Abstract
Transmission, analytical and high-voltage electron microscopy have been used to study the distribution of phases in the naturally occurring silicate minerals hornblende and grunerite. These two minerals belong to the amphibole group and previous analytical work has suggested the existence of a miscibility gap between them involving the segregation of calcium. The minerals in the specimens studied are coarse-grained, having crystallized slowly during metamorphism.
Large lamellar precipitates are present in both phases. These precipitates form on (100) and (¯101) planes of the matrix and have remained coherent. Steps are present on the broad faces of these lamellae, suggesting that they have thickened by the propagation of ledges as observed in metal alloy systems. These features are common to both phases, but the grunerite matrix contains an additional set of fine, coherent precipitates which have formed at a later stage and show distinct precipitate-free zones around the earlier precipitates. Analytical microscopy has been used to confirm qualitatively that the precipitation involves segregation of calcium.
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Gittos, M.F., Lorimer, G.W. & Champness, P.E. An electron-microscopic study of precipitation (exsolution) in an amphibole (the hornblende-grunerite system). J Mater Sci 9, 184–192 (1974). https://doi.org/10.1007/BF00550940
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DOI: https://doi.org/10.1007/BF00550940