Chapter 2 - Advances in the Crystal Chemistry of Sepiolite and Palygorskite

Abstract

Abstract The structure and chemical composition of sepiolite and palygorskite are known from the first half of the twentieth century. Ideal palygorskite Si 8 O 20 (Al 2 Mg 2 )(OH) 2 (OH 2 ) 4 ⋅H 2 O) 4 has dioctahedral character, and sepiolite Si 12 O 30 Mg 8 (OH) 4 (OH 2 ) 4 (H 2 O) 8 is a pure trioctahedral mineral. In the two minerals, isomorphic substitutions in both tetrahedral and octahedral sheets are frequent. In addition, Mg-rich palygorskites, Fe-rich palygorskites and Al-rich sepiolites have been reported and it seems that the compositional limits accepted until now could be too narrow. Therefore, sepiolite and palygorskite can be classified into different types depending on the composition. In this chapter, the variations in the chemical composition of natural sepiolites and palygorskites as well as the limit of chemical composition of both minerals, if it exists, using the data from the literature available are established. Sepiolite can be classified into two types: sepiolite and Al-sepiolite. A limit for these two types can be established from the octahedral occupancy, and Al-sepiolites are those that have more than 10% of octahedral positions vacant and more than 0.5% VI Al atoms. On the other hand, palygorskite is classified into Ideal palygorskite with an octahedral composition near to the ideal palygorskite. Common palygorskite : where VI Al content is less than in the ideal formula and as a consequence that the Mg content is higher, but the number of octahedral cations is close to 4. Magnesic palygorskite is the most trioctahedral extreme, and Aluminic-palygorskite , which is defined by a total number of octahedral cations (p.h.u.c.) 3 ) > Mg. Magnesic palygorskite and aluminic sepiolite can have very similar chemical composition. There is no a compositional gap between the two minerals. Palygorskite can be so rich in Mg and sepiolite so rich in Al that is possible to affirm that a continuous composition series exist and all the intermediate compositions between the two extremes corresponding to the two pure minerals can be found. The intermediate compositions can be explained by intergrowths of sepiolite and palygorskite ribbons or polysomes.