(K,SR)-FELDSPAR SOLID-SOLUTIONS - THE VOLUME BEHAVIOR OF HETEROVALENTFELDSPARS

A series of (K,Sr)-feldspars has been prepared by hydrothermal cation exchange methods according to the reaction 2KAlSi3O8+SrCl2 = SrAl2Si2O 8 + 4SiO2 + 2KCl. The distribution of K and Sr between feldspar and fl uid is non-ideal. The excess free energy of mixing in the (K,Sr)-felds par series has been modelled by an asymmetric Margules expression with W(K-fsp)G = 11.1 (8) kJ/mol and W(sr-fsp)G = 8.5(1.1) kJ/mol. The beh aviour of cell parameters is similar to that of (K,Ca)-feldspars, incl uding excess volumes which are negative. This is in contrast to other binary feldspar solid solutions, where excess volumes are either close to zero (Na,Ca-feldspars) or are positive (Na,Sr-, Na,Ba-, K,Ba-felds pars). In an attempt to rationalize the volume behaviour of heterovale nt feldspars we used the misfit strain energy model of Christian (1981 ) and found that experimental data and model predictions qualitatively agree. A positive excess volume results when both the molar volume an d the bulk modulus in one end-member are larger than in the other one, whereas a negative excess volume is found when in one end-member the molar volume is larger, but the bulk modulus is smaller than in the ot her end-member.