Characterization of laboratory weathered labradorite surfaces using X-ray photoelectron spectroscopy and transmission electron microscopy

Altered surfaces of labradorite resulting from laboratory weathering at pH 4 and 25°C were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). SEM micrographs showed nonuniform surface alteration of labradorite weathered at pH 3.7 for 415 days. TEM micrographs showed exsolution lamellae of a more calcic-rich and more sodic-rich phase, each averaging approximately 700 å thick. The more calcic phase was preferentially weathered to average depths of 1350 Å in excess of the more sodic phase, producing a corrugated surface. The surface roughness caused by preferential weathering of the more calcic phase affects the relative exposure of calcic and sodic phases to the XPS detector. A three-dimensional analysis of possible surface exposures was used to predict the influence of a corrugated surface on XPS measurements. Actual XPS data showed significant Ca depletion, slight Al depletion, slight Si enrichment, and slight Na enrichment relative to unweathered labradorite. Sputter depth profiling with an Ar ion gun showed that surface alteration was significant up to depths of 500 Å, similar to the depth of preferential weathering of the more calcic lamellae observed with TEM. Predicted XPS data accounting for the topographic effects of a corrugated surface showed similar trends of Ca depletion, slight Al depletion, slight Si enrichment, and moderate Na enrichment. Furthermore, predicted XPS sputter depth profiles of Ca, Al, Si, and Na were similar to actual sputter depth profiles, indicating that a significant amount of the surface alteration on labradorite can be explained by preferential weathering of the more calcic lamellae, and the subsequent surface roughness effects this has on XPS spectra. Other surface processes such as H⁺ or H₃O⁺ exchange for Ca, Na, or Al and preferential weathering at sites of excess surface energy (dislocations, twin boundaries, etc.) not accounted for in the predicted XPS data may also contribute to the surface composition of weathered labradorite. Results showing preferential weathering of more calcic-rich lamellae and its effect on XPS spectra indicate the importance of understanding the micro-structure of feldspars used for laboratory weathering studies.