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1.
Magnetization, susceptibility and Mössbauer spectra are reported for representative chlorite samples with differing iron content. The anisotropy of the susceptibility and magnetization of a clinochlore crystal is explained using the trigonal effective crystal-field model developed earlier for 1:1 and 2:1 layer silicates, with a splitting of theT 2g triplet of 1,120K. Predominant exchange interactions in the iron-rich samples are ferromagnetic withJ=1.2 K, as for other trioctahedral ferrous minerals. A peak in the susceptibility of thuringite occurs atT m=5.5 K, and magnetic hyperfine splitting appears at lower temperatures in the Mössbauer spectrum. However neutron diffraction reveals no long-range magnetic order in thuringite (or biotite, which behaves similarly). The only magnetic contribution to the diffraction pattern at 1.6 K is increased small angle scattering (q<0.4 Å?1). A factor favouring this random ferromagnetic ground state over the planar antiferromagnetic state of greenalite and minnesotaite is the presence of pairs of ferric ions on adjacent sites, in conjunction with magnetic vacancies in the octahedral sheets. Monte Carlo simulations of the magnetic ground state of the sheets illustrate how long range ferromagnetic order may be destroyed by vortices forming around the Fe3+-Fe3+ pairs.  相似文献   

2.
Natural barbosalite Fe2+Fe3+ 2 (PO4)2(OH)2 from Bull Moose Mine, South Dakota, U.S.A., having ideal composition, was investigated with single crystal X-ray diffraction techniques, Mössbauer spectroscopy and SQUID magnetometry to redetermine crystal structure, valence state of iron and evolution of 57Fe Mössbauer parameter and to propose the magnetic structure at low temperatures. At 298?K the title compound is monoclinic, space group P21/n, a o ?= 7.3294(16)?Å, b o ?=?7.4921(17)?Å, c o ?=?7.4148 (18)?Å, β?=?118.43(3)°, Z?=?2. No crystallographic phase transition was observed between 298?K and 110?K. Slight discontinuities in the temperature dependence of lattice parameters and bond angles in the range between 150?K and 180?K are ascribed to the magnetic phase transition of the title compound. At 298?K the Mössbauer spectrum of the barbosalite shows two paramagnetic components, typical for Fe2+ and Fe3+ in octahedral coordination; the area ratio Fe3+/Fe2+ is exactly two, corresponding to the ideal value. Both the Fe2+ and the Fe3+ sublattice order magnetically below 173?K and exhibit a fully developed magnetic pattern at 160?K. The electric field gradient at the Fe2+ site is distorted from axial symmetry with the direction of the magnetic field nearly perpendicular to Vzz, the main component of the electric field gradient. The temperature dependent magnetic susceptibility exhibits strong antiferromagnetic ordering within the corner-sharing Fe3+-chains parallel to [101], whereas ferromagnetic coupling is assumed within the face-sharing [1?1?0] and [?1?1?0] Fe3+-Fe2+-Fe3+ trimer, connecting the Fe3+-chains to each other.  相似文献   

3.
Three iron-rich 1:1 clay minerals, greenalite [Si2]{Fe 3 2+ }O5(OH)4, berthiérine [Si, Al]2{Fe2, Mg, Fe3+, Al}3 O5(OH)4 and cronstedtite [Si, Fe3+]2{Fe2+, Fe3+}3O5(OH)4 have been studied by Mössbauer spectroscopy, magnetization measurements and neutron diffraction to determine their magneticproperties. The predominant magnetic coupling is ferromagnetic for pairs of ferrous ions in the octahedral sheet, but antiferromagnetic for ferric pairs. The crystal field at Fe2+ sites in greenalite and berthiérine is effectively trigonal with an orbital singlet l z=0 as ground state. These mainly ferrous minerals order magnetically at 17K and 9K respectively. The magnetic structure of greenalite consists of ferromagnetic octahedral sheets, with the moments lying in the plane, coupled antiferromagnetically by much weaker interplane interactions. The ratio of intraplane to interplane coupling is of order 50, so the silicate has a two-dimensional aspect, both structurally and magnetically. Although the overall magnetic order is established as antiferromagnetic by neutron diffraction, the magnetization curves resemble those of a ferromagnet because of the very weak interplane coupling. Cronstedtite orders antiferromagnetically around 10K. Moments within the planes are antiferromagnetically coupled. The magnetism has no particular two-dimensional character because exchange paths between the layers are provided by the ferric cations present in the tetrahedral sheets.  相似文献   

4.
Mössbauer spectra of biotite at 4 K are reported. The biotite crystals were oriented with the c-axis parallel to the γ-ray direction and some spectra were recorded with external magnetic fields of 40 kOe applied at right angles to the c-axis. Decrease of the magnetic-hyperfine field of both Fe2+ and Fe3+ ions on application of the external field shows that both Fe3+-Fe3+ pairs and Fe2+-Fe2+ pairs are coupled ferromagnetically.  相似文献   

5.
Ilvaite, Ca(Fe2+, Fe3+)Fe2+Si2O8(OH), a black mixed valence iron silicate shows considerable Fe2+?Fe3+ electron delocalization above 400 K, reminiscent of magnetite. A crystallographic phase transition from orthorhombic (Pnam) to monoclinic (P2 1/a) symmetry takes place on cooling at 343 K induced by electron ordering. In both phases, Fe2+ and Fe3+ occur in double octahedral chains parallel to the c axis. The thermal characteristics of the magnetic susceptibilities and their anisotropies in different crystallographic planes have been measured in the temperature range 400?21 K. Below 343±1K, a continuous rotation of the molar susceptibility K ∥ in the ab plane down to 90±2 K is observed, where the symmetry of the magnetic ellipsoid remains unchanged. X a, X b and X c increase abruptly below 123±0.5 K, although antiferromagnetic ordering of Fe2+ and Fe3+ spins on A sites was suggested in previous Mössbauer and neutron powder diffraction studies. In addition, 1/X a shows an antiferromagnetic maximum at 50±3 K, whereas 1/X b and 1/X c at first increase sharply below 123 K, followed by antiferromagnetic curvatures in the lowest temperature region. This behavior is consistent with the antiferromagnetic ordering of Fe2+ spins in the B sites. The observed magnetic phenomena suggest charge delocatization effects between adjacent Fe2+(A)-Fe3+(A) pairs not only along c, but also along a and b directions. The negative sign of the molar anisotropy (K ∥-K⊥) suggests a singlet ground State 5A1 for the Fe2+ ions, in agreement with previous Mössbauer studies.  相似文献   

6.
Mössbauer spectra of biotites (1) and (2) with relative iron concentrations ～1:1·6 and of their oxidation products are recorded at 4 K in zero field and in applied fields up to 5 T. Magnetic susceptibility data are also reported. The results show that Fe III spins are in a ferromagnetic configuration in the c-plane in both biotites. Partial oxidation of biotite (1) leads to a canted ferromagnetic structure, while complete oxidation of biotite (2) yields an antiferromagnetic spin configuration. Nearest-neighbour antiferromagnetic Fe III-O-Fe III, and ferromagnetic Fe III-O-Fe II and Fe II-O-Fe II superexchange can account for the results. For biotites with higher concentrations of iron, Fe II and Fe III seem to be distributed randomly in the triangular lattice. From susceptibility results in biotites dilute in iron, an estimate of the ratio of nearest-neighbour and next-nearest neighbour magnetic interaction in the triangular lattice is derived.  相似文献   

7.
The magnetic properties of two samples of acmite, one natural and the other synthetic, were determined using magnetization and susceptibility measurements, Mössbauer spectroscopy and neutron diffraction. Exchange interactions are quite strongly antiferromagnetic, the paramagnetic Curie temperature being -46 K for a purely ferric synthetic sample, but its Néel temperature is only 8 K. The principal magnetic mode has the periodicity of the crystallographic structure and is made of ferromagnetic chains, coupled antiferromagnetically. Moments are oriented in a direction close to the chain axis, c. The antiferromagnetic exchange between adjacent Fe3+ ions in the same chain is overcome by their coupling to a common Fe3+ neighbour in the next chain. This indicates that the whole (SiO4) group can act as a superexchange ligand in silicates.  相似文献   

8.
Two samples of cronstedtite, a mixed valence serpentine with ideal formula {Fe 2 2+ ,Fe3+}[Si,Fe3+]O5(OH)4, have been examined by X-ray and neutron diffraction, thermopiezic analysis, magnetization and susceptibility measurements and Mössbauer spectroscopy. The conductivity is thermally activated, with activation energies of 0.25 eV in the basal plane and 0.37 eV in the perpendicular direction. The shape of paramagnetic Mössbauer spectra above 200 K is influenced by charge fluctuations in octahedral sites and fits of spectra at temperatures up to 410 K with a stochastic relaxation model give an activation energy of 0.19 eV. Static charge ordering sets in progressively below about 100 K. Cronstedtite orders antiferromagnetically below 12 K in a structure with antiferromagnetic octahedral sheets and moments perpendicular to the a-axis. Magnetic and charge-ordered structures are proposed for the ideal composition.  相似文献   

9.
Mn2+Sb2S4, a monoclinic dimorph of clerite, and benavidesite (Mn2+Pb4Sb6S14) show well-individualized single chains of manganese atoms in octahedral coordination. Their magnetic structures are presented and compared with those of iron derivatives, berthierite (Fe2+Sb2S4) and jamesonite (Fe2+Pb4Sb6S14). Within chains, interactions are antiferromagnetic. Like berthierite, MnSb2S4 shows a spiral magnetic structure with an incommensurate 1D propagation vector [0, 0.369, 0], unchanged with temperature. In berthierite, the interactions between identical chains are antiferromagnetic, whereas in MnSb2S4 interactions between chains are ferromagnetic along c-axis. Below 6 K, jamesonite and benavidesite have commensurate magnetic structures with the same propagation vector [0.5, 0, 0]: jamesonite is a canted ferromagnet and iron magnetic moments are mainly oriented along the a-axis, whereas for benavidesite, no angle of canting is detected, and manganese magnetic moments are oriented along b-axis. Below 30 K, for both compounds, one-dimensional magnetic ordering or correlations are visible in the neutron diagrams and persist down to 1.4 K.  相似文献   

10.
Ilvaite, Ca(Fe2+,Fe3+)Fe2+Si2O8(OH) shows two magnetic phase transitions, which have been studied by Mössbauer spectroscopy within the temperature range 120–4 K. The continued charge localization between Fe2+ and Fe3+ ions in octahedral A-sites causes the Fe2+-Fe3+ interaction to be ferromagnetic, although the overall magnetic order is antiferromagnetic. The thermal evolution of the hyperfine fields at the Fe2+ (A) and Fe3+ (A) sites indicates B hf: 328 and 523 kOe respectively at 0 K and T N1= 116K. The corresponding values for Fe2+ (B) site are: B hf 186 kOe and T N2=36K. An additional hyperfine field exists at the Fe2+(B) site within the temperature range 116–36K due to short-range order induced by the spin ordering in A sites. The considerable difference between the two magnetic transition temperatures is due to spin frustration, because the Fe2+ (B) site occurs on a corner common between two triangles with respect to two sets of Fe2+ (A) and Fe3+ (A) sites with opposite spin directions.  相似文献   

11.
Reduction of Fe3+ to Fe2+ by heating in hydrogen reduces the absorbance of the bands at 9,000 and 13,800 cm?1 in the E ⊥ c spectrum of tourmaline, and the 9,000 and 11,000 cm?1 bands in the E ∥ (001) spectrum of biotite. This behaviour is consistent with the presumed d-d origin of these bands (which seems well established) only if they gain much of their intensity from exchange-coupling with neighbouring Fe3+ ions. Intensification of spin-forbidden bands in sapphire by Fe3+-Fe3+ exchange-coupling was recognized by Ferguson and Fielding (1971, 1972), but exchange-coupling has not previously been thought to intensify spin-allowed d-d bands. Spin-allowed exchange-coupled bands resulting from Fe2+-Fe3+ pairs have features in common with both normal single ion d-d bands, which they resemble in energy, width and pressure dependence, and Fe2++Fe3+→Fe3++Fe2+ charge transfer bands, which they resemble in temperature-, heat treatment-, composition-, and polarization-dependence. Distinction between normal d-d, charge transfer, and pair d-d absorptions is thus complicated, and criteria for assigning these bands are discussed. Spin-allowed exchange-coupled pair bands should be sought in the spectra of transition metal clusters (trimers and polymers as well as pairs may be involved) whenever geometry favours their origin. It is possible that the bands near 10,000 and 11,500 cm?1 in blue sapphire, and at about 5,000 cm?1 in titanian garnets are of this type, but many other examples are likely to occur. Exchange-coupling may involve ions other than Fe3+ (e.g., Mn2+, also d 5), although Fe2+-Fe2+ coupling is unlikely to be important at laboratory temperatures.  相似文献   

12.
Practically identical Mössbauer spectra have been obtained for 40 ferromanganese nodules from a wide variety of marine and fresh-water locations. None of the nodules examined contains more than one weight percent Fe2+, so no more than a few percent of the total iron in these nodules can be Fe2+. Most of the iron is present as Fe3+ in paramagnetic or superparamagnetic oxide phases, although hysteresis loops show the presence of small amounts of ferromagnetic phases not detected by the Mössbauer technique.  相似文献   

13.
Mössbauer spectra of glauconite and nontronite recorded at temperatures down to 1.3K and in applied fields up to 4.5 T show that Fe III spin configurations are respectively ferromagnetic and antiferromagnetic. It is shown that in a particular material depending on the distribution and concentration of Fe III in the silicate sheet either mode might occur. A new model of competing nearest-neighbour (J 1) and next-nearest-neighbour (J 2) magnetic exchange interactions in the triangular lattice is introduced to account for the results. From available magnetic susceptibilities we estimate ∣J 1∣～6∣J 2∣. The results lead to the conclusion that the Fe III cations are highly ordered in glauconite and occupy cis sites so as to maximize their mutual separations.  相似文献   

14.
The color and spectroscopic properties of ironbearing tourmalines (elbaite, dravite, uvite, schorl) do not vary smoothly with iron concentration. Such behavior has often been ascribed to intervalence charge transfer between Fe2+ and Fe3+ which produces a new, intense absorption band in the visible portion of the spectrum. In the case of tourmaline, an entirely different manifestation of the interaction between Fe2+ and Fe3+ occurs in which the Fe2+ bands are intensified without an intense, new absorption band. At low iron concentrations, the intensity of light absorption from Fe2+ is about the same for E∥c and E⊥c polarizations, but at high iron concentrations, the intensity of the E⊥c polarization increases more than ten times as much as E∥c. This difference is related to intensification of Fe2+ absorption by adjacent Fe3+. Extrapolations indicate that pairs of Fe2+-Fe3+ have Fe2+ absorption intensity ～200 times as great as isolated Fe2+. Enhanced Fe2+ absorption bands are recognized in tourmaline by their intensity increase at 78 K of up to 50%. Enhancement of Fe2+ absorption intensity provides a severe limitration on the accuracy of determinations of Fe2+ concentration and site occupancy by optical spectroscopic methods. Details of the assignment of tourmaline spectra in the optical region are reconsidered.  相似文献   

15.
Mossbauer spectroscopy has been used to determine the redox equilibria of iron and structure of quenched melts on the composition join Na2Si2O5-Fe2O3 to 40 kbar pressure at 1400° C. The Fe3+/ΣFe decreases with increasing pressure. The ferric iron appears to undergo a gradual coordination transformation from a network-former at 1 bar to a network-modifier at higher (≧10 kbar) pressure. Ferrous iron is a network-modifier in all quenched melts. Reduction of Fe3+ to Fe2+ and coordination transformation of remaining Fe3+ result in depolymerization of the silicate melts (the ratio of nonbridging oxygens per tetrahedral cations, NBO/T, increases). It is suggested that this pressure-induced depolymerization of iron-bearing silicate liquids results in increasing NBO/T of the liquidus minerals. Furthermore, this depolymerization results in a more rapid pressure-induced decrease in viscosity and activation energy of viscous flow of iron-bearing silicate melts than would be expected for iron-free silicate melts with similar NBO/T.  相似文献   

16.
An attempt has been made to interpret natural sapphire optical absorption spectra with regard to the exchange-coupled pairs of iron impurities in their ferric and ferrous states. Level schemes have been calculated for Fe3+-Fe3+ and Fe2+-Fe2+ pairs of all the orders, their symmetry being observed. The selection rules are discussed. EPR and infrared spectroscopy information has been used to assist the optical spectroscopy data. The sapphire color has been interpreted as a function of Fe2+/Fe3+ ratios and of the presence of their pairs in the stone. The various types of charge compensation in Fe2+→Al3+ isomorphism are discussed.  相似文献   

17.
Magnetization and neutron diffraction measurements have been made on grunerite, Fe7Si8O22(OH)2, a monoclinic double-chain silicate with Fe2+ octahedral bands. The mineral orders antiferromagnetically at 47K into a collinear structure with a second transition at 8K to a canted arrangement. The magnetic susceptibility follows a Curie-Weiss Law above 120K, with a paramagnetic Curie temeprature ?p=67K. Magnetization measurements below 47K indicate a spin-flop or metamagnetic transition in an applied field of about 12KOe. Powder neutron diffraction measurements between 8–45K reveal that all the Fe2+ spins within an octahedral band are ferromagnetically coupled parallel to the b axis, with each band antiferromagnetically coupled to neighboring bands. Below 8K Fe2+ spins at the M1 and M4 sites are canted away from the b axis, whereas those at the M2 and M3 sites are not significantly affected. The ordered Fe2+ moment on the M4 site is substantially lower than those on the other sites, most likely indicating strong covalency effects, i.e. considerable spin transfer to neighboring oxygen atoms.  相似文献   

18.
Ilvaite samples from six different localities in Japan are found to be members of a solid-solution series varying from Ca(Fe2+,Fe3+)2Fe2+(OH)O Si2O7 to approaximately Ca(Fe2+,Fe3+)2Fe 0.5 2+ Mn 0.5 2+ (OH)O Si2O7, and have been studied by Mössbauer spectrometry and magnetic measurements. The variation in intensity of Mössbauer doublets confirms that Mn substitutes for Fe2+ in the M(B) cation site. An temperatures decreasing from 300 K to 4K, an abrupt change in the reciprocal mass magnetic susceptibility, 1/x g, occurs about 120 K; 1/x g depends linearly upon temperature above 120 K. This change, which is characterized by an unusual mode of decrease in 1/x g, has been interpreted based on Mössbauer spectra at 80 K: the spectra of Fe2+ and Fe3+ in the M(A) site show Zeeman splitting, whereas those of Fe2+ in the M(B) site do not show the effect. This Mössbauer evidence suggests that magnetic spins of Fe in M(A) are in an ordered state, very likely of antiparallel coupling, whereas those of Fe in M(B) are randomly oriented, showing that below 120 K ilvaite has two different magnetic states for Fe ions. As there is a line of evidence that the spins of Fe in M(B) would take an ordered state at extremely low temperatures, ilvaite magnetism may be regarded as basically antiferromagnetic. The magnetic spins of Fe in M(A) and M(B) undergo magnetic transitions at different specific temperatures, thus giving as a whole unusual features of magnetism.  相似文献   

19.
The Mössbauer spectra of ilvaite CaFe 2 2+ Fe3+[Si2O7/O/OH] and their temperature dependence between 298 K and 455 K can be satisfactorily least-squares fitted by a superposition of the resonances for Fe2+(8d), Fe3+(8d) and Fe2+(4c). The relative areas under the three resonances are nearly equal and vary only weakly with temperature. No additional resonances or line broadenings have to be introduced, if we assume that the hyperfine interactions of Fe2+(8d) and Fe3+(8d) fluctuate between their values due to electron hopping between the iron ions at the 8d sites. Hopping can be assumed to occur homogeneously among nearly equivalent sites. The fluctuation rate is described by an Arrhenius law with a pre-exponent of about 9 × 108 s?1 and an activation energy of 0.11 eV indicating non-adiabatic hopping. In addition to the intersite hopping process, the strong decrease of the quadrupole splitting and the isomer shift of Fe2+(8d) between 298 K and 360 K suggests the occurrence of intrinsic charge delocalization from Fe2+(8d) which does not involve the neighbouring Fe3+(8d) ions.  相似文献   

20.
Criteria used to identify Fe2+-Fe3+ and Fe2+-Ti4+ intervalence charge transfer absorption bands in electronic spectra are reviewed and compared to the characteristics of unperturbed Fe2+ crystal field bands and those that are intensified by interaction with Fe3+. Band energy is the least definitive diagnostic criterion. Changes in band intensity with temperature are also of limited value. Large widths are the most reliable characteristic of charge transfer bands. New optical absorption spectra are presented for euclase, as well as 80 K spectra of rockbridgeite, babingtonite and lazulite. Comparison of optical spectra to magnetic susceptibility measurements for rockbridgeite and babingtonite provides support for recent theories regarding the effect of magnetic coupling on the variation of charge transfer intensity with temperature.  相似文献   

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