氣相-液相-固相の相平衡についての実験-初期太陽系星雲におけるガスの分別凝縮への応用

抄録

Experimental studies have been conducted on the vaporization of silica, forsterite and enstatite at pressures between 10^-10 and 10^-4 bar and at temperatures between 1350° and 1870°C. The results indicate that silica and forsterite vaporize congruently, whereas enstatite vaporizes incongruently to forsterite and silica-rich vapor. The ' triple point ' where solid, liquid and vapor coexist together lies at about 5×10^-6 bar-1600°C, 10^-5 bar-1700°C, and 2 × 10^-6 bar-1500× for silica, forsterite and enstatite, respectively. These 'triple points' shift to higher pressures with increasing hydrogen in the vapor phase. In the case of enstatite, the, triple point ' lies at about 2×10^-2 bar and 1550°C when the H₂/MgSiO₃ ratio is 10⁴ which is considered to be close to that of the primitive solar nebula. It is suggested that liquid was not stable in most part of the primitive solar nebula and that chondrule melts were not the products of direct condensation of gas. It is most probable that such melts were produced by metastable melting of pre-existing minerals such as olivine and pyroxene. Fractional condensation of the nebular gas would have produced locally silica-enriched residual gas because of the existence of the reaction relation of forsterite and gas to form enstatite. This process is somewhat similar to the fractional crystallization of basaltic magma to produce silica-rich residual liquids. The fractionation would, however, be more efficient in the fractional condensation of gas than in the fractional crystallization of magma.