A new capsule technique for hydrothermal experiments using the piston-cylinder apparatus

Abstract

AssrRAcr A new capsule technique has been developed to maintain a constant sample geometry at the high pressures and temperatures obtainable with a piston-cylinder apparatus. The capsule consists of a thick-walled transition metal cylinder, open on one end, and an inner noble metal sleeve. A noble metal lid welds to the capsule during cold pressurization, eliminating the need for arc welding to seal the loaded capsule. The high strength of the thick-walled capsules protects delicate samples such as single crystals during pressurization. At high temperature and pressure capsules shorten in the direction of compression but deform homogeneously. Pressure calibrations show that the pressure correction for the assembly is negligible. Ni powder can be used to buffer the forin an aqueous charge at NNO for >24 h at 1000 .C. Applications of the capsule technique to hydrothermal experiments on single crystals are discussed. These experiments allow measurement of mineral solubilities, partition coefficients, and diffusion coefficients in minerals. Thick-walled capsules are also useful for experiments that require constant sample geometry, such as diffirsion couples and textural studies. INrnonucrrox There are very few data on the properties of COH fluids at pressures above 1.0 GPa. Because the piston cylinder can achieve pressures of 5.0 GPa (Boyd and England, 1960), it can be used to increase the range of conditions accessible for the study of fluids. However, the difficulties inherent in hydrothermal experiments on small samples have limited the use of the piston cylinder in studies of fluids. The usual procedure of sealing a fluidbearing capsule shut by arc welding is unreliable because it may cause volatilization of an unknown mass of sample. Volatilization of caustic or radioactive fluids during welding is not only undesirable but also may change the bulk composition of the sample. Also, welded seals often fail during the experiment or the quenching process. The irregular shapes of capsules made in this fashion require that each has a custom-made receptacle, resulting in a different geometry in every experiment. We have developed a capsule technique for mineral solubility measurements that avoids these problems. The capsule consists of an open-ended thick-walled transition-metal cylinder and a noble metal sleeve. During cold pressurization a noble metal lid seals the capsule shut, avoiding the step of welding the loaded capsule. The