<p>Kremersite [(NH₄)₂FeCl₅·H₂O] is a rare and structurally unstable iron phase, scarcely described in the mineralogical literature and unexplored at the nanoscale. Here, it is reported a systematic investigation of nano-kremersite (KreNPs), obtained through a green and sustainable recovery route from akaganeite (AkaNPs) synthesis residues. This circular approach not only reduces synthetic waste but also enables access to a metastable and underexplored phase. To ensure reliable identification, KreNPs were compared with hematite, goethite, and magnetite synthesized under controlled conditions. A comprehensive characterization was carried out using SEM/TEM, XRD, FTIR, EDS, DLS, NTA, zeta potential, and TGA/DTA analyses. KreNPs exhibited well-defined prismatic morphologies, orthorhombic crystalline order, and chloride-rich composition consistent with their unit cell, distinguishing them from AkaNPs and other iron oxides. Thermal analysis further confirmed the existence of unique dehydration and transformation pathways. Structural refinement based on XRD confirmed enhanced crystallinity and reduced amorphous contribution compared to other nanoparticles. By demonstrating that a scarcely occurring mineral can be stabilized in nanoparticulate form through residue valorization, this work expands mineralogical knowledge while reinforcing principles of green chemistry. Beyond advancing mineralogical knowledge, these findings highlight the potential of sustainable synthetic strategies to access metastable phases, paving the way for future studies on their functional properties and technological applications.</p> Graphical Abstract <p></p>