Exsolution microstructures

In a number of important mineral systems, solid solution exists at elevated temperatures, but exsoiution into chemically distinct phases takes place on cooling. In general, exsolution proceeds either by a nucleation and growth mechanism or by spinodal decomposition (as defined by Cahn 

More complex exsolution microstructures are observed in cryptoper-thites of similar composition when the K-feldspar lamellae become triclinic (see, e.g.. Brown and Parsons 1984 Champness and Lorimer 1976 Willaime, Brown, and Gandais 1976). 

A somewhat more complex exsolution microstructure involving albite twin lamellae in a peristerite of bulk composition Ab9i.3An4.oOr4.T was studied by Gjonnes and Olsen (1974) using a TEM fitted with an energy dispersive x-ray spectrometer (Chapter 7). In this specimen, the twin 

Nissen et al. (1973) and Cliff et al. (1976) found that the composition of the two types of lamellae in a labradorite (An54) differed by at least 12 percent An. 

3 Exsolution in the pyroxenes. The variety of exsolution microstructures that develop in these minerals have been studied in great detail by TEM, and excellent reviews have been given by Champness and Lorimer (1976), Champness (1977), and Buseck, Nord, and Veblen (1980). Many of the exsolution features observed in the pyroxenes, such as the tweed texture, are essentially the same as those already discussed and, therefore, need not be considered further. However, some important characteristics of pyroxene exsolution microstructures warrant special mention. 

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Yund, R. A., McLaren, A. C., Hobbs, B. E. (1974). Coarsening kinetics of the exsolution microstructure in alkali feldspar. Contrib. Mineral. Petrol., 48, 45-55. 

Minerals are often riddled with microstructures when observed under the electron microscope. Although the observation and classification of microstructures such as twin boundaries, anti-phase boundaries, exsolution lamellae etc., has been a longstanding activity of mineralogists and crystallographers it has only been very recently that we started to understand the enormous importance of microstructures for the physical and chemical behaviour of minerals. 

Yund, R.A. (1975b) Microstructure, kinetics, and mechanisms of alkali feldspar exsolution, in Feldspar Mineralo. Short Course Notes, Mineral Society of America, vol. 2, pp. Y29-Y57. 

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