結晶の生いたち

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Nakaya wrote a poem in which he says “Snowflake is a letter to us from the sky”. If expressed similarly, mineral crystals in nature are the letters to us from the depth of the earth. They convey the message telling us what has happened under the earth, what was the conditions when they were grown, and what they have experienced since they born. It is clearly worth doing our best to read these letters. As Nakaya tried to read the message that snowflakes convey by studying their crystal morphology, we can obtain much information through detailed studies of the nature of crystals. Surface microstructure of crystal faces, zonal structures inside a crystal, distribution of foreign crystals and lattice defects, as well as impurity atoms, in addition to crystal morphology, will serve as useful key codes to decode the letters.<BR>In this paper, diamond crystals are used as an example how to decode the letter from deep under the earth. Analysis of crystal morphology of diamond suggest that natural diamonds were grown under near-equilibrium conditions, whereas synthetic crystals under the conditions apart from the equilibrium. Universal occurrence of rounded crystals in natural diamonds suggests that they have experienced dissolution process while they were transported to the earths surface from the depth, and that the speed of transportation was much higher than our general conjecture. It is also conjectured that diamond will occur only in continental shield and not in the orogenic zones. Observations on the surface microstructures of diamond crystals shows that diamonds were grown from solution phase by condensation and neither from melt phase nor by solid state recrystallization. Zonal structures revealed by chemical etching show that the growth of diamonds was carried out by layer spreading mainly on {111} and that the growth history of individual crystals was very complicated. During the whole history of growth, crystals have experienced partial dissolution, changes in growth speed as well as the rate of impurity adsorption. Recent observations on the nitrogen platelets in natural diamond crystals give an idea that diamond crystals have experienced annealing stage after they were formed, and that the possible origin of carbon of diamond would be carbonaceous material deposited on the ocean bed, which was then transported by an active thrust or heat flow to the depth of 5 to 7 hundreds km, where it was crystallized as diamond. These considerations are mainly based on the hypothesis on the origin of diamond which has recently been proposed by F. C. Frank.
社団法人東京地学協会の論文

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砂川一郎
地質調査所

結晶の生いたち

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