High-angle annular dark field scanning transmission electron microscopic (HAADF-STEM) study of Fe-rich 7 Å–14 Å interstratified minerals from a hydrothermal deposit

The distribution of octahedral cations in the two component layers of a 7 A–14 A interstratified mineral with a bulk chemical composition (Fe 2+ 4.12 Mg 0.07 Mn 0.01 Al 1.69 □ 0.11 )(Si 2.56 Al 1.44 )O 10 (OH) 8 was investigated using high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) in combination with the image simulations. In the 14 A component layers, comparison between the observed and simulated images revealed that the M4 sites of the interlayer sheets were occupied preferentially by Al together with a small amount of Fe; the other M1, M2 and M3 sites were occupied by dominant Fe and residual Al in equal proportions. Two types of octahedral sheets with disordered and ordered cation distributions were recognized in the 7 A component layers. The two types of sheets were similar to the octahedral sheet of the 2:1 layer and the interlayer sheet in the 14 A layer above, respectively. Irregular vertical stacking and lateral contact of the different component layers in structure and chemistry characterized the interstratification, which may be caused by rapid precipitation and accretion of the component layers in hydrothermal environments.