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

Structure superstructure relationships in lithium metaborate meta-aluminate compounds

Journal URL: http://www.societyofglasstechnology.org.uk/cgi-bin/open.cgi?page=journal&sessionid=85597106; http://www.ingentaconnect.com/content/sgt/pcg; http://www.societyofglasstechnology.org.uk/cgi-bin/open.cgi?page=%20PC/PC-00a&sessionid=8559710

Electrical-Conduction in Cadmium Germanate Glasses

Journal URL: http://www.sciencedirect.com/science/journal/0038109

Borate Glass Structure by Raman and Infrared Spectroscopies

Journal URL: http://www.sciencedirect.com/science/journal/0022286

Combined near-infrared and x-ray diffraction investigation of the octahedral sheet composition of palygorskite

The octahedral composition of palygorskite in more than 300 samples from the Pefkaki deposit, W. Macedonia, Greece, has been studied by near-infrared (NIR) and X-ray diffraction (XRD), and evaluated according to the formula yMgSiO (OH)·xMgFe SiO(OH)·(1-x-y) MgAlSiO (OH). Included in the study were PFI-1 and several commercial palygorskites. Our analysis of 2nd derivative NIR spectra shows that the dioctahedral composition is adequately described by three sharp overtone bands representing AlAlOH, AlFeOH and FeFeOH in M2 dioctahedral sites, and that the summed intensity of these bands is proportional to the amount of dioctahedral component present (1-y). The samples show large variations in the degree of dioctahedral Fe-for-Al substitution with Fe occupying up to 70% of the dioctahedral M2 sites. Ternary analysis shows that the distribution of dioctahedral Al and Fe is not random, but displays a tendency towards homoionic pairing. An overtone band at 7214 cm and several combination bands are indicative of a trioctahedral MgOH component (y), and their appearance correlates with a distinct palygorskite signature in thermogravimetric analysis. Nevertheless, these bands cannot be used reliably for the quantification of a trioctahedral palygorskite component due to their close similarity to those of sepiolite. To circumvent this problem, we have evaluated y indirectly by calculating the difference between 1-y and the total concentration of palygorskite determined by the normalized intensity of the d XRD peak of palygorskite at 10.4 Å. Using this methodology, we have found that the samples conform to a trioctahedral limit of y ≈ 0.55, although within this limit they display large variations in octahedral character. Finally, we extend the above methodology to PLS chemometrics and show how NIR can be used to determine palygorskite content routinely in multimineralic geological samples

On the structure of palygorskite by mid- and near-infrared spectroscopy

Journal URL: http://www.minsocam.org/MSA/AmMin/AmMineral.htm

Structure of fast-ion-conducting AgI-doped borate glasses in bulk and thin film forms

Journal URL: http://prb.aps.org

Far-Infrared Spectra of Magnesium Sodium-Borate Glasses

Journal URL: http://www.sciencedirect.com/science/journal/0038109