REDOX CONDITIONS IN PISTON-CYLINDER APPARATUS - THE DIFFERENT BEHAVIOR OF BORON-NITRIDE AND UNFIRED PYROPHYLLITE ASSEMBLIES

Piston-cylinder experiments with C-H-O fluids in equilibrium with grap hite as redox sensor were performed at 900 degrees C and 10 kbar. With this technique, it is possible to investigate the hydrogen permeabili ty of the material surrounding the samples and to determine the hydrog en fugacity. The furnace assembly consisted of boron nitride or unfire d pyrophyllite. Solid organic compounds (C4H4O4, C9H10O2, and C14H22O) were used as starting materials. The different WO ratios of these com pounds (1:1, 5:1, 22:1) result in different initial hydrogen fugacitie s. The fluid formed during the experiments was analyzed by gas chromat ography. The experimental data demonstrate that boron nitride is nearl y impermeable to hydrogen at the investigated conditions. The hydrogen fugacity observed in the gold capsules is controlled by the compositi on of the C-H-O starting compound. After approximately 1 h it is 340 b ar for experiments with C4H4O4, 2400 bear for experiments with C9H10O2 , and 3200 bar for experiments with C14H22O and remains constant for a t least 8 d. The situation is different with unfired pyrophyllite as t he pressure-transmitting material. In these experiments, the same hydr ogen fugacity was determined irrespective of the compositions of the o rganic compounds. The hydrogen fugacity seems to be controlled only by the furnace assembly. It is approximately 470 bar after 3 d. The hydr ogen fugacity in two samples placed side-by-side in the boron nitride assembly and containing different starting compounds (H/O=1 and 5) bec omes identical during experimental durations of less than 2 d. This is because of the exchange of hydrogen through the adjacent capsule wall s. This observation can be used to measure the hydrogen fugacity of an y piston-cylinder experiment. In boron nitride assemblies, a second ca psule with a C-H-O fluid and graphite can also be applied to control t he hydrogen fugacity within given limits.