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 fO2 in 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.