The Mössbauer spectra of several micas and related minerals

Abstract

SUMMARY.Well-resolved M6ssbauer spectra of several dioctahedral and trioctahedral micas have been obtained and subjected to detailed computer analysis. Most of the spectra can be resolved into two ferrous quadrupole doublets and one ferric quadrupole doublet. In dioctahedral micas, ferrous iron is seen to occupy the larger, more symmetric octahedral site in preference to the smaller, less symmetric site, confirming predictions made on the basis of structure alone. In trioctahedral micas there is still a tendency for ferrous iron to occupy preferentially the more symmetric octahedral site even though the two octahedral sites are no longer distinguished by size. A lithium-rich biotite gives a spectrum typical of that expected from a mica with a zinnwaldite structure, but the spectrum of a more authenticated zinnwaldite could not be resolved sufficiently for detailed structural interpretation. The spectra of pyrophyllite and talc give results that can be related to the corresponding micas by considering their respective structural differences. In none of the micas studied was there evidence of ferric iron in tetrahedral coordination. The ferrous:ferric ratios obtained from the M6ssbauer spectra do not always agree with the chemical values. In several cases the spectra show more ferrous iron than found chemically. It is suggested that the spectral values are more accurate, oxidation on chemical analysis being the most likely source of error. THE MSssbauer spectra of various micas have been obtained by several authors (Pollak et al., I962; Herzenberg and Toms, t966; Weaver et al., I967; Malden and Meads, I967; Herzenberg et al., I968; Bowen et al., I969) but nowhere has a detailed analysis of the spectra in terms of mica structures been presented. This is probably due to the low iron content of many micas coupled with orientation effects, which give confusing spectra. Bowen et al. (I969) have obtained well-resolved spectra of several micas, but they found no evidence for more than one ferrous and one ferric quadrupole doublet, and have concentrated on the change in oxidation state of the iron in the micas under artificial weathering conditions. Bancroft et al. 0967, 1968) have shown how detailed computer analysis of M6ss-bauer spectra in minerals can lead to accurate interpretations of the spectra in terms of crystal structure. Similar techniques have been applied here to well-resolved spectra of a number of micas and related sheet silicates, and the spectra interpreted in terms of structure and site occupation.Sample description and preparation. Of the twelve sheet silicates investigated, five were

Mössbauer Spectra of Glauconites Reexamined

“…Even an iron-rich smectite, which was effectively reduced, gave rise to only one Fe 2+ doublet (Rozenson and Heller-Kallai, 1976). Two Fe z+ doublets were observed for two muscovites studied by Hogg and Meads (1970) and for six samples investigated in our laboratory (unpublished). According to Annersten the Fe 2+ in glauconites is disordered, in contrast to Fe 3+, which tends to be ordered.…”

Mössbauer Spectra of Dioctahedral Smectites

“…The points corresponding to sample l saturated with various cations fall on the straight lines, but, for clarity, are not included in the figures. Points corresponding to metabentonite (MB, sample 15) and to pyrophyllite (P), for which the parameters were taken from the literature (Wardle and Brindley, 1970;Hogg and Meads, 1970) and which contain Fe 3+ in M(2) sites only, fall on the straight line of Figure 4, as shown.…”

A Mössbauer Investigation of Glauconite and its Geological Significance

“…On a size basis it would be expected that Fe 2+ would favor the larger trans site; however, preference may be shown for the more readily distorted cis site. Hogg and Meads (1970) suggested that most Fe 2+ in dioctahedral micas occupies the trans sites; it is probable that the same relationship applies in the case of the glauconite (e.g., Rolf et al, 1977) which, although predominantly dioctahedral, may have some trioctahedral character (McRae, 1972). Because most of the octahedral Fe 2+ in glauconite is in the site with the larger QS (see Table 3), the present data suggest that the Fe 2+ occurs preferentially in the trans M(1) site.…”