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

Toshihiro Kogure and Massimo Nespolo
Mineralogical Institute, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-Ku, Tokyo 113-0033, Japan
Present address: National Institute for Research in Inorganic Materials. 1-1 Namiki, Tsukuba-shi, Ibaraki 305-0044 Japan. E-mail of corresponding author: kogure@min.s.u-tokyo.ac.jp

Abstract: Transmission electron microscopy (TEM) observation shows narrow regions in a Ti-containing Mg-rich annite of composition (K0.90Na0.02)(Mg0.72Fe2+1.78Mn0.03Ti0.27Al0.05)(Si2.77Al1.23)O10(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 1M or 2M1 annite. The stacking sequence in one of these regions was determined by two atomic-resolution images recorded along [1¯10] and [010] of the same crystal. Stacking sequences with ±120° or 180° rotations are dominant, although those with ±60° 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 2M1 stacking sequence. The origin of these unusual stacking sequences in annite is discussed.

Key Words: Annite • Biotite • HRTEM • Mica • Polytype-2O • Polytypism • Stacking Disorder

Clays and Clay Minerals; December 1999 v. 47; no. 6; p. 784-792; DOI: 10.1346/CCMN.1999.0470614
© 1999, The Clay Minerals Society
Clay Minerals Society (www.clays.org)