Extended Data Fig. 7: HRTEM lattice fringe image illustrating the glauconitization pathway.

(a), Orientation relations between mature glauconite and ferric smectite. The central glauconite sheets consist of a defect-free regular sequence of 10 Å layers; whereas, the collapsed Fe-smectite show anastomosing d-spacings with abundance of edge dislocations. Both minerals are disposed in a subparallel way, with clear discontinuities between glauconite and smectite layers. The amorphization features of smectite around the border of grains (areas enclosed as circles) suggest a solvent-mediated growth of glauconite at expenses of smectite, in a process similar to Ostwald ripening. There are also appreciable dissolution features and d-spacings of 0.45 nm corresponding to (020) not perpendicular to (001) indicative of rotation between layers. (b), Partially dissolved glauconite after attack by 1 M HCl for 24 h showing the amorphization of the structural framework of tetrahedral and octahedral sheets, although the 10 Å basal repeat is still observable. c) SAED diagram of natural glauconitic mineral showing rotational disorder in the a*b* section. Glauconite samples were obtained from the sediment-water interface at a depth of 200 m from the Galician continental shell (NW of Spain).