角閃石の塩素導入機構

抄録

Characteristic structure of Cl-amphibole is reviewed, and its application to chemical zoned amphibole from the Ramnes cauldron is summarized.
Large ionic radius of Cl⁻ brings geometrical and chemical effects to the amphibole structure. In the Cl-amphibole, the double chain is significantly deformed to fit expanded octahedral strips, which is achieved by substitution of ^IVAl for Si (geometrical constraint). The Cl⁻ projects toward the A site and there is a interaction between Cl⁻ and cations in the A site (A site constraint). The short range order of Fe²⁺-Cl⁻ is observed in Cl-amphibole (chemical constraint). These constraints simultaneously control the Cl⁻ content in amphibole and imply that Cl⁻ substitutes for OH⁻ only in the ferro-paragasite or ferro-hastingsite component.
Based on these structural considerations on Cl-amphibole, Cl⁻ variations in the zoned amphibole from the Ramnes cauldron were interpreted. The zoning were developed under different fluid conditions in chemistry and temperature.