Thermodynamics of ordering/clustering in silicates

Pyroxenes and feldspars are important rock-forming minerals in various metamorphic and magmatic rocks. In these solid-solution systems, ordering processes operate as a function of temperature, i.e., cations that are randomly distributed over crystallographic sites at high temperature (like Na, Al, Ca, Mg in pyroxenes) start to order on distinct crystallographic sites on cooling. This causes alternating layers of unlike atoms in the structure (as in omphacite, a pyoxene). The opposite mechanism of intracrystalline ordering is clustering. Here, domains enriched in like atoms are generated upon cooling (e.g., Na- and K-rich clusters in alkali-feldspars), which then leads to exsolution processes. The aim of the present project is to study the thermodynamics of these ordering/clustering processes in pyroxenes and feldspars. This will result in a comprehensive thermodynamic description of these mineral groups as a basis for deriving so-called phase-relations of minerals in rocks. This enables geoscientists to decipher the pressure-temperature history of rocks, experienced during their geological evolution in orogens like the Alps. The relevant thermodynamic quantities, which will be determined during the course of the project, are the enthalpy and the vibrational and configurational entropies associated with these ordering and clustering processes. The calorimetric methods applied are low-temperature relaxation calorimetry (from the heat capacities, measured on well characterised samples in the temperature range close to 0 up to 300 K, the vibrational entropy can be determined) and differential scanning calorimetry (above room temperature). Infrared spectroscopy will be used to derive the enthalpy of the ordering/clustering processes (through evaluation of the so-called line-broadening). These methods are supplemented by quantum-mechanical calculations using the density-functional theory.

Pyroxenes and feldspars are important rock-forming minerals in various metamorphic and magmatic rocks. In these solid-solution systems, ordering processes operate as a function of temperature, i.e., cations that are randomly distributed over crystallographic sites at high temperature (like Na, Al, Ca, Mg in pyroxenes) start to order on distinct crystallographic sites on cooling. This causes alternating layers of unlike atoms in the structure (as in omphacite, a pyroxene). The opposite mechanism of intracrystalline ordering is 'clustering'. Here, domains enriched in like atoms are generated upon cooling (e.g., Na- and K-rich clusters in alkali-feldspars), which then leads to exsolution processes. For the first time, the vibrational entropy related to ordering/exsolution in pyroxene and alkali-feldspar was calorimetrically determined during the course of this FWF-project. Data from infrared spectroscopy and calculations using the density functional theory served to quantify the enthalpy changes due to these microscopic processes. The results of this FWF project thus contribute to an improved thermodynamic description of these mineral groups. Furthermore, the project outcomes lead to a better understanding of ordering/exsolution processes in general, as relevant in alloys or in other solid solution systems.