Transport properties of low-sanidine single-crystals, glasses and melts at high temperature

Thermal diffusivity (D) was measured using laser-flash analysis from oriented single-crystal low-sanidine (K_0.92Na_0.08 Al_0.99Fe³⁺ _0.005Si_2.95 O₈), and three glasses near KAlSi₃O₈. Viscosity measurements of the three supercooled liquids, in the range 10^6.8 to 10^12.3 Pa s, confirm near-Arrhenian behavior, varying subtly with composition. For crystal and glass, D decreases with T, approaching a constant near 1,000 K: D_sat ∼ 0.65 ± 0.3 mm² s^-1 for bulk crystal and ∼0.53 ± 0.03 mm² s^-1 for the glass. A rapid decrease near 1,400 K is consistent with crossing the glass transition. Melt behavior is approximated by D = 0.475 ± 0.01 mm² s^-1. Thermal conductivity (k_lat) of glass, calculated using previous heat capacity (C_P) and new density data, increases with T because C_P strongly increases with T. For melt, k_lat reaches a plateau near 1.45 W m^-1 K^-1, and is always below k_lat of the crystal. Melting of potassium feldspars impedes heat transport, providing positive thermal feedback that may promote further melting in continental crust.