Synthetic pyroxenes for technological applications

Pyroxenes are a group of important rock-forming inosilicate minerals of special interest to Earth and Planetary Sciences. They are major constituents in the lower crust and upper mantle, as well as of evolved and undifferentiated solid bodies of the Solar System. One of the most intriguing features of this family of silicates is the wide range of chemical substitutions that occur in a relatively simple structure. Extended chemical substitutions may be studied in synthetic pyroxenes to provide a simple model in order to interpret the structural modifications that occur as a function of temperature, pressure and composition, and to reconstruct the sequence of natural processes of cooling and crystallization of the igneous rocks. Such informations can be gathered by careful laboratory investigations by means of microprobe analysis, electron microscopy, X-ray diffraction and spectroscopic techniques. Synthetic pyroxenes with wide compositional ranges, that go beyond the compositions commonly found in those natural, are also studied in order to investigate the flexibility of the pyroxene structure, the occurrence of new phase transitions and the effect of cation substitutions on such phase transitions, as well as the effect of the electronic transitions on the structural changes for those pyroxenes containing transition metals in their octahedral sites. More recently the properties of synthetic pyroxenes containing transition metals were investigated in view of the discovery of a multiferroic behaviour, as well as of their properties as ceramic pigments, since transition metals are often chromophores, capable of imparting specific colors to the pigment. This dissertation reports the results derived from the project aimed to explore some pyroxene structures used as a basis for the production of advanced materials with technological applications, such as, for example, production of ceramic pigments with colorimetric properties enhanced with respect to the commercial colorants, i.e. cobalt-olivine Co2SiO4 and cobalt-spinel CoAl2O4. Solid solutions of synthetic pyroxenes were obtained by doping their octahedrally coordinated sites with transition metal ions (i.e. Co2+ and Zn2+). Pyroxenes containing cobalt were synthesized at ambient pressure by using method of growth from melt and flow, while those containing zinc were obtained in high pressure conditions (5 GPa) by using a multi-anvil apparatus. The analytical techniques adopted to investigate the chemical, crystallographic and physical properties of the synthesized compounds are: single crystal and powder X-ray diffractions, visible, near infrared (VIS-NIR) and infrared (IR) spectroscopies, colorimetric analysis and technological tests. The main results presented in this thesis, concern: 1) the synthesis of pyroxenes along the CaZnSi2O6-Zn2Si2O6 series, in order to explain how a predominantly ionic bond becomes covalent affecting the crystallographic characteristics of the zinc pyroxene, and its geochemical behaviour in silicate melts. The crystal structure and chemical behaviour of pyroxenes belonging to the CaZnSi2O6-Zn2Si2O6 series, was then compared with those belonging to CaCoSi2O6-Co2Si2O6, diopside-enstatite, hedenbergite-ferrosilite series. Preliminary data of IR spectroscopy performed on CaZnSi2O6-Zn2Si2O6 solid solution are also presented; 2) the synthesis of a new pigment based on the CaCoxMg1-xSi2O6 pyroxene structure, low in cobalt content, whose colorimetric behaviour is similar to that observed in conventional ceramic pigments higher in cobalt content (i.e. Co-olivine an Co-spinel); 3) the determination of the "averaged" crystal structure in crystals of CaCoxMg1-xSi2O6 pyroxene series; 4) the definition of the optical properties along the different directions in crystals of CaCoxMg1-xSi2O6 pyroxene series; 5) the discovery of an antiferromagnetic ordering at TNéel= 10 K in CaMgSi2O6-CaCoSi2O6 solid solution that occurs when the cobalt content in the octahedral sites of the pyroxene crystal structure is higher than 0.7 atoms per formula unit. The final goal of this work is the characterization of CaCoxMg1-xSi2O6 pigment from the technological viewpoint by simulating the production process of ceramic tiles, in order to assess, in terms of color stability and strength, its performances under industrial conditions, and compare them with those exhibited by conventional ceramic pigments (CoAl2O4 or Co2SiO4).