AN APPROACH TO THE CHEMISTRY OF PYROXENES FORMED DURING THE FIRING OFCA-RICH SILICATE CERAMICS

Carbonate-bearing ceramic bodies are frequently used in the manufactur e of bricks, roofing tiles, wall and floor tiles, pottery and tablewar e. During the firing of these bodies, clinopyroxene is usually formed in very small crystals, 1-5 mu m in diameter or less. In the literatur e this phase is generally referred to as diopside, but no quantitative data are available. In order to chemically characterize these 'cerami c' pyroxenes, nine industrial products were analysed by XRF and XRD (b ulk sample) and SEM-EDS (fracture surface). Quantitative ZAF analyses of pyroxene crystals showed a certain chemical variability: SiO2 35-50 %, Al2O3 9-20%, Fe2O3 1-15%, MgO 3-14%, and CaO 16-25%. Sodium, K and Ti are always <1%, while ferrous iron is always <0.2% in the bulk samp le. Overall,'ceramic' clinopyroxenes present wide chemical analogies w ith 'fassaite', e.g. the abundance of aluminium and ferric iron, and t he excess of wollastonite molecules with respect to the diopside-heden bergite series.