To explain the large values of the quadrupole splittings and line widths in the Mössbauer spectrum of oxybiotite, a formalized analysis of the distribution of effective charges in the anion network was conducted using an idealized model of the biotite octahedral layer. Based on this analysis, a qualitative physical model has been proposed, according to which the Mössbauer spectrum consists of a superposition of six quadrupole doublets. The proposed model explains the appearance of residual magnetization well below the biotite decomposition temperature. Annealing the biotite at 1070°K leads to the formation of nuclei of the magnetic phase in chains consisting of R3+ and vacancies. This phase is the decomposition product of biotite at 1370°K, i.e., a ferrispinel having Fe2−xMgAlxO4 (x = 0.5–0.6) composition.

The 57Fe Mössbauer spectra of an iron-rich montmorillonite, an illite, and two glauconites were measured and computer-fitted with appropriate Fe3+ and Fe2+ doublet resonances. The broad experimental Fe3+ resonance of montmorillonite probably arises from Fe3+ in the octahedral sites and a trans-arrangement of OH groups; however, a large variation in the neighboring environment of these sites exists. In illite this Fe3+ resonance is similar but shows less broadening; it arises from Fe3+ located predominantly in trans-OH octahedral sites, with some Fe3+ being located in cis-OH octahedral sites. Because of the increased iron content less variation exists, compared with montmorillonite, in the neighboring octahedral sites. The Fe3+ resonance is narrower still for the glauconites and represents Fe3+ substituting primarily into cis-OH octahedral sites, similar to that previously reported for nontronite.

The tetrahedral Fe3+ content is very low for montmorillonite and increases progressively for illite and glauconite, suggesting that a higher tetrahedral Fe3+ content directs Fe3+ in the octahedral layer into cis-OH sites. In montmorillonite, the Fe2+ is located only in trans-OH sites; in illite Fe2+ is largely in trans-OH sites and only slightly in cis-OH sites; and in glauconite, Fe2+ is located largely in cis-OH sites and only slightly in trans-OH sites. These assignments suggest that for Fe2-, the doublet with the larger quadrupole interaction arises from Fe2+ in trans-OH sites and the doublet with the smaller quadrupole interaction, from Fe2+ in cis-OH sites.

57Fe Mössbauer spectra obtained at room temperature and 78 K for natural Mg-saturated, Ca-saturated, and benzidinium-intercalated vermiculite having a high tetrahedral iron content are presented. For all samples the spectra were computer-fitted with five overlapping doublets, representing Fe in both tetrahedral and octahedral sites. One doublet has parameters consistent with the weathered ilmenite known to be present as inclusions. For the intercalated vermiculite, the δ value of the doublet assigned to the tetrahedral Fe3+ increased with respect to the untreated sample, suggesting that the electron densities about the Fe sites had decreased following intercalation. A charge movement from the silicate layers towards the interlayer monovalent benzidinium ions is also implied. The direction of this charge movement is opposite to that found when blue monopositive radical cations form on montmorillonite surfaces. The Mössbauer evidence suggests the absence of an interlayer-Fe3+ complex.

A sample of ferripyrophyllite, a mineral with ideal composition Fe2SiO10(OH)2, was analyzed by Mössbauer spectroscopy, magnetization and magnetic susceptibility measurements. The iron is entirely ferric, and the main feature of the Mössbauer spectrum at room temperature is a single absorption line with isomer shift δ = 0.36(1) mm/sec due to iron in M2 sites. The line is broadened by a slight quadrupole splitting Δ = 0.18(1) mm/sec, the smallest yet reported for a sheet silicate. Weaker features corresponding to quadrupole doublets with relative intensities of 7% and 8% having δ = 0.43(1) mm/sec; Δ = 1.22(2) mm/sec and δ = 0.14(1) mm/sec; Δ = 0.59(2) mm/sec are assigned to ferric iron in nonequivalent octahedral and tetrahedral sites, respectively, giving the following formula, based on published chemical analysis: Ca0.05(Fe3+1.87Mg0.11Al0.09)VI(Si3.80Fe3+0.16Al0.04)IVO10(OH)2, where Ca is in interlayer sites. Ferripyrophyllite orders antiferromagnetically at 18(2)°K. The Mössbauer spectrum at 4.2°K consists of a single, resolved magnetic pattern with hyperfine field Bhf = 51.8 T. The relatively high Néel temperature compared with other dioctahedral ferric phyllosilicates provides further evidence that Fe3+ cations tend to be ordered on M2 sites within the octahedral sheet.

A new approach is described to computer simulate cation distribution in octahedral sheets of dioctahedral 2:1 layer silicates with vacant trans-octahedra. This approach makes use of the information on cation distribution at the one-dimensional level provided by integrated IR optical densities for the region of OH-stretching frequencies. By using this program it is possible to show that (1) the Mössbauer spectrum of glauconite B. Patom conforms to the structural model composed of celandonite-like and illite-like domains whose dimensions are limited by approximately 2 or 4 unit cells; (2) non-equivalency of “left” and “right” cis-positions (with fixed b-direction) with respect to R2+ and R3+ occupancy is a characteristic feature of a celadonite-like domain.

Seven Al-containing lepidocrocite samples, γ-Fe1−xAlxOOH, prepared from FeCl2/Al(N03)3 solutions with initial Al/(Al + Fe) mole ratios Ci of 0.0025, 0.01, 0.025, 0.05, 0.075, 0.10 and 0.15 mol/mol, were examined by means of Mössbauer spectroscopy at room temperature (RT) and at various temperatures in the range of 8 to 80 K. The spectra at RT and 80°K consist of broadened quadrupole doublets and were analyzed in terms of a single doublet and of a model-independent quadrupole-splitting distribution, the latter yielding the best fit. The observed variations of the quadrupole-splitting parameters with increasing Ci are inconclusive as to whether the Al cations are substituting into the structure. The temperature at which the onset of magnetic ordering is reflected in the spectra, was measured by the thermoscan method with zero source velocity. A gradual shift from 50 K for Ci = 0.0025 mol/mol to 44 K for Ci = 0.10 mol/mol was observed for that temperature. As compared to earlier studies of Al-free γ-FeOOH samples with similar morphological characteristics, the fractional doublet area in the mixed sextet-doublet spectra at 35 K is significantly higher for the present lepidocrocites. This observation is ascribed to the substitution of Al cations into the lepidocrocite structure. A similar conclusion is inferred from the variation with Ci of the maximum-probability hyperfine field derived from the spectra recorded at 8 K and fitted with a model-independent hyperfine-field distribution. The magnetic results suggest that for the sample corresponding to Ci = 0.15 mol/mol, not all of the initially present Al has been incorporated into the structure.

The Mössbauer and infrared spectra of Li- and Rb-saturated nontronites from Washington, USA (SWa-1), heat treated at different temperatures, were measured. The Mössbauer results show that diffusion of Li+ into the 2:1 layer by heat treatment at 250-300°C does not affect the magnetic hyperfine interactions of the octahedral iron centers. In contrast, significant changes were observed by calcination at temperatures >300°C for Li-saturated nontronites. The main features of the Mössbauer spectra recorded at different temperatures reveal superparamagnetic behavior with a blocking temperature of ~5 K. The superparamagnetic behavior is related to small magnetic domains created by partially broken Fe-O-Fe bonds upon heat treatment. The infrared spectra of Li-rich nontronite, heat treated at 300°C, show changes attributed to Li migration into the hexagonal cavities. Heating to higher temperatures, produced changes in the spectra of the Li- and Rb-saturated nontronites owing to the dehydroxylation of the layer structure. The dehydroxylation process begins with the loss of OH groups in the FeOHFe bridges and is completed with the disruption of the Al-O bonds at >600°C.

A Mössbauer investigation has been carried out on garnets from the Ivrea-Verbano zone and the results are compared with those obtained on the same samples by X-ray absorption spectroscopy (XAS). The problem addressed is the precise structural characterisation of the local environment of iron in garnets with Fe/Ca ratio variable between ∼3.0 and ∼18.0. Ferric iron is octahedrally coordinated and ferrous iron is in the dodecahedral site in all the samples. Mössbauer results are in agreement with those obtained by XAS and show that, at least in the compositional range of the garnets examined (0.1-0.5 calcium atoms p.f.u.), the iron environment is not significantly modified by the larger calcium cations sharing the same dodecahedral site. It is confirmed that the Mössbauer technique is more sensitive than XAS in detecting low percentages of iron, especially when the cation is present in more than one oxidation state and coordination number.

Fitted Mössbauer spectra are presented for a set of anthophyllites and gedrites. Despite their complexity it is possible to obtain fits for gedrite spectra and the Mössbauer parameters are consistent with results for anthophyllites from this and previous studies. At least three doublets are required to fit the Fe2+ absorption in gedrites; there is no single model at present to explain the behaviour of the spectra. Fe3+ absorption is also resolved.

Chromites from two horizons of the Sukinda area (India) marked as ‘grey ore’ and ‘brown ore’ zones have been studied by 57Fe Mössbauer spectroscopy, which revealed that both chromite types are oxidised and have a type of disordered spinel structure in which octahedral sites are occupied by Fe2+ ions.

The spectra of the grey ore sample can be fitted to three doublets corresponding to Fe2+ (A), Fe3+ (A) and Fe2+ (B) sites. This sample is less oxidised than the brown ore, in which progressive oxidation in the magmatic (?) stage led to the complete conversion of Fe2+ in A sites to Fe3+. The spectra of the brown ore are characterised by two doublets correpsonding to two tetrahedral (A) sites of Fe3+ with different next-nearest neighbour configurations and a third doublet for Fe2+ at the B site. The brown ores have higher chromium and Fe3+ content and have lesser amounts of Ni and Al in comparison to the grey ores. Megascopically, the former shows larger crystal sizes. The high Fe3+ content in the brown ore suggests that this type of chromite was formed in a region of high ƒO2 in the magmatic environment. This perhaps occurred at the part of the mantle where the temperature was higher and the rate of cooling was slower than that of the grey ores which crystallised in the magmatic melt.