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⁺_x
Mg_y
□_z
Si₂
O_(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.