Fluctuation Electron Microscopy (FEM) of disordered mineral phases.

Not all minerals are crystalline. Obsidian, tectites, and several carbon minerals are disordered or glassy.

FEM has shown how paracrystalline order can be detected in amorphous tetrahedral semiconductors

One interesting mineral is shungite, which is a glassy carbon phase found in the former Soviet Union. Peter Buseck and others reported TEM evidence for ordered C60 regions within the sample. Such regions are not common, amorphous and graphitic regions being much more prevalent. However, such observations hint that shungite is rich in structural possibilities and is an ideal candidate for FEM.

The goals of this project are to determine the structural nature of the disorder in the amorphous carbon phases which occur in some carbon-rich minerals such as shungite.

Carbon exhibits a rich short-range structural variability (< 1 nm) that ranges from graphitic (3-connected) to diamond-like (4-coordinated) bonding which makes it a fascinating material to study. Interesting 3-coordinated possibilities are shown in the image on the right. While there is not evidence that simple conical structures occur in shungite, it is clear that a rich diversity of curved graphitic structures is possible. Medium range (paracrystalline) order, at length scales < 2nm, in amorphous carbon is hard to detect by ordinary TEM bright field imaging or by x-ray diffraction. At such length scales, disordered structures all tend to resemble fully disordered glasses. The problem is simply that, on average, one region of the sample looks like every other region, and subtle structural variations are hard to detect.

Fluctuation electron microscopy is an emerging technique that is sensitive to structural fluctuations between regions. It has been applied with success to the study of paracrystallinity in amorphous silicon and germanium films. Silicon and germanium are ideally tetrahedrally-coordinated (i.e. 4-coordinated) materials. Amorphous carbon poses a richer, more interesting, and perhaps more challenging structural variability because it can adopt both diamond-like (4-connected) and graphitic (3-coordinated) characteristics. Long range graphitic ordering (> 3 nm) is readily detected in TEM micrographs. Such regions are readily visible in some TEM micrographs of shungite. However, other regions may contain what appear to be ordered regions of C60 buckyballs, and other regions appear amorphous, resembling amorphous tetrahedral silicon. Suing fluctuation microscopy techniques we plan to investigate such amorphous regions in detail to determine of paracrystallinity is present (length scales < 2 nm) and if so, what is the structural nature of the disorder. Possible outcomes are that such regions are (i) true continuous random networks with diamond-like coordination; (ii) incipient graphitic regions containing aromatic rings; (iii) disordered arrays of fullerenes such as C60; (iv) a compact of nanometer-scale diamond particulates. Fluctuation microscopy can in principle resolve this issue. Additional issues are the effect of local impurities on the local structure.

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Last updated July 4th, 2005.

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