ORIENTED DENDRITIC MAGNETITE CRYSTALS IN QUENCHED SILICATE MELT SPHERULES

In the crystallization processes of silicate melt, extraordinary high cooling rate brings quench crystals with quite characteristic shape and habit. Characteristic dendritic or skeletal magnetite crystals occur in quench rims of effusive rocks (e.g. Szramek et. al., 2010).

We have constructed a fine particle free fall apparatus in a high temperature furnace and carried out crystallization experiments of fine particles with quick heating and quenching (Isobe and Gondo, 2013). The falling particles in the furnace can reach 1400°C within 2 seconds, keep above 1400°C more than 1 second, and are quenched within 1 second.

With this apparatus, we carried out quick heating / quenching experiments of volcanic ash particles from Sakurajima volcano. Volcanic ash particles are sieved to 60 to 160 μm in diameter, and contain plagioclase, pyroxene, magnetite and groundmass glass in various proportions. Some particles contain quite high volume fraction of magnetite. After the experiments, more than half of the volcanic ash particles were well melted to spherical shape. Groundmass glass and plagioclase particles formed clear glass spherules.

SEM images of the spherules with iron-rich melt show dendritic magnetite with a uniform crystallographic orientation and almost constant spacing on the whole spherule. Oriented dendritic magnetites also occur in thin layer of iron-rich melt on spherules. Crystal shape and electron backscattered diffraction analysis revealed that area of dendritic magnetite with uniform orientation expands much larger than spacing of limbs of dendrites.

The texture of the dendritic magnetite occurred in quenched fine spherules with extraordinary high cooling rate suggests that nucleation of magnetite was initiated at a single or few points on the surface of tiny melt spherules. The crystallographic orientation of the nuclei can propagate much faster than diffusion rate in silicate melt which regulate dendritic texture.