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

Abstract

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.

Synthesis and characterization of ferrimagnetic glass‐ceramic frit from waste

“…The glass‐ceramic frit effectively consists in small crystals embedded in a glass matrix, as shown in Figure . The morphology of particles observed by TEM micrographs corresponds to both faceted particles (Figure A) and, in a major proportion, dendritic agglomerated particles (Figure B‐D), this last structure usually generated during fast cooling or quenching step as reported before by other researchers . The size range is found between 10 and 30 nm for smaller faceted grains, the same order of magnitude as the estimated by the Scherrer equation, and between 100 and 400 nm for dendrites.…”

“…The morphology of particles observed by TEM micrographs corresponds to both faceted particles ( Figure 3A) and, in a major proportion, dendritic agglomerated particles (Figure 3B-D), this last structure usually generated during fast cooling or quenching step as reported before by other researchers. 29 The size range is found between 10 and 30 nm for smaller faceted grains, the same order of magnitude as the estimated by the Scherrer equation, and between 100 and 400 nm for dendrites. The SAED patterns of the different particles analyzed exhibit a polycrystalline habit, shown in Figure 3A-C, also in accordance with XRD data.…”

Extraterrestrial dust flux monitoring at Antarctic Vostok station: New collection of extraterrestrial spherules fallen from May to September 2017

“…Spherical crystalline iron oxide particles form when iron oxides, iron sulfides, or iron in the presence of oxygen, are heated to the melting temperatures (Isobe & Gondo, 2013). Therefore, it is necessary to take into account all the possibilities of the origin of these particles, that is, the anthropogenic, natural terrestrial, and extraterrestrial one.…”

Silicate, Iron Oxide and Copper-Gold-Silver Microspherules in Ores and Pyroclastics of the Kostenginskoye Iron Ore Deposit (Russian Far East)

“…Nyström et al [76] предположили, что агрегаты кристаллов магнетита в лавовых потоках Эль Лако представляют собой капли железо-оксидного расплава, в то время как магнетитовые сферулы, особенно с субпараллельными структурами нарастания, наиболее вероятно, являются продуктом кристаллизации из перегретого газа в эволюционирующей эруптивной колонне. Закаленные микроскопические железо-оксидные сферулы со структурами скелетного роста магнетита, очень похожими на таковые в Костеньгинских микросферулах, образовались при экспериментах по охлаждению в печи с контролируемым газовым потоком, условия которых были приближены к условиям в типичной эруптивной вулканической колонне [52].…”