Chlorite trioctahedral

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... 

Figure 2. Illustration of the possible displacement of a sedimentary bulk composition upon reduction of Fe + to Fe + during diagenesis (after Velde, 1968). I = illite Chi = chlorite Kaol = Kaolinite M03 = expanding trioctahedral phases 1 = initial bulk composition 2 = reduced bulk composition. Upper diagram shows initial assemblage and the lower diagram those after reduction of Fe +.

Roy and Romo (1957) and Boettcher (1966) performed high pressure experiments on natural vermiculites. They observed the production of a 14 X chlorite between 300 and 550°C, talc + enstatite and an unidentified phase above 650°C. The experiments on natural minerals indicate that vermiculite will occur when alkali content or activity in solution is low. This trioctahedral expanding phase is relatively stable at high pressures and temperatures as are interlayered minerals which are composed in part by such layers. It is not stable relative to montmorillonite at low emperature. 

Table 2 gives temperatures of montmorillonite stability which are established by the experiments reported. The most important criteria used is reaction reversal this lacking, length of the experiments and variety of starting material was taken into consideration. Two points are important among micas and other phyllosilicates only kaolinite, serpentine and muscovite are stable to very low temperatures. All trioctahedral 2 1 structures break down to expandable phases at low temperatures (bio-tites) or to 1 1 structures plus expandable phase (chlorites). 

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]...

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. 

In sum, one can say that 14 8 trioctahedral brucitic chlorite is largely unstable in most weathering environments, but aluminous soil chlorites are common under acid conditions. The bulk of chlorite found in sediments is certainly detrital in origin. 7 and 14 8 chlorites can be formed from 50°C upward in temperature until above 100°C where 14 8 chlorite becomes one of the most common minerals in sedimentary rocks. 

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

Because the compositions are basic, the expanding minerals are trioctahedral and they are apparently associated in all facies with chlorite. The occurrence of a regularly interstratified montmorillonite (saponite) -chlorite mineral, corrensite, is typified by an association with calcic zeolites and albite. Temperature measurement in the "hydrothermal" sequences at several hundred meters depth indicate that the ordered, mixed layered mineral succeeds a fully expandable phase between 150-200 C and this ordered phase remains present to about 280°C. In this interval calcium zeolites disappear, being apparently replaced by prehnite. The higher temperature assemblage above corrensite stability typically contains chlorite and epidote. 

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 32. Results of experiments on natrual minerals are schematically shown in Mr3-2R -3R coordinates. Kaol = kaolinite ML j = mixed layered beidellitlc mineral MLj, = mixed layered montmorillonitic mineral I = illite compositional field chi = chlorite Exp3 trioctahedral expandable-chlorite mixed layered mineral (expanding chlorite and corrensite).

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"... 

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... 

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.)... 

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 ... 

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 ... 

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