Mössbauer studies of synthetic and natural micas on the polylithionite-siderophyllite join

Mössbauer studies of micas on the polylithionite-side-rophyllite join show the existence of a relation between the quadrupole splitting (ΔE _Q) values of Fe²⁺ high spin doublets and both cationic and anionic composition of micas. This linear relation is positive as Li₂O content increases and negative as F content increases. In the lithium iron micas, the inner ferrous quadrupole doublet is assigned to the cis-site M(2), while the outer doublet is assigned to the trans-site M(1). A random distribution of Fe²⁺ is observed in fluorine-rich compositions, while slight enrichment of the M(1) site is noticed in hydroxyl compositions, perhaps due to a more sensitive oxidation in situ in M(2) than M(1) sites. The Mössbauer spectrum of siderophyllite K₂(Fe ₄ ²⁺ Al₂)(Si₄Al₄)O₂₀(OH)₄ shows the presence of only one ferrous doublet, which is assigned to M(2) sites. Hence from Mössbauer data we must consider a clintonite (“xanthophyllite”) structure for this mica. The ordered octahedral layer has two distorted ferrous cis-sites and one, more symmetrical, aluminum trans-site.     

57Fe mössbauer study of clinopyroxenes in the join CaFe3+ AlSiO6 - CaTiAl2O6

Synthetic clinopyroxenes of compositions between CaFe³⁺AlSiO₆ and CaFe _0.85 ³⁺ Ti_0.15Al_1.15Si_0.85O₆ have been studied by ⁵⁷Fe Mössbauer spectroscopy. The spectra consist of two doublets assigned to Fe³⁺ in M1 and T sites. From the area ratios of the doublets the site occupancies of Fe³⁺ and Al were determined. Si decreases from 1.00 to 0.85 and Al+Fe³⁺ increases from 1.00 to 1.15 per formula unit with increasing CaTiAl₂O₆ component of the clinopyroxene. The atomic ratio of Fe³⁺(T)/Fe³⁺(total) is 0.11–0.16; 4.5–7.5 percent of the T sites are occupied by Fe³⁺. Thus the presence of Si-O-Fe³⁺, Al-O-Fe³⁺, and Fe³⁺-O-Fe³⁺ bonds is expected in addition to Si-O-Si, Si-O-Al and Al-O-Al bonds. However, the possibility of the former bonds being present would be small, because the amount of Fe³⁺(T) is far less than that of Si and Al. The isomer shift of Fe³⁺(T) is one of the largest in the values found previously for Fe³⁺(T) in silicates. It increases with increasing CaTiAl₂O₆ component and seems to be correlated to the ionic character of the cation — anion bonds calculated from electronegativity. The quadrupole splittings of Fe³⁺(M1) and Fe³⁺(T) decrease with the substitution of Fe³⁺?Ti⁴⁺ in the M1 and of Si?Al in the T sites.     

Thermally activated electron delocalization in deerite

The ⁵⁷Fe Mössbauer spectra of deerites of different chemical composition, taken at several temperatures, show that Fe²⁺ and Fe³⁺ occupy all the six-coordinated lattice sites with a preference of Fe³⁺ probably for the M(1) to M(3) positions, and a preference of Fe²⁺ probably for the M(4) to M(6) and the M(7) to M(9) sites. The room and high temperature spectra reveal absorption patterns due to thermally activated Fe²⁺ → Fe³⁺ electron delocalization. The extent of electron delocalization is dependent on the chemical composition, e.g., the amount of ions (Mg, Mn, Al) substituting for Fe.     

57Fe mössbauer study of synthetic Fe3+-melilites

Synthetic Fe³⁺-melilites containing NaCaFe³⁺-Si₂O₇-, Ca₂Fe³⁺AlSiO₇- or Sr₂Fe³⁺AlSiO₇-components have been studied by ⁵⁷Fe Mössbauer spectroscopy. The spectrum of åkermanite containing an NaCaFe³⁺Si₂O₇-component consists of one doublet identified to belong to Fe³⁺ in T1 sites. The spectra of åkermanite and gehlenite containing Ca₂Fe³⁺ AlSiO₇- or Sr₂Fe³⁺ AlSiO₇-component consist of two doublets. The inner and outer doublets are identified to belong to Fe³⁺ in the less distorted T1 and that in the more distorted T2 sites, respectively. The area ratios of the spectra show that the site occupancy of Fe³⁺ (T1) in gehlenite is less than that in åkermanite in which the distribution of Fe³⁺ in T1 and T2 sites is apparently random. The different distributions can be explained in terms of competition between minimizing the deficiency in the electrostatic valence and the preference of Al for T1 sites which the isomer shift measurements show to be more ionic.     

Minerals with Brackebuschite-Like Structures: A Novel Solid Solution System Involving Cr<Superscript>6+</Superscript> and V<Superscript>5+</Superscript>

A novel complex continuous system of solid solutions involving vauquelinite Pb₂Cu(CrO₄)(PO₄)(OH), bushmakinite Pb₂Al(VO₄)(PO₄)(OH), ferribushmakinite Pb2Fe³⁺(VO₄)(PO₄)(OH), and a phase with the endmember formula Pb₂Cu(VO₄)(PO₄)(H₂O) or Pb₂Cu(VO₄)(РО₃ОН)(ОН) is studied based on samples from the oxidation zone of the Berezovskoe, Trebiat, and Pervomaisko-Zverevsky deposits in the Urals, Russia. This is the first natural system in which chromate and vanadate anions show a wide range of substitutions and the most extensive solid solution system involving (CrO₄)^2– found in nature. The major couple substitution is Cr⁶⁺ + Cu²⁺ ? V⁵⁺ + M³⁺, where M = Fe, Al. The correlation coefficients calculated from 125 point analyses are: 0.96 between V and (Fe + Al), 0.96 between Cr and (Cu + Zn),–0.96 between V and (Cu + Zn),–0.97 between Cr and (Fe + Al), and–0.97 between (Fe + Al) and (Cu + Zn). The substitutions V⁵⁺ ? Cr⁶⁺ (correlation coefficient–0.98) and to a lesser extent P⁵⁺ ? As⁵⁺ (correlation coefficient–0.86) occur at two types of tetrahedral sites, whereas the metal–nonmetal/metalloid substitutions, i.e., V or Cr for P or As, are minor. The substitution Fe³⁺ ? Al³⁺ is also negligible in this solid solution system.     

A contribution to the crystal chemistry of the voltaite group: solid solutions, Mössbauer and infrared spectra, and anomalous anisotropy

Voltaite is a mineral of fumaroles, solfatares, coal-fire gas vents, and acid-mine drainage systems. The nominal composition is K₂Fe₅ ²⁺Fe₃ ³⁺Al(SO₄)₁₂·18H₂O and the nominal symmetry is cubic, $Fd\overline{3}c$ . The tetragonal (I4₁/acd) superstructure of voltaite is known as the mineral pertlikite. In this study, we investigated 22 synthetic voltaite samples in which Fe²⁺ was partially or completely replaced by Mg, Zn, Mn, or Cd, by single-crystal and powder X-ray diffraction (both in-house and synchrotron). Two samples contained NH₄ ⁺ instead of K⁺. The structure of voltaite is based on a framework defined by kröhnkite-like heteropolyhedral chains which host both M³⁺ and M²⁺ in octahedral coordination. Unit cell dimensions of the end-members scale almost linearly with the size of M²⁺. In the Fe²⁺-Mg-Zn solid solutions, the Fe²⁺-Mg and Fe²⁺-Zn solutions are linear (ideal) in terms of their lattice-parameter variations. The Mg-Zn solid solution, however, is strongly non-ideal. A detailed analysis of the topology of the chains showed that this behavior originates in expansion and contraction of individual M²⁺-O bonds within the chains. In the Mg-Zn solid solution, some of the M²⁺-O bonds expand while none contract. In the other solid solutions, expansion of some M²⁺-O bonds is always compensated by contraction of the other ones. Parts of the nominally cubic crystals are optically anisotropic and their symmetry is found to be tetragonal by single crystal X-ray diffraction measurements. The coexistence of cubic and tetragonal sectors within a single crystal without any detectable difference in their chemical composition is difficult to explain in terms of growth of such composite crystals. Mössbauer and infrared spectra collected on our synthetic crystals conform with previously published data.     

A reassignment of the optical absorption bands in biotites

The electronic absorption spectra of three biotites with largely differing Fe²⁺/Fe³⁺ ratios were studied before and after thermal dehydration and oxidation of divalent iron. Three absorption bands near 17,100, 20,500 and 24,100 cm^?1 and an absorption edge at slightly higher energies are assigned to trivalent iron present in clusters of strongly interacting ions. The presence of additional broad absorption bands due to intervalence transfer between Fe²⁺ and Fe³⁺ or Ti⁴⁺ in this region cannot be excluded for biotites with high Fe²⁺ concentrations. Three bands at lower energies show a satisfactory correlation with concentration of divalent iron and decrease in the same proportions with oxidation. We therefore assign them to split components of the spin-allowed ligand field transition of Fe²⁺ at the M ₁ and M ₂ sites. This contradicts the assignment of one of these bands to an intervalence charge transfer between Fe²⁺ and Fe³⁺ by previous authors. It is shown that there is no indisputable evidence against our assignment.     

Mössbauer spectra of Fe x O (x>0.95)

The Mössbauer spectra of five samples of Fe _x O with compositions in the range 1.00>x>0.95 have been recorded at 298 K and 4.2 K. The spectrum of Fe _x O at 298 K consists of an asymmetric doublet which was fitted to one Fe²⁺ singlet, two Fe²⁺ doublets and 1 Fe³⁺ singlet. The Mössbauer parameters vary consistently with the increasing density of defects as x decreases. The Mössbauer spectrum of Fe _x O at 4.2 K consists of a large number of unresolved lines. The data were fitted to a series of singlets to enable the rough calculation of quantities relating to the mean Fe²⁺ and Fe³⁺ environments. The results of the fits to the 298 K spectra are briefly discussed in terms of a physical model for the defect structure of Fe _x O.     

57Fe Mössbauer study on compositions of the series Fe3+TaO4-Fe2+Ta2O6

Fe⁵⁷ Mössbauer spectra were measured on compositions of the series Fe_1?x/3Ta_1+x/3O₄, 0≤x≤1. The spectra are characterized by mixed valencies of Fe²⁺ and Fe³⁺ ions for 0<x<1. Starting from x=0 with rutile structure, a trirutile structure forms towards x=1. Quadrupole splitting QS of Fe³⁺ is QS(Fe³⁺)≈0.55 mm/s and isomer shift IS is IS(Fe³⁺)≈0.40 mm/s (referred to Fe); both quantities exhibit minor variations along the series. The Fe²⁺ subspectra for x>0.5 were fitted using one symmetrical doublet; however, for x<0.5 two symmetrical doublets were necessary to describe these patterns. QS(Fe²⁺)=2.0–3.2 mm/s and IS(Fe²⁺)=0.90–1.15 mm/s for all compositions. In the case x<0.5, marked temperature dependent QS values appear to exist. This feature may be related to short range order effects and possibly also in part to intervalence electron transfer betwee Fe²⁺ and Fe³⁺ ions.     

Mössbauer and infrared spectroscopic studies of Belgian chloritoids

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 v _O-H absorption bands. Furthermore, the specific nature of the infrared spectra enables a fast identification of chloritoid samples.     

Magnetic order in grunerite,Fe7Si8O22(OH)2A quasi-one dimensional antiferromagnet with a spin canting transition

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.     

Mössbauer spectra of kyanite,aquamarine, and cordierite showing intervalence charge transfer

The blue colors of several minerals and gems, including aquamarine (beryl, Be₃Al₂Si₆O₁₈) and cordierite (Al₃(Mg, Fe)₂Si₅AlO₁₈), have been attributed to charge transfer (CT) between adjacent Fe²⁺ and Fe³⁺ cations, while Fe²⁺→Ti⁴⁺ CT has been proposed for blue kyanites (Al₂SiO₅). Such assignments were based on chemical analyses and on polarization-dependent absorption bands measured in visible-region spectra. We have attempted to characterize the Fe cations in each of these minerals by Mössbauer spectroscopy (MS). In blue kyanites, significant amounts of both Fe²⁺ and Fe³⁺ were detected with MS, indicating that Fe²⁺→Fe³⁺ CT, Fe²⁺→Ti⁴⁺ CT, and Fe²⁺ and Fe³⁺ crystal field transitions each could contribute to the electronic spectra. In aquamarines, coexisting Fe²⁺ and Fe³⁺ ions were resolved by MS, supporting our assignment of the broad, relatively weak band at 16,100 cm^?1 in E∥c spectra to Fe²⁺→Fe³⁺ CT between Fe cations replacing Al³⁺ ions 4.6Å apart along c. A band at 17,500 cm^?1 in E⊥c spectra of cordierite is generally assigned to Fe²⁺ (oct)→Fe³⁺ (tet) CT between cations only 2.74 Å apart. However, no Fe³⁺ ions were detected in the MS at 293K of several blue cordierites showing the 17,500 cm^?1 band and reported to contain Fe³⁺. A quadrupole doublet with parameters consistent with tetrahedral Fe³⁺ appears in 77K MS, but the Fe³⁺/Fe²⁺ ratios from MS are much smaller than values from chemical analysis. These results sound a cautionary note when correlating Mössbauer and chemically determined Fe³⁺/Fe²⁺ ratios for minerals exhibiting Fe²⁺→Fe³⁺ CT.     

Synthetic and natural chromium-bearing spinels: an optical spectroscopy study

Four samples of synthetic chromium-bearing spinels of (Mg, Fe²⁺)(Cr, Fe³⁺)₂O₄ composition and four samples of natural spinels of predominantly (Mg, Fe²⁺)(Al, Cr)₂O₄ composition were studied at ambient conditions by means of optical absorption spectroscopy. Synthetic end-member MgCr₂O₄ spinel was also studied at pressures up to ca. 10 GPa. In both synthetic and natural samples, chromium is present predominantly as octahedral Cr³⁺ seen in the spectra as two broad intense absorption bands in the visible range caused by the electronic spin-allowed ⁴ A _2g  → ⁴ T _2g and ⁴ A _2g  → ⁴ T _1g transitions (U- and Y-band, respectively). A distinct doublet structure of the Y-band in both synthetic and natural spinels is related to trigonal distortion of the octahedral site in the spinel structure. A small, if any, splitting of the U-band can only be resolved at curve-fitting analysis. In all synthetic high-chromium spinels, a couple of relatively narrow and weak bands of the spin-allowed transitions ⁴ A _2g  → ² E _g and ⁴ A _2g  → ² T _1g of Cr³⁺, intensified by exchange-coupled interaction between Cr³⁺ and Fe³⁺ at neighboring octahedral sites of the structure, appear at ~14,400 and ~15,100 cm^?1. A vague broad band in the range from ca. 15,000 to 12,000 cm^?1 in synthetic spinels is tentatively attributed to ^IVCr²⁺ + ^VICr³⁺ → ^IVCr³⁺ + ^VICr²⁺ intervalence charge-transfer transition. Iron, mainly as octahedral Fe³⁺, causes intense high-energy absorption edge in near UV-range (ligand–metal charge-transfer O^2? → Fe³⁺, Fe²⁺ transitions). As tetrahedral Fe²⁺, it appears as a strong infrared absorption band at around 4,850 cm^?1 caused by electronic spin-allowed ⁵ E → ⁵ T ₂ transitions of ^IVFe²⁺. From the composition shift of the U-band in natural and synthetic MgCr₂O₄ spinels, the coefficient of local structural relaxation around Cr³⁺ in spinel MgAl₂O₄–MgCr₂O₄ system was evaluated as ~0.56(4), one of the lowest among (Al, Cr)O₆ polyhedra known so far. The octahedral modulus of Cr³⁺ in MgCr₂O₄, derived from pressure-induced shift of the U-band of Cr³⁺, is ~313 (50) GPa, which is nearly the same as in natural low-chromium Mg, Al-spinel reported by Langer et al. (1997). Calculated from the results of the curve-fitting analysis, the Racah parameter B of Cr³⁺ in natural and synthetic MgCr₂O₄ spinels indicates that Cr–O-bonding in octahedral sites of MgCr₂O₄ has more covalent character than in the diluted natural samples. Within the uncertainty of determination in synthetic MgAl₂O₄ spinel, B does not much depend on pressure.     

Oxidation Experiment of Natural Megacrystal Clinopyroxene: Implications for Assignment of Mössbauer Spectra

The Mössbauer spectra of natural megacrystal clinopyroxene are usually fitted by 4 sets of symmetric doublets, A‐A', B‐B', C‐C' and D‐D', respectively, in terms of increasing Qs value in literature. But the assignments of those doublets are quite different, except the D‐D' doublet assigned to Fe³⁺at the lattice site M***1 in previous papers. Particularly, the assignment and interpretation of the C‐C' doublet are diverse. The oxidation experiments of natural megacrystal clinopyroxene collected from the Hannuoba basalt, North China, were performed under controlled conditions of temperature at 1000°C and oxygen fugacity of FMQ buffer in 1, 2, ***3 and 5 days respectively. The oxidized samples were then measured by X‐ray diffraction spectrometry and Mössbauer spectrometry. The oxidation of clinopyroxene is characterized by Fe²⁺ → Fe³⁺at M1 under the subsolidus conditions, which is consistent with the increase of the area of the D‐D' doublet when the heating time increases. Accordingly, the area of the A‐A' and B‐B' doublets decreases with the increasing heating time. However, the area of C‐C' keeps almost constant. Therefore, the 4 sets of doublets can be assigned correctly as follows: A‐A' doublet to Fe⁺²at M1, B‐B' Fe²⁺at M1, C‐C' Fe²⁺at M2, and D‐D' Fe³⁺at M1. The M1 site splitting is due to the NNN (Next Nearest Neighbour) effect of the M2 site. The amount of Fe³⁺at the tetrohedral site of megacrystal clinopyroxene is negligible according to this study.     

The structural behavior of ferric and ferrous iron in aluminosilicate glass near meta-aluminosilicate joins

Iron-57 resonant absorption Mössbauer spectroscopy was used to describe the redox relations and structural roles of Fe³⁺ and Fe²⁺ in meta-aluminosilicate glasses. Melts were formed at 1500 °C in equilibrium with air and quenched to glass in liquid H₂O with quenching rates exceeding 200 °C/s. The aluminosilicate compositions were NaAlSi₂O₆, Ca_0.5AlSi₂O₆, and Mg_0.5AlSi₂O₆. Iron oxide was added in the form of Fe₂O₃, NaFeO₂, CaFe₂O₄, and MgFe₂O₄ with total iron oxide content in the range ∼0.9 to ∼5.6 mol% as Fe₂O₃. The Mössbauer spectra, which were deconvoluted by assuming Gaussian distributions of the hyperfine field, are consistent with one absorption doublet of Fe²⁺ and one of Fe³⁺. From the area ratios of the Fe²⁺ and Fe³⁺ absorption doublets, with corrections for differences in recoil-fractions of Fe³⁺ and Fe²⁺, the Fe³⁺/ΣFe is positively correlated with increasing total iron content and with decreasing ionization potential of the alkali and alkaline earth cation. There is a distribution of hyperfine parameters from the Mössbauer spectra of these glasses. The maximum in the isomer shift distribution function of Fe³⁺, δ_Fe³⁺, ranges from about 0.25 to 0.49 mm/s (at 298 K relative to Fe metal) with the quadrupole splitting maximum, Δ_Fe³⁺, ranging from ∼1.2 to ∼1.6 mm/s. Both δ_Fe³⁺ and δ_Fe²⁺ are negatively correlated with total iron oxide content and Fe³⁺/ΣFe. The dominant oxygen coordination number Fe³⁺ changes from 4 to 6 with decreasing Fe³⁺/ΣFe. The distortion of the Fe³⁺-O polyhedra of the quenched melts (glasses) decreases as the Fe³⁺/ΣFe increases. These polyhedra do, however, coexist with lesser proportions of polyhedra with different oxygen coordination numbers. The δ_Fe²⁺ and Δ_Fe²⁺ distribution maxima at 298 K range from ∼0.95 to 1.15 mm/s and 1.9 to 2.0 mm/s, respectively, and decrease with increasing Fe³⁺/ΣFe. We suggest that these hyperfine parameter values for the most part are more consistent with Fe²⁺ in a range of coordination states from 4- to 6-fold. The lower δ_Fe²⁺-values for the most oxidized melts are consistent with a larger proportion of Fe²⁺ in 4-fold coordination compared with more reduced glasses and melts.     

OH in zoned amphiboles of eclogite from the western Tianshan,NW-China

Chemically-zoned amphibole porphyroblast grains in an eclogite (sample ws24-7) from the western Tianshan (NW-China) have been analyzed by electron microprobe (EMP), micro Fourier-transform infrared (micro-FTIR) and micro-Raman spectroscopy in the OH-stretching region. The EMP data reveal zoned amphibole compositions clustering around two predominant compositions: a glaucophane end-member ( ^B Na₂ ^C M²⁺ ₃ M³⁺ ₂ ^T Si₈(OH)₂) in the cores, whereas the mantle to rim of the samples has an intermediate amphibole composition ( ^A _0.5 ^B Ca_1.5Na_0.5 ^C M ²⁺ _4.5 M _0.5^{3+ T} Si_7.5Al_0.5(OH)₂) (A = Na and/or K; M ²⁺ = Mg and Fe²⁺; M ³⁺ = Fe³⁺ and/or Al) between winchite (and ferro-winchite) and katophorite (and Mg-katophorite). Furthermore, we observed complicated FTIR and Raman spectra with OH-stretching absorption bands varying systematically from core to rim. The FTIR/Raman spectra of the core amphibole show three lower-frequency components (at 3,633, 3,649–3,651 and 3,660–3,663 cm⁻¹) which can be attributed to a local O(3)-H dipole surrounded by ^{M(1) M(3)}Mg₃, ^{M(1) M(3)}Mg₂Fe²⁺ and ^{M(1) M(3)} Fe²⁺ ₃, respectively, an empty A site and ^T Si₈ environments. On the other hand, bands at higher frequencies (3,672–3,673, 3,691–3,697 and 3,708 cm⁻¹) are observable in the rims of the amphiboles, and they indicate the presence of an occupied A site. The FTIR and Raman data from the OH-stretching region allow us to calculate the site occupancy of the A, M(1)–M(3), T sites with confidence when combined with EPM data. By contrast M(2)- and M(4) site occupancies are more difficult to evaluate. We use these samples to highlight on the opportunities and limitations of FTIR OH-stretching spectroscopy applied to natural high pressure amphibole phases. The much more detailed cation site occupancy of the zoned amphibole from the western Tianshan have been obtained by comparing data from micro-chemical and FTIR and/or Raman in the OH-stretching data. We find the following characteristic substitutions Si(T-site) (Mg, Fe)[M(1)–M(3)-site] → Al(T-site) Al[M(1)–M(3)-site] (tschermakite), Ca(M4-site)□ (A-site) → Na(M4-site) Na + K(A-site) (richterite), and Ca(M4-site) (Mg, Fe) [M(1)–M(3)-site] → Na(M4-site) Al[M(1)–M(3)-site] (glaucophane) from the configurations observed during metamorphism.     

A Mössbauer study of ferric iron in olivine

Natural and synthetic olivines with ferrous and ferric iron have been studied by Mössbauer spectroscopy. Their spectra exhibit three superimposed quadrupole-split doublets corresponding to Fe²⁺ at M1 and M2 sites and Fe³⁺ at an unidentified position, which is probably M2. The hyperfine parameters of Fe³⁺ at temperatures between 300°C and 450°C are: Δ=0.67–1.23 mm/s, δ=0.04–0.23 mm/s relative to metallic iron, and the full width at half height, HW=0.15–0.43 mm/s. The Fe²⁺ populations of M2 decrease with increasing Fe³⁺ content. However, Fe²⁺ prefers M1 in synthetic olivines, even at high temperatures (800°–1,400°C). In kirschsteinite, Fe²⁺ and Fe³⁺ are exclusively in M1. Fe²⁺/Fe³⁺ ratios estimated from the peak areas are consistent with chemical analyses.     

57Fe Mössbauer spectroscopy and magnetization of cation deficient Fe2TiO4 and FeCr2O4. Part II: Magnetization data

Magnetic properties are reported for synthetic cation deficient Fe₂TiO₄ and FeCr₂O₄ particles (<1 μm). Cation deficiency, achieved by oxidation, is characterized by the oxidation parameter z which represents the fraction of Fe²⁺ ions converted to Fe³⁺ in the spinel lattice (0≤z≤1). Fe₂TiO₄ (z=0.85) has a Curie temperature T _c that can only approximately be given with a value in the range 400–700 K and it has a magnetic moment per formula unit M≈0.50 μ_B (μ_B=Bohr's magneton) at 4.2 K, for FeCr₂O₄ it is T _c≥520 K and M (4.2 K) ≈0.16 μ_B. Magnetic hysteresis parameters at various temperatures show in part characteristic features due to relaxation phenomena. In the Ti-spinel, the latter are caused by a superposition of superparamagnetism and spin relaxation and in the Cr-spinel by superparamagnetism, in agreement with Mössbauer data (part I of this paper). The cation and vacancy distribution and magnetic coupling are discussed in both compositions with respect to magnetic moment data considering magnetic dilution by incorporated vacancies, and in the Ti-spinel also by non magnetic Ti⁴⁺.     

<Superscript>57</Superscript>Fe Mössbauer spectroscopy,X-ray single-crystal diffractometry,and electronic structure calculations on natural alexandrite

Natural alexandrite Al₂BeO₄:Cr from Malyshevo near Terem Tschanka, Sverdlovsk, Ural, Russia, has been characterized by ⁵⁷Fe Mössbauer spectroscopy, electron microprobe, X-ray single-crystal diffractometry and by electronic structure calculations in order to determine oxidation state and location of iron. The sample contains 0.3 wt% of total iron oxide. The ⁵⁷Fe Mössbauer spectrum can be resolved into three doublets. Two of them with hyperfine parameters typical for octahedrally coordinated high-spin Fe³⁺ and Fe²⁺, respectively, are assigned to iron substituting for Al in the octahedral M2-site. The third doublet is attributed to Fe³⁺ in hematite. Electronic structure calculations in the local spin density approximation are in reasonable agreement with experimental data provided that expansion and/or distortion of the coordination octahedra are presumed upon iron substitution. The calculated hyperfine parameters of Fe³⁺ are almost identical for the M1 and M2 positions, but the calculated ligand-field splitting is by far too large for high-spin Fe³⁺ on M1.     

Polarized electronic absorption spectra of synthetic (Mg,Fe)-orthopyroxenes,ferrosilite and Fe3+-bearing ferrosilite

The assignment of spin-allowed Fe²⁺-bands in orthopyroxene electronic absorption spectra is revised by studying synthetic bronzite (Mg_0.8 Fe_0.2)₂Si₂O₆, hypersthene (Mg_0.5 Fe_0.5)₂Si₂O₆ and ferrosilite (Fe₂Si₂O₆). Reheating of bronzite and hypersthene single crystals causes a redistribution of the Fe²⁺-ions over the M1 and M2 octahedra, which was determined by Mössbauer spectroscopy and correlated to the intensity change of the spin-allowed Fe²⁺ d-d bands in the polarized absorption spectra. The 11000 cm^-1 band is caused by Fe²⁺ in M1 (⁵B_2g→⁵A_1g) and Fe²⁺ in M2 (⁵A₁→⁵A₁), the 8500 cm^-1 band by Fe²⁺ in M1 (⁵B_2g→⁵B_1g) and the 5000 cm^-1 band by Fe²⁺ in M2 octahedra (⁵A₁→⁵B₁). The Fe²⁺-Fe³⁺ charge transfer band is identified at 12500cm^-1 in the spectra of synthetic Fe³⁺ -Al bearing ferrosilite. This band shows a strong γ-polarization and therefore is caused by Fe²⁺ -Fe³⁺-ions in edge-sharing octahedra.