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Xue Jiyue Zhang Gendi(Department of Geology, Nanjing University)  
In this paper the method proposed by Camentsev et al. (1977) is adopted to deduce the Al-occupancies in K-feldspars. More than 200 samples collected from 53 granitic bodies of various localities in SE China have been studied and their t_1o, t_1m, t_2o, and t_2m have also been calculated. Meanwhile a t_1o —t_1m—(t_2o+t_2m) triangular diagram and the histogram of t_1 for K-feldspars are also constructed.The values of t_1 for all these K-feldspars range from 0.60 to 1.00; microcline with t_10.9 and orthoclase with t_1=0.80±0.10 are dominant in granitic rocks.There is a distinct relationship between the origin of granitic rocks and the Al/Si ordering of K-feldspars. The Al-occupancies in two T_1 sites of those from granitic rocks of metasomatic origin are0.9 with t_1o0.00 as well. Those crystallized from granodiorite quartz monzonite, tonalite plutons are orthoclases with t_1 values within the range 0.70—0.80, while those from hypabyssal—abyssal granite plutons are either monoclinie or triclinic with t_1= 0.85 —0.90. Sometimes both of them may coexist even in a single grain. In such case there will be a 131 reflection peak of orthoclase appearing between the 131 and 11 reflection peaks of microcline on a X-ray powder diffraction chart.The relationship between various varieties of K-feldspars and the origin of granitic rocks is also discussed. It is believed that the maximum microcline(t_1o0.90) is formed from hydrothermal solution or melt under 500℃, so that it occurs commonly in metasomatic granite or crystallizes from magma rich in volatile components under lower temperatures. Monoclinic K-feldspars occur in magmatic granitic rocks and their t_1 values increase with decreasing temperatures of rock formation. This variance of t_1 indicates continuous ordering of Al/Si in monoelinie K-feldspars. The discontinuous transformation between monoclinic orthoclase and triclinic microcline is supported by(1) a zoned K-feldspar, discovered in the Xiuning pluton, Anhui Province, is made up of orthoclase as its core and microcline as its rim without any transition variety with median Al/Si ordering configuration between them and(2) there is a gap between monoclinic and triclinic K-feldspars as shown in Fig.1.
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