Pyrophyllite structure

The derivation of clays from talcs and micas provides a direct way to understand the structures of the clays. The infinite-sheet mica pyrophyllite, Al2(Si40io)(OH)2, serves as an example. If one of six AI ions in the pyrophyllite structure is replaced by one Mg ion and one Na ion (which together carry the same charge), a type of clay called montmorillonite, MgNaAl5(Si40io)3(OH)g, results. This clay readily absorbs water, which infiltrates between the infinite sheets and hydrates the Mg and Na ions there, causing the montmorillonite to swell (Fig. 22.5). 

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... 

Pyrophyllite - If the kaolin structure is boimd through shared oxygens to a layer of silica rings on its alumina side, the pyrophyllite structure of Figure 9 results. Because both faces of a pyrophylHte platelet are composed of silica oxygens, interlaminar bonding is by relatively weak van der Waals forces. Pure pyrophylhte is therefore soft with talc-like slipperiness, because its laminae will slide past each other or separate fairly easily. 

Smectites are water swellable clays that have a platy structure. Smectite is the mineralogical term for a group of clays, which includes montmorillonite, hectorite, and saponite. Most smectites are more commonly known under the geological term bentonite. By convention, bentonite is understood to be an ore or product with a substantial smectite content. The range of possible chemical variations in the basic smectite trilayer lattice starts with montmorillonite, the high-aluminum end member. Montmorillonite is composed of a central alumina octahedral layer sandwiched between tetrahedral silica layers. This is identical to the dioctahedral pyrophyllite structure except for small... 

It is interesting to note that both minerals lose about 5% of their weight on heating this is the theoretical value for talc, but a little high for pyrophyllite. It may be, therefore, that the pyrophyllite structure can accommodate some excess hydroxyl groups. 

Figure 9.11 Alternative representations of the layer structures of (a) kaolinite, (b) pyrophyllite, and (c) talc. (After H. J. Emeleus and J. S. Anderson, 1960 and B. Mason and L. G. Berry, 1968.)...

The minerals of the pyrophyllite [(Al2Si40io(OH)2)]-ferripyrophyllite [(Fe 2Si40io(OH)2)] series are related to the mica group. The structure of these minerals closely approximates that of muscovite with the two tetrahedral sheets and octahedrally coordinated intercalated ions, a 2 1 layering (Fig. 2.13) but there is little substitution of Si " by Al Only two-thirds... 

Fig. 2.13 The 1 1 and 2 1 layer arrangements in the sheet structure minerals and the (010) view of the structures of the serpentine, clay, talc, pyrophyllite, mica, and chlorite minerals. X = layer charge per formula unit. [From Bailey (1980), Fig 1.1, p. 3 Fig 1.2, p. 6.1...

Montmorillonite, one of the most commonly encountered smectites, is similar to pyrophyllite (2 1) but has some interlayer cations and extra water. In pyrophyllite the layers are neutral because Si " in the tetrahedral sheet is not replaced by Al. In the smectites there is substitution of Al for Si " in the tetrahedral sheets, and occasionally Al appears in octahedral locations as well (for the names assigned to the end members, see Brindley and Brown, 1980, pp. 169-170.) The charge imbalances of the substitutions are compensated by interlayer cations, usually Na or Ca. These cations are easily exchangeable. The hydration level of the smectites is also variable. These minerals are very responsive to changes in water content as well as to the salt contents of the water. Other liquids that might be associated with the minerals and temperature can also effect changes in the chemical and crystal structure. 

Several insulating inorganic solids possessing sheet structures, for example, silicates belonging to the pyrophyllite family (Thomas, 1982), and acid phosphates (Alberti Constantino, 1982 Clearfield, 1981) of some tetravalent metals form intercalation compounds with a variety of donor molecules in these cases, intercalation does not involve a redox process, unlike in the cases of transition metal chalcogenides and... 

The 2 1 structures are rare in clay mineral assemblage. But, the occurrence of pyrophyllite is a useful indication of the attainment of 300°C temperatures. 

There are more complicated structures intermediate between pyrophyllite and talc with variable substitution of A1J and Mg2. Electroneulrality is maintained by hydrated cations between layers. Thus the montmorillonites arc unusual days forming thixotropic aqueous suspensions that arc used as well-drilling muds and in nondrip puints. They are derived from the formulation AU(OH)jSi40 ,-x-H2o with variable amounts of water, Mg3+ (in place of some Al5 ), and compensaUng cations. M"+ (M = Ca in fuller s earth, which is converted to bentonite, M = Na). Vermiculite likewise has variable amounts of water and cations, (t dehydrates to a talc-like structure with much expansion when heated (see page 750). 

Montmorillonites (smectite clays) have structures resembling that of pyrophyllite but the structure is not electrically neutral. Exchangeable cations are located in interlamellar regions of the clay and, furthermore, the clay can be flocculated such that the plate-like crystals compact with parallel c-axes to give coherent layers. The smectites are then attractive materials with which to modify electrodes. 

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 ... 

In a T-O-T layer (also termed a 2 1 layer), tetrahedral sheets attach to both sides of an octahedral sheet, thereby giving a structural formula of M2T40io(°h)4 to M3T4O10 (OH) 4. The T-O-T layer forms the basis of the sheet silicates pyrophyllite (M Al) and talc (M - Mg). 

Pyrophyllite is the simplest layer aluminosilicate in which two tetrahedral Si04 layers are condensed on to the octahedral A106 layer to produce a three-sheet layer, the composition of the unit cell being [Al2(OH)2(Si205)2]2. Another ideal structure in which A1 is replaced by Mg is that of talc. In both cases the three-sheet layer is electrically neutral and the layers are stacked in the ABAB... sequence. Because of the cohesive strength of this ideal structure, neither pyrophyllite nor talc occurs in the form of the very fine particles which generally characterize clay minerals. 

Figure 1.4 Side elevation structures of talc and pyrophyllite...

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