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Abstract: Mssbauer spectra (MS) of anapaite (Ca2 Fe2+(PO4)2  4H2O) and of a sample after being immersed in a 4% H2O2 solution at room temperature (RT) over 12 days (hereafter an4ox) were collected at temperatures in the range 4.2 to 420 K and 11 to 300 K respectively. All MS consist of symmetrical doublets, hence magnetic ordering was not observed. The temperature dependencies of the Fe2+ centre shifts of anapaite and an4ox were analysed with the Debye model for the lattice vibrations. The characteristic Mssbauer temperatures were found as 370 K  25 K and 340 K  25 K, and the intrinsic isomer shifts as 1.427  0.005 mm/s and 1.418  0.005 mm/s respectively. From the external-field (60 kOe) MS recorded at 4.2 and 189 K for the non-treated sample, the principal component V zz of the electric field gradient (EFG) is determined to be positive and the asymmetry parameter η ≈ 0.2 and 0.4 respectively. The temperature variations of the quadrupole splittings, ΔE Q(T), cannot be interpreted on the basis of the thermal population of the 5 D electronic levels resulting from the tetragonal compression of the O6 co-ordination. The low-temperature linear behaviour of ΔE Q(T) is attributed to a strong orbit-lattice coupling. A field of 60 kOe applied to anapaite at 4.2 K produces magnetic hyperfine splitting with effective hyperfine fields of −136, −254 and −171 kOe along the principal axes Ox, Oy and Oz of the EFG tensor respectively. Additional oxidation treatments in solutions with various H2O2 concentrations up to 20% and subsequent Mssbauer experiments at room temperature, have revealed that the anapaite structure is not sensitive to oxidation since eventually only a small amount of Fe2+ (∼6.5%) is converted into Fe3+.

Mössbauer spectra for two riebeckite minerals were collected at temperatures in the range 4.2 to 500 K. The magnetic-ordering temperatures were found to be 33±1 and 31±1 K respectively. Fitting the paramagnetic spectra with a discrete number of doublets (three or four) did not lead to consistent results. Instead, a superposition of an Fe3+ (M2) doublet and one distributed ferrous component was found to produce adequate fits with reasonable parameter values. For both samples, a minor fraction of ferrous ions was observed to be present at the M4 sites and for one of the samples at the M2 sites as well. The temperature variations of the center shifts were well reproduced using the Debye model of the lattice vibrational spectrum to evaluate the second-order Doppler shift. The characteristic Mössbauer temperatures were calculated to be in the range 340–390 K for Fe2+, and 520 K for Fe3+. The temperature dependences of the various ferrous quadrupole splittings could not be explained in terms of the point-charge model and assuming a temperature-independent energy-level scheme for the 5D term. It is suggested that a gradual change with temperature of the orbital-level splittings takes place. All calculations yielded a positive sign for the principal component of the electric field gradient (EFG). The spectrum recorded at 4.2 K for one of the riebeckites was fitted with a superposition of an Fe3+ and a Fe2+ hyperfine-field distribution, the latter one primarily characterizing the Fe2+ (M1) cations. The following relevant hyperfine data were calculated: Hhf=161 kOe, ?EQ=3.11 mm/s, and Vzz<0, all referring to the maximum-propability values. For the second riebeckite at 4.2 K, an additional distributed ferrous component could independently be resolved. The two maximum-probability hyperfine fields were found to be 189 and 98 kOe and the corresponding ?EQ values 3.10 and 2.67 mm/s. Both components exhibit a negative Vzz. The subspectra were attributed to M1 and M3 sites respectively. The Fe3+ hyperfine fields are 548+-2 kOe for both riebeckites. The different values found for the Fe3+ quadrupole shift 2?Q for the two samples is explained by a different angle between the hyperfine field and the EFG's principal axis. The magnetic spectra recorded at 15 K and higher, could not be reproduced adequately with reasonable parameter values.

Synthetic aluminum-substituted maghemite samples, ?-(AlyFe1-y)2O3 with y=0, 0.032, 0.058, 0.084, 0.106 and 0.151 have been studied by Mössbauer spectroscopy at 8 K and in the range 80 K to 475 K at steps of 25 K. The spectra have been analysed as a superposition of two sextets composed of asymmetrical Lorentzians. The A- and B-site isomer shifts were constrained as: dA=dB-0.12 mm/s. From the temperature dependence of dB it was possible to determine the characteristic Mössbauer temperature and the intrinsic shift. Both quantities clearly increase with increasing Al content, at least up to 10 mole%. The temperature dependence of the A-and B-sites hyperfine fields could be satisfactorily reproduced using the molecular-field theory assuming an antiparallel spin configuration. The exchange integrals were found as: JAB=-25 K; JAA=-18 K and JBB= -3 K. The hyperfine fields show a crossing in the vicinity of 300 K as a result of the relatively strong A-A interaction. The Curie temperature for the non-substituted sample was calculated to be 930 K and decreases to 765 K for the sample with 15 mol% Al. The gradual decrease of the saturation value of the A-site hyperfine field with increasing Al substitution and the constancy of this quantity for the B sites, suggest that the Al cations occupy the B sites.

The Mössbauer fractions f for various ferrous- and/or ferric-containing oxides and oxyhydroxides, silicates and carbonates were evaluated from the experimental temperature dependence of their center shifts, using the Debye approximation for the second-order Doppler shift. It is concluded that ferrous ions exhibit a lower fraction as compared to ferric ions. Using standard mixtures of a-Fe2O3 with selected Fe2+ or Fe3+ compounds, it is found that the calculated Fe3+f values are somewhat overestimated with respect to those of Fe2+. Possible explanations for this shortcoming are discussed and it is suggested that a different temperature dependence of the intrinsic isomer shift is the most likely reason. This suggestion is corroborated by analyses of hematite and hedenbergite data which are available for temperatures up to 900 K and 800 K respectively.

The lepidocrocite (?-FeOOH) to maghemite (?-Fe2O3), and the maghemite to hematite (a-Fe2O3) transition temperatures have been monitored by TGA and DSC measurements for four initial ?-FeOOH samples with different particle sizes. The transition temperature of ?-FeOOH to ?-Fe2O3 and the size of the resulting particles were not affected by the particle size of the parent lepidocrocite. In contrast, the ?-Fe2O3 to ?-Fe2O3 transition temperature seems to depend on the amount of excess water molecules present in the parent lepidocrocite. Thirteen products obtained by heating for one hour at selected temperatures, were considered. Powder X-ray diffraction was used to qualify their composition and to determine their mean crystallite diameters. Transmission electron micrographs revealed the particle morphology. The Mössbauer spectra at 80 K and room temperature of the mixed and pure decomposition products generally had to be analyzed with a distribution of hyperfine fields and, where appropriate, with an additional quadrupole-splitting distribution. The Mössbauer spectra at variable temperature between 4.2 and 400 K of two single-phase ?-Fe2O3 samples with extremely small particles show the effect of superparamagnetism over a very broad temperature range. Only at the lowest temperatures (T?55 K), two distributed components were resolved from the magnetically split spectra. In the external-field spectra the ?mI=0 transitions have not vanished. This effect is an intrinsic property of the maghemite particles, indicating a strong spin canting with respect to the applied-field direction. The spectra are successfully reproduced using a bidimensional-distribution approach in which both the canting angle and the magnetic hyperfine field vary within certain intervals. The observed distributions are ascribed to the defect structure of the maghemites (unordered vacancy distribution on B-sites, large surface-to-bulk ratio, presence of OH- groups). An important new finding is the correlation between the magnitude of the hyperfine field and the average canting angle for A-site ferric ions, whereas the B-site spins show a more uniform canting. The Mössbauer parameters of the two hematite samples with MCD104 values of respectively 61.0 and 26.5 nm display a temperature variation which is very similar to that of small-particle hematites obtained from thermal decomposition of goethite. However, for a given MCD the Morin transition temperature for the latter samples is about 30 K lower. This has tentatively been ascribed to the different mechanisms of formation, presumably resulting in slightly larger lattice parameters for the hematite particles formed from goethite, thus shifting the Morin transition to lower temperatures.

Four synthetic lepidocrocite samples with a significant difference in crystallinity have been studied by the 57Fe Mössbauer effect technique at temperatures ranging between 12 K and 360 K. In the magnetically ordered region, the spectra display broad hyperfine field distributions, the profile of which could be derived numerically. In a broad transition region of approximately 20 K, an additional doublet must be included in the fitting model. From Mössbauer thermoscanning measurements at zero-velocity and with and without the application of an external magnetic field, it is found that the doublet is due to Fe3+ species in a paramagnetic state. Neither the hyperfine field nor the Néel temperature distributions are markedly affected by the crystallinity. In the pure paramagnetic state, the broadened doublets are best described by a distribution of quadrupole splittings in the range 0.3–1.9 mm/s. By numerical manipulations of the derived probability profiles it was possible to distinguish between the iron species in the outer surface layers of the γ-FeOOH particles and those in the innermost layers, the relative amount of each being correlated with the measured surface areas. The centre shift is quite uniform and its temperature dependence, determined in detail for one of the samples, is perfectly described by the Debije approximation for the second-order Doppler shift. From the obtained Debije temperature, a Mössbauer fraction f of 0.79 at room temperature was calculated, which is in good agreement with the results obtained for a reference lepidocrocite/hematite mixture. Finally, line shape simulations aimed to explain the external field spectrum at 180 K provided strong indications that both the sign and the asymmetry parameter of the electric field gradient are non-uniform as well.

It is shown that under ambient atmospheric conditions heating causes the crystal structure of ßFeOOH (synthetic akaganeite) to degenerate gradually into a quasi amorphous intermediate state, before the final phase transformation to aFe2O3 (hematite) takes place. Using X-ray diffraction (XRD), transmission electron microscopy (TEM), thermogravimetric analysis (TGA) and Mössbauer Spectroscopy, this amorphization process is monitored and the structural, morphological and magnetic features of the intermediate phase as a function of the isochronal heating temperature are discussed: the crystallites develop macropores on their surface, the adsorption capacity raises up to 10 percent of the initial mass, a third type of Fe3+ coordination, having an extremely large quadrupole splitting, is created and the Néel temperature, after an initial decrease, exhibits a sharp increase at higher heating temperatures. The magnetic behaviour of this intermediate phase at low temperatures and in high external fields suggests this antiferromagnet undergoes magnetic phase transitions (metamagnetism and spin flop) at unusually low critical fields.

A natural sample of ankerite has been characterized by chemical analysis, X-ray diffraction and differential thermal analysis. The composition was found to be (Ca1.11Mg0.50Fe0.33Mn0.09) [Co3]1.99.57Fe Mössbauer effect measurements were performed at temperatures between 4.2 and 400 K. At low temperatures (T < 25 K) relaxation effects are clearly dominant. The temperature dependence of the center shift is remarkably well reproduced by a model based on the Debije approximation of the lattice vibrations. In contrast, the temperature dependence of the quadrupole splitting cannot be described by any reasonable crystal field model. It is argued that an orbit-lattice coupling might explain the observed quadrupole splittings. A spectrum recorded in an applied field of 6 T reveals a positive electric field gradient from which an orbital doublet ground state is concluded. Highly anisotropic field reductions are derived but cannot be quantitatively explained due to a lack of knowledge concerning the magnetic structure of ankerite. The line widths decrease significantly with increasing temperature which is only partly due to the decreasing Mössbauer fraction.

Three chloritoid samples from the Stavelot massif (Belgium) and one sample from the Serpont massif (Belgium) have been characterized by chemical analyses and differential X-ray diffraction. A classification of chloritoid is proposed. Mössbauer spectra at temperatures between 78 and 360 K and in external magnetic fields were obtained for a triclinic and for a monoclinic specimen. The spectra show a superposition of a weak Fe3+ doublet (less than 10%) and an intense Fe2+ doublet. It is found that a decomposition of the ferrous absorption into two distinct quadrupole doublets leads to smaller deviations between experimental and calculated line shapes, especially at low temperatures. This suggests that Fe2+ is present in both cis and trans O2(OH)4 octahedral positions in the trioctahedral layer. However, the structural data derived from the temperature dependence of isomer shifts and quadrupole splittings, are found to be inconsistent with known crystallographic data. It is therefore concluded that Fe2+ is present in only one type of lattice site and that the numerically imposed decomposition into two ferrous doublets is merely an artifact due to thickness saturation effects and to the distributive character of the hyperfine parameters. The negative sign of the electric field gradient further confirms the assignment of the Fe2+ doublet to a cis octahedral configuration. Finally, only minor differences between the Mössbauer results for triclinic and monoclinic chloritoid are observed. The infrared absorption spectra of the four samples are almost identical except in the region around 600 cm-1 at which the monoclinic phase exhibits two absorption bands instead of one band for the triclinic samples. All absorption bands can be well assigned to the different vibrations. Inter-layer hydrogen bonding is evidenced by the occurrence of two vO-H absorption bands. Furthermore, the specific nature of the infrared spectra enables a fast identification of chloritoid samples.

57Fe Mössbauer measurements were performed on two natural orthopyroxenes in the temperature range between 17 and 490 K. The temperature variations of the center shifts and of the quadrupole splittings have been interpreted using the Debye model for the lattice vibrations and the crystal-field model respectively. Two approaches have been applied to evaluate the crystal field. The first one, which is commonly used by Mössbauer spectroscopists, emanates from the approximative and simplified symmetry of the ferrous sites, whereas the second one takes into account the real C1 symmetry of the ferrous sites, thus leading to a pointcharge calculation. For comparison, analogous calculations have been carried out on literature data for an iron-rich orthopyroxene (specimen XYZ).

Three natural hematites, a-Fe2O3, from the region of Elba have been investigated by means of 57Fe Mössbauer spectroscopy at variable temperatures between 80 and 400K. The samples were selected on the basis of their different morphology as observed from powder X-ray diffraction and transmission electron microscopy. The mean crystallite diameters (MCD) along [104] are 1000, 280 and 40 nm respectively. Energy-dispersive analyses of X-rays revealed the presence of minor amounts of Si impurities in those two hematites having the largest MCD. All three hematites show the coexistence at low temperatures of antiferromagnetic-like (AF) and weakly-ferromagnetic-like (WF) spin states. The saturation values of the AF and WF magnetic hyperfine fields and quadrupole shifts have been determined, from which conclusions are drawn concerning the spin structure in relation to the crystallinity of the samples. The variations of the hyperfine parameters in the Morin-transition region indicate a gradual reorientation of both AF and WF spins towards the basal plane. As expected, the Morin transition itself is affected by the particle size. The two hematites exhibiting the largest particle dimensions still show an AF contribution for T>270K. It is suggested, and argued that this unusual behaviour is due to the presence of Si4+, and hence Fe2+, in the lattice. The characteristic Mössbauer temperatures and the intrinsic isomer shifts were evaluated from the temperature variation of the observed isomer shifts. Both parameters are not significantly affected by the morphology and are in excellent agreement with data obtained for synthetic hematites.

Five chlorite samples have been studied by 57Fe Mössbauer spectroscopy at different temperatures. The spectra are consistently described by a superposition of one ferric and three ferrous quadrupole doublets. All hyperfine data exhibit only minor variations from sample to sample, meaning that the structural and electronic properties of chlorite are rather insensitive to compositional changes and local cation disorder. Two of the ferrous doublets are ascribed to cis and trans co-ordinations in the T-O-T layers. The third ferrous doublet is interpreted as being due to ferrous ions in the hydroxide sheets. Its hyperfine parameters are close to those of the former doublets and the observed deviations from the parameter values found for pure brucite are discussed. Approximately 20 to 25 percent of the total iron is situated in the hydroxide sheets. All five chlorites contain about 10 percent Fe3+ which is most probably located on octahedral sites. The temperature dependence of the ferrous quadrupole splittings for two chlorites has been interpreted on the basis of the thermal populations of the two lowest electronic states of the Fe2+ ions. From this, the deformations of the different octahedral co-ordinations could be estimated.