Trioctahedral entities in palygorskite; near-infrared evidence for sepiolite-palygorskite polysomatism
Trioctahedral entities in palygorskite; near-infrared evidence for sepiolite-palygorskite polysomatism
European Journal of Mineralogy (August 2011) 23 (4): 567-576
The mixed dioctahedral-trioctahedral character of Mg-rich palygorskite has been previously described by the formula yMg (sub 5) Si (sub 8) O (sub 20) (OH) (sub 2) (OH (sub 2) ) (sub 4) .(1-y)[xMg (sub 2) Fe (sub 2) .(1-x)Mg (sub 2) Al (sub 2) ]Si (sub 8) O (sub 20) (OH) (sub 2) (OH (sub 2) ) (sub 4) , where y is the trioctahedral fraction of this two-chain ribbon mineral with an experimentally determined upper limit of y nearly equal 0.5 and x is the Fe (super III) content in the M2 sites of the dioctahedral component. Ideal trioctahedral (y=1) palygorskite is elusive, although sepiolite Mg (sub 8) Si (sub 12) O (sub 30) (OH) (sub 4) (OH (sub 2) ) (sub 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 (sub 2) O in the tunnels and surface silanols of palygorskite, as well as their response to loss of zeolitic H (sub 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 (super III) /(Al+Fe (super 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 (sub 200) in palygorskite follows the regression d (sub 200) (Aa)=6.362+0.129 x(1-y)+0.305y, R (super 2) =0.96, sigma = 0.013 Aa. When extrapolated to y=1, d (sub 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 (sub 8) Si (sub 12) O (sub 30) (OH) (sub 4) (OH (sub 2) ) (sub 4) .(1-y')[x'Mg (sub 2) Fe (sub 2) .(1-x') Mg (sub 2) Al (sub 2) ]Si (sub 8) O (sub 20) (OH) (sub 2) (OH (sub 2) ) (sub 4) , with 0<x'=x<0.7 and 0<y'=y/(2-y)<0.33.