RT Journal Article
T1 Trioctahedral entities in palygorskite: Near-infrared evidencefor sepiolite-palygorskite polysomatism
A1 Stathopoulou, Elizabeth T.
A1 Súarez Barrios, Mercedes
A1 García Romero, Emilia
A1 Sánchez del Río, Manuel
A1 Kacandes, Georges H.
A1 Gionis, Vassilis
A1 Chryssikos, Georgios D.
AB The mixed dioctahedral-trioctahedral character of Mg-rich palygorskite has been previously described by the formulayMg5 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-chainribbon mineral with an experimentally determined upper limit of y  0.5 and x is the FeIII content in the M2 sites of the dioctahedralcomponent. 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 andwith 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 octahedralcomposition on the structure of the TOT ribbons, H2O in the tunnels and surface silanols of palygorskite, as well as their response toloss 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). Incontrast, the trioctahedral entities have fixed octahedral composition or ribbon structure and are spectroscopically identical tosepiolite. 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-zerotrioctahedral character should be considered as members of a polysomatic series of sepiolite and (dioctahedral) palygorskite describedby 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.
PB Mineralogical Society of America.
SN 0003-004X
YR 2011
FD 2011
LK https://hdl.handle.net/20.500.14352/43841
UL https://hdl.handle.net/20.500.14352/43841
LA eng
DS Docta Complutense
RD 29 abr 2024