Formation of iron mineral fine particles by acidic hydrothermal alteration experiments of synthetic martian basalt

Abstract

The constituent and formation process of the Martian surface soil are fundamental problems to understand the evolution and current environment of the Mars. At the Martian volcanoes, iron-rich basaltic rocks should be subject to the hydrothermal alteration by sulfuric acid-bearing solutions. In this work, we carried out alteration experiments of the synthetic iron-rich basaltic material simulated the typical Martian basalt to elucidate the soil formation processes on the Martian surface. In the run products at 100 °C with initial concentration of 0.1N sulfuric acid solution, quite characteristic, snowflake-like hematite fine particles of submicrometers in diameter were produced continuously from 7 days to 112 days. In the run products at longer than 28 days at 100 °C and all run durations at 150 °C, initial precipitation of hydrous and low-crystalline iron oxide minerals form nuclei for crystalline aggregates of hematite with several micrometers in diameter. The ferric iron oxide particles are not produced with 0.001N sulfuric acid solutions and distilled water. Ferrous iron in olivine and glass phase can be oxidized to ferric iron by reaction with sulfuric acid in the low temperature hydrothermal conditions in anoxic conditions. Hematite particles produced in this study are small enough to be blown up by the Martian winds to disperse from alteration sites in volcanic regions to the whole planet. The iron oxide minerals which brought reddish color of the Martian dust may be products of sulfuric acid-bearing hydrothermal alteration with volcanic activities rather than a low temperature weathering related to the ancient Martian climate.