Tetrahedral layers

The extent of substitution of magnesium and siUcon by other cations in the chrysotile stmcture is limited by the stmctural strain that would result from replacement with ions having inappropriate radii. In the octahedral layer (bmcite), magnesium can be substituted by several divalent ions, Fe ", Mn, or Ni ". In the tetrahedral layer, siUcon may be replaced by Fe " or Al ", leaving an anionic vacancy. Most of the other elements which are found in vein fiber samples, or in industrial asbestos fibers, are associated with interstitial mineral phases. Typical compositions of bulk chrysotile fibers from different locations are given in Table 3. 

Clay minerals that are composed of two tetrahedral layers and one octahedral layer are referred to as 2 1 clay minerals or TOT minerals. The apical oxygens of the two tetrahedral sheets project into the octahedral sheet. The 2 1 stmcture has a basal spacing (nominal thickness) of 1.0 nm (10 E). Pyrophjlhte [12269-78-2] Al2Si40 Q(0H)2, is the dioctahedral mineral, ie, AF" in the octahedral sites, and talc [14807-96-6], Mg3Si402Q(0H)2, is the trioctahedral, ie, in the octahedral sites. Both these minerals are essentially free of substitution in the octahedral site and therefore do not have a net... 

The multitude of variation in clay minerals is caused by substitution in the octahedral and tetrahedral layers resulting in charge deficits. The manner in which the charge deficit is balanced leads to many of the useful and unique properties of clay minerals. 

Smectites are stmcturaUy similar to pyrophylUte [12269-78-2] or talc [14807-96-6], but differ by substitutions mainly in the octahedral layers. Some substitution may occur for Si in the tetrahedral layer, and by F for OH in the stmcture. Deficit charges in smectite are compensated by cations (usually Na, Ca, K) sorbed between the three-layer (two tetrahedral and one octahedral, hence 2 1) clay mineral sandwiches. These are held relatively loosely, although stoichiometricaUy, and give rise to the significant cation exchange properties of the smectite. Representative analyses of smectite minerals are given in Table 3. The deterrnination of a complete set of optical constants of the smectite group is usually not possible because the individual crystals are too small. Representative optical measurements may, however, be found in the Uterature (42,107). 

Palygorskite and sepioHte are different from other clay minerals in the manner in which the 2 1 layers are joined. Rather than being joined in a continuous manner, the tetrahedral sheets are joined to an adjacent inverted tetrahedral layer, making the octahedral layers noncontinuous and leaving an open channel in the mineral stmcture (37,38,148). The dimension of palygorskite is teI.S nm (18 E) the dimension of sepioHte is 9e2.7 nm (27 E) (37). 

Fig. 2. A tetrahedral layer from fl-cristobalite or fl-tridymite. A silicon ion is located at the center of each tetrahedron, and an oxygen ion at each comer.

Fig. 3. A tetrahedral layer in which all the tetrahedra point in the same direction. Fig. 4. A complete layer of octahedra (brucite layer).

Figure 1. Cross-sectional diagram of an expanding 2 1 layer silicate showing the octahedral layer, tetrahedral layer, and hydrated exchange cations in the interlayer.

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). 

Fundamental structural units of detrltal silicates, (a) octahedron, (b) octahedral layer found in sheet silicates, (c) tetrahedron, and (d) tetrahedral layer found in sheet silicates. Source From Grim, R. E. (1968). Clay Mineralogy, 2nd ed., McGraw-Hill Publishing Company, p. 52. 

As shown in Figure 14.4, each clay mineral exhibits a large range in the type and degree of isomorphic substitution. The central silicon atom in the tetrahedral layers can be replaced by aluminum, alkali, alkaline earth, and trace metal atoms. In the octahedral layers, the central Al and Mg atoms can be similarly replaced. The large range in composition within each mineral type reflects variability in the environmental conditions under which crystallization and chemical weathering occur. Thus, the... 

The cations in the octahedral sheet may have a distorted octahedral coordination, depending on the cation size. B site ions, such as Ca, can be in either six- or eight-fold coordination with oxygen. Such variations can cause local structural distortion. The cation occupying the A site can bond to as many as 12 oxygen atoms—6 in the tetrahedral layer above and 6 below. 

Figure 5.25 (a) Vacancy ordering in the ab plane of CajFejOj of brownmillerite structure (b) alternating sequence of octahedral and tetrahedral layers in the c direction. Small filled circles, iron large filled circles, oxygen open squares, oxygen vacancy. 

Fig. 1 Chemical structure of MMT showing tetrahedral layer sandwiched between two octahedral sheets...

Other forms, corresponding to more complex arrangements of the tetrahedral layers, also occur in nature C. Frondel and C. Pal che, Am. Mineralogist 35, 29 (1950). 

Fig. 13-18.—The fundamental layers of the clays, micas, and chlorites, (a) A hydrargillite layer of ootahedra. The light circles indicate oxygen atoms, the heavier ones hydroxide ions in mica, (b) A tetrahedral layer from /9-eristobalite or / -tridymite. A silicon (continued on opposite page)...

Clay minerals that are composed of two tetrahedral layers and one octahedral layer are referred to as 2 1 clay minerals or TOT minerals. 

Serpentines. Substituting 3 Mg21 lor the 2 Al + in Lhe kaolin slruclttre results in the serpentine minerals. Mg.SrO.-tOHU In serpentines all three possible octahedral cation sites are filled. Most serpentine minerals are tubular lo fibrous in structure presumably because of misfit heiween Mg octahedral and tetrahedral layers. 

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