A direct lattice-resolution study of precipitation (exsolution) in orthopyroxene

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Abstract
A direct lattice technique in which a number of diffracted beams from the same systematic row were used to form a lattice image has been used to study the detailed distribution of phases in orthopyroxenes from two igneous intrusions. The samples contained coarse, heterogeneously-nucleated, lamellar augite precipitates on (100), which have stepped interfaces with the matrix. Direct lattice images showed that the steps varied from one to several lattice parameters in height; they appeared to be growth ledges associated with the thickening of the lamellae. The study has also resolved fine, homogeneously-distributed, coherent precipitates one matrix lattice-parameter (18 Å) thick on (100). A third heterogeneously-nucleated, coherent lamellar precipitate parallel to (100) with monoclinic symmetry and an α-parameter of 9–6 Å was also identified. Both of the coherent phases are thought to be metastable precipitates which formed at low temperatures after precipitation of the equilibrium augite.