One goal of earth scientists is to determine the temperature and pressure of formation of rocks that, by one way or another, are brought to the surface of the earth for study. Minerals in rocks from outcrops, mines, and drill cores may have crystallized at temperatures and pressures considerably above those prevailing when they are studied. Certain physical and chemical characteristics of these minerals may be examined to determine what conditions prevailed during crystallization or during recrystallization. The resulting estimates of preexisting temperatures and pressures are known as geologic thermometry and as geologic barometry, respectively (also geothermometry and geobarometry ; see Thermometry, Geologic ).
If, from experimental studies of phase equilibria (see vol. IV A), a certain mineral or mineral assemblage is determined to be in equilibrium only within a defined pressure range, the presence of that mineral or assemblage in a rock indicates that it was once subjected to...
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