First occurrence of a stacking sequence including (+/- 60 degrees,180 degrees) rotations in Mg-rich annite

Transmission electron microscopy (TEM) observation shows narrow regions in a Ti-containing Mg-rich annite of composition (K0.90Na0.02)(Mg0.72Fe1.782+M n0.03Ti0.27Al0.05)(Si2.77Al1.23)O-10(OH,F)(2) from a granitic rock, where t he +/-60 degrees and 180 degrees stacking angles occur extensively. These r egions are a few hundreds of nanometers thick along the [001]* direction an d are within 1M or 2M(1) annite. The stacking sequence in one of these regi ons was determined by two atomic-resolution images recorded along [(1) over bar 10] and [010] of the same crystal. Stacking sequences with +/-120 degr ees or 180 degrees rotations are dominant, although those with +/-60 degree s rotations occur also. Locally 2O and more complex sequences exist. Compos itional analysis by TEM indicated no difference in the chemical composition s between these regions and the adjacent ones with regular 1M or 2M(1) stac king sequence. The origin of these unusual stacking sequences in annite is discussed.