The hydrothermal alkaline alteration of potassium feldspar: A nanometer-scale investigation of the orthoclase interface

Potassium feldspars (KAlSi3O8) are ubiquitous minerals in the Earth's upper crust. This family of has been subject numerous experimental and theoretical investigations concerning their dissolution kinetics mechanisms controlling chemical alteration at acid neutral pH, temperatures ranging from ambient to hydrothermal conditions. On other hand, considerably less research on behavior K-feldspars carried out alkaline conditions, particular pH > 9 elevated temperatures. Filling this gap knowledge is major motivation for study. More specifically, we wanted document understand how K-feldspar interface structurally chemically evolves during order determine mechanism dissolution. In study examined interfaces orthoclase samples that were altered separate experiments a Ca(OH)2-H2O solution (pH25°C 12.4) 190 °C 24 h. We used combination focused ion beam (FIB) milling advanced analytical transmission electron microscopy (TEM) techniques investigate structure chemistry near surface region post-reaction grains, with attention being given fluid-solid interface. Even though each grain diminishes volume due dissolution, high-resolution TEM imaging indicates feldspar itself remains completely intact crystalline, as evidenced by lattice fringes abruptly terminate edge. Nanometer-scale composition measurements mapping TEM-EDXS (energy dispersive X-ray spectroscopy) EFTEM filtered TEM) show parent also unchanged particular, there no evidence incursion Ca fluid solvent into structure, either interdiffusion or replacement process. Taken together, observations point sharp reaction front characterized congruent (i.e. stoichiometric) release all elements structure. high resolution reveal layer (SAL) amorphous material forms situ expense The demarcates spatially coincident nm-sharp structural discontinuity between phase. SAL variable thickness, under 10 nm up ~200 nm. likely one first observed occurrences significant an solvent. lack two phases points interfacial dissolution-reprecipitation process continuously operates alteration, mostly right onset contact fluid. After initial formation layer, 1–2 μm-thick porous amalgam secondary crystalline comprised calcite, tobermorite, hydrogrossular, well minor phases, precipitated over SAL. These authigenic formed experiment (hydrothermal followed loss evaporation) classical thermodynamically-controlled precipitation reactor bulk became increasingly concentrated. propose coupled (CIDR) best explains properties layer. fact, many recent studies postulate CIDR controls SALs circumneutral wide range Combining these previous results our new supports idea unique unifying aqueous fluids.