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 D _oct/D _tetr 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.     

Hydrogen and carbon isotope studies on the graphite-bearing metapelites in the northern Kiso district of central Japan

Measurements were made of the hydrogen isotope ratios of hydrous silicates (mica and amphibole) and whole rocks, and the carbon isotope ratios of graphite and carbonaceous matter in the metamorphic rocks from the northern Kiso district in central Japan.D values of hydrous silicates in the graphite-bearing metapelites are always higher than those in graphite-free schists, even though the sample localities of the two rock-types are very close. Hydrogen isotopic equilibrium has been attained between the coexisting minerals.D/H ratios of water in the metamorphic fluids seem to depend strongly on the presence or absence of graphite and seem to be not constant throughout the district. The district is divided into three areas of low (metamorphic zones I, II), medium (zones IIIa–V) and high ¹³C_gr value (zones VIa–VII) areas. In the high ¹³C_gr values area, the carbon contents of the graphite-bearing rocks decrease slightly from zones VIa to VII, whereas the ¹³C_gr values increase sharply from the upper part of zone VIa to VIb. TheD values of biotite in these graphite-bearing rocks are higher than those in the medium ¹³C_gr area. This suggests that methane enriched inH and¹²C is produced and liberated by the devolatilization reactions between muscovite, graphite and water. The fluid produced is composed of water, methane and a subordinate amount of carbon dioxide, and its logfO₂ value is deduced to be about 1.2 lower than that defined by the FMQ buffer. In the medium ¹³C_gr area, the ¹³C values of graphite are nearly constant (–20.8), while the Fe₂O₃/(Fe₂O₃ + FeO) ratio of the graphite-bearing rocks apparently decreases with increasing metamorphic grade.D differences in hydrous silicates between graphite-bearing and graphite-free rocks are observed. These facts are interpreted to mean that methane was produced in addition to water and carbon dioxide, and that its generation ( ratio of the fluid was about 2) had practically no isotope effect on the graphite. In the low ¹³C_gr area, the carbon contents of the rocks decrease clearly from zones I to IIIa. TheD and ¹³C_gr values of the non-metamorphosed shales are much lower than those of the low grade graphite-bearing metapelites. This suggests that methane is produced and liberated from the rocks even at the incipient stage of metamorphism.     

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.     

Comparison of carbon cycle between the western Pacific subarctic and subtropical time-series stations: highlights of the K2S1 project

A comparative study of ecosystems and biogeochemistry at time-series stations in the subarctic gyre (K2) and subtropical region (S1) of the western North Pacific Ocean (K2S1 project) was conducted between 2010 and 2013 to collect essential data about the ecosystem and biological pump in each area and to provide a baseline of information for predicting changes in biologically mediated material cycles in the future. From seasonal chemical and biological observations, general oceanographic settings were verified and annual carbon budgets at both stations were determined. Annual mean of phytoplankton biomass and primary productivity at the oligotrophic station S1 were comparable to that at the eutrophic station K2. Based on chemical/physical observations and numerical simulations, the likely “missing nutrient source” was suggested to include regeneration, meso-scale eddy driven upwelling, meteorological events, and eolian inputs in addition to winter vertical mixing. Time-series observation of carbonate chemistry revealed that ocean acidification (OA) was ongoing at both stations, and that the rate of OA was faster at S1 than at K2 although OA at K2 is more critical for calcifying organisms.