Mineral Crystallisation in Chondrules (An - SiO2 - Fo including binary systems)

Описание: Chondrules are mafic, magmatic systems. Chondrule crystallisation can be described in e.g., the petrologic ternary phase diagram Forsterite-Anorthite-Quartz. In brief, and only the most typical path: most chondrules start deep in the forsterite primary phase field. A melt then develops towards the peritectic line, where the peritectic reaction olivine + silica produces pyroxene from the olivine over-saturated melt. The melt develops along the peritectic line during this reaction and towards the peritectic point. There, the olivine and pyroxene are joined by anorthite, and the peritecic reaction plus anorthite crystallisation continues until all melt is consumed. This is equilibrium crystallisation, more likely is fractional crystallisation, in which the first formed olivine does not completely reacts to pyroxene druing the peritectic reaction. Then, also enstatite crystallises, and the melt develops to either the En+An or En+Qz cotecic, and finally crystallises either as glass or 3 phases, when the eutectic point is reached. Olivine is because of the fractional crystallisation also still present. Chondrules likely experienced fractional crystallisation, as their feldspar-like mesostases often is glassy or fine-crystalline, indicating fast cooling. The typically observed paragenesis of chondrules is olivine+pyroxene+feldspar, and in rare cases also silica. For a more complete description the ternary phase diagram Diopside-Anorthite-Forsterite should also be consulted.