In-situ FE-SEM observation of the growth behaviors of Fe particles at magmatic temperatures

•The first direct observation of crystallization in silicate magma using FE-SEM.•Non-classical growth processes of Fe and Pt up to a few hundred nanometers.•High-temperature increased metal diffusivity and decreased melt viscosity.•Melt viscosity and metal wettability to melt control crystal mobility. To investigate the nucleation and growth behavior of nano- to micro-meter sized crystals in silicate melts, we attempted the high-temperature in-situ observation using field emission-scanning electron microscopy with a heating stage under high- and low- vacuum (9.6 × 10–5 and 60 Pa, respectively). The sub-micrometer sized particles of Fe slightly above the experimentally determined melting point of nanocrystal moved, coalesced via particle attachment, and recrystallized to form grains with sizes of up to a few hundred nanometers in a rhyolitic silicate melt at ~950 °C under high vacuum. In this study, we 1) conducted the first observation of non-classical growth processes of metals up to a few hundred nanometers above the recrystallization temperature of the bulk crystals, and 2) found the melt viscosity and wettability control of crystal mobility by comparing the behavior of Fe and Pt particles on the sample surface originating from the focused ion beam deposits. The increased diffusivity of metals and decreased melt viscosity could have induced the coalescence of bulk-sized crystals at high-temperature. The crystal growth of silicates and oxides via classical pathways was also observed.