A temperature-composition diagram has been constructed for the system MgSiO3 FeSiO3 at 1 atm, using a combination of experimental data obtained at 1100 and 1250° and thermodynamic calculations. From the information contained in this T-X diagram, and that provided by experimental work on the P-T stability field of orthoferrosilite, and by studies of the distribution of iron between olivine and orthopyroxene, a series of T-X sections of the system have been calculated for pressures of 0 (× 2.5) to 15 kbars. The ferrosilite content of a metamorphic orthopyroxene coexisting with quartz and olivine increases by a constant 2.6% Fs per kbar over this pressure range, and decreases by approximately 1% for each 100° increase in temperature. The T-X sections are thought to be sufficiently accurate to give the absolute pressure of crystallisation to within 1 kbar if the temperature of formation of an olivine-pyroxene-silica assemblage is known to ± 100°C. Application of this geobarometer to the orthopyroxenes of metamorphic rock suggests pressures of crystallisation ranging up to 8.5 kbars. Intermediate values of 3 to 5 kbars are indicated for contact metamorphosed rocks and deep-seated acid intrusions. The corresponding depths of crystallisation of orthopyroxene-bearing metamorphic rocks are 15 to 35 km if g has remained constant. In geologically favourable circumstances the geobarometer is sufficiently accurate to permit secular variations of 10% in the universal gravitational constant (G) to be detected.