FIRST OCCURRENCE OF A STACKING SEQUENCE INCLUDING (±60°, 180°) ROTATIONS IN MG-RICH ANNITE

Abstract

Transmission electron microscopy (TEM) observation shows narrow regions in a Ti-containing Mg-rich annite of composition (K (sub 0.90) Na (sub 0.02) ) (Mg (sub 0.72) Fe (super 2+) (sub 1.78) Mn (sub 0.03) Ti (sub 0.27) Al (sub 0.05) ) (Si (sub 2.77) Al (sub 1.23) ) O 10 (OH,F) 2 from a granitic rock, where the + or -60 degrees and 180 degrees stacking angles occur extensively. These regions are a few hundreds of nanometers thick along the [001] (super *) direction and are within 1M or 2M 1 annite. The stacking sequence in one of these regions was determined by two atomic-resolution images recorded along [110] and [010] of the same crystal. Stacking sequences with + or -120 degrees or 180 degrees rotations are dominant, although those with + or -60 degrees rotations occur also. Locally 2O 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 1M or 2M 1 stacking sequence. The origin of these unusual stacking sequences in annite is discussed.