Takamitsu Yamanaka

Infrared absorption spectra and cation distributions in (Mn, Fe)3O4

Abstract Infrared spectra of the solid solution, (Mn, Fe)3O4, were examined and interpreted based on the spectra of the end members, Fe3O4 and Mn3O4. Local lattice distortions due to the Jahn-Teller effect were revealed in the still cubic region in comparison with the X-ray data.

Structure and lattice vibrations of Mg-Al spinel solid solution

X-ray structure refinements have been made for nonstoichiometric (MgO · 3Al2O3) and stoichiometric Mg-Al spinels. Several structure variations with chemical composition have been observed and are discussed in relation to Al substitution in tetrahedral sites. Infrared reflection and Raman spectra of the single crystal of the nonstoichiometric spinel (MgO · 3Al2O3) have been measured and analyzed. The results obtained are compared with those reported for the stoichiometric sample. From the infrared and Raman frequencies reported for the stoichiometric Mg-Al spinel, which are partly complemented with our results, the effective ionic charges of the ions in MgAl2O4 have been estimated on the basis of the rigid ion model.

Mössbauer spectra and magnetic features of ilvaites

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+)2Fe0.52+Mn0.52+(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/xg, occurs about 120 K; 1/xg depends linearly upon temperature above 120 K. This change, which is characterized by an unusual mode of decrease in 1/xg, 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.

Crystal structures of Ni2SiO4 and Fe2SiO4 as a function of temperature and heating duration

X-ray structure refinements of Ni2SiO4 and Fe2SiO4 spinels have been made as a function of temperature and heating duration by intensity measurements at high temperatures and room pressure. The lattice parameters of Ni2SiO4 spinel linearly increased with temperature up to 1,000° C. However, Fe2SiO4 spinel exhibited a nonlinear thermal expansion and was converted to a polycrystalline mixture of spinel and olivine by heating of less than one-hour at 800° C.The ratios between the octahedral and tetrahedral bond lengths Doct/Dtetr and between the shared and unshared edge distances (O-O)sh/(O-O)unsh in Fe2SiO4 spinel were both much larger than those in Ni2SiO4. These ratios increase with temperature. The Fe2SiO4 spinel more readily approached a activation state which facilitated the transition to the olivine structure than the Ni2SiO4 spinel.The lattice parameter of Ni2SiO4 spinel decreased with heating period at constant temperatures of 700° C and 800° C. The parameter of the quenched sample after heating for 52 h at 700° C was smaller than that of the nonheated sample. The refinements of the site occupancies at each heating duration indicated an increase in the cation deficiency in both tetrahedral and octahedral sites. Electron microprobe analysis, however, proved no significant difference in the chemical compositions between the quenched and nonheated samples. Si and Ni atoms displaced from normally occupied spinel lattice sites are assumed to settle in vacant sites defined by the cubic close packed oxygen sublattice in a manner which preserves the electric neutrality of the bulk crystal.

Raman scattering and lattice vibrations of Ni2SiO4 spinel at elevated temperature

Raman spectra of Ni2SiO4 spinel (Oh7Z=8) have been measured in the temperature range from 20 to 600 °C and the Raman active vibrations (A1g+Eg+3F2g) have been assigned. A calculation of the optically active lattice vibrations of this spinel has been made, assuming a potential function which combines general valence and short range force constants. The values of the force constants at 20 and 500 °C have been calculated from the vibrational frequencies of the observed Raman spectra and infrared (IR) spectral data.The Ni spinel at 20 °C has a prominently small Si-O bond stretching force constant of K(SiO)=2.356 ∼ 2.680 md/Å and a large Ni-O bond stretching constant of K(NiO)=0.843 ∼ 1.062 md/Å and these force constants at 500 °C decrease to K(SiO)=2.327 ∼ 2.494 md/Å and K(NiO)=0.861 ∼ 0.990 md/Å. The Si-O bond is noticeably weakened at high temperatures, despite the small thermal expantion from 1.657 Å (20 °C) to 1.660 Å (500 °C).These changes of the interatomic force constants of the spinel at high temperatures are in accord with the thermal structure changes observed by X-ray diffraction study. The weakened Si-O bond is consistent with the fact that Si atoms in the spinel lattice can diffuse at significant rates at elevated temperature.

Pressure and temperature dependence of cation distribution in Mg-Mn olivine

Synthetic (Mg0.51, Mn0.49)2SiO4 olivine samples are heat-treated at three different pressures; 0, 8 and 12 GPa, all at the same temperature (∼500° C). X-ray structure analyses on these single crystals are made in order to see the pressure effect on cation distribution. The intersite distribution coefficient of Mg and Mn in M1 and M2 sites, KD = (Mn/Mg)M1/(Mn/Mg)M2, of these samples are 0.192 (0 GPa), 0.246 (8 GPa) and 0.281 (12 GPa), indicating cationic disordering with pressure. The small differences of cell dimensions between these samples are determined by powder X-ray diffraction. Cell dimensions b and c decrease, whereas a increases with pressure of equilibration. Cell volume decreases with pressure as a result of a large contraction of the b cell dimension. The effect of pressure on the free energy of the cation exchange reaction is evaluated by the observed relation between the cell volume and the site occupancy numbers. The magnitude of the pressure effect on cation distribution is only a fifth of that predicted from the observed change in volume combined with thermodynamic theory. This phenomenon is attributed to nonideality in this solid solution, and nonideal parameters are required to describe cation distribution determined in the present and previous experiments. We use a five-parameter equation to specify the cationic equilibrium on the basic of thermodynamic theory. It includes one energy parameter of ideal mixing, two parameters for nonideal effects, one volume parameter, and one thermal parameter originated from the lattice vibrational energy. The present data combined with some of the existing data are used to determine the five parameters, and the cation distribution in Mg-Mn olivine is described as a function of temperature, pressure, and composition. The basic framework of describing the cationic behavior in olivine-type mineral is worked out, although the result is preliminary: each of the determined parameters is not accurate enough to enable us to make a reliable prediction.

Structural change of minerals.

The mini-symposium on structural change of minerals was organized at the 50th Anniversary Meeting of the Mineralogical Society of Japan. Ten papers by oral, including three invited papers, and 4 papers by poster were presented at the meeting. Presented papers are characterized by the dominance of experimental works related to high pressure phase transformations and the theoretical works related to computer simulations. Abstracts of three invited papers and the list of the presented papers are given.