Dendritic magnetite crystals in rapid quenched fine spherules produced by falling experiments through the high temperature furnace with controlled gas flow

摘要

Magnetite is a common accessory mineral in various rocks. Crystal shapes and habits of magnetite show diversity depending on crystallization conditions, especially cooling rate. Characteristic dendritic or skeletal magnetite crystals occur in quench rims of effusive rocks. The dendritic magnetite also occur in micrometeorites undergone quick heating and quenching at atmospheric entry. In this study, we constructed a fine particle free fall apparatus in a high temperature furnace to carry out crystallization experiments with controlled rapid heating and quenching. Experiments were carried out in a high-temperature vertical tube furnace with H_(2), CO_(2) and Ar mass flow controllers to control oxygen partial pressure and total gas flow rate. At the top of the furnace, a silica glass tube with an orifice with approximately 0.5 mm in diameter was set to keep falling rate of particles. Particles were retrieved in an alumina crucible at the bottom of the furnace tube. Terminal velocity of silicate particles with 100 μm in diameter in the static Ar gas at 1200 °C is approximately 0.18 m/s. Gas ascent rate at 1200 °C is approximately 0.11 m/s in the furnace tube when gas flow rate is approximately 1 l/min at standard condition. The falling velocity of the particles with 100 μm in diameter, therefore, is reduced to approximately 0.07 m/s. When the highest temperature in the furnace tube set to 1520 °C, the falling particles reach 1400 °C within 2 s, keep above 1400 °C more than 1 s, and are quenched within 1 s. For the fine particles with 100 μm in diameter, time scale of thermal equilibrium by radiation can be achieved within 0.1 s. In the experiments with volcanic ash particles, we found quite characteristic dendritic magnetite crystals in rapid-quenched spherules. From particles with high volume fraction of magnetite, we can see quite characteristic texture in which dendritic magnetite cover almost whole surface of the spherule. Magnetite dendrite crystals with particular crystallographic orientation also occur. The rapid quenching experiments for fine particles can be applied to reproduce atmospheric entry heating processes of micrometeorites.