Polytypes chlorite

Bailey, S. W., and B. E. Brown (1962). Chlorite polytypism—regular and semirandom one-layer structures. Amer. Min. 47 819-850. 

Brown and Bailey (1962) examined 300 chlorites from different localities and found that approximately 80% had the lib structure but found examples of the orthorhombic lb, monoclinic lb and la structural type. The relative abundance of the polytypes was related to structural stability. Composition influences to some extent the stability of the chlorites through its effect on the cation charge and amount of distortion of the hexagonal network caused by size adjustments. Increasing tetrahedral A1 substitution is accompanied by an increase in octahedral Fe to maintain a reasonable degree of fit between the two types of layers. 

There is considerable overlap in the compositions of the various structural polytypes. The la and lb chlorites have relatively distinct compositions, both being low in tetrahedral Al. Average compositions as calculated by Brown and Bailey (1962) are. ... 

The lib chlorite is the stable poly type in normal chlorite-grade metamorphism and in medium and high-temperature ore deposits. Brown and Bailey suggest that when sufficient energy is available, the most stable polytype (lib) will form. They found that the orthohexagonal and monoclinic lb types were the ones most likely to be considered diagenetic chlorites. These are the chlorites with the lowest amount of tetrahedral Al and these polytypes are most apt to be stable with a small amount of... 

Brown, B.K. and Bailey, S.W., 1962. Chlorite polytypism, 1. Regular and semi-random one-layer structures. Am. Mineralogist, 47 819-850. 

Hayes, J.B., 1970. Polytypism of chlorite in sedimentary rocks. Clays Clay Miner., 18 285-306. 

Brown BE, Bailey SW (1963) Chlorite polytypism. II. Crystal stmcture of one layer Cr-chlorite. Am Mineral 48 42-61... 

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. 

Recently we have investigated these mechanisms by analyzing the change of periodicity of polytypes owing to the formation chlorite layers (Kogure and Banfield... 

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. 

Thus, twenty-four chlorite layers were formed by mechanism 2 and two layers by mechanism 1 in this region, accompanying a volume decrease of about 25%. Investigation of other areas in this crystal with the 9-layer polytype indicated similar results, suggesting that mechanism 2 was dominant. 

Figure 19. Filtered HRTEM images of ordered biotite polytypes containing a few chlorite layers, recorded down [100]/[110]/[110]. The unfiltered portions of the images are inserted around the center of (a) and (b). The signs at the bottom of each figure indicate the direction of lateral shift between two tetrahedral sheets in a 2 1 layer. The long square brackets in (b), (c) and (d) indicate 9-layer polytype units without chlorite layers. The signs in the parentheses are the shift directions of removed 2 1 layers via mechanism 2 (see text), expected from polytypic sequences in biotite. (Kogure and Banfield 2000).

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

Kogure T, Banfield JF (2000) New insights into biotite chloritization mechanism via polytype analysis. Am Mineral 85 1202-1208... 

Cathelineau M, Nieva D (1985) A chlorite solid solution geothermometer. The Los Azufres (Mexico) geothermal system. Contrib Mineral Petrol 91 235-244 Dalla Torre M, Frey M (1997) The evolution from disordered Ad to ordered 2Mi white K-mica polytype in low-temperature metamorphosed sedimentary rocks. Schweiz mineral petrogrMitt 77 149-159 Dalla Torre M, Stem WB, Frey M (1994) Determination of white K-mica polytype ratios Comparison of different XRD methods. Clay Minerals 29 717-726... 

Walker JR (1993) Chlorite polytype geothermometry. Clays Clay Minerals 41 260-267... 

The kaolinite mineral species studied are kaolinite, kaolinite d (disordered kaolinite), dickite and nacrite. These polytypes have been described by Bailey (1963) on the basis of sense and degree of displacement of 1 1 layers and the position of vacant octahedral positions in the layer sequence. For the hydrated kaolinitic minerals, we have used the terminology of Keller and Johns (1976) which is based on endellite as the completely hydrated species and halloysite as the partly or completely dehydrated species. The polytypes of chlorite have been described by Bailey and Brown (1962) and Hayes (1970). In Fig. 8.3b it is shown that montmorillonite, the mixed-layer clays and illite are located between pyrophyllite without interfoliar charge and the dioctahedral... 

Baer AJ (1981) Geotherms - evolution of the lithosphere and plate tectonics. Tectonophysics 72 203-227 Bailey SW (1963) Polymorphism of kaolin minerals. Am Mineralogist 48 1196-1209 Bailey SW, Brown BE (1962) Chlorite polytypism. I. Regular and semirandom one-layer structures. Am Mineralogist 47 819-850... 

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. 

Figure 10 (a) Projection on XZ plane of the II6 chlorite structure. Anion planes are numbered to facilitate discussion in the text of the Bailey and Brown [1962] method of poly type derivation (b) initial 2 1 layer orientation assumed for all one-layer polytypes. The fixed axes of this layer do not necessarily coincide with the resultant axes of the crystal. 

Drits and Karavan [1969] also have derived the possible two-layer chlorite polytypes and their diffraction characteristics. They recognize only 148 different trioctahedral structures, in contrast to the results of Lister and Bailey cited above. The structures are described by the analytical symbols developed by Zvyagin. Dioctahedral and di,trioctahedral chlorites are included. 

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