Relationships between tetrahedral kink angles and site occupations in C2/c pyroxene from volcanic rocks

The relationships between kinking of the T tetrahedral chain and the site configuration of C2/c pyroxenes from volcanic rocks were investigated, and about two hundred samples were examinated. Kink angle (KA) is defined by the equation KA = arccos [1-c 2/(2d 2)], in which c is the cell edge andd the O3A1–O3A2 distance; in the pyroxenes studied here, it varies from about 164° to about 167°.The ratio betweenc cell edge and O3A1–O3A2 distance depends on the various M1, M2 and T site configurations which, in turn, are related to their different mechanisms of cation substitution. Thec/d ratio varies if isomorphic substitutions involve cations with different charge, while it is quite constant if the substitutions involve cations with the same net charge.Thec/d ratio variation is related to the amount of cation substitution in each crystallographic site, and reaches its maximum range in synthetic and natural end-member pyroxenes.