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|Title:||Trioctahedral entities in palygorskite: Near-infrared evidence for sepiolite-palygorskite polysomatism|
|Author/Creator:||Stathopoulou, Elizabeth T.|
Del Rio, Manuel Sanchez
Kacandes, George H.
Chryssikos, Georgios D.
|Type:||Journal Article (Scientific Journal article)|
|Abstract:||The mixed dioctahedral-trioctahedral character of Mg-rich palygorskite has been previously described by the formula yMg(5)Si(8)O(20)(OH)(2)(OH(2))(4)center dot(1-y)[xMg(2)Fe(2)center dot(1-x) Mg(2)Al(2)]Si(8)O(20)(OH)(2)(OH(2))(4), where y is the trioctahedral fraction of this two-chain ribbon mineral with an experimentally determined upper limit of y approximate to 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 Mg(8)Si(12)O(30)(OH)(4)(OH(2))(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, H(2)O in the tunnels and surface silanols of palygorskite, as well as their response to loss of zeolitic H(2)O. 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 d(200) in palygorskite follows the regression d(200)(angstrom) 6.362 + 0.129 x(1-y) + 0.305y, R(2) = 0.96, sigma = 0.013 angstrom. When extrapolated to y = 1, d(200) 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'Mg(8)Si(12)O(30)(OH)(4)(OH(2))(4)center dot(1-y')[x'Mg(2)Fe(2)center dot(1-x')Mg(2)Al(2)]Si(8)O(20)(OH)(2)(OH(2))(4), with 0 < x' = x < 0.7 and 0 < y' = y/(2-y) < 0.33.|
|Publisher:||East Schweizerbartsche Verlags|
|Journal Title:||European Journal of Mineralogy|
|Keywords:||palygorskite; sepiolite; XRD; near-infrared spectroscopy; dioctahedral; trioctahedral; polysomatism; Mineralogy|
|Other Identifiers:||DOI: http://dx.doi.org/10.1127/0935-1221/2011/0023-2112|
|Rights holder:||© E SCHWEIZERBARTSCHE VERLAGS|
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