HRTEM-study of stacking faults and polytypism in kyanite
Grobéty, Bernard H.; Veblen, David
European Journal of Mineralogy Volume 7 Number 4 (1995), p. 807 - 818
published: Aug 1, 1995
manuscript accepted: Feb 16, 1995
manuscript received: Sep 10, 1994
ArtNo. ESP147050704008, Price: 29.00 €
Abstract Digitally recorded HRTEM images of kyanite containing planar defects have been processed by Fourier optics techniques using Bragg filters with lens-shaped holes. The enhanced image contrast together with computer contrast simulations allowed determination of the position of (100) stacking faults within the unit cell. The stacking faults are located within the aluminum octahedral layer of the kyanite structure regardless of their formation mechanism. Polytypes in kyanite are the result of the ordering of these stacking faults. A new monoclinic 4-layer polytype was found in kyanite of the Rieserferner aerea, Italy. This 4-layer polytype has the same stacking order as the 4-layer polytype in sapphirine. A modified ANNNI model, derived from a model for wollastonite, successfully predicts the stacking sequence for the 4-layer polytype in kyanite.