Crystal Chemical and Thermodynamic Properties of Garnet

Investigating the crystal-chemical, thermodynamic and physical properties of minerals plays a central role in the fields of mineralogy, petrology and geochemistry. Garnet is an important rock-forming silicate and is typically a complex solid solution in nature. Considerable research of a variety of types has been done on garnet, but nonetheless much is still not understood. This is especially true for garnets in the system grossular(Gr)- andradite(An)-uvarovite(Uv)-knorrinigite(Kn). A major goal of this project is to investigate their macroscopic thermodynamic (i.e. Cp and S) and microscopic crystalchemical and physical (i.e. magnetic and electronic) properties and to study how atomistic factors influence the free energy (i.e. stability) of garnet. We want to focus on those properties that are or could be affected by the presence of the transition metal cations namely Fe3+ and Cr3+. Another important goal will be to construct a thermodynamic mixing model that will describe the behavior of natural garnet solid solutions. We propose a multi-methodological investigation on Gr-An-Uv-Kn garnets using synthesis experiments, a variety of characterization measurements (X-Ray, IR, Mössbauer, microprobe) and calorimetric Cp determinations from 2 K to 900 K. The latter will be done on mg-sized samples using relaxation calorimetry and DSC methods, thus providing fundamental thermodynamic data on Cp and S for three end- members An, Uv and Kn and Smix for the An-Gr solid solution. A detailed analysis of these thermodynamic properties in terms of magnetic and electronic behavior will be made, a new field of research in mineralogy, petrology and geochemistry and something that has not been done fully before for rock-forming silicates.

Investigating the crystal-chemical, thermodynamic and physical properties of minerals plays a central role in the Earth Science disciplines mineralogy, petrology and geochemistry. Garnet is an important rock- forming mineral with a variety of species (e.g. grossular - Ca3Al2Si3O12, andradite - Ca3Fe3+2Si3O12, uvarovite - Ca3Cr3+2Si3O12, pyrope - Mg3Al2Si3O12, almandine - Fe3+3Al2Si3O12). Thus, considerable academic research of a variety of types has been undertaken on garnet, but nonetheless much is still not understood. Experimental approaches can be broadly divided into two types namely applied and basic in nature with considerable overlap. In terms of the former an important goal is to investigate the macroscopic thermodynamic properties of various garnet compositions. This is because many processes on Earth and extraterrestrial bodies are governed by chemical equilibrium. Even disequilibrium processes can be driven by differences in thermodynamic potentials. Thus, precise knowledge of the thermodynamic functions Cp (constant pressure heat capacity), S (standard third-law entropy), ?Hf (enthalpy of formation at P ? 1 bar and T ? 298.15 K), V(P,T) (volume) and ?Gf(P,T) (Gibbs free energy) of crystals is essential. These functions can be used for constructing, for example, so-called geothermobarometers, which can be used to determine the paleo-temperatures and -pressures at which igneous rocks crystallized from a magma or metamorphic rocks recrystallized in the Earth. The second type of research can be considered more basic in nature. That is, various scientists have been working for many years to understand the underlying microstructural-macroscopic property relationships of many different materials. The nature of the problem is complex. Here, for example, mineralogists and mineral physicists are investigating the atomic level or microscopic scale structural, crystal-chemical, magnetic and electronic properties of minerals to determine how they can affect the different thermodynamic functions and ultimately mineral P-T stability. In garnet, investigating the crystal-chemical, electronic and magnetic properties deriving from the presence of the transition-metal-cations such as Fe2+, Fe3+ and Cr3+ is important. Both natural and synthetic minerals need to be studied and their properties compared.In this project, the microscopic crystal-chemical, electronic and magnetic properties of different garnets were investigated. It started with the acquisition of natural samples and garnet synthesis experiments at elevated P-T conditions. Then, a variety of measurements (IR, UV/VIS, Mössbauer and NMR spectroscopy as well as standard microprobe and X-ray methods) were undertaken to study the garnets microscopic properties. In terms of the macroscopic thermodynamic properties, Cp was measured from about 2 K to 300 K using relaxation calorimetry and 300 K to 900 K using differential scanning calorimetry (DSC) techniques. Cp and S behavior for end-member An and An-Gr (Ca3Fe3+2Si3O12-Ca3Al2Si3O12) solid solutions were measured. Low-temperature magnetic phase transitions, resulting from the presence of Fe3+,were measured and analyzed. Furthermore, a detailed study of Cp and S for the six common silicate garnets, in terms of their crystal chemical, magnetic and electronic behavior, was made.