COMBINED NEAR-INFRARED AND X-RAY DIFFRACTION INVESTIGATION OF THE OCTAHEDRAL SHEET COMPOSITION OF PALYGORSKITE V ASSILIS G IONIS 1 ,G EORGE H. K ACANDES 2 ,I OANNIS D. K ASTRITIS 2 AND G EORGIOS D. C HRYSSIKOS 1, * 1 Theoretical & Physical Chemistry Institute, National Hellenic Research Foundation, 48 Vass. Constantinou Ave., Athens, Greece 11635 2 Geohellas S.A., 60 Zephyrou Str., Athens, Greece 17564 Abstract—The octahedral composition of palygorskite in more than 300 samples from the Pefkaki deposit, W. Macedonia, Greece, has been studied by near-infrared (NIR) and X-ray diffraction (XRD), and evaluated according to the formula y Mg 5 Si 8 O 20 (OH) 2 · x Mg 2 Fe III 2 Si 8 O 20 (OH) 2 ·(1 Àx Ày ) Mg 2 Al 2 Si 8 O 20 (OH) 2 . Included in the study were PFl-1 and several commercial palygorskites. Our analysis of 2 nd derivative NIR spectra shows that the dioctahedral composition is adequately described by three sharp overtone bands representing AlAlOH, AlFe III OH and Fe III Fe III OH in M2 dioctahedral sites, and that the summed intensity of these bands is proportional to the amount of dioctahedral component present (1Ày). The samples show large variations in the degree of dioctahedral Fe III -for-Al substitution with Fe III occupying up to 70% of the dioctahedral M2 sites. Ternary analysis shows that the distribution of dioctahedral Al and Fe III is not random, but displays a tendency towards homoionic pairing. An overtone band at 7214 cm À1 and several combination bands are indicative of a trioctahedral Mg 3 OH component (y), and their appearance correlates with a distinct palygorskite signature in thermogravimetric analysis. Nevertheless, these bands cannot be used reliably for the quantification of a trioctahedral palygorskite component due to their close similarity to those of sepiolite. To circumvent this problem, we have evaluated y indirectly by calculating the difference between 1Ày and the total concentration of palygorskite determined by the normalized intensity of the d 110 XRD peak of palygorskite at 10.4 A ˚ . Using this methodology, we have found that the samples conform to a trioctahedral limit of y & 0.55, although within this limit they display large variations in octahedral character. Finally, we extend the above methodology to PLS chemometrics and show how NIR can be used to determine palygorskite content routinely in multimineralic geological samples. Key Words—Greece, Near-infrared Spectroscopy, Octahedral Composition, Palygorskite, X-ray Diffraction. INTRODUCTION Palygorskite/sepiolite minerals are unique among clays in that they consist of 2:1 units with a modulated tetrahedral sheet (Brindley and Brown, 1980; Jones and Gala ´n, 1988; Singer, 1989). The octahedral sheet is thus discontinuous, giving rise to alternating 2:1 ‘slabs’ that are linked to each other through Si ÀOÀSi bonds between adjacent SiO 4 tetrahedra. The gaps between slabs form tunnels that contain both zeolitic and crystalline water, the latter completing the coordination of Mg atoms at the edges of each octahedral sheet. While sepiolite is thought to be uniformly tri- octahedral and of orthorhombic symmetry (Post et al., 2007), palygorskite can be either orthorhombic or monoclinic (Christ et al. , 1969; Artioli and Galli, 1994; Giustetto and Chiari, 2004) and can be assigned a variable dioctahedral Àtrioctahedral character expressed by the general formula [M II 3x+2 M III 2(1Àx) & 1Àx ]Si 8 O 20 (OH) 2 (OH 2 ) 4 .4H 2 O where M II = Mg and M III = Al, Fe (Paquet et al., 1985; Gala ´n and Carretero, 1999). The end-member structure of the octahedral sheet at x = 0 (dioctahedral) and x =1 (trioctahedral) is shown in Figure 1 (adapted from Gu ¨ven et al., 1992). Regardless of dioctahedral-trioctahedral character and symmetry considerations, the detection of palygors- kite by XRD is based on the presence of a strong 110 line at ~10.4 A ˚ , which is unaffected by either mild heat treatment (<200ºC) or ethylene glycol solvation (Moore and Reynolds, 1997). All other palygorskite diffraction peaks are weak and of little diagnostic value in the presence of other clay minerals. Vibrational spectroscopy is suitable for determining the speciation of the octahedral sheet of smectites by taking advantage of the sensitivity of the hydroxyl stretching and bending modes on the nature and extent of cationic substitutions in the octahedral sheet (Zviagina et al., 2004; Gates, 2005; Madejova ´ and Komadel, 2005 and references therein). However, plausible interpreta- tions of IR data addressing the composition of the octahedral sheet of palygorskite have been reported only very recently. Sua ´rez and Garcı ´a-Romero (2006) studied the Fourier transform infrared (FTIR) spectra of six palygorskite samples of variable composition and Clays and Clay Minerals, Vol. 55, No. 6, 543–553, 2007. Copyright # 2007, The Clay Minerals Society 543 * E-mail address of corresponding author: gdchryss@eie.gr DOI: 10.1346/CCMN.2007.0550601