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 μm 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% SiO 2 , 32%–42% CaO, 10%–19% Al 2 O 3 , 2%–7% MgO, and 4%–10% Fe 2 O 3 . The Na 2 O, K 2 O, and TiO 2 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%).