(K,Sr)-feldspar solid solutions: the volume behaviour of heterovalent feldspars
Kroll, Herbert; Kotelnikov, Alexej R.; Göttlicher, Jörg; Valyashko, Elena V.
European Journal of Mineralogy Volume 7 Number 3 (1995), p. 489 - 500
published: May 19, 1995
manuscript accepted: Jan 20, 1995
manuscript received: Jun 17, 1994
ArtNo. ESP147050703003, Price: 29.00 €
Abstract A series of (K,Sr)-feldspars has been prepared by hydrothermal cation exchange methods according to the reaction 2KAlSi3O8 + SrCl2 = SrAl2Si2O8 + 4SiO2 + 2KCl. The distribution of K and Sr between feldspar and fluid is non-ideal. The excess free energy of mixing in the (K,Sr)-feldspar series has been modelled by an asymmetric Margules expression with WGk-fsp = 11.1(8) kJ/mol and WGSr-fsp = 8.5(1.1) kJ/mol. The behaviour of cell parameters is similar to that of (K,Ca)-feldspars, including 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-feldspars). In an attempt to rationalize the volume behaviour of heterovalent 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 and 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 other end-member.