Order-Disorder Phenomena Accompanying the Dehydroxylation of Dioctahedral Phyllosilicates

Dehydroxylation of Dioctahedral Phyllosilicates

“…No migration of octahedral cations was observed under the thermal regime employed in this study (I-3 hr heating; T ~< 700~ in contrast to samples subjected to more prolonged heat treatment, described by Rozenson and Heller-Kallai (1980). The XRD, XRF, IR, and M6ssbauer data show that dehydroxylation does not occur uniformly throughout the structure.…”

“…Fe 3+ in 5-coordination in BaFe11019 shows a doublet with a very small I.S. which, like the large Q.S., may be attributed to the very considerable distortion of the sites (Rozenson and Heller-Kallai, 1980). The I.S., therefore, is not a reliable criterion for site assignment.…”

“…Although the M6ssbauer spectra of montmorillonite and muscovite do not, therefore, disprove the migration of cations on heating the samples for one hour, or even three hours, the spectra of nontronite, beidellitic montmorillonite, celadonite, and phengite strongly suggest that under these conditions cations maintain their original sites. Moreover, cation migration after more prolonged periods of heating was clearly shown by the M6ssbauer spectra (Rozenson and Heller-Kallai, 1980). Lindqvist (1962) and Bailey (1975) suggested that the increase in the b parameter with dehydroxylation of dioctahedral layer silicates indicates redistribution of the octahedral cations over all the octahedral positions.…”

“…Only two minerals are known in which Fe 3+ may occur in 5-coordination: replacing A1 in andalusite (Burnham and Buerger, 1961) and m yoderite (Fleet and Megaw, 1962). Halenius (1978) assigned two doublets in the spectra of andalusite to Fe 3 § in 6-and 5-coordination, but in view of the small isomer shift of the latter, the question arises whether some Fe 3+ may be present in tetrahedral coordination (Rozenson and Heller-Kallai, 1980). Abu Eid et al (1978) obtained only one Fe z+ doublet for yoderite, and the assignment is uncertain.…”

“…of 2.01 mm/sec (Kreber et al, 1975), but the site is so distorted that the Structure cannot serve as a general model. Several structures have been reported in which Fe s § occurs in very distorted 6-coordination, and the associated doublets have a very large Q.S., e.g., 1.8 ram/ sec for andalusite (Rozenson and Heller-Kallai, 1980) and 1.97 mm/sec for epidote (Dollase, 1971). By analogy with the structure of pyrophyllite dehydroxylate (Wardle and Brindley, 1972) it is to be expected that the 6-coordinated sites in dehydroxylated phyllosilicates are very distorted.…”