Trioctahedral entities in palygorskite: Near-infrared evidence for sepiolite-palygorskite polysomatism

The mixed dioctahedral-trioctahedral character of Mg-rich palygorskite has been previously described by the formula yMg5 Si8 O20(OH)2(OH2)4(1–y)[xMg2Fe2(1–x)Mg2 Al2] Si8 O20(OH)2(OH2)4, where y is the trioctahedral fraction of this two-chain ribbon mineral with an experimentally determined upper limit of y 0.5 and x is the FeIII content in the M2 sites of the dioctahedral component. Ideal trioctahedral (y ¼ 1) palygorskite is elusive, although sepiolite Mg8Si12O30(OH)4(OH2)4 with a similar composition, three-chain ribbon structure and distinct XRD pattern is common. A set of 22 samples identified by XRD as palygorskite and with variable composition (0 , x , 0.7, 0 , y , 0.5) were studied to extrapolate the structure of an ideal trioctahedral (y ¼ 1) palygorskite and to compare this structure to sepiolite. Near-infrared spectroscopy was used to study the influence of octahedral composition on the structure of the TOT ribbons, H2O in the tunnels and surface silanols of palygorskite, as well as their response to loss of zeolitic H2O. All spectroscopic evidence suggests that palygorskite consists of discrete dioctahedral and trioctahedral entities. The dioctahedral entities have variable structure determined solely by x=FeIII/(Al+FeIII) and their content is proportional to (1–y). In contrast, the trioctahedral entities have fixed octahedral composition or ribbon structure and are spectroscopically identical to sepiolite. The value of d200 in palygorskite follows the regression d200 (A°)= 6.362 + 0.129 x(1–y) + 0.305y, R2 = 0.96, σ = 0.013A°. When extrapolated to y = 1,d200 is identical to sepiolite. Based on this analysis, we propose that palygorskite samples with non-zero trioctahedral character should be considered as members of a polysomatic series of sepiolite and (dioctahedral) palygorskite described by the new formula y'Mg8 Si12 O30(OH)4(OH2)4.(1–y')[x'Mg2Fe2(1–x')Mg2Al2]Si8O20(OH)2(OH2)4, with 0 < x'= x < 0.7 and 0 < y' = y/(2–y) < 0.33

Octahedral cation distribution in palygorskite

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