著者

Atsushi Ishihara Hiroaki Ohfuji

出版者

Japan Association of Mineralogical Sciences

雑誌

Journal of Mineralogical and Petrological Sciences (ISSN:13456296)

巻号頁・発行日

pp.230327, (Released:2023-10-13)

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In this study, we investigated the origin and formation process of zeolitized gastropod fossils in Neogene sediments (Shiote Formation) in Minamisoma, Fukushima, Japan using powder X-ray diffraction, SEM-EDS and micro-Raman spectroscopic analysis. The formation of zeolites was particularly pronounced in the upper chamber, which was not filled with detrital particles, of the gastropod fossils, where tabular crystals of heulandite were observed growing directly from the shell wall. The heulandite crystals are often covered by large euhedral crystals of calcite and occasionally by acicular crystals of mordenite. The formation of zeolite (heulandite) was also observed in the matrix of the host sandstone together with clay minerals (mostly montmorillonite), suggesting that the Shiote Formation experienced moderate metamorphism equivalent to zeolite facies during burial diagenesis. The Si/Al ratio of heulandite was found to decrease gradually from the bottom (∼4.5) to the top (∼3.1) within single crystals across the threshold (4.0) for clinoptilolite/heulandite classification boundary. This may reflect the increase in temperature of the surrounding environment with increase in the burial depth. The extensive growth of zeolites and calcite inside the gastropod fossils indicates that the shell provided semi-closed spaces in which pore fluid could be retained and condensed during diagenesis, thus promoting the crystal growth from the supersaturated solution.

著者

Keisuke MITSU Tetsuo IRIFUNE Hiroaki OHFUJI Akihiro YAMADA

出版者

Japan Association of Mineralogical Sciences

雑誌

Journal of Mineralogical and Petrological Sciences (ISSN:13456296)

巻号頁・発行日

pp.210319, (Released:2021-09-30)

被引用文献数

1

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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.

著者

Hiroaki OHFUJI Motosuke NAKAYA Alexander P. YELISSEYEV Valentin P. AFANASIEV Konstantin D. LITASOV

出版者

一般社団法人日本鉱物科学会

雑誌

Journal of Mineralogical and Petrological Sciences (ISSN:13456296)

巻号頁・発行日

vol.112, no.1, pp.46-51, 2017 (Released:2017-03-18)

参考文献数

19

被引用文献数

6

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This study revealed for the first time the microtexture and crystallographic features of natural polycrystalline diamond, yakutite found in placer deposits in the Siberian Platform, Russia. Yakutite consists of well–sintered nanocrystalline (5–50 nm) diamond and small amount of lonsdaleite showing distinct preferred orientations. Micro–focus X–ray and electron diffractions showed a coaxial relationship between lonsdaleite 100 and diamond 111, suggesting the martensitic formation of yakutite from crystalline graphite. These textural and crystallographic features are well comparable to those of the impact diamonds from the Popigai crater located in the central Siberia and strongly support the idea that yakutite is a product of long–distance outburst from the Popigai crater, which has been inferred merely from the geochemical signatures.

著者

Atsushi ISHIHARA Hiroaki OHFUJI

出版者

Japan Association of Mineralogical Sciences

雑誌

Journal of Mineralogical and Petrological Sciences (ISSN:13456296)

巻号頁・発行日

vol.118, no.1, pp.230327, 2023 (Released:2023-11-21)

参考文献数

34

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In this study, we investigated the origin and formation process of zeolitized gastropod fossils in Neogene sediments (Shiote Formation) in Minamisoma, Fukushima, Japan using powder X-ray diffraction, SEM-EDS and micro-Raman spectroscopic analysis. The formation of zeolites was particularly pronounced in the upper chamber, which was not filled with detrital particles, of the gastropod fossils, where tabular crystals of heulandite were observed growing directly from the shell wall. The heulandite crystals are often covered by large euhedral crystals of calcite and occasionally by acicular crystals of mordenite. The formation of zeolite (heulandite) was also observed in the matrix of the host sandstone together with clay minerals (mostly montmorillonite), suggesting that the Shiote Formation experienced moderate metamorphism equivalent to zeolite facies during burial diagenesis. The Si/Al ratio of heulandite was found to decrease gradually from the bottom (∼ 4.5) to the top (∼ 3.1) within single crystals across the threshold (4.0) for clinoptilolite/heulandite classification boundary. This may reflect the increase in temperature of the surrounding environment with increase in the burial depth. The extensive growth of zeolites and calcite inside the gastropod fossils indicates that the shell provided semi-closed spaces in which pore fluid could be retained and condensed during diagenesis, thus promoting the crystal growth from the supersaturated solution.

著者

Hiroaki OHFUJI Masashi YAMAMOTO

出版者

一般社団法人日本鉱物科学会

雑誌

Journal of Mineralogical and Petrological Sciences (ISSN:13456296)

巻号頁・発行日

vol.110, no.4, pp.189-195, 2015 (Released:2015-08-29)

参考文献数

15

被引用文献数

1 14

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This study demonstrates the validity of a thin osmium coating for quantitative energy–dispersive spectroscopic (EDS) analysis, particularly for light elements such as O (and potentially C and N) in natural/synthetic minerals. An osmium coating prepared by chemical vapor deposition provides an extremely thin and uniform layer whose thickness can be controlled simply by coating time. Because of the high reproducibility and reliability of the osmium coating process, users have no difficulty in evaluating the actual coating thickness, which enables strict and precise absorption corrections (for the coating layer), even for low–energy characteristic X–rays, which are susceptible to attenuation by the coating layer itself. Our results show that oxygen concentrations in silicate and oxide minerals can be quantified correctly when using the osmium coating, whereas quantification using a carbon coating afforded values that were a few wt% lower than stoichiometry, probably due to the uncertainty of the actual coating thickness (i.e., the absorption correction was incorrect). The ability to accurately quantify oxygen may stimulate new analytical applications, such as the estimation of Fe2+/Fe3+ concentrations and water content in minerals. Furthermore, the Os–coated samples prepared for EDS analysis are also suitable for electron back–scattered diffraction (EBSD) analysis without re–polishing and re–coating, which are usually routine but time–consuming tasks in the case of carbon–coated samples.