Trioctahedral minerals chlorites

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... 

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. 

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).

Berthierine, as shown by Brindley (1982) is essentially a trioctahedral mineral, following the line of trioctahedral chlorites in Figure 7. In our simulations of the XRD spectra of odinite, we use a ferrous serpentine and a ferric dioctahedral smectite component. Translated into constituent ions of a mineral structure, this mineral combination will give a bulk average composition between nontronite (ferric, dioctahedral smectite) and berthierine (trioctahedral chlorite). 

If we look back to the singular chemical features of odinite as having large amounts of ferric iron present and having an overall low occupation of the octahedral site for a trioctahedral mineral, a mixed layered mineral of ferric, dioctahedral smectite (nontronite), and berthierine (ferrous 7 A chlorite) would give the overall chemical characteristics of odinite. Thus, one can fit the XRD data by using a ferric... 

There is not much resistance to weathering in these minerals because of the relative lack of Si—O—Si bonding, especially in island silicates such as olivine. Layer silicate minerals rich in Mg (e.g., trioctahedral smectites, chlorite, serpentine) may form from the siliceous residue if leaching does not deplete in the weathering zone. 

Modal spaces for some common pelitic schist assemblages characteristic of the lower grades of metamorphism will be presented below. The minerals involved will be quartz, dioctahedral mica, trioctahedral mica, chlorite (14-., garnet, and chloritoid. The formulas of the additive components for these will, to save space, be written in the equations as Qtz, Dim, Bio, Chi, Gar, and Ctd, respectively. The additive components may then all be chosen so as to lie in the space defined by K20-Mg0-Al203-Si02-H20, as follows ... 

Crystallochemical and typomorphic data enable us to propose a scheme for the formation of the main compound of this complex on the following path Mg-mont-morillonite trioctahedral swelling mineral chlorite with structural defects —1 Mg-chlorite (see model in Fig. 2.10). The factor responsible for the diversity of the intermediate phases of this series is the degree of establishment and perfection of the brucite layers and not the ordering in distribution of the interfoliar intervals between mont-mordlonite and swelling chlorite. 

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. 

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. 

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. 

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... 

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)... 

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. 

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.)... 

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. 

The definition of the verdine facies is largely due to the work of Odin (1988). Bailey (1988) defined the mineral odinite. Verdines and odinite span much of the range of octahedral site occupation from 2.5 to 2.0 ions, containing much ferric iron and alumina. Assuming that all of the minerals are 7 A structures, the difference in observed composition of the different phases can be illustrated by their octahedral cation occupancy AI2 kaolinite (dioctahedral) (Mg, Al)3 2.5 7 A trioctahedral chlorite and (Mg, Al)2 2.5 odinite (di-, trioctahedral). 

Hazen RM, Finger LW (1978) The crystal stractures and compressibilities of layer minerals at high pressure. 11. Phlogopite and chlorite. Am Mineral 63 293-296 Hazen RM, Finger LW, Velde D (1981) Crystal stracture of a sihca- and alkali-rich trioctahedral mica. Am Mineral 66 586-591... 

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... 

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