Attapulgite structural

With small Es/clay loadings, (1/1000-1/100 of the clay s capacity, with the capacity being 10 -10 meq./mg clay) it was difficult to detect changes in the clay after 1-2 weeks by electron microscopy. However, when the clays were loaded to their capacity with Es, extensive destruction of the clay structures were noted in 2-4 days. In Figure 1 are micrographs of kaolin and attapulgite clays with and without exposure to Es. 

Attapulgite and sepiolite are clay minerals with a chain structure. The former has five octahedral positions and the latter either eight or nine. Both have relatively little tetrahedral substitution. The octahedral positions in sepiolite are filled largely with Mg and those in attapulgite with approximately half Mg and half Al. 

Attapulgite and palygorskite have a fibrous texture and a chain structure. The structure proposed by Bradley (1940) is that of a 2 1 layer structure with five octahedral positions (four filled) four Si tetrahedra occur on either side the octahedral sheet with their apices directed towards the octahedral sheet. These structural units alternate in a checkerboard pattern leaving a series of channels between the structural units. These channels contain water molecules. 

The average Al203/Mg0 ratio for 24 illites is 9.6 and for 101 montmorillonites 6.7. Attapulgite values range from 2.5 to 0.48. The ratios of octahedral Al/octahedral Mg are respectively 5.4, 4.3 and 1.8-0.4. Radoslovich (1963b) found that the 2M muscovite structure required a minimum of 1.7 of the three octahedral sites be filled with Al. The Al occurs in the two symmetrically related sites and the larger divalent cation occurs in the distinctive or unoccupied site. The lower limit of 1.7 Al is equivalent to 85% of the two symmetrically related or occupied sites being filled in a stable muscovite structure. A similar restriction is reported for the trioctahedral micas where an upper limit of 1.00 (R3++ R4+)per three sites was found by Foster (1960). 

The structure proposed by Bradley (1940) has three forms of water a zeolitic water, bound water (at the edges of the octahedral sheet), and structural hydroxyls. Using thermogravimetric curves, Caillere and Henin (1961a) attempted to measure the amount of these three types of water for several attapulgites (Table LV). The amount of bound water and hydroxyls differs considerably from that calculated on the basis of the ideal structure (column 5) and suggests there are more structural hydroxyls than proposed for the ideal structure. 

Fig.28. The relation of percent octahedral occupancy to RJ+/(R3+ +R2 +) for layer structure and chain structure clays, = saponite = attapulgite x = sepiolite (nine octahedral positions) o = sepiolite (eight octahedral positions).

Attapulgite and sepiolite data plot on both sides of the neutral line.. The calculated structural formulas of these two clays are more subject to error than those of the sheet structure clays. As is suggested by their low cation exchange capacity octahedral sheets tend to approach neutrality. The amount of R3+ required to maintain neutrality is less if OH substitues for O2. ... 

Bentonite and attapulgite clays are used to modify viscosity of coatings. Mica clay has a platelet structure and can be useful in reducing permeability of paint films. 

Wollastonite is a natural calcium metasilicate mineral with an acicular structure. Silanized and uncoated versions with maximum fiber lengths up to 25 pm are used as inexpensive secondary reinforcements in friction materials. Short fiber attapulgite clays have also been developed as secondary reinforcements. 

The paligorsquita or attapulgite and sepaolite are the only representatives of the fibrous structure of sihcates. 

Shen L, Lin YJ, Du QG, Zhong W, Yang YL (2005) Preparation and rheology of polyamide-6/attapulgite nanocomposites and studies on their percolated structure. Polymer 46 5758-5766... 

Chen J, Chen J, Zhu S, Cao Y, Li H (2011) Mechanical properties, morphology, and crystal structure of polypropylene/chemically modified attapulgite nanocomposites. J Appl Polym Sci 121 899-908... 

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