Meteoritics & Planetary Science 42, Nr 1, 49–61 (2007) Abstract available online at http://meteoritics.org Hydrothermal alteration experiments of enstatite: Implications for aqueous alteration of carbonaceous chondrites Ichiro OHNISHI1* and Kazushige TOMEOKA1, 2 1Graduate School of Science and Technology, Kobe University, Kobe 657-8501, Japan 2Department of Earth and Planetary Sciences, Faculty of Science, Kobe University, Kobe 657-8501, Japan *Corresponding author. E-mail:
[email protected] (Received 28 March 2006; revision accepted 04 November 2006) Abstract–Enstatite is one of the major constituent minerals in carbonaceous chondrites. Hydrothermal alteration experiments (26 in total) of enstatite were carried out at pH 0, 6, 7, 12, 13, and 14, at temperatures of 100, 200, and 300 °C, and for run durations of 24, 72, 168, and 336 h in order to provide constraints on the aqueous-alteration conditions of the meteorites. The recovered samples were studied in detail by using powder X-ray diffraction, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Under acidic and mildly acidic conditions (pH 0, 6), no significant alteration occurred, whereas under neutral to alkaline conditions (pH 7–14), serpentine and saponite formed in various proportions by replacing enstatite. At 300 °C for 168 h, serpentine formed under neutral to moderately alkaline conditions (pH 7, 12), and serpentine and saponite formed as unit cell-scale coherent intergrowths under highly alkaline conditions (pH 13, 14). The amounts of phyllosilicates have a tendency to increase with increasing pH, temperature, and run duration. There is also a tendency for saponite to form at higher pH and temperature and under longer run-durations than serpentine.