Chlorite octahedral sheet

Vermiculites exist in various stages of dehydration. Because of the similar dimensions of the water-cation layer in vermiculite and the brucitelike layer in chlorite, vermiculites can be confused with the chlorites. The common substitutions of Fe" or Fe for Mg (in either the water or octahedral sheet of vermiculites), and AF for Si (in the tetrahedral sheets), as well as the hydration variations, present enormous potential for structural distortion in these types of minerals. Fibrous vermiculite was described by Weiss and Hofmann (1952). 

The chemistry of the chlorites has been reviewed by Hey (1954), Foster (1962) and Deer et al. (1962). Hey and Foster have presented classification schemes. All chlorites have replacement of Si by Al which affords the tetrahedral sheets a net negative charge. This charge is balanced by the substitution of Al and Fe3+ for Mg and Fe2+ in the two octahedral sheets. 

A classification of the chlorites was devised by Foster (1962) based on ionic replacement of Al by Si in the tetrahedral sheet and Mg by Fe2+ in the octahedral sheet. Fig. 17 shows the classification with the location of 150 chlorites. The dividing lines are arbitrary and imply no genetic significance in fact, they probably have some. 

In three of the four structures that were determined in detail the 2 1 octahedral sheet and the hydroxide octahedral sheet show a difference in cation population. In three of these specimens all or most of the octahedral charge (R3+) is located in the hydroxide sheet. In the other the positive charge is split between the two octahedral sheets. In two of these chlorites (Steinfink, 1958a Brown and Bailey, 1963) the A1 in the tetrahedral sheet appears to have an orderly rather than a random distribution. 

The 060 reflections of trioctahedral chlorite range from 1.53 to 1.55 A and vary linearly with compositional variations in the octahedral sheet. Two formulas for the b parameter are ... 

A number of Al chlorites in which both octahedral sheets are dioctahedral have recently been described. Dioctahedral Al chlorites have been reported in bauxite deposits (Bardossy, 1959 Caillere, 1962). These chlorites appear to have been formed by the precipitation-fixation of Al hydroxide in the interlayer position of stripped illite or montmorillonite. A similar type of chlorite, along with dioctahedral chlorite-vermiculite, occurs in the arkosic sands and shales of the Pennsylvanian Minturn Formation of Colorado (Raup, 1966). Bailey and Tyler (1960) have described the occurrence of dioctahedral chlorite and mixed-layer chlorite-montmorillonite in the Lake Superior iron ores. Hydrothermal occurrences have been described by Sudo and Sato (1966). 

The hydrothermal dioctahedral chlorites have considerably less tetrahedral substitution than those formed in sediments. The former would appear to be a stable phase and the latter a metastable phase. The tetrahedral composition of the hydro-thermal specimens is similar to that for the other dioctahedral clays and represents a reasonable fit between the tetrahedral and octahedral sheets. 

K is obtained from associated K-feldspars and micas. The layer charge is increased by the reduction of iron in the octahedral sheet and incorporation of Al, entering through the ditrigonal holes in the basal oxygen plane, into the tetrahedral sheets (Weaver and Beck, 1971a Pollard, 1971). Weaver and Beck have presented evidence that indicates mixed-layer clays formed in this manner contain 20—30% chloritic layers and are actually mixed-layer illite-chlorite-montmorillonite clays. 

If the silica sheets in a 2 1 Fe-rich clay had enough A1 substitution to adjust to the size of the octahedral sheet, this would most likely provide sufficient layer charge to cause contraction and the formation of a biotite mica, or the bonding of a positively charged brucite sheet and the formation of a chlorite. Low-temperature micas of this composition may well exist, but have not yet been recognized. It is not known if low-temperature chlorites of this composition can form. 

Several of the iron-rich clays as well as an amesite have a larger R3+ value than that allowed for the biotites. Nelson and Roy (1958) were not able to synthesize a 1 1 (amesite) or a 2 2 (chlorite) mineral with more than 33% A1 substituting for Mg (Mg8Al4) in the octahedral sheet. However, if the analyses in Fig. 24 are correct, it appears that when Fe2+ rather than the smaller Mg is the dominant cation in the octahedral sheet, up to 50% R3+ can occur in the octahedral position. Foster s (1960) biotite data show a similar relation. 

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. 

There are few low-temperature octahedral sheets with compositions that would plot in the center area of the compositional triangle. The low-temperature triocta-hedral chlorites and perhaps Fe2+-rich attapulgites contain octahedral sheets with compositions of this type. 

The amount of substitution in the tetrahedral and octahedral sheets and the ratio of octahedral to tetrahedral sheets are the primary differentiating characteristics between the many clay minerals (Fig. 3.6). For example, clays that have one tetrahedral sheet and one octahedral sheet are known as 1 1 clay minerals (e.g., kaolin group) (Fig. 3.7) clays that have two tetrahedral sheets and one octahedral sheet are known as 2 1 clay minerals (e.g., smectite group) (Fig. 3.8) or mica and vermiculite (Fig. 3.9), while clays that have two tetrahedral sheets and two octahedral sheets are known as 2 2 clay minerals (e.g., chlorite) (Fig. 3.10). These sheet arrangements give rise to various mineral surface identities such as magnitude (specific surface), functional groups, and interactions with solution species. 

An additional structural variant for clay minerals is the chlorite-type structure. Chlorites are similar to the pyrophyllite-type structures with two tetrahedral sheets and an octahedral sheet making up each layer. Instead of alkali or alkaline earth interlayer cations, chlorites contain a brucite (Al-Mg hydroxide) layer between successive pyrophyllite-type layers [18]. 

The tetrahedral sheet of chlorites contains isomorphic substitution by Al(III), and sometimes Fe(III) or Cr(III) is found (Kohut and Warren 2002). The octahedral sheet normally contains Mg(II), Al(III), Fe(II), and Fe(III), but some substitution by Cr(III), Ni(II), Mn(II), or other metals occurs as well. Chlorites, in contrast with vermiculites and smectites, are hardly expandable, because the hydroxide cations are not easily exchangeable, nor is water easily incorporated to the intermediate octahedral sheet. 

The structural unit of a chlorite mineral consists of a 2 1 layer with the negative charge balanced by a positively charged octahedral hydroxide sheet in the interlayer. Two different types of octahedral layers are therefore present, one within the 2 1 TOT layers and the other between them. In di- and trioctahedral chlorites, both types of octahedral sheets are di- or trioctahedral, respectively. Di,trioctahe-dral chlorites have 2 1 dioctahedral layers and trioctahedral interlayers (the reverse mineral is not known). A detailed discussion of specific chlorite minerals can be found in Ref 12. Refer to Figure 2 for the [010] crystallographic view of a chlorite and to Table 2 for the composition of clinochlore, one chlorite mineral. 

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. 

Foster found two series of ionic replacements in the chlorites, replacement of Mg by Fe " " and replacement of tetrahedral and octahedral Al by Si and Mg. Replacement of Mg by Fe in the octahedral sheets is ion for ion and does not cause any change in the layer charges. 

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. 

Von Engelhard et al [1962] have reported dioctahedral chlorite, mixed with quartz, kaolinite, and considerable interstratified illite-montmorillonite, in clay-marl sediments of the middle Keuper of Wurttemburg. The material does not expand on solvation. The rf(060) values for all the clay components are between 1.490 and 1.504 A. The tetrahedral Al content is 1.1 atoms, judged by the stated definitely whether the chlorite has two dioctahedral sheets or mixed di,tri-octahedral sheets. The 001 reflection at 14.23 A intensifies on heating at 550°C and decreases in spacing to 13.73 A. The interlayer material appears to be unstable, because /(OOl) decreases further to 11.9 A on heating to 700°C. 

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