Chemical 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 pm or less in size). In the literature, this phase is generally called gehlenite; however, no precise c omposition is available. To achieve a chemical characterization of this "ce ramic" melilite, six industrial products have been analyzed via irt situ te chniques (scanning electron microscopy coupled with energy-dispersive spect roscopy) 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, a nd TiO2 contents are almost always <1%, and FeO is always practically absen t in the bulk sample. Overall, "ceramic" melilite seems to be actually a so lid solution with the following composition range, in terms of end members: 32%-56% gehlenite, 20%-49% akermanite, pins a significant fraction of ferr i-gehlenite (14%-37%).