Activity–composition relationships and pressure–temperature determinations in metamorphic rocks
Published:January 01, 2001
Thomas M. Will, 2001. "Activity–composition relationships and pressure–temperature determinations in metamorphic rocks", Solid Solutions in Silicate and Oxide Systems, Charles A. Geiger
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Mineral assemblages are only stable in specific pressure and temperature ranges in the Earth. At fixed external variables such as, for example, pressure, temperature or the composition of an infiltrating fluid phase, the compositions of minerals in a given assemblage are fixed as well. However, changes in, say, pressure (P) and/or temperature (T) lead to compositional variations in solid solutions between specific end-member components or compositions. This interdependence between the chemistry of mineral solid solutions and P and T forms the basis of most pressure and temperature estimations (i.e., geothermobarometry). Assuming that the compositions of at least some of the minerals preserved in a rock reflect equilibrium, the thermodynamic equilibrium condition can be used to calculate the PT conditions which the rock experienced at some stage during its tectono-metamorphic history. ΔG0 is the standard state Gibbs energy change of a reaction and depends on pressure and temperature, R is the universal gas constant, and K, the equilibrium constant, is a function of the activities of the components in a solid solution which, in turn, are related to mineral compositions. In order to apply the fundamental equation, the thermodynamic properties of the mineral end-member components and the appropriate form of the activity–composition (a−x) relationships of the solid solution must be known. For most minerals, however, the activities of the components in a solid solution are a complex function of pressure, temperature and mineral composition.
There are several possibilities to decipher quantitatively the conditions of formation of rocks through thermodynamic calculations.
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Solid Solutions in Silicate and Oxide Systems
The EMU book series or notes, as they are called, were introduced to provide university teachers with up-to-date reviews in important, rapidly evolving areas of mineralogy, petrology and geochemistry. They are also meant to introduce scientists into special and often interdisciplinary fields of research. In this regard, a volume on solid solutions is current and sorely needed. The solid Earth, as well as many meteorites and the other solid planets, consists for the most part of mineral solid solutions. Research on solid solutions is extremely broad encompassing work in physics and chemistry, metallurgy, materials science and, last but not least, mineralogy and petrology. Hence, because the theme is so strongly interdisciplinary in nature, the workshop was organised to include solid state physicists, physical chemists, crystallographers, mineralogists and petrologists. The various chapters reflect some of this diversity and show what mineralogy has become. Experimental investigations in mineralogy now routinely include different types of spectroscopies along with more traditional phase equilibrium, X-ray diffraction, calorimetry, and TEM methods. There have also been new and impressive developments in theory and computation. Many computational approaches relating to the study of solid solutions, for example, the Cluster Variation Method or Monte Carlo simulations, have been brought in from materials science, chemistry and physics. It can be concluded that the traditional or historical, and perhaps artificial, boundaries between the various disciplines are disappearing. Many current research efforts in mineralogy are similar to those in chemistry, materials science and physics.