Ionic conductivity in gem-quality single-crystal alkali feldspar from the Eifel: temperature,orientation and composition dependence

We measured the ion conductivity of single-crystal alkali feldspar originating from two different locations in the Eifel/Germany, named Volkesfeld and Rockeskyller sanidine and having potassium site fractions (C_mathrm{K}) of 0.83 and 0.71, respectively. The dc conductivities resulting from electrochemical impedance spectroscopy over the temperature range of 300–900 (^{circ }hbox {C}) show a weak composition dependence but pronounced differences between the b-direction [(perp (010))] and (c^{*})-direction [(perp (001))] of the monoclinic feldspar structure. Conductivity activation energies obtained from the observed linear Arrhenius plots are close to 1.2 eV in all cases, which is closely similar to the activation energies of the (^{22}mathrm{Na}) tracer diffusivity in the same crystals. Taking into account literature data on K tracer diffusion and diffusion correlation effects, the present results point to a predominance of the interstitialcy mechanism over the vacancy mechanism in mass and charge transport on the alkali sublattice in potassium-rich alkali feldspar.