The reactions best for pressure (geobarometers) are ones that have a large change in molar volume during the reaction. Higher pressures cause the reaction to decrease in total volume, and lighter pressures allow reaction to increase in total volume. Therefore, based on the proportion of minerals that have larger volumes versus the proportion of minerals that have smaller volumes, the pressure of the environment during the reaction can be calculated, as a function of temperature. Experiments must be done to calibrate each reaction and determine the rate at which the volume changes with changes in pressure.