Here, we focus on the obtaining of mesocrystalline submillimeter-sized (150/50 mu m) rhombohedral hematite (-Fe2O3) by thermal treatment in air of single crystalline submillimeter-sized (150/50 mu m) rhombohedrons of ferrous carbonate (FeCO3). Mass spectrometer-coupled thermogravimetric analysis and TGA-MS revealed the chemical reactions occurring during the thermal treatment of ferrous carbonate sample. The X-ray Diffraction (XRD) data sustain that the final product is hematite. The XRD line-profile analysis indicates that the resulted hematite is built of individual ordered crystallites with 66 +/- 5nm average sizes, confirmed by scanning electron microscopy and transmission electron microscopy images. Small-angle x-ray scattering investigation of hematite sample was presented. The log-log plot of scattering intensity decay showed the same slope, =-3.76, corresponding to both high and low scattering vector regions; the fractal surface is D-s=2.24. This fractality is extended over a range of sizes and can touch high molecular dimensionality. The internal morphology and the synthesis mechanism of the obtained hierarchical superstructure were described.