Étude par Diffraction X des Modes d'Empilement de la Nacrite Hydratée et Deshydratée

Abstract

X-ray diffraction based on the comparison of experimental and calculated powder profiles enabled the determination of the structural characteristics of hydrated and dehydrated Tunisian nacrite. Using the concept describing the structure of natural nacrite, the stacking mode of the layers in the hydrated and dehydrated nacrite has been determined. The hydrate is characterized by an 8.42 Å basal distance; one water molecule per Si 2 Al 2 O 5 (OH) 4 is intercalated in the interlamellar space, located above the vacant octahedral site of the layer at z = 6.5 Å and inserted inside the ditrigonal cavity of the tetrahedral sheet of the upper layer. The dehydrated nacrite obtained by heating of the hydrate at 423 K has the same interlayer shift t = −0.35 a as the natural nacrite. Coherence domain sizes along c ^{\ast} and in the ab plane are the same as those in the hydrate but different from those of the natural mineral. After dehydration, 5% of the layers had an interlayer shift similar to that obtained from the hydrate.