Volume and surface oxidation of basalt

Natural, Fe²⁺-rich basalt glass (quenched lava) was heat treated as glass pieces and glass powder in air, in 6.0 Ar and in a 9×10^?6 mbar vacuum below temperatures of significant crystallization to access volume and surface oxidation by ⁵⁷Fe Mössbauer spectroscopy. While no oxidation occurs upon heating in vacuum, the amount of Fe³⁺ formed in powder (surface oxidation) is about 10 times higher than in pieces (volume oxidation), and surface oxidation is of the same order in air and Ar. This effect is assigned to chemisorption of water or CO₂. Crystalline basalt, investigated by wet chemistry, includes five glass pieces treated above T of crystallization in air and in 6.0 Ar, and three lava samples of increasing depth up to 9 cm of lava lobes. The high Fe₂O₃ of all these crystalline samples is explained as a stabilization of Fe³⁺ due to the change of the local electronic environment in the course of crystallization; volume oxidation therefore appears to be independent on the environmental atmosphere.