Synthesis and structural state of K-feldspars in the system K[AlSi3O8]K[FeSi3O8

Abstract

K-feldspars in the system K[AlSi 3 O 8 ]–K[FeSi 3 O 8 ] were synthesized hydrothermally at P H 2 O = 1 kbar and T = 400 °C, 500 °C, and 600 °C to investigate their ordering behaviour, the extent of Al, Fe substitution, and the influence of oxygen fugacities on the run products (Ni–NiO compared to Cu–Cu 2 O buffer conditions). Starting materials were mixtures of Al 2 O 3 , Fe 2 O 3 and SiO 2 in non-stoichiometric ratios together with KOH solution as the transport medium. In the Ni–NiO buffered runs Fe 3+ was partly reduced to Fe 2+ . Abundant growth of mica with the approximate composition K 0.8 □ 0.2 (Ni 2.75 Fe 2+ 0.25 )[(Fe 3+ 0.3 Al 0.5 Si 3.2 )O 10 ](OH) 2 left the K-feldspar strongly depleted in Fe 3+ relative to the starting material. In the Cu–Cu 2 O runs with their larger oxygen fugacities, growth of delafossite (Cu 1+ Fe 3+ O 2 ) took up Fe 3+ so that the Fe 3+ content of the K-feldspars, varying between 20 at% and 100 at%, roughly correlated with the amount of delafossite present. Considering that the phase transition in both K-feldspar endmembers is first order, an approximate phase diagram has been calculated from known transition temperatures (480 °C and 704 °C, respectively) and known (7300 J/mol; Hovis, 1988) or derived transition enthalpies (11500 J/mol). The calculation results in a narrow two-phase field separating high-temperature (Al, Fe)-sanidine from low-temperature (Al, Fe)-microcline. In the Al-rich half of the diagram, metastable monoclinic K-feldspar occurred in the stability field of microcline, whereas Fe-rich compositions attained the triclinic ordered state. No indication of a miscibility gap has been detected. Fe-rich microcline that was found coexisting with less Fe-rich monoclinic K-feldspar in a few runs at 600 °C and 500 °C (not at 400 °C, however) is interpreted as the result of a conditional metastable phase separation, in the sense that the separation only occurs on the condition of triclinic ordering in the Fe-rich K-feldspars. b – c diagrams show that pure KAl-feldspars, even after long annealing at 600 °C to 400 °C, keep a high degree of disorder corresponding to reference high sanidine. This agrees with slow ordering kinetics predicted from the Mueller rate equation, when using an updated calibration of ln K D vs. 1/ T : − RT ln K D (J/mol) = 4047(668) − 1.80(47) T + 10955(929) Q . Structural incorporation of Fe 3+ leads to faster ordering kinetics. For example, substitution of 27 at.% Al by Fe enhances the ordering rate constant at 600 °C by a factor of ≈ 170. Further, the observed degrees of order increase at increasing Fe contents and decreasing temperatures ruling out ordering in the solid state subsequent to growth. One possibility would be initial growth with the observed degrees of order driven by atomic radii effects and the proton concentration in the structure (Graham & Elphick, 1991, 1994) having a larger effect on Fe, Si than Al, Si interchanges. A second possibility would be solution-reprecipitation becoming more effective at increasing Fe contents and increasing deviation of the metastable monoclinic crystals from their (Al, Fe)-sanidine stability fields, i.e. the lower the temperature.