• journal_title：European Journal of Mineralogy
• Contributor：Elizabeth T. Stathopoulou ; Mercedes Suárez ; Emilia García-Romero ; Manuel Sánchez Del Río ; George H. Kacandes ; Vassilis Gionis ; Georgios D. Chryssikos
• Publisher：E. Schweizerbart'sche Verlagsbuchhandlung Science Publishers
• Date：2011-
• Format：text/html
• Language：en
• Identifier：10.1127/0935-1221/2011/0023-2112
• journal_abbrev：Eur J Mineral
• issn：0935-1221
• volume：23
• issue：4
• firstpage：567
• section：Articles

The mixed dioctahedral-trioctahedral character of Mg-rich palygorskite has been previously described by the formula yMg₅Si₈O₂₀(OH)₂(OH₂)₄·(1–y)[xMg₂Fe₂·(1–x)Mg₂Al₂]Si₈O₂₀(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 Fe^III content in the M2 sites of the dioctahedral component. Ideal trioctahedral (y = 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 = 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 = Fe^III/(Al+Fe^III) 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 (Å) = 6.362 + 0.129 x(1–y) + 0.305y, R² = 0.96, σ = 0.013 Å. 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′Mg₈Si₁₂O₃₀(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.