Ordered but Disordered: The Peculiar Crystal Structure of Greenalite

Greenalite is a chemically simple but structurally complex sheet silicate with a general formula of Fe2+(3−x−y−z)Fe3+xMgy☐zSi2O(3.5+x−2z)(OH)(6−x+2z). Originally characterized as a serpentine from X-ray powder diffraction data, detailed interrogation of its structure through electron microscopy has revealed complex yet systematic disorder within tetrahedral-octahedral layers, and disorder in the stacking patterns of those layers along the crystallographic c-axis. These features arise from the misfit in lateral dimensions between oxygens coordinating relatively large Fe2+ octahedra and those forming the basal plane of Si tetrahedra, and result in a composition that deviates significantly from that of an ideal serpentine-group mineral. Continued interrogation of greenalite’s structure and chemistry will be fundamental to resolving problems related to its formation and stability in natural systems.