Abstract
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.
