• journal_title：American Mineralogist
• Contributor：Georgios D. Chryssikos ; Vassilis Gionis ; George H. Kacandes ; Elizabeth T. Stathopoulou ; Mercedes Suárez ; Emilia García-Romero ; Manuel Sánchez Del Río
• Publisher：Mineralogical Society of America
• Date：2009-
• Format：text/html
• Language：en
• Identifier：10.2138/am.2009.3063
• journal_abbrev：American Mineralogist
• issn：0003-004X
• volume：94
• issue：1
• firstpage：200
• section：Letters

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.