Vermiculite layer charge

Lagaly G (1982) Layer charge heterogeneity in vermiculites. Clays Clay Miner 30 215-222... 

Smectites and vermiculites have a lesser tendency to agglomerate because their layer charge is smaller than that of illite... 

Vermiculites have a 2 1 layer structure similar to smectites, but expand less freely in water, presumably because of the higher layer charge in the former minerals. Most of this structural charge resides in the tetrahedral layers of the vermiculite platelets. Even when fully wetted, vermiculites do not expand beyond the two water-layer stage ( " 1.5 nm c-spacing). 

This fact may explain the superiority of montmorillonite over vermiculite as an adsorbent for organocations (3, 4). Complicating this description, however, is the fact that a sample of any particular layer silicate can have layer charge properties which vary widely from one platelet to another (j>). By measuring the c-axis spacings, cation exchange capacity, water retention, and other properties of layer silicates, one obtains the "average" behavior of the mineral surfaces. 

Most of the vermiculites listed by Foster apparently were formed by the leaching of K from biotite. Biotite has a negative layer charge near 1.00 per Oi0(OH)2 units. Foster s vermiculites have charges from 1.08 to 0.38 with only five of 25 values... 

High-iron biotites are not likely to alter to vermiculite. When the ferrous iron content is sufficiently high, oxidation will result in a positive layer charge and the extension of the octahedral sheet to such an extent that the basic layer would be unstable and break up quite rapidly. 

Some of expanding clays in soils have the attributes of vermiculite, some of smectite, and some have features of both. The variation in properties is largely related to the layer charge. The charge is dependent on the original charge on the 2 1 layer of the parent mineral and the amount of ferrous iron in the octahedral sheet. The oxida-... 

Recently, it was noted that the addition of poly(vinyl methyl ether) to a clay-salt-water system induced the contraction of the interlayer separation between the clay platelets [18], The K-butylammonium vermiculite used in those experiments provided an ideal structure of regularly spaced and parallel charged colloidal platelets in a salt solution [19,20], Swenson et al. [21-23] studied the effect of the addition of poly(ethylene oxide) (PEO) on the distance between the vermiculite layers and observed that, with increasing PEO concentration, the distance between the vermiculite layers decreased. They estimated that the bridging force per bridge was 1.4 pN. 

As indicated in Table 1, the three 2 1 groups differ from one another in two principal ways. The layer charge decreases in the order illite > vermiculite > smectite, and the vermiculite group is further distinguished from the smectite group by the extent of isomorphic substitution in the tetrahedral sheets. Among the smectites, those in which substitution of Al for Si exceeds that of Fe2+ or Mg for Al are called beidellite, and those in which the reverse is true are called montmorillonite. The sample chemical formula in Table 1 for smectite thus represents montmorillonite. In any of these 2 1 clay... 

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