Chryssikos, Georgios D.Gionis, VassilisKacandes, Georges H.Stathopoulou, Elizabeth T.Súarez Barrios, MercedesGarcía Romero, EmiliaSánchez del Río, Manuel2023-06-202023-06-2020090003-004Xhttps://hdl.handle.net/20.500.14352/50678The OH speciation of 18 palygorskite samples from various localities were evaluated by near infrared spectroscopy (NIR) and compared to the corresponding octahedral composition derived from independent, single-particle analytical electron microscopy (AEM). NIR gives evidence for dioctahedral-like (AlAlOH, AlFe3+OH, Fe3+Fe3+OH) and trioctahedral-like (Mg3OH) species. Therefore, palygorskite can be approximated by the formula yMg5 Si8O20(OH)2·(1 – y)[xMg2Fe2·(1 – x)Mg2Al2]Si8O20(OH)2, where x is the Fe content of the dioctahedral component, and y is the trioctahedral fraction. The values of x estimated from the NIR data are in excellent agreement with the Fe/(VIAl + Fe) ratio from AEM (R2 = 0.98, σ = 0.03), thus suggesting that all octahedral Al and Fe in palygorskite participate in M2M2OH (dioctahedral-like) arrangements. Furthermore, y values from AEM can be compared to NIR (R2 = 0.90 and σ = 0.05) after calibrating the relative intensity of the Mg3OH vs. (Al,Fe)2OH overtone bands using AEM data. The agreement between the spectroscopic and analytical data are excellent. The data show that Fe3+ for Al substitution varies continuously in the analyzed samples over a broad range (0 < x < 0.7), suggesting that fully ferric dioctahedral palygorskites (x = 1) may exist. On the other hand, the observed upper trioctahedral limit of y = 0.50 calls for the detailed structural comparison of Mg-rich palygorskite with sepiolite.engjournal articlehttp://www.minsocam.org/MSA/AmMin/TOC/open access549.6PalygorskiteFe-richMg-richstructurenear infrared SpectroscopyTrioctahedralDioctahedralCompositionAEMSepioliteMineralogía (Geología)2506.11 Mineralogía