Trioctahedral structure

Drits and Karavan [1969] also have derived the possible two-layer chlorite polytypes and their diffraction characteristics. They recognize only 148 different trioctahedral structures, in contrast to the results of Lister and Bailey cited above. The structures are described by the analytical symbols developed by Zvyagin. Dioctahedral and di,trioctahedral chlorites are included. 

Fig. 8.2 PXRD pattern of ethlyenediamine-functionalized magnesium phyllosilicate showing reflections indexed according to the 2 1 trioctahedral phyl losi I icate structure of talc.

Fig. 20. Polyhedral representation of the structure of [H2W12O42]10 (paratungstate B) showing the location of the protons in the center. The structure is built from two different types of trioctahedral subunits.

Table 5.52 Structural parameters for some trioctahedral and dioctahedral micas (from Smyth and Bish, 1988).

The 2-3 subscript for the B site in the formula expresses the fact that there are two families of mica structures, the dioctahedral and trioctahedral micas, based on the composition and occupancy of the intralayer octahedral sites. The trioctahedral micas have three divalent ions—for example, Mg or a brucitelike [Mg(OH)2] intralayer, and the dioctahedral group—two tri-valent ions—for example, Al or a gibbsitelike [AlfOHfa] intralayer, between the tetrahedral sheets. In the dioctahedral micas, therefore, one-third of the octahedral sites are vacant or unoccupied (Fig. 2.12C). 

Fig. 2.12 Structural components and variations in the micas. (A) Plan view of the continuous aluminosilicate sheet (T), [Si,Al205] , a portion of the mica structure. (B) Stereographic representation of an idealized mica. The structure is composed of continuous layers containing two tetrahedral aluminosilicate sheets (T) that enclose octahedrally coordinated cations, or Mg (O). This layer or sandwich," the T-O-T or 2 1 aggregate, is held together by or Na ions. (C) The two possible positions (I and II) of octahedral cations in the micas. Sets of three locations for each are superimposed on the tetrahedral hexagonal aluminosilicate sheet. (D) The three possible directions of intralayer shift when octahedral set I (upper) or II (lower) are occupied. The dashed lines and circles represent ions below the plane of the paper. (E) Distorted hexagonal rings of apical oxygens in the tetrahedral sheet of dioctahedral micas compared with the undistorted positions of the apical oxygens in the tetrahedral sheet of trioctahedral micas.

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 ... 

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). 

The type phengite suggested by Foster (1956) has a composition almost identical to that of the Belt illite (No.8). When the H20 content is appreciably higher and the K20 content lower than muscovite, the minerals have been called hydromicas or hydromuscovites. The excess H20 in some instances is present as interlayer water, particularly in the trioctahedral hydrobiotites. Table XII contains a selection of sericite and hydromuscovite analyses and Table XIII the structural formulas. The H20 and K20 values of these minerals are similar to those reported for the illite minerals however, the MgO content of the sericites and hydromuscovites is lower and the NazO contents higher than for the illites (Table XIV). 

The maximum amount of Al3+ tetrahedral substitution that 2 1 clays minerals formed at low temperatures can accommodate appears to be 0.80—0.90 per four tetrahedra. While this appears to place an upper limit on the amount of R3+ octahedral substitution, it is not clear why the limit should be such a low value. The dioctahedral smectites can accommodate more substitution (R2 + for R3+) in the octahedral sheet than can the dioctahedral micas. The reverse situation exists for trioctahedral equivalents. In the latter clays octahedral R3+ increases as tetrahedral Al increases. Thus, as one sheet increases its negative charge, the other tends to increase its positive charge. This is likely to introduce additional constraints on the structure. In the dioctahedral clays substitution in either sheet affords them a negative charge and substitution in one sheet is not predicted by substitution in the other sheet thus, one might expect more flexibility. 

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.)... 

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 ... 

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 ... 

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. 

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