Chlorites structures

Rohrlich, et al.. 1969 Gaertner and Schellmann, 1965 Leclalre, 1968 Giresse and Odin, 1973) at fairly shallow depths (< 80 meters). These reactions take place under saline or estuarine conditions. The transformation of sediment into berthierine is apparently progressive the initial sedimentary material found in shell tests becomes gradually transformed into a single phase, 7 chlorite structure. 

The most important character of all of the berthierine compositions is their low silica content. The variation of compositions found for pellets from the recent sediments appears to be the result of the crystallization of a chlorite structure with full octahedral occupancy. The meta-berthierines fall within the limits deduced for synthetic magnesian 7 X chlorites, limits which are also near full octahedral occupancy. 

The chlorite clays appear to have octahedral sheets with compositions that are largely intermediate between the 2 1 and 1 1 octahedral sheets. The chloritic structure allows for a wider range of substitution than the other clays. In part this is because most data on the octahedral composition are an average of two octahedral sheets, each of which could have relatively restricted compositions. 

Brindley, G.W. and Cillery, F.H., 1954. A mixed-layer kaolin-chlorite structure. Proc. Natl. Conf. 

Figure 3.10. Schematic of the chlorite structure showing two gibbsite sheets between two silicate sheets (from Jackson, 1964, with permission).

The formula Dim is that of an idealized Al-celadonite, Bio is that of an idealized phlogopite, and Chi is that of an idealized clinochlore. (Dim + tk) is, of course, the idealized muscovite formula (Bio + tk) is that of an idealized eastonite, and (Chi + tk) is that of an idealized corundophyllite. The last two are taken as the extreme tk+ limits on the basis of the aluminum avoidance rule. The choice of the clinochlore formula as the tk- limit makes good crystallo-chemical sense for 14-Aigstrdn chlorites, the ones found in pelitic schists. The original chlorite structure described by Pauling (1930) was, in fact, that of a clinochlore found in a blackwall skarn in close proximity to a lizardite-bearing serpentinite near Chester, Vermont, U.S.A. The idealized lizardite formula may be taken... 

The clay mineral spectrum is notably less differentiated than in the other facies, the dominant minerals being trioctahedral chlorites and dioctahedral illites. In the chlorite structure, non-swelling layers predominate whereas the alternation of layers of different types exhibits a trend towards ordering. The proportion of mixed-layer minerals of the iUite-montmorillonite type decreases especially as one approaches the massive layers of rock salt. 

Bailey, S. W. (1988a) Chlorites Structures and Cr3retal Chemistry In Hydrous Phyllosilicates (exclusive of micas), Vol 19, S. W. Bailey, ed., Mineralogical Society of America, Washington, DC, 347-403. 

In 1934, the first insights into the structure of vermicuUte were obtained by two independent workers using X-ray powder methods. Kazantzev, on the one hand, reported that the unit cell is analogous to that of biotite, but of slightly larger dimensions, with K partly replaced by H and Fe by Mg. The other, Gruner, showed that the structure consists of silicate layers resembling those of mica or talc, with double sheets of water molecules between them. These so-called interlayer water molecules occupy a space very nearly equal to that occupied by a brucite layer in the chlorite structure, with the result that the X-ray diffraction effects obtained from vermiculites and chlorites have certain similarities. 

The trioctahedral chlorite structure consists of 2 1 talclike layers of composition (R, R " )3(Si4 j.AyOio(OH)2 that alternate in the structure with octahedral brucitelike interlayer sheets of composition (R ", R )3(OH)6. The tetrahedral portion of each 2 1 layer has a negative charge x due to substitution of x ions of AP, or occasionally of Fe or Cr, for Si . The interlayer sheet has a positive charge due to substitution of R " " ions for R and serves to neutralize the negative charge on the 2 1 silicate layer. In most cases, it is not possible to determine if the tetrahedral charge is compensated entirely within the interlayer sheet or whether the octahedral portion of the 2 1 layer also acquires a positive charge. The main constituents of the two octahedral sheets are Mg, Fe, Al, and Fe , but with important substitutions of Cr, Ni, Mn, V, Cu, or Li in certain varieties. Any medium-sized cation will fit in the octahedral sites. 

Figure 6. Diagrammatic sequence of micalike layers and brucitelike interlayers in the chlorite structure according to Pauling [1930].

Relative to the results of later workers, it can be pointed out that both of McMurchy s final structures were based on what has proved to be the most common type of chlorite layer superpositions (life in the terminology of Bailey and Brown [1962]). It is unlikely that he was dealing with two-layer structures, however, because of the rarity of regular two-layer sequences. Most of his other theoretical structures do not correspond to true chlorite structures, because they do not allow optimum hydrogen bondihg between adjacent 2 1 layer and interlayer sheet surfaces. 

Brindley and Gillery observed that the 00/ intensities from an Fe-rich daphnite specimen from Cornwall were not in accord with a true chlorite structure. One-dimensional Fourier syntheses indicated the tetrahedral Si and O peaks to be unexpectedly low and broad and to extend closer to the interlayer sheet than normal. A model was postulated in which approximately one-third of the tetrahedra is inverted to link with the interlayer sheet instead of with the silicate octahedral sheet. This has the effect of changing a chlorite 14 A unit into a 7 A layer at the point of inversion, so that the structure can be described as a mixed layer 7-14 A structure. The inversion can be accomplished physically by shifting a Si atom to the opposite side of its basal triad and completing tetrahedral coordination with an interlayer anion. The reverse of this process may be the mechanism of the hydrothermal transformation of 7 A aluminian serpentines to chlorites. 

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. 

In regular two-layer chlorite structures, the second 2 1 layer no longer adopts the same orientation as that of the initial layer. Within each 2 1 layer, the upper tetrahedral sheet can be... 

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. 

Warshaw [1960] and Koizumi and Roy [1959] have reported the hydrothermal synthesis of 14 A Al-chlorite between 475° and 600°C. No Mg was present in the systems investigated, so that the chlorite would have to be dioctahedral in both sheets. Koizumi and Roy point out that this phase was not observed in the pure Al203-Si02-H20 system by Roy and Osborn [1954] and that some K" or Na" seem to be essential for its formation. The 00/ intensities listed by Koizumi and Roy for their material do not agree with a true chlorite structure. The absence of a 7 A (002) reflection suggests that the material is either vermiculitic or does not have complete interlayers. 

Figure 16. Projection on XY plane of four chlorite structural types showing ditrigonal rings after tetrahedral rotations. (From Shirozu and Bailey [1965].)...

The different layer-interlayer-layer sequences that are possible in the chlorite structure create varying amounts of cation-cation repulsion and cation-anion attraction as a result of the different superpositions of sheets. Bailey and Brown [1962] and Shirozu and Bailey [1965] have attempted to explain the observed relative abundances of the chlorite structural types according to the relative stabilities indicated by these interatomic forces. Repulsion between the superposed interlayer and tetrahedral cations in the la and Ila structural units is considered the most important single factor in reducing the stability of these two unit types relative to the lb and lib units. Three other factors considered are ... 

It is necessary to consider not only the interatomic forces within each type of chlorite structural unit, but also how the manner of stacking of individual layers to form the six semirandom structures or the several regular polytypes may affect these forces. Figure 17 illustrates in [010] projection the layer sequences and some of the vertically superposed attractive and repulsive forces in these structures. It can be seen from this figure that the interatomic forces are symmetrical for four of the structures, so that the stability ratings above for the structural unit types should apply equally well to the three-dimensional structures. In terms of the distribution of interatomic forces, however, the lb (j8 = 97°) and Ila (/3 = 90°) structures are best considered as regular alternations of la and lb units and of Ila and lib units, respectively. The stabilities of these two structures should be functions of those for the unit types involved. The chart below summarizes the stacking sequences for the six structures and compares their observed abundances with the stabilities of the structural units involved. 

Figure 18. Powder photographs of four different chlorite structural types. Monochromatized FeKa radiation, 114.6 mm camera, (a) II6, = 97° (b) la, p = 9T (c) lb, P = 9T (d) lb, p = 90°.

Chlorite Structure B G Petruk Schoen 0,S,S Analysis B G Petruk Schoen 0,S,S... 

Hydrothermal syntheses at low temperature and pressure by Yoder [1952], Roy and Roy [1955], Nelson and Roy [1954, 1958], and Cillery [1959] have demonstrated that the 7 A serpentine structure can be formed for any composition between chrysotile and amesite. At higher temperatures and pressures, above 400 to 500°C and 10,000 to 15,000 Ib/in., aluminian serpentines between penninite and amesite in composition invert slowly to the 14 A chlorite structure. It is not certain whether the lower-temperature 7 A structures are thermodynamically stable or metastable. Phase equilibrium diagrams for both possibilities have been given by Nelson and Roy [1958], shown here as Figure 26. 

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