Dehydroxylation of Dioctahedral Phyllosilicates

Abstract

Abstraet--Mrssbauer spectroscopy of dioctahedral phyllosilicates showed that on dehydroxylation iron which originally occupied M(2) and M(1) sites became, respectively, 5-and 6-coordinated. The 6-coordinated sites are very distorted 9 No migration of cations occurs in the course of heating the specimens for 1-3 hr at 600~176By using a combination of several physicochemical methods, different successive stages of the dehydroxylation process could be distinguished: (1) migration of protons; (2) localized dehydroxylation of individual associations without significant change in the overall configuration of the octahedral sheets; and (3) loss of most of the hydroxyl groups with concomitant changes in the cell dimensions. Penetration of Li into the octahedral sheets does not affect the course of the reaction, but reduces the dehydroxylation temperature and the stability of the products.Dehydroxylation was preceded by or associated with the oxidation of any divalent iron present 9 Fe n+ derived from Fe ' 2+ was indistinguishable by MSssbauer spectroscopy from iron initially present in the trivalent form 9 High concentrations of Fe lower the dehydroxylation temperature and reduce the stability of the dehydroxylate to the extent that partial disintegration may precede complete dehydroxylation.

Order-Disorder Phenomena Accompanying the Dehydroxylation of Dioctahedral Phyllosilicates

“…Table 1 shows the M6ssbauer parameters of montmorillonite and beidellitic montmorillonite heated at 600 ~ and 700~ for various periods of time. The assignments were discussed by Heller-Kallai and Rozenson (1980). It is evident that prolonged heating did not change the parameters of the doublets of montmorillonite dehydroxylate, but affected their relative intensity.…”

“…Samples of montmorillonite and beidellitic montmorillonite, described by Heller-Kallai and Rozenson (1980), were heated in open crucibles at 600 ~ or 700~ for various periods of time. Their M6ssbauer and IR spectra were recorded and interpreted, and the wavelength of the AIK~ emission was measured as previously described.…”

“…Fe ions occupying M(1) and M(2) positions in dioctahedral phyllosilicates become 6-and 5-coordinated, respectively, on dehydroxylation of the minerals (Heller-Kallai and Rozenson, 1980). No migration of cations was observed in the course of the dehydroxylation, which involved heating the specimens for 1 hr at temperatures up to 700~ One specimen of phengite was heated for 3 hr at 700~ and for longer periods at lower temperatures.…”

Mössbauer and Infrared Study of Heat-Treated Nontronite

“…Because Rb + ions are too large to penetrate into the hexagonal cavities, we conclude that Li + migration is responsible for the observed changes in the OH vibrations. Another explanation for the diminution of the FeFeOH band at 814 cm 1 after heating natural nontronites (with Mg 2+ and Ca 2+ as interlayer cations) is based on proton migration from the FeFeOH bridges (Heller-Kallai and Rozenson, 1980).…”

“…Also, the environment at the octahedral sites can be modified by thermal treatment above 300~ where dehydroxylation reactions may occur. These reactions may induce modifications in the immediate environment about the iron centers, such as a change from octahedral coordination to five-coordination (Heller-Kallai and Rozenson, 1980) or the creation of iron-rich clusters. Migration of Fe 3. into vacant octahedral sites upon heating was demonstrated also by Tsipursky and Drits (1984).…”