Program: K4 Minerals
K4 Minerals

K4.1 Mössbauer Spectroscopy for Study of Oxidation Processes in Iron-Containing Minerals
M. Mashlan1, R. Zboøil1 and K. Barèová2
1) Faculty of Sciences, Palacky University, Olomouc, Czech Republic
2) Institute of Physics, Technical University of Ostrava, Ostrava, Czech Republic

Thermal transformations of iron-containing minerals in an oxidizing atmosphere result in the formation of different Fe3+ phases including iron(III) oxides. a-Fe2O3 with the well-known hexagonal corundum structure, b-Fe2O3 with the rare cubic bixbyite structure, g-Fe2O3 with cubic spinel structure, and orthorhombic e-Fe2O3 are the polymorphs, which can be found in the mixture of transformation products. Moreover, the thermally induced isochemical structural changes of the thermally less stable polymorphs (b-, g-, and e-Fe2O3) to a-Fe2O3 accompany the primary oxidation process.

The mechanism of thermally induced oxidation of Fe2+ cations in natural orthosilicates (almandine, pyrope, olivine) was studied in the range of 600-1200° C using 57Fe Mössbauer spectroscopy and XRD. Electron-microprobe analysis and XRF were used for the determination of the chemical composition of the minerals used.

g-Fe2O3 nanoparticles appear as the primary Fe3+ phase in Mössbauer spectra of all orthosilicates heated at 600-800° C. These nanoparticles are thermally unstable and they are transformed to a-Fe2O3 with an increase of the heating period. In the case of the thermal treatment of almandine, e-Fe2O3 was identified as the intermediate of the isochemical structural change of g-Fe2O3 to a-Fe2O3. Transformation mechanisms related with the decomposition of the silicate structure have been observed at higher temperatures. Mixed oxide Mg(Al,Fe)2O4 with the spinel structure and enstatite (Mg,Fe)SiO3 were identified as iron-bearing transformation products after high temperature treatment of pyrope and olivine.


K4.2 Mössbauer Studies of the iron Borates Fe2BO4 and Fe3BO5
V. Papaefthymiou
Physics Department, University of Ioannina, Ioannina, Greece

The iron borates Fe2BO4 and Fe3BO5 are mixed valence iron (Fe2+-Fe3+) oxides and members of the large family of oxyborates with very interesting electronic and magnetic properties [1,2,3]. Mössbauer spectroscopy is particularly suited for the study of these mixed valence iron phases, because this technique provides information on the detailed electronic and spin structure of the iron ions.

We report in the present work detailed Mössbauer studies in the temperature range 4.2 to 650 K. Fe2BO4 exhibits ferrimagnetic transition at TN =155 K. The charges are localized at the iron sites at temperatures below 270 K while above this temperature charge delocalization starts between the ions of the mixed valence iron pairs. Above 425K we observed only fully delocalized states where the excess electron is spread about equally over the two sites of a mixed valence pair Fe2+-Fe3+ and thus all irons are in the oxidation level Fe2.5+. Fe3BO5 exhibits electron delocalization in the Fe2-Fe3-Fe2 triads between pairs of Fe2+ and Fe3+ ions at all temperatures between 4.2 and 180 K with localized ferric and ferrous states while above 180 K the delocalization procedure changes and includes the rest ferric ions. Magnetic hyperfine splitting is also observed in the Mössbauer spectra of Fe3BO5 below TN = 114 K although 1/3 of the localized ferrous ions continues to remain paramagnetic down to TC = 75 K. This unusual magnetic behaviour known as 'idle spin behaviour' is due to competitive magnetic interactions with the neighbouring ordered ions.

[1] J.P. Attfield,et al., Physica B, 180&181 (1992) 581.
[2] J.P. Attfield et al., Nature 360 (1998) 655.
[3] A.P. Douvalis, et al., NATO ASI series, E338 (1997) 761; J. Phys.: Condens. Matter 12 (2000) 177; J. Phys.: Condens. Matter, (2002).


K4.3 Processes of the Synthesis and Formation Conditions of Iron Sulphide: Mössbauer Study
N.I. Chistyakova1, V.S. Rusakov1, S.V. Kozerenko2 and V.V.Fadeev2
1) Lomonosov Moscow State University, Moscow, Russia
2) Vernadsky Institute of Geochemistry and Analytical Chemistry Russian Academy of Sciences, Moscow, Russia

Interest in sulphides is connected with the widespread occurrence of these minerals on the Earth and in the Universe. Sulphide forming reactions are considered an important factor in the global carbon, sulphur and iron cycles in Earth crust conditions. The kinetics and formation conditions were investigated by use of Mössbauer spectroscopy and X-ray phase analysis. Synthesis temperature range was 200 C < t < 2000 C. Investigated samples were exposed to aging from several days to several years. It was shown that the reaction proceeded in different ways depending on redox conditions. The hydrotroilite ® pyrite transformation was realized when elemental sulphur was present in the solution. In the absence of an excess of elemental sulphur in the mineral forming media several metastable sulfides formed in succession: hydrotroilite and mackinawite ® greigite ® pyrite and marcasite. Mackinawite content was practically unaffected by time interval of synthesis. The nucleation barrier of mackinawite is not high, so details of its crystallization were primarily determined by the sedimentation conditions (pH, pS).

Mössbauer investigation of synthetic tochilinites in the compositional range of pure iron-bearing up to iron-magnesium (Fe:Mg " 5:1) was performed. An increase in the Mössbauer line shift in the subspectrum of the brucite layer of thochilinite against that in the spectrum of mackinawite has been shown to result from the mackinawite unite cell expansion caused by its conjugation with brucite cell. Magnesium atoms in tochilinite have been found to occur only in the brucite layer, largely in one of two observed nonequivalent sites. The suggestion that, in the structure of tochilinite, conjugation involves equal numbers of mackinawite and brucite layers has been substantiated. The comparative analysis of Mössbauer investigation results of different sulphides with layered structure (iron-magnesium, sodium tochilinites and valleriite) was carried out.

This work was supported by RFBR, Grant 00-05-65372.


K4.4 Archaeology and Mössbauer Spectroscopy
J. Lipka
Department of Nuclear Physics and Technology, Slovak University of Technology, Bratislava, Slovakia

The introduction of Mössbauer spectroscopy in the study of archaeological artifacts is one of the most additions to physical analytical techniques that have been used in elucidation of archaeological problem. The importance of these techniques is based on the presumed elimination of subjectivity in criteria used by archaeologists to classify and extract information from ancient objects. This method is convenient for determining the provenance and manufacture of pottery. Transformation, induced by firing the clay and characterized by Mössbauer spectroscopy, give valuable information regarding the manufacture as, for instance, the final temperature of firing in it and age dating etc. The relative abundance of Fe2+ and Fe3+ determines the atmosphere used to fire a pottery. The results obtained on the samples collected from Slovakia will be presented.


K4.5 Pyrite: Linking Mössbauer Spectroscopy to Mineral Magnetism
E.A. Ferrow
Institute of Geology, Lund University, Lund, Sweden

Gold-bearing pyrite ores are refractory and must be pre-treated to break down the sulphides by oxidation, an undertaking that presents serious problems on the environmental impact of acid mine drainage. In order to improve separation of pyrite from other metals as well as for the development of new strategies to inhibit oxidation of pyrite, it is necessary to understand its thermal and magnetic properties during weathering. Pyrite and its oxidation products were studied with RT Mössbauer spectroscopy and mineral-magnetic methods, including RT hysteresis measurements, demagnetisation of IRM and ARM, thermomagnetic analyses, and low-T-magnetic property measurements.

The pyrite concretion comes from the Limhamn lime quarry in southern Sweden and it has not been exposed to weathering. Ten samples with a diameter of 10 mm and a length of 8.5 mm were drilled from the concretion. The mean natural remanent magnetization of the samples was about 0.18 x 10-3 Am-1, slightly less than the maximum values measured in the marly layers.

The acquisition and demagnetisation curves of the isothermal remanent magnetization indicate the presence of a remanence-carrying mineral in very small amounts. The acquisition curves of the IRM reach saturation magnetization at about 0.15 T for the unheated sample and at about 0.25 T for the sample heated to 4400 C. The shape of the acquisition and demagnetisation curves suggests that the magnetic phases present are similar for the two samples. Comparisons of the IRM data from the sample heated to 4400 C with the unheated one, as well as with the Mössbauer and XRD data, indicate hematite as the possible remanence-carrying mineral.

The thermo-magnetic measurements of pyrite grain show that susceptibility increase between 150 C and 320 C, in the range of hexagonal and monoclinic pyrrhotite, an observation confirmed by ME. Thereafter alterations to highly magnetic phases occur, ending in a peak susceptibility value. This behavior seems to be dependent upon particle size.

The hysteresis loops are well developed for samples heated at 3700 C for 18 or more hours, reflecting the increase in the concentration of the magnetic phases during prolonged heating. The coercivity has increased from an indeterminate value at 1 hour heating to 26.5 mT at 18 hours and remains constant at 45.5 hours. Mössbauer spectroscopy shows that for samples heated for 9 hours or less, only a single pyrite doublet could be observed; but with prolonged heating pyrite is oxidized to iron sulphate anhydrate and eventually to hematite.

The hysteresis measurements of grains confirm the susceptibility behaviour. Grains heated to 500, 550 and 6500 C for 1-hour show a magnetite-type curve, an intermediate wasp-shaped type curve and a hematite-type curve, an observation supported by Mössbauer spectroscopy.