Smectite 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. 

The chemical composition of vermiculite can be quite variable (145). The megascopic varieties are generally trioctahedral, and the clay-si2e varieties contain both dioctahedral and trioctahedral varieties (144). Smectite minerals do not commonly occur as macroscopic single crystals. 

Chatteijee, A., Ebina, T., and Iwasaki, T. 2001. Best dioctahedral smectite for nitrogen heterocyclics adsorption—A reactivity index study. J. Phys. Chem. A 105 10694-10701. 

Drits VA, Manceau A. 2000. A model for the mechanism of Fe + to Fe + reduction in dioctahedral smectites. Clays and Clay Minerals 48 185-195. 

Stucki JW, Golden DC, Roth CB. 1984. Effect of reduction and reoxidation of strucmral iron on the surface charge and dissolution of dioctahedral smectites. Clays and Clay Minerals 32 350-356. 

The smectite group of clay minerals is also poorly crystalline but perhaps better known because of their cation exchange capacity and their occurrence in the bentonite clays. A general formula for montmorillonite, which is one of the dioctahedral smectites is... 

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. 

The dioctahedral subgroup is by far the most abundant. The layer charge on the expanded clays ranges from 0.3 to 0.8 per Oi0 (OH)2 unit of structure. The low-charged (0.3-0.6), expanded minerals are called montmorillonite, montmorillonids, and smectites, among others. Subdivision of the expanded clay group is still a problem. 

The analyses of dioctahedral smectites indicate that octahedral Fe and Al form a continuous isomorphous series and any subdivisions must be arbitrary. As the octahedral Al/Fe ratio increases, there is a tendency for octahedral Mg to increase (although... 

The upper limit for the number of R3+ cations in octahedral positions is essentially 0.50 or approximately 85% occupancy by R2+ cations. This compares with a minimum R2+ occupancy value of about 75% for trioctahedral micas (Radoslovich, 1963b) and 65% for dioctahedral smectites and illites. The composition of the octahedral sheet of the trioctahedral smectites falls within the compositional limits established for the trioctahedral micas (Foster,1960). All samples fall within the... 

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. 

Octahedral Mg, Al, and Fe3+ for the dioctahedral clays were totalled and the relative proportion calculated (atomic percent). Octahedral Mg, Al, Fe3+ and Fe2 + were totalled and the percent Fe2+ calculated. The distribution values for the 2 1 minerals are summarized in Fig.25. For comparison purposes, values for attapulgite and some trioctahedral smectites are also shown. 

Gjems, O., 1963. A swelling dioctahedral clay mineral of a vermiculite-smectite type in the weathering horizons of podzols. Clay Miner., 5 183-193. 

IHite/Smectite. Another common intergrowth of sheet silicates is the mixed-layering of illite and smectite. As discussed above, illite and smectite are clay minerals whose basic structures resemble the mica muscovite. Their compositions may differ significantly from muscovite, but they generally have a lower occupancy of the interlayer sites than mica. Numerous other compositional differences are possible for smectite however, this discussion will be restricted to a dioctahedral illite and a dioctahedral smectite containing potassium and vacancies in the interlayer sites as given above. 

Figure 1. Composition and classification of known end members of dioctahedral (A) and trioctahedral (B) smectites ...

Evidences for electron transfer from surface tg bulk. According to the nature of the clay, variable amounts of Fe are present in the lattice. Glauconite, which is an iron rich member of the dioctahedral smectite family, reacts at pH 7 with sodium... 

The creation of Bronsted acid sites in a dioctahedral clay such as the beidellite, or a trioctahedral smectite, such as the saponite, is carried out using the same methodology previously explained for zeolites ... 

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