 There are numerous extra factors to consider such as oxygen fugacity and water activity (roughly, the same as concentration).The distribution of component elements between the mineral assemblages is then analysed using an electron microprobe or scanning electron microscope (SEM).Data on the geothermometers and geobarometers is derived from both laboratory studies on artificial mineral assemblages, where minerals are grown at known temperatures and pressures and the chemical equilibrium measured directly, and from calibration using natural systems.