Estudo cristaloquímico de minerais: modelos de desordem e análise de sistemas iônicos utilizando Superfícies de Hirshfeld

Abstract

Single-crystal samples of tantalum and niobium minerals and other associated minerals found in nature often show high crystallographic disorder due to their geological formation history. Several methods have been employed to reduce system disorder to make the structure solution and crystallographic refinement more stable and representative. In this case, the most used procedure is heating the single crystals – annealing. Despite generating ordered single crystals that allow more direct data treatments, the procedure causes the loss of information about the disorder about the system and partially decontextualizes it from the characteristics and dynamics of evolution from its parental system. In our approach we performed a study at room / low temperature; in contrast to the typically performed annealing procedures, we employ an adjustment strategy based only on restraints / constraints supported by other analytical techniques in crystallographic refinement to obtain the structure with the system’s disorder information and its correlation with the mineralogical evolution. A structured strategy of block refinements with charge / mass balance constraints with systematic disorder refinement proved to be an interesting tool to obtain consistent structural information from tantalite-columbite mineral samples with high level of substitutional and positional disorder. The translational disorder has a model with a more complex profile, which can change the system’s symmetry observed on data collection and structure resolution. In our work, we used the multiple reflection’s profile over the directions of systematic extinctions to support the elaborated model of disorder. In addition to the structure resolution and refinement, the structures were submitted to structural analysis using a new methodology based on Hirshfeld Surfaces developed on this work, aiming the estimation of ionic charges and orbital radii involved on bonds between the system’s ions. The use of Hirshfeld surfaces to evaluate ion characteristics has open challenges in the methodology classically employed in the analysis and construction of these surfaces. In this work we also propose a new model for using this approach for ionic systems.