Oscillatory zoning with respect to the albiteand orthoclase components was observed in the feldspar megacrysts from the Weinsberg granite (Moldanubian Zone). All growth zones show perthitic exsolutions in the form of albiterich precipitates in an orthoclase-rich host. The structure of the precipitates depends on the mean bulk composition. Namely, an intermediate bulk composition (Or50Ab41An09 Or80Ab18An02) results in a large number of relatively small precipitates, whereas in more orthoclase-rich zones (Or88Ab11An01) the precipitates are smaller in number but larger in size.
Phase separation in a binary system that results from the uphill diffusion of chemical components is investigated theoretically. We reconsider the original spinodal decomposition model of Cahn and Hilliard taking into account possible intrinsic anisotropy of the underlying system. Specifically we address the effects of orientation dependent interfacial energy. Such anisotropy is of particular importance for mineral systems. The corresponding evolution equation is systematically derived. We show how the anisotropy affects the interfacial energy per unit area, the interface width, scaling laws, and the critical system size at which spinodal decomposition starts to develop. We further obtain numerical solutions and discuss coarsening dynamics for the case where the decomposition products have pronounced shape anisotropy.
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 $$850\,^{\circ }\hbox {C}$$ 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 $$~ \hbox {J}\,\hbox {m}^{-2}$$ 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.
Abstract A thermodynamic analysis of coherent lamellar intergrowth resulting from the exsolution of initially homogeneous alkali feldspar is presented. In contrast to earlier treatments, where the simplifying assumption of zero strain in the lamellar interfaces was used, our treatment is more general. The elastic stresses and strains associated with coherent lamellar intergrowth of Na-rich and K-rich alkali feldspar are calculated by minimising the overall elastic energy of the lamellar microstructure. At given pressure and temperature, the elastic energy depends on the volume proportions of the two lamellar types, and thus on the composition of the homogeneous precursor feldspar. As a consequence, there is no single coherent solvus for alkali feldspar, but coherent solvi are different for different compositions of the homogeneous precursor phase. Experimentally observed lamellar orientations agree with those predicted by minimising the strain energy on a set of all possible lamellar orientations.
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