Montmorillonite structural formulas

Chemical analyses and structural formulas of some montmorillonites of various origins... 

Fig. 14. Histograms showing the distribution of the cations of 101 montmorillonite-beidellite structural formulas.

A source of error in chemical analyses of montmorillonites (and in other clays) that is not commonly checked is the presence of amorphous material, particularly Si and Al. Table XXXII lists structural formulas given by Osthaus (1955) for montmorillonites which were purified by size fraction and by extraction with 0.5 N NaOH to remove amorphous Si and Al. In six analyses dissolved silica ranged from 3.6 to 8.4% and alumina from 0.6 to 2.25%. Amorphous silicon dioxide should be expected in most montmorillonites derived from volcanic material. The source glass has more Si than is required for the 2 1 layer and the excess must be leached from the glass. Much of the Si is deposited in the sediments underlying the bentonite bed in the form of chert but it is to be expected that the extraction would not be complete and a portion of the colloidal Si would remain in the bentonite bed. 

Structural formulas of montmorillonites before and after extraction of easily soluble Si and Al (After Osthaus, 1955)... 

Soil scientists (Sawhney, 1958 Rich and Cook, 1963 and others) have established that Al and Fe hydroxides are commonly present in the interlayer position of soil montmorillonites. This material is not readily exchangeable and is probably responsible for a great deal of error in calculated structural formulas. 

Structural formulas for montmorillonites from Cretaceous bentonites of Black Hills region (Wyoming, South Dakota)... 

Structural formulas of some mixed-layer illite-montmorillonite clays... 

There is considerable overlap between minerals named montmorillonite and nontronite. Many of the montmorillonite samples that overlap the nontronite field are relatively high in iron but so are a number of samples that lie in the restricted montmorillonite zone. The structural formulas indicate that there is a complete gradation between montmorillonite-beidellite and nontronite so that any boundary is arbitrary. 

Preferred bentonite clays are those whose chief constituent is mont-morillonite, a mineral of the composition corresponding to the empirical formula, 4Si02-Al203 H20. The principal sources of raw clay for the manufacture of the presently most widely used natural catalyst (Filtrol Corporation) are deposits in Arizona and Mississippi. The clay from these deposits contains appreciable amounts of impurities, principally CaO, MgO, and Fe203, which replace part of the A1203 in the ideal montmorillonite structure. The catalyst is prepared by leaching the raw clay with dilute sulfuric acid until about half of the alumina and associated impurities is removed. The resulting product is then washed, partially dried, and extruded into pellets, after which it is activated by calcination. A typical analysis of the finished catalyst is as follows (Mills, 12). 

The computation can be illustrated with a montmorillonite whose structural formula is Nao.66 Si8(Al3,o.s Feo.29Mgo.66)02o(OH)4. The relative molecular mass of the unit cell is calculated by multiplying the relative molecular mass of each clement by its stoichiometric coefficient and summing over all such products to obtain Mr 742.4 for the total. Note... 

Typical results of specific surface area determinations on phyllosilicates by nitrogen gas/water vapor or nitrogen gas/CPB adsorption are listed in Table 1.7. For Mg-vermiculite and Na-montmorillonite, the measured adsorption specific surface area is close to that calculated from the unit cell dimensions and structural formula. For illitic mica, the area is about 14 per cent of the ideal crystallographic value, indicating that this mineral forms particles containing about seven phyllosilicate layers that cannot be penetrated by water vapor or CPB. 

Montmorillonite acquires its charge by the replacement of octahedral A1 by Mg atoms. The structural formula is written as... 

Hectorite is the trioctahedral equivalent of montmorillonite and owes its charge to octahedral replacements of Mg by Li. The structural formula is... 

Brydon et al. [1961] report dioctahedral chlorite occurring with quartz and an inter-stratified chlorite-montmorillonite mineral in the AB horizon of the Alberni soil series in British Columbia. The d(060) value of the chlorite is 1.496 A. The 001 reflection at 14.15 A increases in intensity on heating at 550°C and is stable in position to 700°C. Two alternative structural formulas, depending on the estimated amount of free quartz, are given for the clay fraction containing both the chlorite and the interstratified mineral ... 

The coordinates are based upon calculated formulas for the 2 1 layers and subsequent assignment to the M+ as equivalent to the total charge on the structure. This eliminates the problems of H+ interlayer ions and others, which frequently arises in formula calculations for montmorillonites. 

From the point of view of structure, the above minerals belong to the so-called two-layer ones. Another clay mineral, montmorillonite, has a three-layered structure. It has the formula Al2(Si205)2 (OH)2,and exists as finer particles than the previously mentioned minerals (up to 60% below 0.1 pm). The gaps between the individual layers are readily penetrated by water which produces swelling when wetted and shrinkage when dried. Montmorillonitic clay is called bentonite. It has a considerable ion-cxchange capacity as described below. 

Clay minerals are formed when igneous rocks weather. These minerals are the main constituent of fine-grained (<63 rm) particles in mud. In general these minerals are less cation-rich than their igneous precursors. Kaolinite has the simplest clay mineral formula because it is pure aluminosilicate. It is the mineral that held the secret to making porcelain, which was greatly valued by the emperors of China before AD 1000, after they discovered how hard and clear kaolin becomes when heated to 1300-1400 °C. Other, more complicated clay minerals, e.g. iUite and montmorillonite, have various amounts of cations added to their structures. 

Smectites, which are based on either the trioctahedral 2 1 (talc) or dioctahedral 2 1 (pyrophyllite) structure, differ from these neutral structures by the presence of isomorphous substitution in the octahedral or tetrahedral sheet. For example, the dioctahedral smectite, montmorillonite, has the general formula... 

Mostly focused on cationic clays, and particularly on montmorillonite and hectorite, smectite-type layered silicates and clay-based nanofillers have recently been extended to the family of LDH. Hydrotalcite-like LDH materials are described according to the ideal formula, [M1/ xM"l(OH)2]frl(lra [A H20]inter, where Mn and Mm are metallic cations, A the anions, and intra and inter denote the intralayer and interlayer domain, respectively. The structure consists of brucite-like layers constituted of edge-sharing octahedra. The presence of trivalent cations induces positive charges in the layers that are counterbalanced by interlamellar anions (Scheme 15.16). 

Natural talc is not pure, but contains Al, Ca and Fe. As already described (see section on montmorillonites) talc is a hydrated aluminium silicate having the formula Mg3Si40io(OH)2. Structurally it is composed of a layer of brucite, Mg(OH)2, sandwiched between two silica-type layers (see section on montmorillonites). 

The 2 1 layer type has two tetrahedral sheets sandwiching an octahedral sheet. The three clay groups with this structure are (illitic) mica, vermicu-lite, and smectite (montmorillonite), each with the general unit cell chemical formula ... 

If the octahedral Al is partially replaced by Mg " ) additional cations with charges of 1+ or 2+ are also added to the structures, and montmorillonites are the result. These clays swell on the absorption of water, act as cation exchangers, and have thixotropic properties they are gels when undisturbed but become liquid when stirred, making them useful as oil field muds and in paints. Their formulas are variable, with Nao.33[Mgo,33Ali 67(OH)2(Si40io)] n H2O as an example. The cations can include Mg, Al, and Fe in the framework and H, Na, K, Mg, or Ca in the exchangeable positions. 

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