Physical Aspects of Exsolution in Natural Alkali Feldspars

Abstract

At high temperatures alkali feldspars form a complete series of solid solutions with monoclinic symmetry between albite and K-feldspar. On cooling, different intracrystalline processes may occur and we consider that the evolution of perthitic textures is determined by minimizing strain energy. It is possible to calculate the strain energies involved in these processes:

• a) diffusive segregation of potassium and sodium atoms giving exsolution with K-rich and Na-rich lamellae either by spinodal decomposition or by homogeneous or heterogeneous nucleation;

• b) symmetry change from C2/m to C l̄ and twinning due to a reversible displacement of silicon tetrahedra in sodium-rich feldspars, and/or to an ordering of aluminum and silicon atoms of the framework in either lamella. The reversible displacement is instantaneous, whereas the Al/Si ordering, a diffusive process, is geologically slow.