IN-SITU ATOMIC-FORCE MICROSCOPY STUDY OF THE DIFFERENTIAL DISSOLUTION OF FAYALITE AND MAGNETITE

Atomic force microscopy (AFM) was used to observe differential dissolution of a rock sample in nitric acid in realtime. The sample consisted predominantly of fayalite intergrown with veins of magnetite and serpentine. Serpentine areas were recessed compared to fayalite and magnetite areas immediately after polishing, but both serpentine and magnetite areas protruded from the fayalite surface with prolonged exposure to nitric acid. The textures of all mineral surfaces appeared smoother as dissolution progressed, and no pitting was observed. A dissolution rate law for fayalite was determined by measuring the difference in height between magnetite features and surrounding fayalite as a function of time and nitric acid concentration and assuming the dissolution of magnetite was negligible compared to that of fayalite. The rate law from this study is Rate = 2.8 x 10(-10) [HNO3](0.45) mol cm(-2) s(-1) and is in good agreement with previously reported rate laws for olivine dissolution. AFM in solution is an important high resolution in situ technique for exploring mineral surface dissolution which can closely approximate real systems.