Crystalstructure of natural materials
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An important part of the soil properties, such as pollutant sorption, is related to the fine mineral fraction less than 2 µm, corresponding to the clay minerals. To understand the behavior of such clay minerals in the environment, it is necessary to determine their structure at the atomic level. X-ray powder diffraction and the new transmission electron microscopy (TEM) techniques are used to explore the variability of the crystalline structure of the natural minerals at the nano or sub-nano scale.
Several minerals are studied: birnessite, illite, smectite, kaolinite and palygorskyte.
Keywords: Layered materials, transmission electron microscopy, crystallography
Crystalchemical study of birnessite polytypes (XRD, SAED, HRTEM, EELS)
Despite their low occurrence on soil, manganese oxides are ubiquitous in nature and play a major role in the fate of metallic pollutants. In the soil, manganese oxides are present as highly disordered minerals. To better understand their sorbent properties, work on the crystalchemistry of layered manganese oxides, for which birnessite is the major form, where undergone to understand their crystal structure as a function of the chemical parameters and the Mn mean oxidation state. An analysis of the seven existing polytypes and the nature of the crystallographic defects were performed using powder X-ray diffraction (XRD, electron diffraction (SAED), high-resolution transmission electron microscopy (HRTEM) and electron energy loss spectroscopy (EELS).
Identification method of illite polytypes by SAED
Dioctahedral 2:1 aluminosilicates are the dominant fraction the clay minerals in the soil, and their presence has an important impact on the mechanical but also oil properties of the soil. Illite, the most abundant potassic aluminosilicate, display a morphological variability (laths or isomorphic plates) but also a structural variability (trans-vacant or cis-vacant 1M or 2M1 polytypes, mainly). If a statistic relationship (by X-ray powder diffraction, or thermal analyses) as a function of the burial depth was reported, no direct relationship at the particle scale was demonstrated. The transmission electron microscopy (TEM) offers a unique opportunity to determine for each given particle its morphology, its chemistry and its crystal structure. A method by selected area-electron diffraction (SAED) was developed to identify the crystallographic polytype of the individual particles of illites. If the [001] zone axis is the most characteristic for such an identification, a method based on the observation of diffraction patterns along several zone axes was proposed.
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| Figure 1 : Selected-area electron diffraction of single crystal illite oriented along the [001] zone axis : a) tv-1M polytype ; b) cv-1M polytype ; c) tv-2M1 polytype |
References
Collaboration
ISTERRE Grenoble France, Moscow Russia, Univ Crete Grece
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