ATTAPULGITE AND PALYGORSKITE

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. 

Christ et al. (1969) found that X-ray diffraction powder data for palygorskite samples show both orthorhombic and monoclinic structures and suggested that the variations in symmetry reflect variations in chemical composition. The present data suggest the most likely difference is octahedral Fe. 

Only a few analyses have been made of palygorskite but it is enough to indicate that the composition is probably as variable as that of the 2 1 minerals. Some of the variations are due to the presence of montmorillonite which is commonly intimately associated with the attapulgite and is difficult to remove. 

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). 

For 24 illites an average of 1.53 Al per three octahedral sites was found. This is 77% occupancy of the two occupied sites. Total trivalent ions (A1+ Fe3+) averaged 1.76. Thus, most illites have near the minimum or less than the minimum number of Al per three sites required to maintain a stable 2M, muscovite structure. 

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Abdul-Latif, N. and Weaver, C.E., 1969. Kinetics of acid-dissolution of palygorskite (attapulgite) and sepiolite. Clays Clay Miner., 17 169-178. 

In addition to the mineralogical distinction between palygorskite (attapulgite) and sepiolite, hormite clay products are generally classified by function and water content. 

The adsorption of water from the vapor phase takes place on the exterior surfaces of the clay particles. In addition, for minerals of the group of expanding clays— the montmorillonites including the vermiculties—water is adsorbed between the unit layers. Furthermore, in certain minerals—attapulgite or palygorskite and sepiolites—water adsorption occurs in the channels of molecular dimensions, which are a characteristic of the crystal structure of these minerals. 

Attapulgite-palygorskite clays lie between the Mg-rich and R3+-rich sheet clays. Thus, a continuous series exists between Mg and R3+. The Mg/R3+ ratio in this series is related to the relative amount of filled octahedral positions per total positions available. The dioctahedral clays theoretically have 67% of their total octahedral positions filled the attapulgites 80% the sepiolites 90% and the trioctahedral clays approximately 100%. Fig.28 shows a plot of percent octahedral occupancy versus R2+/r3+ + r2+ inustrating this relation. As R2 + is replaced by R3 + there is a gradual increase in unoccupied positions to satisfy charge requirements. 

Polymeric compositions contain clay nucleating agents. The clays are preferably sepiolite, palygorskite/attapulgite, or their combinations. The resulting products llud particular application as insulation and packaging materials. ... 

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