The mechanism of Cl incorporation in amphibole

Abstract

The crystal structure of three calcic C2/m amphiboles with different Cl contents have been refined to R indices of ∼2% using MoKα X-radiation. The monovalent anion site is split into two distinct sites, O3 and O3', that are separated by ∼0.5A along [100] and are occupied by (OH,F) and Cl, respectively. The variation in bond lengths tho the O3 and O3' sites as a function of cation and anion site populations can be interpreted in terms of a strong preference for Fe 2+ -Cl short range order. Bond strength arguments and observed interatomic distances indicate a significant bonding interaction between K at the A site and Cl at the O3' site