Sodium-potassium interdiffusion in potassium-rich alkali feldspar I: Full diffusivity tensor at 850 °C

Anisotropic diffusion is described by a tensor of diffusivities, Dαβ, which may be composition dependent leading to nonlinear anisotropic diffusion. In this work the practical problem of reconstructing such a tensor for Na-K interdiffusion in potassium-rich alkali feldspar in the composition range 0.85 ≤ XOr ≤ 1.00 is addressed. Gem quality sanidine with an initial composition of XOr = 0.85 was exchanged with KCl salt melt at 850 °C and ≈1 bar. The diffusivity tensor Dαβ(XOr) and its composition dependence was reconstructed from composition profiles produced by cation exchange in six different crystallographic directions using a generalization of the Boltzmann approach. Na-K interdiffusion in potassium-rich alkali feldspar is considerably anisotropic and composition dependent. The principal axes of the diffusivity tensor representing the directions of highest and lowest diffusivity lie in the a-c plane with highest diffusivity parallel to the [101] direction, lowest diffusivity perpendicular to the (101̄) plane, and the direction with intermediate diffusivity parallel to the crystallographic b-axis. All diffusivities Dαβ(XOr) increase as XOr tends to unity. Our main result is given in the form of numerical values for all components of the Dαβ(XOr) tensor for 0.85 ≤ XOr ≤ 1

Sodium-potassium interdiffusion in potassium-rich alkali feldspar II: Composition- and temperature-dependence obtained from cation exchange experiments

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Diffusion-controlled crack propagation in alkali feldspar

The chemically driven propagation of interacting parallel cracks in monoclinic alkali feldspar was studied experimentally. Single crystals of potassium-rich gem-quality sanidine were shifted towards more sodium-rich compositions by cation exchange with a NaCl–KCl salt melt at a temperature of 850∘C and close to ambient pressure. Initially, a zone with elevated sodium content formed at the crystal surfaces due to the simultaneous in-diffusion of sodium and out-diffusion of potassium, where the rate of cation exchange was controlled by sodium–potassium interdiffusion within the feldspar. A chemical shift of potassium-rich alkali feldspar towards more sodium-rich compositions produces highly anisotropic contraction of the crystal lattice. This induced a tensile stress state in the sodium-rich surface layer of the crystals, which triggered the formation of a system of nearly equi-spaced parallel cracks oriented approximately perpendicular to the direction of maximum shortening. Crack propagation following their nucleation was driven by cation exchange occurring along the crack flanks and was controlled by the intimate coupling of the diffusion-mediated build-up of a tensile stress state around the crack tips and stress release by successive crack propagation. The critical energy release rate of fracturing was determined as 1.8–2.2 Jm−2 from evaluation of the near-tip J-integral. The mechanism of diffusion-controlled crack propagation is discussed in the context of high-temperature feldspar alteration.© The Author(s) 201

Chemically induced fracturing in alkali feldspar

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