TEM study of the transformation of augite to sodic pyroxene in eclogitized ferrogabbro

Abstract

Transmission and analytical electron microscopy has been used to study relicts of augite that occur in various stages of transformation to sodic pyroxene. The augite relicts are characterized by a hatching produced by two sets of former “001” exsolution lamellae that possess high dislocation densities and were altered completely to sodic pyroxene, even where the augite matrix is still fresh. With further alteration, the sodic pyroxene in these “001” lamellae recrystallized and grew into the augite matrix, resulting in irregular lamellae that consist of subgrains having low dislocation densities. Needles and thin (100) lamellae of sodic pyroxene developed on the “001” lamellae. Alteration of the augite matrix proceeded by growth of areas with defects (dislocations, stacking faults). All sodic pyroxenes in these microstructures have the same orientation as the precursor augite, indicating a topotactic reaction mechanism. The reactions occurred at roughly constant Si and mainly involved replacement of Ca and Mg by Na and Al. Dislocations may have played a prominent role in the transformation by acting as diffusion pathways and by migrating into untransformed augite, leaving sodic pyroxene in their wake. At the grain boundaries of the augite, discrete grains of sodic pyroxene formed without any fixed orientation relation with the augite, consistent with a non-topotactic reaction. The predominance of the topotactic reaction inside the augite over the non-topotactic grain-boundary reaction is attributed to the scarcity of fluids during eclogite metamorphism.