Exoskeletons of diatoms are constituted of amorphous silica and organic matter, called frustules. They are composed of two valves fit each other like a petri-dish, and connected by girdle bands. In this study, structural analyses of the silica-based compound in frustules were carried out using Fourier transform polarized infrared microspectroscopy (pol-mFTIR). Unpolarized spectrum of diatoms show strong peaks at ν≈1100 cm−1 and small peaks at 800 cm−1 in the region of ν=700-1500 cm−1. The feature is similar to those of low-quartz and fuzed silica glass indicating the existence of the SiO4 network structure. The slight difference of three band shapes may indicate different linkage ordering of SiO4 tetrahedra in these materials. Polarized IR spectra of a pennate diatom show an anisotropy of absorption band, showing the existence of two Si-O bonds with different distances. Polarized IR spectra of Si-O band with long distances shows a sharp peak in the region ν=1051-1081 cm−1. When the long-axis of pennate diatoms was set parallel to IR light oscillation, the absorbance took a maximum. Whereas the polarized IR spectra of short Si-O bonds shows a broad peak at ν≈1227 cm−1. When the long-axis of pennate diatoms was set parallel to IR light oscillation, the absorbance took a minimum value. These results indicate that Si-O bonds with long distances exists along the long-axis of the diatom and short Si-O bonds exist along the short-axis. We applied models suggested by Seifert et al. (1982) to interpret Si-O-Si angle in the frustules. Si-O-Si angle from long Si-O bonds was smaller than ones from short Si-O bonds. The structural distortion generated with the anisotropic SiO4 network may be stabilized by the presence of organic matters in diatoms. The pol-mFTIR technique used in this study has a possibility for the application to investigate structural anisotropy in other amorphous materials that show a broad amorphous profile in X-ray diffraction method.
The Mizoguchi area, Southwest Japan, has been interpreted as a western continuation of the Hida terrane, based on the similarity of the mineralogy and radiometric age of metamorphic rocks. However, new geochemical data show metapelite and metabasite from the Mizoguchi area differ from those of the Hida terrane in the following points. i) Metapelite is relatively rich in Sc compared to La and Th, and show gently inclined chondrite-normalized REE patterns. ii) In discrimination diagrams for basaltic rocks, metabasite plots in the fields of arc tholeiites or N-MORBs; however, they are relatively depleted in light REEs but much richer in LIL elements than N-MORBs. From these geochemical features the Mizoguchi metamorphic rocks are interpreted to have formed in the fore-arc region of an oceanic island arc. This is in contrast to generally accepted continental setting for the Hida area and suggests the two areas are geologically distinct.
Thermal diffusivity of synthetic forsterite was measured in the temperature range from 350 to 1500 K at 0 and 6 GPa. Reciprocal thermal diffusivity cannot be regarded as a linear function of absolute temperature. Thermal diffusivity of forsterite increases by ca. 40% over 6 GPa.
Magmatic andalusite is found from the metatexite and from the pelitic rock with boudin necks filled with leucosome in the garnet-cordierite (Grt-Crd) zone of the Aoyama area, Ryoke metamorphic belt, SW Japan. In general, andalusite crystals in the pelitic schist without leucosome from the sillimanite-K-feldspar (Sil-Kfs) zone and low-temperature part of the Grt-Crd zone are partly transformed into sillimanite, suggesting that the peak P-T conditions of the Aoyama area have reached to the sillimanite grade. On the other hand, andalusite crystals found from the leucosome of the low-temperature part of the Grt-Crd zone are subhedral, and have not transformed into sillimanite at all. Absence of an evidence for andalusite-sillimanite transition implies that these crystals did not experience the prograde andalusite-sillimanite transition and thus are retrograde products. They characteristically occur in the leucosome containing euhedral plagioclase or subhedral cordierite crystals and show subhedral shape different from the euhedral shape usually observed in the prograde andalusite. The P-T conditions for the low-temperature part of the Grt-Crd zone is sufficiently high for the partial melting of pelitic rocks, and the andalusite-bearing leucosome has appropriate composition as a frozen melt. These observations suggest that the retrograde andalusite is of magmatic origin. Combining effects of the presence of boron in the melt, addition of Al2O3 to the subaluminous Qtz-Ab-Or system to saturate in aluminosilicates, and preferential incorporation of Fe2O3 into andalusite may have favored the crystallization of magmatic andalusite in the Aoyama area. Finding of the magmatic andalusite from the low-temperature part of the Grt-Crd zone suggests that the P-T path of this zone went through the overlapping region of water-saturated solidus of peraluminous granite and andalusite stability field. The P-T path of the low-temperature part of the Grt-Crd zone may, therefore, not be a hairpin shaped one but a clockwise one when the aluminosilicate phase diagram of Holdaway (1971) is used. Non-hairpin shaped nature of the P-T path may be partly responsible for the preservation of the textural evidences of partial melting in the Aoyama area.