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The importance of solid solutions is known since prehistory times. The phrase ‘The Bronze Age’ is evidence for this and shows that mankind has for thousands of years engineered and produced materials, or taken those from nature, that consist of atomistic mixtures of elements having certain beneficial physical properties. Thus solid solutions, or mixed crystals as they are sometimes termed, are investigated in the fields of metallurgy, chemistry, physics, materials science and the Earth Science disciplines mineralogy, petrology and geochemistry. The Earth, as well as other solid planets and meteorites, is made up for the most part of minerals, largely silicates and oxides, and with the exception of some of the SiO2 phases, most are substitutional solid solutions. Silicates and oxides are quite extraordinary in their ability to incorporate many different elements and, at the same time, in having extremely large pressure and temperature stabilities. MgO and ‘FeO’ and their solid solutions are stable at ambient conditions and also at pressures and temperatures corresponding to Earth’s lower mantle. Many silicate garnets are stable at 1 atm and compositionally complex garnets are also stable in the Earth’s transition zone.

A description and understanding of the chemical and physical properties of silicates and oxides are essential for investigating geological processes. For example, most geothermobarometers that are employed on metamorphic and igneous rocks are critically dependent on the activity-compositional relationships that are used to describe the thermodynamic mixing properties of the participating solid solution phases. Existing thermodynamic mixing models for most rock-forming

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