Violet Iron Oxide: Precipitated Hematite Calcined at 1000-1100 °C (Nurdini et al. 2022)

Iron hydroxide is thrown down from hot ferric nitrate solution with ammonia, washed, dried and calcined. All of it ends as pure hematite, but the colour moves from bright red through dark red to purple as the calcining heat rises from 1000 °C, because the grains grow larger and more crystalline. Result: pure violet to purple ferric oxide (hematite) after 4 hours at 1000-1100 °C. Source: Nurdini et al., Heliyon 8 (2022) e10377, sections 2.2 and 4. Not given: batch size, how much ammonia, end pH, yield. Picture drawn by Youblob.