Palygorskite is a modulated phyllosilicate mineral with a structure formed by 2:1 layer type polysomes which are linked by periodical inversions of the apical oxygen of the continuous tetrahedral sheet, and the octahedral sheets is the modulated component, having the following structural formula: Si8(Mg2Al2)O20(OH)2(OH2)4.4H2O. In the literature we can find several structural models of palygorskite, belonging all of them either to monoclinic (Bradley, 1940; 1963; Drits and Sokolova, 1971) or orthorhombic (Preisinger, 1963) groups. Christ et al., (1969) performed a comparative study of palygorskites from different localities, concluding that the structure of the samples studied varied from orthorhombic to monoclinic, stablishing two regions of interest comprised between 4.5 – 4.0 Å and 2.6 – 2.5 Å. Chisholm (1992) performed simulations of powder diffraction considering both a monoclinic C2/m space group and an orthorhombic Pbmn space group, defining a new region of interest, along with the ones defined by Christ et al., (1969), comprised between 3.3 – 3.05 Å. Interestingly, looking at the region comprised at 4.5 – 4.0 Å, hereafter referred to as “Chisholm zone”, one may visually determine whether the palygorskite is monoclinic or orthorhombic. Recent studies of the Rietveld refinement (Chiari et al., 2003; Giustetto and Chiari, 2004; Post and Heaney, 2008) agree that palygorskite is a mixture of, at least, two different phases a monoclinic and an orthorombic, as already proposed by Preisinger (1963) and Chisholm (1992) for some palygorskite samples.

Comunicaciones del Congreso SEM 2017, XXXVI Reunión Científica de la Sociedad Española de Mineralogía, Oviedo 4 -7 julio de 2017

Financial support of CGL2016-77005-R MINECO project and BES-2013-065092 grant are acknowledged.

Peer reviewed