Chlorite stacking

Interpretation of two-dimensional images of sheet silicates has previously been addressed. Computer-simulated images of chlorite (22) and biotite and muscovite (2A) show that two-dimensional images of these sheet silicates allow determination of the basal spacing (spacing perpendicular to the sheets) and stacking order (relative rotations and shifts between sheets). Furthermore, these calculations show that at the Scherzer (optimum) focus, many, but not all, aspects of the structures may be interpreted directly. 

In particular, planar defects have been investigated successfully using HRTEM and such defect structures are common in phyllosilicates, including micas. Stacking faults are also regarded as planar defects but they are closely related to polytypism, which has been described above. Here we will discuss other types of planar defects which are important for micas. They include defects related to the initial stages of the transformation of micas to other minerals, e.g., mica to chlorite, mica to vermiculite, mica to kaolinite and the decomposition of mica at high temperature. 

If ehloritization occurs in disorder-free biotite polytypes other than IM, mechanisms 1 and 2 must result in different stacking sequences. If brucite-like sheets are formed by mechanism 1, the original stacking sequences in the biotite polytype are preserved because all 2 1 layers are preserved. In the case of mechanism 2, original stacking sequences must be altered owing to the removal of some 2 1 layers. Thus, it is possible to determine the number of biotite layers consumed by the formation of a chlorite unit cell and hence, its formation mechanism. 

Kogure T, Banfield JF (1998) Direct identification of the six polytypes of chlorite characterized by semirandom stacking. Am Mineral 83 925-930... 

Commercial grades of kaolin are composed primarily of the mineral kaolinite, a sheet sihcate, and may contain greater or lesser quantities of related sheet silicates (mica, illite, chlorite, smectite) and quartz. An individual kaolinite particle has the shape of an hexagonal plate. In nature these plates occur in stacks or books that exhibit varying degrees of staeking regularity. Kaolin is hydrophilic (readily water dispersible) for nonaqueous apphcations matrix compatibility can be improved by siuface treatment. 

Clay minerals (kaolinite, Ulite, montmoriUonite, and chlorite) are characterized by different stacking combinations or architecture of the two building elements (Fig. 1.6). Individual figures always represent one crystal. 

McMurchy examined six varieties of chlorite by the X-ray powder technique. He confirmed Pauling s basic structure and derived a total of 16 possible theoretical stacking arrangements based on a monoclinic unit cell with jS 97°. His proposed structures were equally divided between two-layer structures of space group C2jc and one-layer structures of space group C2/w. He narrowed the possibilities down to one two-layer and one one-layer structure by comparison of observed and calculated intensities. He was unable to decide between these structures because of the absence of the definitive A 3 reflections on his powder films, but he favored the two-layer polytype. 

The six semirandom stacking structures give different k = 3n intensities and can be identified by powder patterns. Bailey and Brown examined some 303 different chlorite specimens with the following results ... 

Lister and Bailey with the aid of stacking models have shown that many of these theoretical two-layer structures are equivalent after +60° or 180° rotation about the normal to (001) or 180° rotation about the Y axis. The probable number of different two-layer chlorite structures is 1134. Of these, 1009 have monoclinic-shaped unit cells, and 125 have orthorhombic-shaped cells. The authors use six symbols to describe a two-layer structure analytically, three for each 14 A unit. For each 14 A unit, the first symbol represents the direction of tetrahedral stagger within the 2 1 layer (X, X2, X, Xi, X2, X ). The second symbol indicates the orientation of the interlayer sheet relative to the 2 1 layer below (la, lb, Ila, IIA). The third symbol describes the position of the upper 2 1 layer relative to the interlayer sheet (1 through 6). The same terminology can be extended to chlorites with more than two layers. 

Because most chlorites appear to have semirandom stacking of layers, only a few regular multilayer chlorites have been described. 

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