Mineral or group of minerals whose existence, coexistence, or element distribution, is stable between known pressure bounds at given temperatures and which is therefore useful as an indicator of the pressure under which a rock equilibriated. For example, the FeS content of the mineral sphalerite, in equilibrium with pyrite and pyrrhotite, is unaffected by temperatures in the range of 300–550 °C, but has been shown to vary according to pressure, and has been used as a geobarometer in regionally metamorphosed terrains.