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This paper describes applications of analytical Transmission Electron Microscopy (TEM) in the geosciences. The topics include: 1) sulphide-mineral oxidation; 2) trace-metal attenuation by secondary Mn oxides; 3) silicate weathering; 4) transition-metal valence in minerals; and 5) secondary Hg minerals in stream sediments. The main advantage of the analytical TEM is the ability to obtain images, chemical information, and electron diffraction patterns at the nanometre scale. With such high spatial resolution, it is possible to observe physical and chemical features in samples that cannot be resolved with most other techniques. This information can lead to signicant improvement in our understanding of the system under investigation. Sample preparation techniques that are used in each study are also described in this paper. The preparation of samples for TEM analysis can be challenging because of the heterogeneity commonly encountered in geological materials, the fragility of some geological samples (e.g., low-temperature minerals), and the need to maintain spatial relationships present in the samples. The sample preparation techniques presented are specic to the needs of the study and the appropriateness of these methods is demonstrated by the high quality analytical TEM data that are obtained.  RESUME  Cet expose decrit des applications de la microscopie electronique a transmission analytique dans les sciences de la terre. Les aspects etudies comprennent : 1) l'oxydation des mineraux sulfures; 2) l'attenuation des metaux-traces par des oxydes de Mn secondaires; 3) la silicatisation meteorique; 4) la valence des metaux de transition dans les mineraux; et 5) les mineraux de Hg secondaires dans les sediments uviatiles. Le principal avantage qu'offre la MET analytique est la possibilite d'obtenir des images, des donnees chimiques et des gures de diffraction des electrons a l'echelle nanometrique. Une resolution spatiale aussi elevee permet l'observation dans les echantillons de proprietes physiques et chimiques impossibles a eclaircir au moyen de la majorite des autres techniques. De tels renseignements peuvent mener a une amelioration marquee de notre comprehension du systeme a l'etude. Cet expose decrit en plus les techniques de preparation des echantillons utilisees lors de chaque etude. La preparation des echantillons a une analyse MET peut s'averer compliquee en raison de l'heterogeneite que presentent communement les matieres geologiques, de la fragilite de certains echantillons geologiques (p. ex. mineraux a basse temperature) et de la necessite de maintenir les liens spatiaux presents dans les echantillons. Les techniques de preparation des echantillons presentees sont propres aux besoins de l'etude; les donnees de haute qualite obtenues des analyses MET temoignent de la pertinence de ces methodes.  [Traduit par la redaction]

Geoscience at the nanometre scale: review of analytical transmission electron microscopy applications