First Occurrence of a Stacking Sequence Including (±60°, 180°) Rotations in Mg-Rich Annite

Transmission electron microscopy (TEM) observation shows narrow regions in a Ti-containing Mg-rich annite of composition (K 0.90 Na 0.02 )(Mg 0.72 Fe 2+ 1.78 Mn 0.03 Ti 0.27 Al 0.05 )(Si 2.77 Al 1.23 )O 10 (OH,F) 2 from a granitic rock, where the ±60° and 180° stacking angles occur extensively. These regions are a few hundreds of nanometers thick along the [001]* direction and are within 1 M or 2 M 1 annite. The stacking sequence in one of these regions was determined by two atomic-resolution images recorded along [1̄10] and [010] of the same crystal. Stacking sequences with ± 120° or 180° rotations are dominant, although those with ±60° rotations occur also. Locally 2 O and more complex sequences exist. Compositional analysis by TEM indicated no difference in the chemical compositions between these regions and the adjacent ones with regular 1 M or 2 M 1 stacking sequence. The origin of these unusual stacking sequences in annite is discussed.