PhD Eloïse Gaillou

"Relationship between nanostructure, physical properties and the formation mode
of opal-A and -CT

Eloïse Gaillou, 22 September, 2006.

We have demonstrated that opal, which is an amorphous (opal-A) or poorly crystallized (opal-CT) material, shows a wide variety of structures, and is built from elementary silica nanograins of about 25 nm in diameter. In opal-A, nanograins arrange themselves in spheres, most often concentrically, sometimes radialy. In opal-CT, they can organize or not, with up to four degrees of order, including several structures described here for the first time. We have established that identification of inclusions is possible by Raman spectrometry, a nondestructive method. Inclusions are often characteristic of a specific environment, helping to identify formation conditions, even sometimes the exact locality. Thus, pink opals, which contain quinones adsorbed on phylliteous minerals, indicate a fossil lake environment. Geochemical criteria have been determined, permitting the identification of the geographical origin of all deposits studied. Geological origin is reflected in the barium content: > 100 ppm for sedimentary opals, < 110 ppm for volcanic opals. Several elements influence physical properties such as color or luminescence. Luminescence is activated by uranium (green emission, when U ≥ 1 ppm and Fe < 1000 ppm) or by oxygen-related defects (blue luminescence, when U < 1 ppm and Fe < 1000 ppm), and inhibited by iron. It was possible to establish a narrow range of relationships between nanostructure, physical properties and the mode of formation of opal.

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