X-Ray Diffraction Study of Structural Transformations of Aluminous 2:l Layer Silicates During Dehydroxylation

Abstract: X-ray powder diffraction was used for the study of structural transformations of dioctahedral Al-rich smectites and illites having different distribution of octaherdal cations over trarrr-vacant (tv) and cis-vacant (cv) sites within 2:l layers. The main problems were to determine a model for dehydroxylated cv structures and to explain why smectites and illites consisting of cv 2:l layers are characterized by dehydroxylated temperatures which are higher by 150-200°C than those for the same ~n~nerals consisting of tv 2:l layers. On set of XRD experiments has been done for sample heated at different temperatures during 1 hour before measurements ("ex situ " samples) and another one has been done for samples heated during the measurements ("in situ " samples). According to the model proposed for cv dioctahedral 2:l layer silicates the dehydroxylation takes place in two main stages. During the first one each two adjacent OH groups are replaced by the residual oxygen atom and the A1 cations which originally occupied cis-and trans sites become 5- and 6- coordinated respectively. The structure corresponding to this stage of the dehydroxylation is unstable and during the second stage the A1 cations migrate from the former trans-sites to vacant pentagonal prisms. After this stage the dehydroxylated stucture of the former cv 2: 1 layers is quite similar to that of the former trans-vacant 2: 1 layers. RbumB: La diffraction des rayons X a BtB utilisee pour Ctudier les transformations structurales dans de