Synthesis and characterisation of dioctahedral smectites.
Résumé
The synthetic clays are used in a lot of studies to analyse clay properties. They permit to have samples without impurities and with a specific chemical composition. In synthetic smectite group, trioctahedral smectites as saponite were generally studied. Because the majority of natural smectites are dioctahedral ones, it can be interesting to product dioctahedral synthetic smectites. This is the objective of this work. The synthesized smectites should be mono-phased with well-known characteristics and chemical composition. In this study we have synthesized beidellites with different tetrahedral charges and different structural Fe3+ content and montmorillonites with different octahedral charges. Synthesis protocol : Gels used for starting materials were prepared according to the usual method from the following compounds : tetraethyl orthosilicate (TEOS), Mg(NO3)2.6H2O, Al(NO3)3.9H2O, Fe(NO3)3.9H2O, HNO3, Na2CO3, NH4OH and ethanol. After dissolution of Al, Mg nitrate and Na carbonate in nitric acid, the TEOS and ethanol were added. A precipitate was obtained after neutralization of the solution; this precipitate was dried and heated to 400°C to obtain Al, Mg, Si, Na oxides gel. The samples were synthesized in an internally heated pressure vessel. Starting gel was sealed with water solution in a gold capsule. The mixture was treated at 350°C under a variable pressure between 200 and 1500 Bars (depending on the sample) during 15 days. Results : First, we present results for synthetic beidellites with different tetrahedral charges. In the reaction products, no additional phases was observed by XRD. Secondly, beidellites with iron content were synthesized with a maximum FeV I/AlV I ratio equal to 1. In this syntheses, a small quantity of iron oxide was present in final product. Infrared analysis permit to assign the principal part of iron to the clay phase. Iron rich beidellites can not be obtained in these conditions. If we increase the FeV I/AlV I ratio above 1, the reacted product is not iron rich beidellite. We obtain a mixture between iron oxides, opal and sometimes smectite phase. Lastly, for montmorillonites synthesized at low pressure, only smectite was identified by XRD but this product is probably a mixture between beidellite and saponite. At more important pressure, the results seem to be different because only one smectite phase was identified by XRD.