Hematite nucleation and growth in the firing of carbonate-rich clay for pottery production

Here we describe the evolution of iron-bearing phases in the firing of carbonate-rich illite and chlorite clay. Breakdown of chlorite forms an amorphous phase, which continues to retain iron in a (distorted) paramagnetic octahedral site, until carbonates start to decompose and react at 750 °C, forming Ca-silicates. Only at this temperature can hematite nucleate and grow in specific micro-domains located in former chlorite flakes. At 950 °C, decomposition of illite in the presence of an amorphous phase and CaO produces a melt in which nano-sized hematite may nucleate, although its growth is inhibited due to low diffusion with respect to soaking time.