCl-H₂O fluids containing 0, 2, 6, 11, 14, 18, 30, and 38 mol% NaCl (0.0, 5.5, 17.0, 28.8, 34.5, 41.7, 58.2, and 66.6 wt% NaCl) to ∼2500 bars and ∼700 °C by high-pressure differential thermal analysis (HP-DTA). At pressures below 150 bars, brucite dehydrates in two separate reactions involving at least one intermediate phase. The dehydroxylation reaction of brucite at pressure to ∼2 kbars follows lnP(bars) = 22.23 2 13620/T(K). In the presence of NaCl- H₂O fluids, the dehydroxylation temperature is lowered because of the reduction of the activity of H₂O in these fluids. At 1 kbar pressure, mixing in the NaCl-H₂O fluids is virtually ideal, assuming a nearly complete association of NaCl, but at 2 kbars, the dissociation of NaCl is substantial. The phase relations in the system MgO-NaCl-H₂O show five univariant reactions emanating from an invariant assemblage, periclase = brucite = halite = liquid = vapor, located at 565 ± 5 °C and 440 ± 30 bars.