<論文>鬼首火山高日向山溶岩ドームから産するホルンブレンド(magnesiohornblende)斑晶に認められる複合イオン置換

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The crystal chemistry of euhedral magnesiohornblende phenocrysts from the Takahinatayama lava dome of Onikobe volcano in Miyagi Prefecture was studied by EPMA to document temperature-dependent cation substitutions. The relationships between the site populations of ^<IV>A1^<3+> and other cations (based on a unit formvla of 23 oxygens) indicate that the following substitutions are active : Mg^<2+>+sSi^<4+>〓Ti^<4+>+2^<IV>Al^<3+>(Ti-tschermakite) Mg^<2+>+Si^<4+>〓Fe^<3+>+<IV>Al^<3+>(ferri-tschermakite)□+Si^<4+>〓Na_A+^<IV>Al^<3+>(edenite) Crystal chemical considerations of ^<IV>Al indicate that iron is present dominantly as ferric iron in this amphibole rather than ferrous iron. Furthermore, the bimodal distribution of ^<IV>Al in the single crystal specimen suggests that the chemical stibstitution of Al in the tetrahedral sites occurred in two stages. Possibly, in the later of the two stages, the introduction of Al in the tetrahedral sites was facilitated by a slight increase in the size of the tetrahedral sites due to thermal expansion of the amphibole lattice.
2002-10-31