Condensation experments using a forsterite evaporation source.

Condensation experiments have been carried out with a new condensation furnace in H2 at low pressures (1.4, 1.4×10−1, and 1.4×10−2 Pa) using an evaporation source of forsterite powder heated at about 1500°C. Large amounts of condensates (up to a few hundreds of mg) were recovered as a function of temperature (from about 1350°C to room temperature) at 1.4 Pa, where the evaporated molecules from forsterite sufficiently collided with H2 molecules. Forsterite, forsterite + silicon, forsterite + silicon + enstatite and amorphous material condensed with decreasing temperature under a reducing condition in a Ta crucible, while forsterite, forsterite + clinoenstatite and amorphous materials condensed near a neutral redox condition in a Mo crucible. These condensation sequences are essentially the same as fractional condensation sequences expected from the phase diagrams in the system Mg–Si–O–H constructed by thermodynamic calculations. Kinetic effects are also superimposed on the fractional condensation. For example, dendritic crystals of forsterite and clinoenstatite, and amorphous materials condensed due to nucleation delay of the silicates. Each whisker of the den-drites was formed by the vapor-liquid-solid growth mechanism with metastable nucleation of melt droplets. The elongation of clinoenstatite whiskers (c-axis) is different from that in interplanetary dust particles (a-axis). Euhedral forsterite crystals (a few μm in size and elongated to the c-axis), which condensed at high temperatures (>1000°C), have similar size and morphologies of olivine in matrix of chondrites. Condensation of clinoenstatite at high temperature (about 900°C) may indicate easy pyroxene condensation in the primordial solar nebula.