Kinetic Model of Al-Si Ordering in Alkali Feldspars under 'Dry' Conditions

摘要

The following kinetic model proposed for K-Na feldspar under dry conditions:C_i=C_0exp{[(K_0RT_0~2)/(Q_g)]exp[-Q/RT_0)] where C_i is the aluminum content in the 2t_1 position after completion of the ordering process,which can be determined from X-ray diffraction data:C_0 is the aluminum content in the 2t_1 position at the temperature of the beginning of ordering,T_0;g is the cooling rate;Q is the activation energy of Al-Si ordering in alkali feldspars;K_0 is the constant;and R is the universal gas constant.The analysis of published high-pressure experimental data allowed us to determine within this model the values of the activation energy of disordering,which is equal within errors to the activation energy of ordering.The relations obtained for natural alkali feldspar samples allowed us to determine T_o for a known cooling rate (g),which can be estimated from the measurements of sizes of primary exsolution perthites in alkali feldspar and C_i.Cooling rate and temperature of the beginning of Al-Si ordering were estimated for the alkali feldspars of the Mama pegmatite on the basis of X-ray diffraction determinations of Al-Si ordering (C_i) and the sizes of primary exsolution perthites.The influence of pressure on the Al-Si order-disorder process in alkali feldspars in discussed.