Chem¬ical composition of melilite formed during the firing of carbonate-rich and iron-containing ceramic bodies

During the firing of carbonate-bearing clay bodies, melilite is frequently formed in the form of very small crystals (1-5 ?m or less in size). In the literature, this phase is generally called gehlenite; however, no precise composition is available. To achieve a chemical characterization of this ''ceramic'' melilite, six industrial products have been analyzed via in situ techniques (scanning electron microscopy coupled with energy-dispersive spectroscopy) and via X-ray fluorescence and X-ray diffractometry (bulk sample). Melilite crystals exhibit the following concentration ranges: 29%-41% SiO2, 32%-42% CaO, 10%-19% Al2O3, 2%-7% MgO, and 4%-10% Fe2O3. The Na2O, K2O, and TiO2 contents are almost always <1%, and FeO is always practically absent in the bulk sample. Overall, ''ceramic'' melilite seems to be actually a solid solution with the following composition range, in terms of end members: 32%-56% gehlenite, 20%-49% åkermanite, plus a significant fraction of ferri-gehlenite (14%-37%).