Trioctahedral clay chlorite

It is difficult to obtain sedimentary chlorites of sufficient purity for reliable chemical analyses however, reasonably accurate estimates of chemical composition can often be made from X-ray data. If the difference in scattering power and size of the various atoms in the chlorite species is sufficiently large, the positions and intensities of X-ray reflections are measurably affected. The b and c parameters are the most useful. 

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  

As might be expected, Fe2+ is considerably more important than the smaller Fe3+ and Al ions. 

Some of the high Fe values may be real. During weathering under neutral to acid conditions, the Mg-rich brucite sheet tends to be stripped out and removed from the immediate environment. If new material is precipitated between the talc layers, it is more apt to be Fe and Al than Mg. As chlorites go through the sedimentary cycle, perhaps several times, their average Fe and Al content will tend to increase. 

Many high-iron chlorites associated with sedimentary iron formations apparently have formed by direct precipitation. These have compositions similar to chamosite (see Table LXX1I) and occur with both a 7 A and 14 A structure (Brindley, 1961a). 

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Trioctahedral clay chlorite is an abundant constituent of soils formed by the weathering of basic volcanic pumice and tuffs in North Wales (Ball,1966). The adjusted chemical analysis (29.35% Si02, 16.82% A1203, 4.42% Fe203, 15.08% FeO, 0.25% MnO, 21.54% MgO, 12.00% H20+, 0.54% H20 ) produces the following structural formula ... 

Foster (1962) calculated the structural formulas for 150 selected chlorite analyses. These formulas indicate that the Si content ranges from 2.34 to 3.45 per four tetrahedral positions. Most samples fall in the 2.40-3.20 range (Fig. 17), the distribution being highly skewed towards the higher Si values. Most chlorites tend to have a much higher tetrahedral A1 content than 2 1 clays. (Some of the 1 1 trioctahedral clays are the only clay minerals with tetrahedral A1 contents as high as that of most chlorites.)... 

There is nothing to indicate whether there is any difference between the tetrahedral and octrahedral sheets of the chlorite layer and the montmorillonite layer. In the trioctahedral clays, Mg is the dominant (60—90%) octahedrally coordinated cation. In the dioctahedral clay, A1 is the dominant octahedral cation. 

The development of vermiculite minerals in soils at the expense of micas is now well established as a common phenomenon, more particularly by the work of Jackson and his collaborators e.g., Jackson et al. [1952], Schmehl and Jackson [1956], Jackson [1959,1963], Brown and Jackson [1958]) as well as by others e.g., Fieldes and Swindale [1954], Rich [1958], Cook and Rich [1962], Millot and Camez [1963], Nelson [1963]). In spite of the frequent occurrence of dioctahedral clay vermiculites in soils, dioctahedral clay micas, in general, appear to resist decomposition better than their trioctahedral counterparts and, where direct comparison is possible, the dioctahedral type may remain unaffected, whereas the trioctahedral mica in the same profile is almost completely altered (Mitchell [1955]). Vermiculitelike minerals, however, may also develop in soils by other routes, for example, from montmorillonite (Bundy and Murray [1959], Jackson [1963]) or from chlorite (Droste and Tharin [1958], Brown and Jackson [1958], Droste et al. [1962], Millot and Camez [1963]). Such alterations are reversible, and they depend on a chemical equilibrium between the mineral and the soil solution. Hence clay chlorites, illites, and montmorillonites may develop from clay vermiculites in an appropriate environment, and intermediate types are common. The alteration of clay vermiculites to kaolinite in podzols has also been proposed (Walker [1950], Brown [1953], Jackson et al. [1954], McAleese and Mitchell [1958a]). 

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. 

With increasing temperature ( 200°C), either due to deeper burial or increase in heat-flow rates, upward migrating K, Mg, and Fe, derived from the underlying sediments, become sufficiently abundant that the remaining expanded layers are lost and some discrete 10A illite (2M) and trioctahedral chlorite are formed however, much of the illite at this stage still contains an appreciable proportion of dioctahedral chlorite and the chlorite contains some 10A layers. This is the typical clay-mineral suite... 

Vermiculite and vermiculite layers interstratified with mica and chlorite layers are quite common in soils where weathering is not overly aggressive. (A few references are Walker, 1949 Brown, 1953 Van der Marel, 1954 Hathaway, 1955 Droste, 1956 Rich, 1958 Weaver, 1958 Gjems, 1963 Millot and Camez, 1963 Barshad and Kishk, 1969.) Most of these clays are formed by the removal of K from the biotite, muscovite and illite and the brucite sheet from chlorite. This is accompanied by the oxidation of much of the iron in the 2 1 layer. Walker (1949) has described a trioctahedral soil vermiculite from Scotland formed from biotite however, most of the described samples are dioctahedral. Biotite and chlorite with a relatively high iron content weather more easily than the related iron-poor dioctahedral 2 1 clays and under similar weathering conditions are more apt to alter to a 1 1 clay or possibly assume a dioctahedral structure. 

Tetrahedral Al3 + values range from 0.10 to 0.76 per two tetrahedral positions and average 0.49 for the ten selected samples. The amount of tetrahedral substitution of the larger Al3+ for the smaller Si4+ is appreciably greater than has been reported for the 2 1 clays, although the higher values are in the range (0.9 1.5 per four tetrahedral positions) reported for the trioctahedral micas (Foster, 1960) and chlorites (Brown and Bailey,1962). 

Brigatti MF, Poppi L (1993) Crystal chemistry of Ba-rich trioctahedral micas-IM Etrr J Mineral 5 857-871 Brigatti MF, Lalonde AE, Medici L (1997) Crystal chemistry of Fe3+-rich phlogopites A combined singlecrystal X-ray and Mossbauer study. Proc. 11 Int. Clay Conf. Ottawa, Canada, 317-327 Brindley GW, Oughton BM, Robinson K (1950) Polymorphism of the chlorites. 1. Ordered stmctrues. Acta Crystallogr 3 408-416... 

Throughout the areas studied we observe in rocks of different granulometry, from sandstones to rocks of the terrigenous-chemical complex, primary clay mineral associations. The main components of the associations are a number of varieties of Mg-chlorites, primary hydromicas grouped together under the term ferruginous illites, trioctahedral swelling minerals and mixed-layer clays. 

The results obtained on the association of the clay minerals as well as on the crystal-lochemical pecularities of the Triassic deposits in the Triassic Province lead us to conclude that the association Mg-chlorite + swelling trioctahedral mineral + Fe-illite may be interpreted as an indication of the dolomite-sulfate stage of the salinization of a sedimentary basin of the terrigenous-chemical type. 

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. 

Trioctahedral Chlorite. This is encountered in virtually all fine-grained fractions of the sections studied. There are no swelling layers in the structure of this mineral which exhibits a high degree of crystallinity. The d(o6o) reflex at 1.532 A indicates that the chlorite is trioctahedral. There are no mixed-layer clays of the chlorite-montmorUlo-nite type, nor swelhng chlorites nor, in view of the lack of any other structure, any minerals containing hydrate-sheets between their layers. 

The components making up the fine-grained fractions of the terrigenous-halitic complex are essentially Mg-rich trioctahedral well-crystallized chlorites without swelling layers as well as Fe-illites, the structure of which does not contain swelling layers (see model in Fig. 2.10). At the same time there are no mixed-layer species of the chlorite-montmorillonite type which were so characteristic of the carbonate-terrigenous complex (dolomite-sulfate facies). Mixed-layer clays of the illite-montmoriUonite type also diminish throughout the complex. 

Also, these authors 1968] have made a detailed study using X-ray, chemical, and IR absorption methods of podzol clays from New Brunswick. They found mica-vermiculite-smectite in the Ae layer, and dioctahedral mica (illite) with trioctahedral chlorite in the C layer. Fourier transforms showed that the proportion of hydrated layers and randomness of stacking in the Ae layer are related to the degree of weathering. 

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