| Abstract
| - This paper is on an application of nucleation theory and an empirical extension of the quasichemical solutiontheory of Guggenheim to an ancient bit of nature and to a long standing problem on the origins of meteorites.Chondritic meteorites, which were formed 4.6 billion years ago, contain solid chondrules, which appear tohave been liquid silicate droplets formed in a rainlike process in the solar nebula with a gas of solar composition.Such droplets have been experimentally subcooled up to 500 °C below their liquidus temperatures. Just as interrestrial clouds of subcooled water, silicate droplets should readily form metastably below their liquidustemperatures. Calculations of the condensation of these metastable droplets from a solar gas with an expectednucleation constraint on the formation of metallic iron lead to calculated compositions of chondrules at twodifferent temperatures for each particular pressure that emulate the compositions of two types of porphyriticchondrules in the well-studied meteorite Semarkona, a minimally metamorphosed (or automorphosed), i.e.,“most primordial”, chondrite. The two types of chondrules, types IA and II, have very different concentrationsof FeO, which are well above the equilibrium concentration because of the nucleation constraint on theformation of metallic iron. The high supersaturation of iron in the gas leads to much higher levels of ironoxide in the chondrules than would be present at equilibrium by driving the reaction H2O + Fe ↔ H2 + FeOto the right. These concepts, if correct, can explain a major part of the chemistries of primordial meteoritesand the different relative sizes of the metallic cores of planets. The physics of nucleation of metallic iron andof the crystallization of silicates governs the chemistry of iron compounds such as FeO and leads to metastableliquid chondrule droplets. These significant physicochemical processes explain the origins of two chondruleclasses.
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