『Abstract
High-resolution transmission electron microscope (HRTEM) imaging
and electron diffraction, combined with powder X-ray diffraction
(XRD) data and a partial powder profile analysis by the Rietveld
method, have been used to determine the parent structure and the
polytypism of bementite, Mn7Si6O15(OH)8, a modulated 1:1
layer silicate from Franklin, New Jersey.
The parent structure is monoclinic P21/c
with a = 14.838(2), b = 17.584(2), c = 14.700(2)Å, and β= 95.54(2)゜,
Z = 4, V = 3814.8Å3, and M = 929 g/mol. It consists
of two hexagonal sheets of octahedra, accommodating the Mn, which
are alternately rotated by 22゜ in the ab plane. These are interlayered
by a continuous tetrahedral sheet containing pairs of six-membered
rings interconnected with five- and seven-membered rings. Inverted
tetrahedra form strips with like orientation (up or down) parallel
to a. Linked pairs of six-membered rings are rotated relative
to pairs across strip boundaries by 22゜ to allow coordination
with adjacent octahedral sheets.
The polytypism, which can be ascribed to an interface modulation
caused by stacking faults, is best visible in HRTEM images and
diffraction patterns of the [010] projection. The displacement
and frequency of the faults were determined from the distribution
of satellite reflections and their displacement relative to the
parent reflections. Bementite can be interpreted as a combination
of two basic stacking varieties of topologically identical octahedral
and tetrahedral sheets. Variety I is considered the parent structure,
with β= 95.5゜. Variety II has a stacking error after every Mn
layer, which results in β= 71.3゜.』
Introduction
Sample
Experimental
High-resolution transmission electron microscopy: HRTEM
Powder XRD
Results
Electron diffraction data
The hk0 diffraction pattern
The h0l diffraction pattern
The 0kl diffraction pattern
HRTEM images
Powder X-ray diffraction pattern
Derivation of the subcell model
Stacking faults and polytypism
Derivation of the supercell model
Symmetry
Topology of the tetrahedral sheet
Powder pattern analysis
HRTEM: Comparison between simulations and experimental images
The [010] projection
The [100] projection
The [110] projection
Discussion and conclusions
Acknowledgments
References cited