Palygorskites thermal

High pressure studies using natural sepiolite and palygorskite (Frank-Kameneckiji and Klockova, 1969) indicate that these minerals can contain variable quantities of silica because they exsolve quartz while retaining their basic structural and mineral identity. These experiments also demonstrate that the natural minerals are compositionally intermediate between talc or montmorillonite and quartz. These latter phases are formed upon the thermal breakdown of sepiolite and palygorskite under conditions of 1 and 2Kb total pressure. Both sepiolite and palygorskite appear to remain stable in sequences of buried rocks, at least up to the depth where fully expandable dioctahedral montmorillonite disappears (Millot, 1964). 

Figure 43. Phase diagram of phyllosilicates found above the thermal stabilities of sepiolite and palygorskite in the system Si-R -R. ...

As a parenthesis, Figure 42 shows the system Si-R -R as it appears above the thermal stability limits of sepiolite and palygorskite. This is based upon the experiments of Velde (1973) in the Mg-Al-Si-H20 system at 2Kb pressure and temperatures above 200°C, and the experiments at high pressure of Frank-Kameneckiji and Klockova (1969) on natural sepiolite. 

Hubbard B, Kuang WX, Moser A, Facey GA, Detellier C (2003) Structural study of Maya Blue textural, thermal and solid state multinuclear magnetic resonance characterization of the palygorskite-indigo and sepiolite-indigo adducts. Clays Clay Miner 51 318-326... 

Subsequently, Sun and Garces [96] also reported the preparation of PP/clay nanocomposites by in situ polymerization with metallocene/clay catalysts. Recently, Wang and coworkers [97] used thermally treated naturally occurring palygorskite to support titanocene (CpaTiCla) catalyst. After activation by MAO, the supported catalyst initiated an in situ ethylene polymerization resulting in the exfoliated dispersion of the nanofibers into the polyethylene matrix. The activity of the supported catalyst was found to be even higher than its solution counterpart and the final PE/clay nanocomposite showed physical properties. 

The differential thermal analysis curve of palygorskites (CaillIre and Henin [1957]) shows an important, essential endothermic reaction, which ranges from 90° to around 240°C, at which a second endothermic reaction begins, terminating at around 350°C. This second peak, distinctly less pronounced than the first, is sometimes followed by a rather broad exothermic reaction. At about 540°C, a third endothermic reaction occurs, often producing a very sharp peak. Lastly, one or two small endothermic inflections are observed rather frequently at around 740° and 900°, and between 900° and 1000°, a clearly defined exothermic peak is obtained. 

Middle East. Typical differential thermal curves of the two minerals are shown in Figure 36 cf Martin Vivaldi and Fenoll Hach-Ali [1970]. Mixtures of palygorskite and kaolinite examined by one of the authors indicate that in concentrations of less than 25 %, palygorskite would be extremely difficult to detect by differential thermal analysis. 

Figure 36. Differential thermal curves for A— palygorskite, Attapulgus, Georgia, U.S.A B— sepiolite, Cornwall, England.

---