Stefan S. Hafner

Ordnung/Unordnung und Ultrarotabsorption IV. Die Absorption einiger Metalloxyde mit Spinellstruktur

Infrared absorption measurements of a number of metal oxides with spinel structure have been carried out in the range of 10 to 30 μ. All spinels show mainly two chief groups of absorption (A and B). By preparing a number of mixed crystal series it was possible to determine the mutual relations of A and Β and the dependence of their wave lengths on chemical composition and lattice constants. The force constants of the normal vibrations were calculated according to the approximation of WAXJDRON (1955). While A corresponds approximately to a valency vibration between the tetrahedral cations and oxygen, Β approximates to a valency vibration between the octahedral cations and oxygen. The following relation exists between the spinel structure (with given composition and cation distribution) and the absorption spectrum: 1. The half width of the absorption is mainly determined by near symmetry (Nahsymmetrie) of the individual cations. Different types of cation in the same lattice points lead to increased half width. 2. The force constants of the cation-anion valency vibrations diminish with increasing values of the Goldschmidt radius of the cation. The influence of covalent bonding can definitely be observed (increase of the force constant in hybrid compounds). 3. The force constants depend upon the distribution between tetrahedral and octahedral sites of the cations present in the structure. (This was examined on CuFe204 and MgAl204-Al203.) Auszug Die Ultrarotabsorptionen einer Reihe von Metalloxyden mit Spinellstruktur wurden im Bereich von 10 bis 30 μ gemessen. Alle Spinelle zeigen im wesentlichen zwei Hauptgruppen von Absorptionen (A und B). Durch die Herstellung einer Anzahl von Mischkristallreihen konnten Α und Β gegenseitig zugeordnet und ihre Wellenlängen in Abhängigkeit von Chemismus und Gitterkonstante bestimmt werden. Die Kraftkonstanten der Normalsehwingungen wurden nach 332 S T E F A N H A F N E R dem Näherungsansatz von W A L D R O N (1955) berechnet. Α entspricht angenähert einer Valenzschwingung zwischen tetraedrischem Kation und Sauerstoff, Β einer Valenzschwingung zwischen oktaedrischem Kation und Sauerstoff. Es besteht folgender Zusammenhang zwischen der Spinellstruktur (mit gegebener chemischer Zusammensetzung und Kationenverteilung) und dem Absorptionsspektrum: 1. Die Halbwertsbreite der Absorptionen wird weitgehend durch die „Nahsymmetrie der einzelnen Kationlagen bestimmt. Verschiedene Kationtypen in einer Punktlage der Struktur erzeugen große Halbwertsbreiten. 2. Die Kraftkonstanten der Kation-Anion-Valenzschwingungen nehmen mit wachsendem Goldschmidt-Radius des Kations ab. Es ist ein deutlicher Einfluß von kovalenten Bindungsanteilen zu erkennen (Erhöhung der Kraftkonstante bei Hybridbildung). 3. Die Kraftkonstanten hängen von der in der Struktur vorhandenen Verteilung der Kationen über die tetraedrischen und oktaedrischen Plätze ab (untersucht an CuFe s04 und MgAlj04—Α1203).

Electric Field Gradients at 57Fe in ZnFe2O4 and CdFe2O4

The nuclear quadrupole coupling constants and isomer shifts of 57Fe in the spinels ZnFe2O4 and CdFe2O4 were measured using the Mossbauer effect. The signs of the quadrupole coupling constants were determined from spectra which were taken in an applied magnetic field. The sign is negative in both spinels. The isomer shifts and Fe–O distances indicate that Fe3+ in ZnFe2O4 is somewhat more covalently bonded than in CdFe2O4. The external field gradients at the Fe3+ positions can be interpreted in terms of the ionic point‐multipole model modified by some charge transfer between oxygen and the cations. The point charge contribution to the field gradient is positive in case of ZnFe2O4 and nearly zero in case of CdFe2O4; the predominant contribution is due to the electric dipole moments of the oxygen ions and is negative. The dipole polarizability of the oxygen ion which gave the best fit is αD = 0.8u2009A3. The effect of charge transfer on the ionic field gradient is small.

Vacancy distribution in synthetic spinels of the series Fe3O4–y-Fe2O3

Auszug Die Überstruktur und Leerstellenverteilung in den Originalproben der Magnetit-Mischkristallreihe FESOI—Y-FejOs von G. H Ä G G wurde mittels RöntgendifEraktion und Mößbauerspektroskopie untersucht. Die Intensität der Überstrukturlinien auf den Diffraktionsfilmen nimmt mit zunehmender Abweichung der Probe vom stöchiometrischen Verhältnis zu. Sämtliche Filme können jedoch unter Annahme eines kubisch-primitiven Gitters indiziert werden. Tetragonale Symmetrie konnte nicht nachgewiesen werden. Mößbauerabsorptionsspektren zeigen, daß die Leerstellen sowohl über tetraedrische als auch oktaedrische Eisenplätze verteilt sind. Alis den Linienverbreiterungen der Absorptionslinien, die den Eisenionen auf den oktaedrischen Plätzen zuzuordnen sind, geht hervor, daß die Häufigkeit des Elektronenaustausches zwischen Fe 2 + und Fe34 auf diesen Plätzen bei gegebener Temperatur mit zunehmender Zahl der Leerstellen abnimmt. Abstract The original samples of the magnetite series F E A O Í — γ F E Î O S of G . HÄGG have been investigated with x-ray diffraction and Mössbauer spectroscopy. X-ray diffraction studies showed that the intensities of the superstructure lines increase with increasing deviation from the stoichiometric ratio. The patterns of all samples exhibited a cubic-primitive lattice. Tetragonal symmetry was not observed. Mössbauer absorption spectra gave evidence for vacancy distributions over tetrahedral as well as octahedral iron sites of magnetite. Broadening of the octahedral iron lines indicated that the relaxation times of the hopping electrons at constant temperature increases as the deviation from the stoichiometric ratio increases. Introduction

Hyperfine Splitting of Fe57 and Mg-Fe Order-Disorder in Orthopyroxenes (MgSiO3-FeSiO3 Solid Solution)

The apparent two-line hyperfine spectrum of Fe57 in some orthopyroxene minerals from metamorphic rocks has been analyzed by the Mössbauer technique. The spectrum consists predominantly of two overlapping two-line patterns. The patterns have been assigned to the two non-equivalent Fe2+ sites, M1 and M2, of the crystal structure. The approximated intrinsic values of the electric quadrupole interaction documentclass{aastex}nusepackage{amsbsy}nusepackage{amsfonts}nusepackage{amssymb}nusepackage{bm}nusepackage{mathrsfs}nusepackage{pifont}nusepackage{stmaryrd}nusepackage{textcomp}nusepackage{portland,xspace}nusepackage{amsmath,amsxtra}nusepackage{wasysym}npagestyle{empty}nDeclareMathSizes{10}{9}{7}{6}nbegin{document}

Mg2+–Fe2+ distribution in metamorphic and volcanic orthopyroxenes

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Iron and magnesium distribution in cummingtonites (Fe,Mg)7Si8O22(OH)2*

end{document} and isomer shifts at M1 and M2 are: The Fe2+ site occupancies have been approximately determined. The high degree of Mg-Fe ordering in an orthopyroxene (Fe/[Mg + Fe] = 0.535) found previously by X-ray diffraction has been confirmed. Heat treatment of this sample at 1,000° C. and 1,100° C. yields considerable Mg-Fe disorder.

Nuclear Quadrupole Coupling Tensors of 27Al in Sillimanite (Al2SiO5)

The distribution of iron over the tetrahedral and octahedral sites in natural and oxidized (kenotetrahedral) magnetites has been studied using the Mössbauer effect. The ratio of the 5 7Fe recoil-free fractions at the two types of sites is 1.00 ± 0.04 at 298 ° K . From the intensity ratios of the two magnetic hyperfine patterns it is concluded that the vacancies in the kenotetrahedral samples are statistically distributed over the tetrahedral and octahedral sites. The Verwey transition which occurs at 119 °K in pure magnetite decreases with increasing vacancy concentration. Its spread over a range of at least 20° Κ in the kenotetrahedral magnetites indicates that these are not homogeneous phases. The vacancies appear to be predominantly concentrated at rims of the crystals. * Dedicated to Professor F . Laves on the occasion of his 65 th birthday. * cf. also abstract (with G. KULLERUD) in Trans. Amer. Geophysical U. 51 (1970) 447. * * Chicago, Illinois 60637. 328 HANS-PETER W E B E R and STEFAN S. HAFNER

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Mg—Fe~ d i s t r i b u t i o n a n d s i t e -occupancy f a c t o r s of o r t h o p y r o x e n e s a t t h e Mi a n d M, s i tes f r o m h igh -g rade m e t a m o r p h i c rocks (granul i tes) in G r e e n l a n d a n d Sweden , a s well a s vo lcanic o r t h o p y r o x e n e s f r o m J a p a n a n d t h e D o m i n i c a n I s lands , h a v e b e e n d e t e r m i n e d f r o m t h e h y p e r f i n e sp l i t t i ng of 5 7 Fe s p e c t r a us ing t h e Mössbauer t s c h n i q u e . T h e degree of Mg +—Fe 2 + o rde r in t h e m e t a m o r p h i c p y r o x e n e s is h igher t h a n t h a t in t h e volcanic p y r o x e n e s . E q u i l i b r i u m c o n s t a n t s h a v e been e s t i m a t e d , a s s u m i n g t h e ideal -solut ion m o d e l for each si te. Introduction The strong fractionation of Fe + between the two non-equivalent sites, and M2 of orthopyroxenes (Fe preferring the M2 site) has been established by x-ray diffraction (GHOSE, 1 9 6 5 ) and Mössbauer resonance (EVANS, GHOSE and H A F N E R , 1 9 6 7 ) . The latter technique * W o r c e s t e r P o l y t e c h n i c I n s t i t u t e , W o r c e s t e r , M a s s a c h u s e t t s 01609. 1 5 8 S U B R A T A G H O S E a n d S T E F A N H A F N E R utilizes the electric hyperfine splitting of the Fe spectra, which consist of a superposition of two doublets arising from the nuclear quadrupole interaction of iron at Mx and M2. The apparent two-line spectra can be resolved into two doublets Av Bv and A2, B2 due to Fe at M2 and Mx respectively. The Fe 2 + fraction at the sites are estimated from the intensity ratios of the doublets. The Fe spectra of four orthopyroxenes from high-grade metamorphic rocks of Greenland and Sweden were investigated in the hope tha t maximum Mg—Fe order would be found in these samples. Likewise three samples from volcanic rocks of J apan and of the Dominican Islands were studied to determine the possible maximum disorder in natural orthopyroxenes. Experimental and results The spectra were fitted by four Lorentzian lines Alt A2, Bl and B2 with center position, half width, intensity, and background counting rate as variables. The fractions of Fe + a t the two sites Mj and M2 were derived from the average of the intensity ratios A2/(A1 A2), BJ(B1 + B2), and A1/{A1 + A2), BJ{B1 + B2), respectively. The site-occupancy factors were obtained by multiplication of the fractions by the total ratio 2Fe/(Fe + Mg) of the mineral according to two formula units (Mg, Fe)Si03 in the unit cell. The total Fe/(Fe + Mg) ratios were taken directly from wet chemical and/or electron probe analyses, since their determination by absolute intensity measurements of the doublets is rather difficult. For further details of the procedure, the reader is referred to E V A N S et al. ( 1 9 6 7 ) . If the site-occupancy factors are relatively high, their determination is relatively precise. In such cases, it is usually sufficient to introduce halfwidth constraints of the form Γ(Α1) = Γ(Α2), and Γ{Β1) = Γ(Β2) in order to obtain good line fits. This was done for each of the orthopyroxenes with the exception of 3 7 2 1 8 and 3 7 6 5 1 . The latter two are found to be highly ordered, with only a very small fraction of Fe at M1. The lines of Mx appear in those cases as a slight disturbance of the outer tails of the M2 doublet, scarcely distinct from the background. They could only be resolved by introducing positional constraints for the most probable line positions of Mx. Therefore the relative precision of the Mj-site occupancy factor is reduced. Possible sources for systematic errors of the employed technique, which are difficult to estimate at the present stage, have been briefly mentioned by E V A N S et al. ( 1 9 6 7 ) . Mg+—Fe d i s t r ibut ion in o r thopyroxenes 159 In our simple four-line fits, the contribution of a minor amount of trivalent iron to the spectra was neglected, although traces of Fe could be observed, particularly in the volcanic orthopyroxenes. Each specimen was carefully hand-picked and should be pure to more than 99 per cent. The Fe site-occupancy factors are listed in Table 1, and plotted in Fig. 1. Previously reported data on orthopyroxenes from metamorphic rocks and heat-treated samples have been included for the sake Table i . F e 2 + distribution and site-occupancy factors at M1 and M2 Orthopyroxene F e F rac t i on of F e 2 + Site-occupancy fac tor F e + Mg M, M2 M, 1 M, Metamorphic xyz Greenland 0.866 0.44 0.56 0.76 0.97 V2 Sweden 0.758 0.37 0.63 0.56 0.96 37651 Greenland 0.406 0.07 0.93 0.06 0.76 37218 Greenland 0.532 0.15 0.85 0.16 0.91 Heated metamorphic 37651 1000°C 0.406 0.26 0.74 0.21 0.60 37218 1000°C 0.532 0.27 0.73 0.29 0.78 37218 1100°C 0.532 0.29 0.71 0.31 0.76 Volcanic 52110301 J a p a n 0.56 0.26 0.74 0.29 0.83 56071001 J a p a n 0.53 0.24 0.76 0.25 0.81 Dominican Is lands 0.46 0.16 0.84 0.15 0.77 1 C h e m i c a l a n a l y s i s is g i v e n in HOWIE a n d SMITH (1966) , xyz = YL. 160 S U B R A T A G H O S E a n d S T E F A N H A F N E R of completeness. Table 2 summarizes the chemical composition of those minerals which had not been presented in the earlier work. The hyperfine parameters for Fe at and M2 are shown in Table 3. ο Metarnorph/c ° Volcanic & Heated metamorphic Fig. i . Fe—Mg order-disorder in me tamorph ic , hea t t r ea t ed me tamorph ic , and volcanic or thopyroxenes . Solid lines indicate e s t ima ted Fe—Mg dis t r ibut ion isotherms a t t h e M j a n d M2 sites for Κ = 48 a n d Κ = 7 (1000°C) Table 2. Chemical compositions of orthopyroxenes Specimen S i0 2 A1203 F e 2 0 3 F e O MgO CaO T i 0 2 MnO 37651Μ Greenland 51.86 1.03 24.48 20.13 0.47 0.05 0.72 52110301 · J a p a n 48.95 0.24 3.93 30.66 13.28 0.85 0.75 0.93 56071001> J a p a n 49.24 0.49 2.03 31.16 15.24 0.17 0.00 0.90 Hess 1 Dominican Is lands 51.15 0.72 26.55 17.71 1.12 0.45 0.72 1 Elect ron-microprobe analysis (Analys t : C. R. KNOWLES); F e O has been calculated on the basis of to ta l Fe . 2 W e t chemical analysis (Analys t : H . H A R A M U R A ) ; K U N O ( 1 9 6 6 ) . 3 Granul i t ic rock. 1 Phenocrys t s in daci te , Nagae near Tagut i , S idara d i s t r . ; K U N O ( 1 9 6 6 ) . 5 Phenocrys t s in b io t i te fer rohypers thene daci te , beh ind the temple o f H o r a i zi, Sidara d is t r . ; K U N O ( 1 9 6 6 ) . Mg 2 + —Fe 2 + d i s t r i b u t i o n in o r t h o p y r o x e n e s 161 T a b i c 3. Hijperfine parameters of 5 7 Fe at ,1/j and JA., .1 F e Absorbern , Spec imen , , „ , , : m m / s e c 1 F e + M g , m g / e n r I s o m e r sh i f t v s coppe r* Mj M, Mt I M,