Synthesis and characterization of CaCoSi2O6 – Co2Si2O6 pyroxenes

Abstract

This thesis concerns cobalt pyroxenes and it is mostly based on the join CaCoSi2O6-Co2Si2O6. This series has an interest both in Mineralogy and Ceramic Science. If we consider Mineralogy it is interesting to stress the analogy with the rock forming quadrilateral pyroxenes formed by diopside - enstatite (CaMgSi2O6 - Mg2Si2O6) and hedenbergite - ferrosilite (CaFeSi2O6 - Fe2Si2O6) series. Moreover, Ca,Co-pyroxenes (CaCoSi2O6-Co2Si2O6) provide a crystal chemical model to study the solid solutions of pyroxenes with a large ionic radius cation in the M2 site, such as diopside (CaMgSi2O6) and hedenbergite (CaFeSi2O6) and those with a smaller cation, such as enstatite, (Mg2Si2O6) and ferrosilite, (Fe2Si2O6). Regarding Ceramic Sciences Ca,Co-pyroxenes may be used as a ceramic pigment, with good chemical stability and colour properties, whose stability field and physical properties are poorly constrained.

This work is dived in three parts.

In an introductory first part, the use of cobalt as an industrial inorganic pigment is reported. Cobalt is an important colour agent, giving different hues, obtained by the different coordination number of this element in the structure. If cobalt is in tetrahedral coordination the colour is deep blue (e.g. blue cobalt pigment, based on CoAl2O4 spinel); otherwise, if the coordination number is higher, six or eight, the hue swings to pink (e.g. Co-olivine Co2SiO4, Ca,Co-pyroxenes (Ca,Co)Si2O6, or Co-garnet Co3Al2Si3O12). The possible use of Ca,Co-pyroxenes as a pigment is then introduced; being Ca,Co-pyroxenes a member of the pyroxene mineral family, and in view of planning subsequent experiments, phase equilibria in the closely related group of quadrilateral pyroxenes, constituents of most rock forming pyroxenes, are presented. The presence of a miscibility gap between Ca-rich and Ca-poor pyroxenes, and the coexistence at low pressure of non-pyroxene phases are reported and discussed. The detailed crystal structure of pyroxenes has been then shown, for monoclinic and orthorhombic pyroxenes, in order to provide the basis for an interpretation of the Ca,Co-pyroxene properties at the atomic level. The different pyroxene varieties are related to the phase diagram stability conditions and crystal chemistry. In the second part, based on a ceramic approach, Co-pyroxenes are synthesized at room pressure, with particular attention to the end member or close to end member CoCaSi2O6: this composition is most promising for a possible use as pigment, being stable also at room pressure. The conditions at which the end member is stable as a single phase are sought, as the coexistence with other phases, mostly Co-ackermanite (Ca2CoSi2O7) significantly modifies the colour response of the mixture, from pink in pure Ca,Co-pyroxene, to blue if some Co-ackermanite is present. The colorimetric properties of the mixtures obtained at room pressure for the starting materials along the CaCoSi2O6-Co2Si2O6 series, are also determined at different temperature, relating the colour effect to the percentage of the phases. In the third part we use a mineralogical crystal chemical approach: a series of pyroxenes with compositions spanning along the CaCoSi2O6-Co2Si2O6 join are synthesized at high pressure and characterized to understand the effect of Ca with Co substitution on the crystal-chemical and physical properties of pyroxenes. A pyroxene only phase-diagram is obtained: the observed miscibility gap and the phase transition between C2/c and P21/c pyroxenes are compared with those of quadrilateral pyroxenes and related to the structural changes obtained by single crystal investigation. The results constrain the chemical properties of pyroxenes, discussing the general point of the substitution of a smaller cation vs a larger one in pyroxenes in view of a simple ionic model. The crystal structure, the symmetry variations, the phase diagram, and the physical properties of the Co-pyroxenes are analyzed by means of a multi-technique approach, using X-Ray Diffraction (powder and single crystal), Transmission and Scanning Electron Microscopy with Energy Dispersive System analysis and Raman micro-spectroscopy.