Vermiculite structural formulas

Partial structural formulas for some vermiculites (selected from Foster, 1963)... 

Structural formulas for vermiculite clays (After Barshad and Kishk, 1969)... 

As we wanted to make a complete structural determination of the mineral, we used a more highly oriented vermiculite from Llano, Texas. This was a special clay (VTx-1) from the Clay Minerals Society s Source Clays Repository it had the structural formula [23]... 

A vermiculite clay has the structural formula K.Mgo.sKMgaoFe sFeJ sXAl, sSi65)02o(OH)4. Calculate its cation-exchange capacity in meq/IOO g and its surface-site density in mol/g. Assuming the surface area of the clay is 30 mVg, what is its surface-site density in nm and /umol/m ... 

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. 

These experiments do not prove, but suggest the existence of a qualitative boundary separating saponites from vermiculites. This boundary cannot be found in the structural formula. If it exists, it can only be found in the distribution (ordered and disordered) of isomorphous replacements. The exact structure of saponites is still poorly known. Vermiculites are most often derived from biotite and phlogopite and could inherit the structure of the tetrahedral layers of micas. The A1 atoms in the tetrahedral layers of micas form unidimensional, ordered ensembles (linear chains of substitution Gatineau [1964] and Gatineau and Merino [1966]). 

Many analyses of vermiculites and hydrobiotites were collected together by Gruner [1934]. More recently, Foster [1963] has listed 38 analyses, and a number of others are given by Walker and Cole [1957]. Data for an Indian vermiculite are given by Mukherjee [1963] and for another by Adhikari and Majumdar [1964]. A vermiculite with an exceptionally high content of Ni has been described by Nikitina [1956] and one with a high Cu content by Bassett [1958]. The structural formula of a fibrous vermiculite from Kropfmiihl is given by Hofmann et al. [1956]. 

The structural formula for a typical vermiculite may be written approximately as... 

A fibrous vermiculite from Kropfmiihl in Bavaria, which gives single-crystal and powder diagrams similar to those of lamellar vermiculites, has been described by Weiss and Hofmann [1952]). The structural formula of this material is given by Hofmann et al. [1956]. The fiber axis coincides with the crystallographic a axis, and the cell constants are a — 5.34 A, b = 9.24 k,c = 29.0 A, jS = 97°. An 11.6 A phase was observed after the specimen had been heated at 135°C. From -Xray and electron micrograph data, the authors conclude that the fibers are solid rods rather than tubes. 

Walker [1951], and Mukherjee [1963]. Powder data for two typical Mg-vermiculites are listed in Table 1. The structural formula of the specimen from West Chester, Pennsylvania, is... 

The primary mineral, chlorite, which occurs in rocks as large crystals, possesses an interlayer sheet composed largely of Mg(OH)2. Since the mineral brucite is composed of magnesia sheets with the same basic structure, the single interlayer sheet in chlorite is termed the brucite layer. Isomorphous substitution of part of the Mg by produces a positively chained hydroxide sheet (see formula above) that props the 2 1 layers apart at a c-spacing of 14 A. This rigid interlayer contrasts with the hydrated interlayer of vermiculite, and even though chlorite and vermiculite have similar c-... 

Micas lose their structural relatively rapidly by a process of cation exchange with or metal ions in the weathering solution. This initial reaction is an alteration that is not necessarily very sensitive to pH. The pH, however, tends to rise during alteration as H" is adsorbed. The secondary mineral formed directly is a vermiculite, which inherits more or less intact the 2 1 layer structure of the mica parent. An ideal biotite mica might have the formula ... 

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