The alkali-feldspar solvus at 1 kilobar water-vapour pressure

Abstract

SummaryThe compositions of feldspar pairs crystallized from stoichiometric alkali feldspar gels with water at PH2O≈ 1 kbar in runs of up to 3000 hours' duration vary with Na:K in the starting material. This is because, for most bulk compositions, rapid nucleation in the gel leads to growth of one feldspar phase outside the solvus. The composition of this initial feldspar phase can be thought of as reflecting the configuration of the liquidus-solidus curves in the system Ab-Or. In general, the attainment of the stable equilibrium solvus (binodal) requires that one phase approach the binodal from outside; for compositions with < 30 mole % Or this phase becomes more potassic with time, with > 30 mole % Or, more sodic. Homogeneous gels of bulk composition Ab70Or30initially crystallize feldspar pairs inside the binodal, which rapidly (< 17 hours) unmix to define a solvus-like curve (probably the spinodal, with an apparent critical point at 602 °C and 31 mole % Or) and then unmix slowly, giving a solvus after 3000 hours with a critical point at 657 °C and 36 mole % Or.Because this curve has been approached from two directions it is the best approximation to the binodal (metastable only with respect to Al-Si order) obtained. It is similar to the 2 kbar ‘peralkaline’ solvus of Orville, 1963, provided an adjustment for pressure of 16 °C/kbar is made. Differences between solvi determined by Luth and Tuttle (1966) for ‘non-stoichiometric’ bulk compositions may reflect differences in initial crystallization behaviour, as may the breaks in the solvus limbs suggested by Luthet al.(1974).