Montmorillonite trioctahedral

In the discussions that follow, the terms which will be used are montmorillonite = tetrasilicic expanding phase, dominantly octahedrally charged montmorillonites = the undefined expanding phase(s) dioctahedral or trioctahedral montmorillonite = smectite or saponite, respectively. 

SiO system under conditions of one atmosphere pressure and 80°C. It was found that the trioctahedral phase had a variable alumina content which was the result of variations in the composition of the chemical system of the experiment. Variations in pH (4-6) and the additions of Na or K ions did not seriously affect the phase relations of the montmorillonites. 

Velde (1963) synthesized several types of montmorillonites in the Si-Al-Mg-H O system at 2Kb pressure near 300°C. The trioctahedral forms have a substitutional series as follows ... 

If we consider the substitutions in the trioctahedral montmorillonite structure which give rise to a charge imbalance on the basic 2 1 structure, they can be considered as being two in kind ... 

Synthesis has produced phases of type 1) at low temperature (80°C). Experiments in the Mg0-Si02 H20 system demonstrate that type 1) is not stable at high temperatures (250°C). The montmorillonites contain either Na, Ca or valent ions, such as aluminum. In contrast, substitutions of type 2) were effected at both low and high temperatures, 80-600°C and to 1,000 atmospheres pressure. The interlayer ions in type 2) montnoril-lonites were Na, Ca or Mg. It appears then that a distinction is possible between the trioctahedral montmorillonites the non-aluminous form (stevensite) Jjeing restricted to low temperatures, aluminous forms can be stable to temperature in the range of metamorphism. 

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]...

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. 

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. 

Pelitic rocks investigated in the same areas where corrensites are formed during alpine metamorphism (Kiibler, 1970) revealed the absence of both montmorillonite and kaolinite but the illite or mica fraction was well crystallized as evidenced by measurement of the "sharpness" of the (001) mica reflection (Kiibler, 1968). This observation places the upper thermal stability of the expandable and mixed layered trioctahedral mineral assemblages at least 50°C. above their dioctahedral correlevants. This is valid for rocks of decidedly basic compositions where no dioctahedral clay minerals are present. 

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 40. Representation of Mg-Si-i O system in terms of the activity of Mg, H+ and aqueous silica (25°C and atmospheric pressure). Solid line boundaries are taken from Wollast, et al., (1968), dashed lines are deduced boundaries based upon the data of Siffert (1962). The appearance of sepiolite is found only above pH 8 and thus the log Mg +/H+ ratio is not valid for all Mg +-H+ values. There are no specified extensive variables or inert components in the system described. Br = brucite M03 = trioctahedral montmorillonites Sep = sepiolite T = talc.

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.

It is important to note that the tie-line between a dioctahedral and trioctahedral montmorillonite becomes possible. 

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. 

Figure 50. "Facies diagram" for phyllosilicates in pelitic rocks and sediments. Zones I to VI are discussed in the text. M02 = dioctahedral montmorillonites M03 = trioctahedral, fully expandable phases ML =...

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 average Al203/Mg0 ratio for 24 illites is 9.6 and for 101 montmorillonites 6.7. Attapulgite values range from 2.5 to 0.48. The ratios of octahedral Al/octahedral Mg are respectively 5.4, 4.3 and 1.8-0.4. Radoslovich (1963b) found that the 2M muscovite structure required a minimum of 1.7 of the three octahedral sites be filled with Al. The Al occurs in the two symmetrically related sites and the larger divalent cation occurs in the distinctive or unoccupied site. The lower limit of 1.7 Al is equivalent to 85% of the two symmetrically related or occupied sites being filled in a stable muscovite structure. A similar restriction is reported for the trioctahedral micas where an upper limit of 1.00 (R3++ R4+)per three sites was found by Foster (1960). 

Siffert (1962) reviewed the literature on low-temperature synthesis and reported the results of his own studies. In a saturated solution of silica with a SiC>2 /MgO mole ratio of 0.702 he was able to crystallize a trioctahedral montmorillonite at pH 11.3. Using a mole ratio of 1.432 he obtained talc at pH 12. From a Si02/MgO mole ratio of... 

MacEwan, D.M.C., 1947. The nomenclature of the halloysite minerals. Mineral Mag., 28 36-44. MacEwan, D.M.C., 1954. Cardenite a trioctahedral montmorillonite derived from biotite. Clay Miner., 2 120. 

What can also be seen in Table 1.2 is that each group is usually further divided into two series dioctahedral (D) and trioctahedral (T) according to the number of central positions of the octahedrons occupied by cations. In the trioctahedral minerals, every central position is filled, usually with Mg2+, while in the dioctahedral minerals, two-thirds of the central position is filled with Al3+ ions (e.g., montmorillonite Chapter 2, Figure 2.1). 

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