Amphiboles are common in metamorphic and igneous rocks. The amphibole structure consists of doubled Si4O11 chains running parallel to c-axis. These chains are bonded to octahedral strips consisting of three regular octahedral sites (M1, M2, M3) and one larger 6- to 8-fold site (M4). In addition there is an even larger 10- to 12-fold A site that is usually empty.