Chlorite dioctahedral

Vermicuhte is an expandable 2 1 mineral like smectite, but vermiculite has a negative charge imbalance of 0.6—0.9 per 02q(0H)2 compared to smectite which has ca 0.3—0.6 per 02q(0H)2. The charge imbalance of vermiculite is satisfied by incorporating cations in two water layers as part of its crystal stmcture (144). Vermiculite, which can be either trioctahedral or dioctahedral, often forms from alteration of mica and can be viewed as an intermediate between UHte and smectite. Also, vermiculite is an end member in a compositional sequence involving chlorite (37). Vermiculite may be viewed as a mica that has lost part of its K+, or a chlorite that has lost its interlayer, and must balance its charge with hydrated cations. 

Trioctahedral chlorite occurs commonly in geothermal and hydrothermal areas, whereas the occurrence of dioctahedral chlorite is very limited. For instance, donbasite... 

H20(a) = diaspore, gibbsite, serpentine, dioctahedral micas H20(b) = chlorites, talc, trioctahedral micas, amphiboles... 

In zones of hydrothermal alteration it is apparent that the formation of dioctahedral montmorillonites is limited by temperature. They almost never occur in the innermost zone of alteration, typically that of sericitization (hydro-mica or illite), but are the most frequent phase in the argillic-prophylitic zones which succeed one another outward from the zone where the hydrothermal fluid is introduced in the rock. Typically, the fully expandable mineral is preceded by a mixed layered phase (Schoen and White, 1965 Lowell and Guilbert, 1970 Fournier, 1965 Tomita, et al., 1969 Sudo, 1963 Meyer and Hemley, 1959 Bundy and Murray, 1959 Bonorino, 1959). However, temperature is possibly not the only control of expandable clay mineral occurrence, the composition of the solutions and the rock upon which they act might also be important. It is possible that high magnesium concentrations could form chlorite, for example, instead of expandable minerals. 

Figure 27. Proposed phase relations for the expanding and mica-like dioctahedral phases, a) low temperatures (less than 100°C) b) moderate temperatures (100-200°C) Kaol = kaolinite ML = mixed layered illite-beidellite or illite-montmorillonite M03 = trioctahedral expandable phases Chi = chlorite I = illite b = beidellite m = montmorillonite (dioctahedral]...

Above 200-220°C only illite or sericite is found, usually with chlorite. No dioctahedral mixed layered phase is present. This is the "illite-chlorite" zone. 

Considering the compositions of the mixed layered minerals found in sedimentary rocks (Figure 25) it is obvious that magnesian-iron expandable dioctahedral minerals will be in equilibrium not uniquely with kaolinite but also in many instances with a magnesian-iron phase—either chlorite or an expanding trioctahedral mineral. In such a situation the slope in... 

If we look back to the experimental studies on natural expandable minerals at high pressures, it can be recalled that the production of a chlorite-phase occurred when interlayering in the natural dioctahedral mineral had reached about 30% interlayering. It is possible that below this transition only expandable phases are present for most magnesium-iron compositions one is dioctahedral, the other would be trioctahedral. Thus, at temperatures below the transition to an ordered allevardite-type phase, dioctahedral mixed layered minerals will coexist with expandable chlorites or vermiculites as well as kaolinite. The distinction between these two phases is very difficult because both respond in about the same manner when glycollated. There can also be interlayering in both di- and... 

In the magnesian system, 7 8 chlorite can coexist with talc, magnesian trioctahedral and dioctahedral montmorillonite, boehmite and brucite. A 14 8 chlorite can coexist with magnesian montmorillonite, talc, quartz, kaolinite, boehmite and brucite. It is important to note that 7 8 aluminous chlorites do not stably coexist with quartz or a free silica phase. 

Corrensite-mixed layered illite montmorillonite-illite Corrensite-chlorite-illite-trioctahedral montmorilIonite Corrensite-chlorite-illite-dioctahedral montmorilIonite-talc. 

Two phase assemblages of any of these minerals are known. It should be noted that aluminous phases, such as kaolinite, have never been reported with corrensite neither are sedimentary phyllosilicates such as 7 8 chlorite or glauconite. Non-phyllosilicates in association with corrensite frequently include diagenetic quartz, albite and dolomite. Pelitic rocks, specially associated with those containing corrensite, contain allevardite and fully expanding montmorillonite (dioctahedral). 

From the experiments it seems that the corrensite type mineral is converted to 14 8 chlorite when a four layer dioctahedral ordered mineral... 

This possibility is due to the non-equivalence of Mg and Fe which segregate into corrensite and chlorite respectively. This effect is discussed in the chlorite chapter. Thus four major phyllosilicate phases could be present in an equilibrium situation. It should be noted that the expanding trioctahedral phase is or can be more aluminous than chlorite. This might lead one to think that some of the layers might in fact be dioctahedral such as those in sudoite. The importance of the differentiation of the two types of mixed layered minerals lies in the segregation of alumina and potassium in one (the dioctahedral mixed layered mineral)... 

Figure 41. Phase diagram for the extensive variables R -R -Si combining the data for synthetic magnesian chlorites and the compositional series of natural sepiolites and palygorskites. Numbers represent the major three-phase assemblages related to sepiolite-palygorskite occurrence in sediments. Chi = chlorite M03 = trioctahedral montmorillonites M02 = dioctahedral montmorillonite Sep = sepiolite Pa = palygorskite Kaol = kaolinite T = talc.

If we consider three components, the phases will be arranged as in Figure 48a at conditions of initial burial. The solid solution series are somewhat abbreviated for simplicity. The phase relations are dominated by fully expanding and mixed layered minerals which cover a large portion of the compositional surface. Notably two dioctahedral expandable minerals exist as does a large undefined series of trioctahedral phases designated as expanding chlorite, vermiculite and trioctahedral montmorillonite. 

The second facies is marked by the instability of the fully expanding dioctahedral phases and the existence of a kaolinite-illite tie-line (Figure 48b). In this facies the siliceous alkali zeolites (other than analcite) become unstable, the compositional range of the trioctahedral expanding phases is reduced and aluminous 14 8 chlorite-"allevardite"... 

V is characterized by kaolinite-illite-chlorite assemblages beyond the stability of an expanding mixed layered potassic dioctahedral mineral and below the thermal stability of pyrophyllite. The establishment of such conditions will be difficult in that the non-appearance of a mineral is a poor diagnostic and, as we have seen, kaolinite is frequently eliminated from sediments before its upper stability limit in the presence... 

Once the illite-chlorite zone is entered, i.e., the facies where dioctahedral mica-montmorillonite mineral solid-solutions are no longer stable, how does the assemblage change into muscovite-chlorite The major... 

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. 

At a temperature near 150°C kaolinite starts to decompose and the Al as hy-droxy-Al moves into the interlayer position increasing the proportion of dioctahedral chlorite layers. At this stage some of the chlorite layers form packets with a sufficient number of layers to diffract as the discrete mineral chlorite. Some additional Al may move into the tetrahedral sheet at this stage and some packets of 10A layers form (the K derived from K-feldspar). Thus, the amount of discrete 10A illite and dioctahedral chlorite has increased slightly but the majority of the clay consists of a mixed-layer illite-chlorite with a lesser amount of montmorillonite. 

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... 

Complete unmixing of the mixed-layered minerals requires temperatures on the order of 400°C or a relatively high degree of metamorphism (phengite -> muscovite + chlorite). Velde (1964), Weaver (1965), Raman and Jackson (1966), and Weaver and Beck (1971a) have presented data to indicate chloritic layers are present in most, if not all, 10A illites. Based on chemical data, Raman and Jackson concluded that the illites they had examined contained 20—29% chlorite layers. Weaver and Beck believe that most of the chloritic layers are dioctahedral. 

Most of the chlorite-like material formed in soils is dioctahedral rather than trioctahedral. In the process of weathering, illite and muscovite are stripped of their potassium and water enters between the layers. In these minerals and in montmoril-lonites and vermiculites, hydroxides are precipitated in the interlayer positions to form a chlorite-like mineral (Rich and Obenshain, 1955 Klages and White, 1957 Brydon et al., 1961 Jackson, 1963 Quigley and Martin, 1963 Rich, 1968). Al(OH)3 and Fe(OH)3 are likely to be precipitated in an acid to mildly basic environments and Mg(OH)2 in a basic environment. The gibbsite sheets in the soil chlorites are seldom complete and the material resembles a mixed-layer chlorite-vermiculite. The gibbsite may occur between some layers and not between others or may occur as islands separated by water molecules. 

Lazarenko (1940) described a hydrothermal aluminum-silicate mineral from the Donetz Basin which he called donbassite. This material is a variety of dioctahedral chlorite and the Nomenclature Committee (Bailey et al., 1971) considers that it has priority. 

Although partially organized dioctahedral chlorites form readily in soils, there are relatively few reported in sedimentary rocks. (More dioctahedral chlorites probably exist than have been recognized.) Swindale and Fan in 1967 reported the alteration of gibbsite deposited in Waimea Bay off the coast of Kauai, Hawaii, to chlorite but no data were obtained on its composition. Dioctahedral chloritic clays have been reported forming in recent marine sediments however, the identification is indirect and the interlayer material is relatively sparse (Grim and Johns, 1954). 

Weaver (1959) noted that the chlorite, which is a common constituent of the Ordovician K-bentonite beds of the eastern United States, has a dioctahedral 2 1 layer and a trioctahedral hydroxide sheet. A partial chemical analysis indicated the chlorite contained less than 2% Fe203. Both layers were probably formed in place from the alteration of volcanic ash in a marine environment. Only one other chlorite of this type had been detected in X-ray patterns of approximately 75,000 samples of sedimentary rocks. The other sample was from a Paleozoic argillaceous limestone at a depth of 24,400 ft. in Oklahoma. Chlorites of this type might well go undetected when chlorite is only a minor component. 

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. 

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