Trioctahedral sheet

The distribution of the cations in the octahedral sheet of both the 1 1 and 2 1 trioctahedral clay minerals is shown in Fig.24. The solid boundary lines are those established by Foster (1960) for biotite. 

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. 

With increased R3+ substitution, the positive charge in the octahedral sheet increases. This would increase cation-cation repulsion. The larger the cations the smaller would be the structural strain imposed by this repulsion. In addition, as the R3+ content of the octahedral sheet is increased, it is matched by an equivalent amount of R3+ in the tetrahedral sheet. As a- result of R3+ substitution, the tetrahedral sheet increases in size and the octahedral sheet decreases. Eventually the tetrahedral sheet 

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. 

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Chlorite can occur as a clay-sized mineral. Most consist of a 2 1 talc layer plus a brucite sheet. This forms a unit 14 A thick. Most chlorites are trioctahedral although a few dioctahedral chlorites have been found. Some chlorites have both dioctahedral and trioctahedral sheets. Because substitution can occur both in the 2 1 layers and in the brucite sheet, the chlorites have a wide range of compositions. The coarser grained chlorites have been analyzed and classified (Hey, 1954) but relatively little is known of the composition of sedimentary chlorites. 

Eggleston and Bailey (1967) published a study on dioctahedral chlorite and gave five examples of chlorites having a pyrophyllite-like layer and a brucite-like sheet (designated di/trioctahedral by the authors with the trioctahedral sheet including all species of chlorite with 5 to 6 octahedral cations per formula unit and dioctahedral 4 to 5 octahedral cations per formula unit). Identification of di/trioctahedral chlorites is indirectly accomplished. Eggleston and Bailey stated that identification depends on the intermediate value of c (060), on chemical analysis of impure material, and on the ideal compositions of the recrystallization products of static heating . The composition of one such chlorite for which they refined the structure is ... 

They explained the fit of the dioctahedral and trioctahedral sheets by the thinning of the dioctahedral sheet (2.05 A) and the thickening of the trioctahedral sheet (2.15 A), giving a mean lateral octahedral edge of 3.02 in both sheets. To compensate for the thinning of the dioctahedral sheet the tetrahedral sheet thickened slightly. The tetrahedral rotation is in the same direction as for the other chlorites, but the angle is somewhat less than expected. 

In some weathering environments, Fe and Mg are ejected from the mica structure to counter the excess positive charge built up in the trioctahedral sheet as Fe " is oxidized. In the process, Mg may form a Mg(OH>2 sheet between the 2 1 layers, forming chlorite as an intermediate weathering product. 

The ideal composition of tungusite differs from that of gyrolite by the presence of six divalent cations, which complete the (001) X sheet of which the dimensions a b 9.72 A, y = 120°, thickness about 2.8 A) correspond to those of a 3 X 3 trioctahedral sheet. 

Micas are layer silicates (phyllosilicates) whose structure is based either on a brucite-like trioctahedral sheet [Mg(OH)2 which in micas becomes Mg304(0H)2] or a gibbsite-like dioctahedral sheet [Al(OH)3 which in micas becomes Al204(0H)2]. This module is sandwiched between a pair of oppositely oriented tetrahedral sheets. The latter sheet consists of Si(Al)-tetrahedra which share three of their four oxygen apices to form a two-dimensional hexagonal net (Fig. 1). In micas, the association of these two types of sheet produces an M layer, which is often referred as the 2 1 or TOT layer. 

Takeuchi Y, Haga N (1971) Structural transformation of trioctahedral sheet sihcates. Shp mechanism of octahedral sheets and polytypic changes of micas. Mineral Soc Japan Spec Paper 1 74-87 (Proc IMA-lAGOD Meetings 70, IMA Vol)... 

In the same structural category, three-layer minerals are found in which the octahedral Al or Mg layer is sandwiched between two Si04 tetrahedral layers, the free apices of which point towards each other (Figure 2.4). Talc, Mg3[(OH)2/Si40,o] is one common example of a three-layer trioctahedral sheet silicate, whereas pyro-phyllite, Al2[(OH)2/Si40io] is the three-layer equivalent of kaolinite with dioctahedral nature (Table 2.2). 

If the cation in a dioctahedral sheet is trivalent aluminum the mineral is kaoli-nite with the formula AI2 Si2 O5 (OH)4 Fig. 3. If the cation in a trioctahedral sheet is magnesium, the resulting mineral is serpentine, Mg3 Si2 O5 (013)4. Because of the structural mismatch between the Mg octahedral layer and the Si tetrahedral layer these minerals are often tubular or fibrous, and in many cases carcinogenic if inhaled in substantial quantities. 

Chemically, the chlorites are hydrous silicates incorporating medium-sized cations, primarily Mg, Al, and Fe, but occasionally Cr, Mn, Ni, V, Cu, and Li. There is a continuous solid solution series between the Mg and Fe varieties. Al substitutes for Si between the approximate limits 0.5 to 1.8 atoms per 4 tetrahedral positions. Most chlorites are trioctahedral and belong to the Mg-Fe series. Dioctahedral chlorites are rare, as are intermediate forms that combine one trioctahedral sheet with one dioctahedral sheet. 

Comparison of observed 00/ and 20/ intensities for the Michigan material with intensities calculated for a lib structural unit shows best agreement for a model having a dioctahedral 2 1 layer and a trioctahedral interlayer (Eggleton and Bailey [1967]). Least-squares refinement of this model shows that structural accommodation between the trioctahedral interlayer and the dioctahedral 2 1 layer is reached by thickening the trioctahedral sheet considerably and by thinning the dioctahedral sheet slightly, so that the lateral dimensions of the two sheets are identical. 

Schultz [1963] has described the occurrence of 20 samples of aluminian chlorite from sediments in Triassic rocks of the Colorado plateau. The rf(060) values are between 1.50 and 1.51 A, and the 003 reflection is considerably more intense than should be the case for a trioctahedral chlorite. A chemical analysis of impure material shows slightly more AI2O3 than MgO and, according to allocation by the present writer, yields an octahedral total close to five atoms. These data suggest the presence of mixed di,trioctahedral sheets, but further details are needed for verification. 

It is likely that the amount of tetrahedral rotation is governed primarily by the misfit within the 2 1 layer, but that the interlayer sheet has a secondary and modifying influence on a and on b (obs.). This influence can be seen especially well in the dioctahedral chlorites. Chlorites with two dioctahedral sheets have b (obs.) values near 8.94 A, similar to b (obs.) for the dioctahedral kaolin minerals. Most chlorites, believed to have a dioctahedral 2 1 layer, but a larger and trioctahedral interlayer sheet, have b (obs.) near 9.05 A so that the interlayer sheet is probably stretching the 2 1 octahedral sheet and reducing the amount of tetrahedral rotation required. The interlayer sheet itself must be compressed within the (001) plane and thickened. This misfit of 2 1 layer and interlayer sheets would be minimized by restricting the amount of tetrahedral substitution, as is often observed. Cookeite also has di,trioctahedral sheets, but b (obs.) is smaller and quite similar to b (obs.) for the kaolin minerals. The reason for this similarity is that the composition of the interlayer sheet in cookeite is 2A1 -1- ILi. The Li is just about the right size to fit in the octahedral site that is normally vacant in a kaolin structure. 

Ilb chlorite (di,trioctahedral sheets). Tracy mine, Michigan. 

Brindley and Gillery [1956] have calculated 00/ structure amplitudes for chlorites with two dioctahedral sheets and with mixed di,trioctahedral sheets (Table 14). Because the values for the two cases are not markedly different, they suggest using them in conjunction with the value of the b parameter. 

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