Exploring the relationships between elasticity and crystal chemistry in oxide spinels

“Mineral physics” is the science of materials (minerals) that compose the Earth’s interior and generally refers to the study of the physical properties of minerals typically at high pressure and/or temperature conditions. “Crystal chemistry” is the study of the relationships between structure and composition of crystalline materials. These two subjects evolved independently of each other and the attempts to relate them were in fact only partial. The aim of this thesis is the study of the elastic properties of a set of oxide spinels with different crystal-chemistry in order to quantify and understand the relationships between elasticity and crystal chemistry. The logic path followed during the work can be schematized as follow: 1) elasticity in the system MgAl2O4 as a function of cation distribution in the non-equivalent polyhedral sites. 2) elasticity in the system (Mg,Mn)Al2O4 as a function of the Mn2+ --> Mg substitution. 3) elasticity in the system (Mg,Fe2+)Al2O4 as a function of the Fe2+ --> Mg substitution. 4) elasticity in the system (Mg,Fe2+)Al2O4 as a function of applied pressure. The results of this thesis highlights the non-trivial effect of chemical substitution which can affect the elastic properties of oxide spinels in complicated way. In particular it has been demonstrated that even a small amount of transition elements (Mn and Fe to a greater extent) may have a large influence on the elastic properties in the (Mg,Mn)Al2O4 and (Mg,Fe)Al2O4. In addition the elastic parameters may behave non-monotonically as a function of chemical substitution. Cation disorder has much less influence on the elasticity with respect to chemical substitution. The elasticity of the system (Mg,Mn)Al2O4 has been (at least in part) explained using the polyhedral approach which allows the description of the bulk properties in terms of polyhedral contribution. This approach however failed to explain the elasticity of the system (Mg,Fe2+)Al2O4 suggesting that the physics behind the Fe2+ --> Mg substitution is more complex than the Mn2+ --> Mg substitution.