Letter: Octahedral cation distribution in palygorskite

Abstract

The 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.