著者

田窪 宏

出版者

Japan Association of Mineralogical Sciences

雑誌

岩石鉱物鉱床学会誌 (ISSN:00214825)

巻号頁・発行日

vol.75, no.9, pp.300-312, 1980-09-05 (Released:2008-08-07)

参考文献数

30

被引用文献数

1 1

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Experiments on the crystal growth of materials belonging to the A12O3-TiO2 system and on the high-temperature behaviour of natural star-sapphire were made with special reference to the genesis of regular arrangement of the ruitle needle crystals in and on corundum crystals. The crystals were grown by using direct fusion, flux-fusion and hydrothermal methods under the various conditions of temperature, chemical concentration, cooling mode, temperature gradient and oxygen partial pressure. The following results were obtained: (1) When a thin layer of melt made from TiO2 powder or from mixture of Al2O3 and TiO2 powder was slowly cooled on (0001) face of corundum substrate, the regular arrangement of acicular rutile crystals was easily formed on the substrate. (2) The βAl2TiO5 crystals grown from the melt on the substrate did not show the regular arrangement. (3) When crystals of corundum and rutile were synchronously grown by the monotonous slow-cooling of flux-melt in air, no regular intergrowth appeared in both as-grown and aged corundum crystals. (4) When the intermitent flucutation of temperature was introduced during the slow-cooling in the above method, the conspicuous zonal structure composed of various imperfections appeared in the as-grown corundum crystals. After aging in air, the regular arrangement of rutile appeared on the (0001) faces and along the imperfection zones of corundum crystals. (5) When rutile was crystallized synchronously with corundum or epitaxially on the (0001) face of corundum from flux-melt in oxygen gas flow or from hydrothermal solution, no regular intergrowth was formed. (6) In the presence of flux-melt, the regular arrangement of rutile on the corundum substrate was promoted in air or in reducing atmosphere. (7) Epitaxial growth of corundum on a rutile single crystal did not occur in any of the growth methods mentioned above. (8) When natural star-sapphires were heated above 1200°C, the regular arrangement of rutile needles fell into disorder from surface to inside and finally disappeared with leaving small gas-bubbles. Ti-atoms tended to migrate predominantly towards the surface and into the small cracks where rutile was recrystallized. Based on these results and those reported by other authors, critical examinations were made as to such problems as solubility of Ti into corundum, stability of βAl2TiO3, removal of Ti from corundum technology for synthesis, mechanical behaviour, epitaxis and natural occurrence. As a result of examinations, it seems that the “precipitation from solidsolution” theory, which is generally accepted as to the genesis of regular intergrowth in star-sapphire, is less plausible. Following two theories were developed on the genesis ofstar-sapphire. (1) Secondary and epitaxial crystallization of rutile along open spaces and imperfection-zones such as shear planes, tension cracks, zonal structure and inclusions, most of which are parallel to (0001) face. TiO2-component is supplied not only from the external sources, but also from the internal sources such as the imperfection-zones with higher Ti-concentration. (2) Synchronous and parallel growth of corundum and dendritic rutile crystals, being followed by survival of the twig crystals which preserve proper orientation for epitaxis.

著者

山中 高光 永井 隆哉 大高 理 植田 千秋 土山 明 田窪 宏

出版者

大阪大学

雑誌

一般研究(C)

巻号頁・発行日

1993

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プレートの流動現象や、沈み込むスラブの運動を把握するためにはダイナミカルな構造研究をする上で、地殻やマントルの構成鉱物の環境変化(温度・圧力・成分等)に伴って、生じる転移・分解・融解・結晶内イオン交換反応等の諸々の構造変化のカイネティックスや機構を究明することが重要な課題である。本実験ではケイ酸塩鉱物や類似鉱物の圧力誘起による構造変化と逐次観察と動的構造の研究を行った。1.マントルの主構成鉱物であるカンラン石(Mg_2SiO_4)について分子動力学(MD)計算を用いて圧力誘起の構造転移のシミュレーションを行った。室温では35〜40GPa圧力領域で圧力誘起非晶質相転移が生じ、95〜100GPaで未記載な結晶構造に再結晶化することが計算から明らかになった。ダイヤモンドアンビル高圧発生装置と放射光X線を用いた高圧実験でカンラン石のGe置換体のMg_2GeO_4の圧力誘起非晶質相転移を実際に確認した。2.マフィックな珪酸塩鉱物が海洋地殻で水和物に変質し、それらがサブダクションでの低温(<500℃)で応力下での構造安定性を調べ水の挙動を研究する。そのためCa(OH)_2の圧力誘起相転移と準安定相の存在領域を放射光X線回折実験で決定しその機構を解明した。その結果水和鉱物は高圧下では脱水反応はせず、非晶質相として地殻内部にもたらされ、これらがマグマなどに重要な水の起源として考えられる。3.マルチアンビル高圧発生装置に装着し、SiO_2の同一の多形構造転移を示すGeO_2の圧力誘起非晶質相転移した物質についてS波とP波の弾性波速度を測定し、体積圧縮率や剛性率を求めた。これらの弾性波速度の温度・圧力変化の研究はサブダクション・ゾーンで生じる深発地震の発生の解明にも貢献した。また分子動力学から求められた温度圧力関数にした弾性常数の変化と比較し検討した。