Parasymplesite and vivianite specimens were obtained from Kiura Mine, Ohita, Japan and Tomigaoka, Nara, Japan, respectively. Empirical chemical formulas of the specimens determined by energy–dispersive X–ray spectroscopy on the scanning electron microscopy were Fe3(AsO4)2·8H2O, and (Fe0.93Mn0.06Mg0.01)3(PO4)2·8H2O, respectively. The crystal structures of parasymplesite and vivianite determined by single–crystal X–ray diffraction method were monoclinic, space group C2/m, with unit–cell parameters: a = 10.3519(10), b = 13.6009(13), c = 4.7998(4) Å, β = 104.816(2)°, V = 653.32(11) Å3 (Z = 4), and monoclinic, space group C2/m, with unit–cell parameters: a = 10.1518(6), b = 13.4327(7), c = 4.7005(3) Å, β = 104.692(2)°, V = 620.03(6) Å3 (Z = 4), respectively. The crystal structure of parasymplesite solved with the ideal chemical formula was refined to the R1 value of 0.0301 (wR2 = 0.0788) for 722 independent reflections with |Fo| > 4σ(|Fo|), whereas that of vivianite was refined to the R1 value of 0.0272 (wR2 = 0.0832) for 664 independent reflections. The hydrogen atom positions determined by the difference Fourier method coincided with the positions where residual electron density peaks appeared. In the edge–sharing Fe2O6(H2O)4 double octahedra in parasymplesite and vivianite, the bond distance of Fe2–O5, where O5 is the oxygen atom of the H2O molecule, is shorter than that of Fe2–O2. In each arsenate and phosphate phase, only the M2–O2 bond distance shows an increase trend with the increase in the average ionic radii of the M2 site, but the M2–O3 bond distance never shows a clear average M2 ionic radius dependence. In vivianite group minerals, a distortion at the isolated M1O2(H2O)4 octahedra increases as a function of the average M1 ionic radius. The respective complex sheets consisting of the TO4 tetrahedra, isolated M1 octahedra, and edge–sharing M2 double octahedra are connected only by the hydrogen bond O5–H52•••O4. In the arsenate phases, the donor–acceptor distance between O5 and O4 exhibits an increase trend as increase of the average M ionic radius, but in the phosphate phases, there is no clear correlation between donor–acceptor distances and the average M ionic radius.
Diatomaceous earth (DE) samples from the Nevada, United States of America and the Noto region, Ishikawa Prefecture, Japan were subjected to heat treatment of up to 1200 °C, and their SiO4 network structurer changes and crystallization were characterized in detail by thermogravimetric and X–ray diffraction analyses, and infrared and Raman spectroscopies. Raman spectra for the Nevada–DE revealed that the SiO4 network structure of the unheated diatom shells in Nevada–DE may be mainly composed of 6–membered rings of SiO4 tetrahedra. In addition, with heat treatment above 600 °C, this rings structure increased and 4– and 3–membered rings of SiO4 tetrahedra appeared, whereby the SiO4 network structure became similar to that of silica glass. The first diffraction peak (FSDP) position of X–ray diffraction patterns showed the size of the medium–range structure of diatom shells in Nevada–DE and Noto–DE may be smaller than that of silica glass but larger than that of silica gel. Since the FSDP position of Nevada–DE and Noto–DE is the same, the medium–range structure of Noto–DE may also compose of the 6–membered rings of SiO4 tetrahedra. Furthermore, the crystallization temperature (1100 °C) from biogenic amorphous silica such as diatom shells in Nevada–DE and Noto–DE to cristobalite was lower than that (1200 °C) of inorganic amorphous silica such as silica gel. Raman spectra show that the SiO4 network structure in diatom shells for unheated Nevada–DE is mainly composed the 6–membered rings of SiO4 tetrahedra such as cristobalite, and the 6–membered rings in SiO4 network structure is increased at a lower temperature than silica gel. It suggested that the diatom shells in Nevada–DE easily crystallize to cristobalite at a lower temperature than silica gel.
Attempts to synthesize transparent polycrystalline jadeite have been made by direct conversion from bulk glass at pressures 10–20 GPa and temperatures 900–1300 °C using Kawai–type multianvil apparatus. The grain size of jadeite tends to decrease with increasing pressure, but we failed to synthesize polycrystalline jadeite with grain sizes in nano–regime (<100 nm) and obtained the sample with the smallest average grain size of ~ 240 nm at 20 GPa and 1300 °C for 20 min. Polycrystalline jadeite of the minimum grain size exhibits high optical transparency with a transmittance of ~ 70% for a typical wavelength in the visible region. The highest Vickers hardness (Hv) of 14.2 GPa was observed for the polycrystalline jadeite sample with the minimum grain size of ~240 nm, which is about 7% higher than the hardness (Hv = 13.3 GPa) of the sample with the largest grain size of ~ 390 nm. Further increases in optical transparency and hardness of polycrystalline jadeite would be realized if we get nano–polycrystalline samples by optimizing pressure, temperature, heating duration, etc. of the ultrahigh–pressure synthesis experiment.
The oxidation process from magnetite to hematite through maghemite was investigated by X–ray diffraction (XRD) and X–ray absorption spectroscopic techniques. The XRD pattern of magnetite heated at 100 °C for 3 h showed small reflections of maghemite with partially ordered distribution of vacancy (space group P4132 or P4332). Thereafter, the XRD pattern of magnetite heated at 250 °C for 3 h exhibited extra reflections corresponding to the tetragonal maghemite with fully ordered distribution of vacancy (space group P41212 or P43212). Diffraction peaks of hematite occurred from the magnetite heated at 250 °C, in which maghemite and hematite coexisted with magnetite. Diffraction peaks of magnetite subsequently disappeared at 300 °C. Instead, maghemite and hematite dominated the XRD pattern, but the amount of maghemite reduced from 300 °C. The maghemite completely disappeared at 500 °C, and hematite finally dominated the XRD pattern. Rietveld fitting results clearly showed that the a lattice parameter and site occupancy factor of Fe at the octahedral site continuously decreased at the temperatures from 25 to 300 °C. The X–ray absorption near edge structure (XANES) result showed that the Fe3+/ΣFe increased up to 300 °C and remained constant until 500 °C, indicating that Fe2+ in oxidized magnetite was completely oxidized to Fe3+ at 300 °C. Furthermore, the intensities of radial structure function (RSF) peaks at 1.7 and 3.1 Å corresponding to the Fe–O bonds in octahedral site and the Fe–Fe interaction between the octahedral sites reduced continuously from 25 to 300 °C. The fitting results of the first shells indicated that the coordination number and site occupancy factor at the octahedral site continuously decreased at the temperature range from 25 to 300 °C, which were approximately consistent with those of Rietveld fitting analysis. The a lattice parameter of the oxidized magnetite displayed a linear trend between stoichiometric magnetite and stoichiometric maghemite with a relationship of a = 0.0985x + 8.3397 (x = Fe2+/Fe3+). It was clearly confirmed that during the magnetite oxidation, Fe was continuously removed from the octahedral sites, which resulted in the formation of maghemite with partially ordered distribution of vacancy. Just after magnetite oxidation was completed, the vacancy ordering further progressed by the diffusion of Fe3+ within the structure, leading to the formation of maghemite with fully ordered distribution of vacancy.
The monzogranite of El Fereyid is one of the rare metal–rich granites, where zircon is one of the ore minerals. Thus, studying zircon here is of vital importance. Most of the studied zircon grains are metamict and thus the loss of radiogenic Pb is detected for them. Nevertheless, our study allowed us to obtain the U–Pb (SHRIMP–II) age of magmatic crystallization for the El Fereyid monzogranite: 626 ± 13 Ma. These data allowed the correction of earlier determined age (K–Ar system in biotite) for the El Fereyid massif.
Zircon grains of El Fereyid monzogranite demonstrate heterogeneous structure in CL images, they are rich in rare earth elements (REE, concentration 3000–22500 ppm), trace elements (U: 2000–14800 ppm, Th: 300–2500 ppm), and has low Th/U ratios (average 0.18). Most zircon grains exhibit multiple internal oscillatory zoning in CL images, indicating a typical magmatic origin. The core of zircon grain has a dark tone on CL imaging and magmatic–type REE spectra. Zircon cores are enriched in REEs, U, Th, and Y, thus recording that the magma was rich in incompatible elements. Light CL rims of zircon grains are depleted, relative to the cores in most of the trace elements, except for P, Ca, and Ti. REE spectra in rims show similar patterns that are more often demonstrated by hydrothermal zircon grains. This explains their crystallization in the late–magmatic stage when magma was rich in fluids but depleted in most of the trace elements.