Feldspars exsolution

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. 

Yund R. A. (1974). Coherent exsolution in the alkali feldspars. In Geochemical Transport and Kinetics, Hofmann, Giletti, Yoder, and Yund, eds. New York Carnegie Inst. Washington and Academic Press. 

Many minerals show exsolution textures. For these minerals, there is complete solid solution at sufficiently high temperatures, but a miscibility gap at lower temperatures. The alkali feldspar, (Na, K)AlSi30s, is an example. Suppose a roughly homogenous mineral of intermediate composition cools down and into... 

Exsolution The unmixing of an initially homogeneous substance into two or more separate crystalline substances. For example, at 1000 °C and one bar pressure, sodium and potassium can readily substitute for each other in the crystalline structure of the aluminum silicate mineral alkali feldspar to form (Na,K)AlSi30g. At much lower temperatures, the sizes of the sodium and potassium atoms are too dissimilar for the crystalline structure to remain stable and the alkali feldspar separates (exsolves) into NaAlSisOg and KAlSisOg components (Klein, 2002), 143. 

Figure 8.39. DF images of exsolution lamellae experimentally induced in an alkali feldspar (<>33). (a) Sample annealed at 600°C for 4 hours, g=400.

Figure 8.40 is a BF image of the exsolution nucrostructure commonly observed in cryptoperthites, together with its SAD pattern. The electron beam is approximately normal to (001). The single spots corresponding to the K-feldspar (monoclinic) and the pairs of spots corresponding to the... 

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). 

Cliff, G., Champness, P. E., Nissen, H.-U., Lorimer, G. W. (1976). Analytical electron microscopy of exsolution lamellae in plagioclase feldspars. In Electron Microscopy in Mineralogy, edited by H.-R. Wenk, pp. 258-65. Berlin Springer-Verlag. 

Gjonnes, J., Olsen, A. (1974). Study of exsolution structures in sodium feldspars by a combination of electron microscopy and energy dispersive x-ray analysis. In Electron Microscopy 1974, Eighth International Congress on Electron Microscopy, Vol. 1, edited by J. V. Sanders D. J. Goodchild, pp. 504-5. Canberra, Australia Australian Academy of Science. 

Willaime, C., Gandais, M. (1972). Study of exsolution in alkali feldspars. Calculation of elastic stresses inducing periodic twins, phys. stat. sol., (a)9, 529-39. 

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. 

Ribbe, P,H. 1983. Exsolution textures in ternary and plagioclase feldspars Interference colors, p. 241-270. In P.H. Ribbe (ed.) Feldspar mineralogy. 2nd ed. Vol. 2. Reviews in mineralogy. Mineralogical Society of America, Washington, DC. 

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