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- ISIDORE Id: 10670/1.6a713d...
- hal: hal-02570678
- doi: 10.1016/j.jnoncrysol.2020.119938
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François Fröhlich, « The opal-CT nanostructure. », HAL SHS (Sciences de l’Homme et de la Société), ID : 10.1016/j.jnoncrysol.2020.119938
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Opal-CT, a silica polymorph in marine sedimentary siliceous rocks, exhibits commonly a form of spheres about ten micrometers in diameter ("lepispheres"). Quantitative infrared analyses were performed with a special attention to the band related to the SieOeSi angle and to the [SiO 4 ] tetrahedral framework. Opal-CT includes about 40% non-crystalline structures mixed with crystalline ones related to tridymite. Under TEM a single opal-CT lepisphere section exhibits a nanostructure constituted of a stacking of domains 1.5 nm thick, alternatively crystalline and non-crystalline. The crystalline domains match with a monoclinic form of tridymite which pass to non-crystalline domains with a distortion of the SieOeSi angle. The regular stacking of crystallites 1.5 nm thick composed with about three tridymite modules is assigned to a tridymite superstructure. The paradoxical crys-tallization at sea water temperature of a high temperature mineral is here explained by the polymerization of silica dimers from sea water enriched with dissolved silica. Opal-CT hydrothermal in origin differs from the sedimentary one by its infrared spectrum and its optical properties that matches with the silica polymorph lussatite. It is inferred that the hydrothermal opal-CT exhibits a structure significantly different from the sedi-mentary one (pseudo-orthorhombic, even orthorhombic).
