Order-disorder alkali feldspars

Nevertheless, immiscibility and Al-Si ordering are minimized when specimens anneal at high temperatures for long periods and then are rapidly quenched. In natural and synthetic specimens that have experienced such cooling histories, the effects of cationic substitutions can be analyzed independent of other factors. For instance, a number of authors have examined transition temperatures from CUm to CT symmetry in completely disordered alkali feldspars (Nai- cK cAlSisOg), and the results (Fig. 26) indicate that the critical temperature decreases linearly with K content, since the larger K cation inhibits structural collapse (Kroll et al. 1980, 1986 Salje 1985, Harrison and Salje 1994... 

Waldbaum and Thompson (1968). A second-degree Margules expression for the excess molar volimie is derived from a statistical analysis of molar volimie data for disordered (high-temperature) alkali feldspars. The two-coefficient equation for binary mixtures is derived from a third-order polynomial expansion in X. ... 

Experimental studies of alkali feldspars in Westerley granite with ordered microcline [63,64] and disordered sanidine [65-68] show that (010) [001] shp is active in both. Other dislocations derive from (010)[101] [Fig. 15(a)], (001) [110], and (121) [101] shp. In naturaUy deformed alkali feldspars, subgrain formation was observed, indicative of climb [357-360]. It has been suggested that shear-induced mechanical Albite and Pericline twinning in potassium feldspar may facilitate ordering [361] but this has been disputed [362]. Dislocations have no effect on diffusion in alkah feldspar [363]. 

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