Congruent and Incongruent Dissolution of Labradorite in Dilute, Acidic, Salt Solutions

Abstract

Polished (001) cleavage faces of Nain Labradorite ($An \approx 56$) were leached at 20°C ($\pm 2^{\circ}$) for 72 days in distilled water ($pH \approx 5.6$), in HCl solution ($pH \approx 4.05$) and in four other HCl solutions containing 1 mg/l or less of Na, K, Ca, Al(aq), and $SiO_{2}(aq)$ ($pH of all four solutions \approx 4.05$). Analysis of the cleavage faces by Secondary Ion Mass Spectrometry (SIMS) revealed no residual surface layer on faces leached by distilled water. By contrast, thick Si-rich residual layers (1500 A) formed on surfaces leached by pure HCl solutions. Much thinner residual layers (700 A) were observed on labradorite leached by HCl solutions containing 1 mg/l of Na, Ca, and K, whereas leached layers <75 A thick formed on faces leached by solutions containing 1 mg/l each of Na, K, Ca, Al(aq) and Si(aq); consequently incongruent dissolution of feldspars occurs in HCl solution, but congruent dissolution of feldspars occurs in mixed electrolyte solutions in which dissolved cations are m...