Porous crystals of magnetite and vacancy-ordered maghemite, rhombohedron-shaped, reaching 150 mu m in size, and having remarkably large specific surface area (88.55 m(2)/g for magnetite, 40.14 m(2)/g for maghemite) were obtained via topotactical conversion, starting from hydrothermally grown siderite single crystals. The increase in density (from 3.9 g/cm(3) for siderite to 5.24 g/cm(3) for magnetite and 4.9 g/cm(3) for maghemite) caused quasi-ordered internal pores-grains pattern, with mesocrystalline appearance. The X-ray Line-Profile Fitting-based microstructure analysis gave 64 +/- 6 nm and 84 +/- 8 nm for the average inner grains size in magnetite and maghemite. Structure and phase content analysis indicated high purity and crystallinity. Magnetic measurements indicated saturation magnetization (92.1 emu/g for magnetite and 85.5 emu/g for maghemite) approaching the upper limits reported for the pure bulk oxides. (C) 2013 Elsevier BY. All rights reserved.