Solid-state reactions in the system talc-CaCo 3 doped with Co(II) at high temperatures

Abstract

Four solid reaction mixtures of talc- x CaCO 3-0.2 CoCl 2·6 H 2O in the molar ratio 1 : x:0.2 ( x = 1, 2, 3 and 4) were prepared and thermally treated at 1000 and 1200°C. The resulting coloured reaction products were identified—after sudden cooling to room temperature—by means of X-ray diffraction and ligand-field absorption spectrum methods. Analysis of the reaction products obtained at 1000°C showed diffracted lines as well as absorption bands characteristic of phases (I) Co-diopside Ca(Mg, Co)Si 2O 6 at x = 1, (II) Co-diopside, Co-monticellite Ca(Mg, Co)SiO 4 and Co-akermanite Ca 2(Mg, Co)Si 2O 7 varying in concentrations at x = 2 and 3, and (III) Co-akermanite and Co-monticellite at x = 4. On the other hand, analysis of the reaction products treated further at 1200°C confirmed the presence of the same phases formed in the reaction mixtures at x = 1 and 4, whereas Co-monticellite seemed to dissociate where x = 2 and 3. However, the intensities of both diffracted lines and absorption bands of the corresponding phases were found to increase on increasing the firing temperature and CaCO 3 concentration. Generally, the results are discussed in view of formation enthalpies as well as spectra of differently coordinated Co(II) in silicate structures.