Introduction to minerals under extreme conditions
The first experimental studies on minerals and rocks under high temperature and high pressure were probably carried out by Sir James Hall (1761–1832). At that time, some geologists still debated the magmatic origin of basalts. In a series of meltingand crystallisation experiments carried out in furnaces used at that time in the ceramic and glass industry, Hall was able to reproduce the textures and mineral assemblages of natural basalts. Experimentally much more demanding were his studies on the recrystallisation of limestone (Eyles, 1961). In order to reach high pressures and high temperatures simultaneously, Hall sealed the sample together with some water into gun barrels. By heating the sealed gun barrels in a furnace (Fig 1), he was probably able to reach pressures close to 0.1 GPa and temperatures around or above 600 °C simultaneously. Limestone subjected to these conditions converted to marble and Hall therefore simulated for the first time metamorphic processes in the laboratory. In order to accurately measure pressure, Hall devised a dead-weight pressure gauge, which determines the force on a piston by balancing it with a weight (Fig 2). In principle, the same type of instrument is still in use today for the high-precision calibration of gauges.
Hall’s ingenious experiments were far ahead of his time and it appears that not much progress was made in experimental mineralogy and petrology for almost one century following his work. At the same time, however, physicists and chemists became interested in high-pressure and high-temperature phenomena such as the behaviour of gases under extreme conditions.