Structures of Brittle Micas
Clays and clay minerals
THE CRYSTAL structures of xanthophyllite, CaMg2Al(Al3Si)Oio(OH)2(a IM structure) and margarite CaAl2(Al2Si2)Oio(OH)2(a 2Mi structure), have been refined by threedimensional least squares. Ordering of Mg and Al has been observed in the octahedral layers of xanthophyllite. There is no signiiicant evidence for ordering among cations in the silicate layers of margarite. The tetrahedra are rotated about 23° in xanthophyllite and about 21° in margarite. The configuration of the octahedral layers in
... ahedral layers in margarite has the same features as those in muscovite and dickite. The mode of deformation of the siUcate layer is roughly similar in both structures, but there are important differences. These differences are caused by the different configurations of the octahedral layers and are a common feature among micas. In the aluminum octahedral layer, the oxygen hexagons whose comers are apices of tetrahedra have short edges of 2.81A and a pair of longer edges of 3.35A (distances are averages obtained from the structures of margarite, muscovite and dickite). Because of these longer edges, the tetrahedra in the dioctahedral micas are tilted in addition to having rotations caused by the short edges. The ^-parameters of the basal oxygen atoms in the tetrahedra thus show maximum deviations of 0.19 ± 0.03A in margarite and 0.12 ± 0.03A for muscovite. On the other hand, in trioctahedral micas, the edges of the oxygen hexagons are almost the same length, with a maximum deviation of 0.05A. The silicate layers in trioctahedral micas are accordingly almost free from tilting of tetrahedra. The difference in ^--parameters in dioctahedral micas causes a corrugation of layers and also causes a shift of interlayer cations. It is known that in trioctahedral micas, the direction of the OH bond is perpendicular to (001), while in muscovite it is inclined to the &-axis. Taking into account this asymmetrical orientation of the OH bond, together with the above-mentioned interlayer cation shifts, it is possible to show that a layer stacking by the operation of the twofold axis in the space group of the 2Mi structure is no longer identical with that produced by rotations of ± 120° about an axis perpendicular to (001). A more restricted number of operations would therefore be possible in the generation of poljrtypes. An index, D, has been defined that is a direct measure of the misfit between octahedral and tetrahedral layers. It will be shown that the layer silicates may be classified into three categories with the aid of this index.