Ca in orthopyroxene; structural variations and kinetics of the disordering process

The substitution of Ca for Mg and Fe in the orthopyroxene structure was investigated. Geometrical parameters obtained by single-crystal XRD on different Ca-rich orthopyroxenes showed that Ca enters the octahedral M2 site, causing an enlargement of this polyhedron. As a consequence, both tetrahedral chains extend to allow matching between the tetrahedral and octahedral layers. A kinetic study at 730 degrees C on a volcanic orthopyroxene (Wo (sub 4) En (sub 60) Fs (sub 36) ) with thin exsolved augite lamellae but no Guinier-Preston zones yielded a disordering rate constant of 2.69 X 10 (super -2) /min. This value is in agreement with that expected for a 'normal' kinetic behaviour.