Solid solution and unmixing in the feldspar system, albite (NaAlSi308) - celsian (BaAl2Si208)
Viswanathan, Krishnamoorthy; Harneit, Otto
European Journal of Mineralogy Volume 1 Number 2 (1989), p. 239 - 248
published: May 3, 1989
manuscript accepted: Dec 7, 1988
manuscript received: Apr 9, 1988
ArtNo. ESP147050102008, Price: 29.00 €
Abstract Hydrothermally synthesized (Na,Ba)-feldspars show limited miscibility on the albite-rich side below ~ 875 °C, while complete solid solutions were observed from about Cn30 to Cn100. Precise lattice parameters of (Na,Ba)-feldspar solid solutions are presented for the first time and their variations with the composition discussed. The volume-composition curve is S-shaped as in many other binary silicate solid solutions. The two-trend variation shown by the b and c parameters appears to confirm the dissimilarities in the structures of the albite-rich and celsian-rich members of the series. The miscibility gap was found to extend from ~ Ab95Cn5 to ~ Ab7OCn30. It was determined by crystallizing gels of bulk compositions Ab89Cn11, Ab83Cn17 and Ab78Cn22 at 700 °C and 1 kbar for 21 days. Experiments to reproduce the gap at 700 °C and 1 kbar by using a mixture of gels, Ab100 and Cn100, with a bulk composition lying inside the solvus showed an unexpected behaviour. This consisted of three stages: 1) Two feldspars, ~ Cn2 and ~ Cn^ were observed after two hours. 2) After 21 days a homogeneous feldspar with the bulk composition appeared instead. 3) After 84 days two feldspars, ~ Cn3 and Cn25-30 had formed. This sequence of observations can be understood from a consideration of hypothetical curves of Gibbs energies of mixing of (Na,Ba)-feldspars. Probably the very fine grain size of the crystals, which initially form from the gels, favours rapid chemical diffusion and reaction ; the large surface area leads to the formation of intermediate metastable phases, until finally the equilibrium assemblage is attained.