Solidification of magma is one of the most important phase transformations that occurs on and within the Earth and other terrestrial planets.

This transition from liquid to solid occurs by either vitrification or crystallization. Vitrification takes place when crystallization is impeded; atoms are unable to organize into structures with long range order, with the result that amorphous or glassy solids, rather than crystals, form. Crystallization is classically viewed like a two-step process below liquidus conditions: 1) nucleation, the formation of nanoscopic molecular clusters (nuclei) with atoms arranged in space similar to corresponding macroscopic crystalline phases and 2) crystal growth, the attachment of chemical nutrient to nuclei.

A firm grasp of crystallization kinetics would allow us to solve for the thermochemical history of igneous rocks by inverting from the resulting sample petrography and microtexture. However, our knowledge of nucleation and crystal growth of silicate, sulfide, and oxide minerals in magmas is inadequate, in the sense that crystallization models are incapable of reproducing many features of natural igneous rocks and synthetic analogues. This has important consequences for a quantitative understanding lava flows, volcanic plumbing systems, magma oceans, and planetary core formation.

Advancing the frontier of magma crystallization may require questioning the assumptions embedded in the classical model of nucleation. For example, evaluating whether metastable precursor phases constitute a nucleation cascade, searching for evidence of nucleation by nanometric particle agglomeration, characterizing the short-range structural ordering of silicate liquid, and considering more thoroughly whether interphase influences such as heterogeneous nucleation dominate.

Macroscopically, solidification processes depend on the time-evolution of the thermodynamic driving force as well as on the physical and chemical attributes of the phases. For example, the interplay between chemical diffusion in the liquid and dT/dt, dP/dt, and dC/dt determines whether crystallization occurs at equilibrium or disequilibrium, with the latter apparently predominating in volcanic and shallow intrusive rocks. All else being equal, crystal nucleation occurs more easily in liquids that have low viscosities and high component diffusivities, and this has important consequences for degassing and thus magma rheology, transport, and eruptive style. For example, bubbles separate from crystal-free, SiO2-poor magma relatively easily, whereas a magma containing a higher viscosity liquid and having higher crystallinity is a suspension with relatively high bulk viscosity; it may move more sluggishly, trap bubbles that increase magmatic overpressure, or drive melt expulsion by gas-driven filter pressing.

Solidification processes are important in petrology, volcanology, geochemistry, mineralogy, and the mining and glass-ceramics industries. This Research Topic welcomes contributions on the broad topic of solidification, encompassing observations across the m to nm spatial scale and including research on diverse materials (magmas, experimental run products, slags, etc.)

We welcome contributions based on experimental, theoretical and numerical studies, including such topics as:
- The kinetics of crystallization and diffusion;
- The textures and crystal-chemistries of magmatic phases;
- The rheology of magmatic suspensions, and;
- The mobility/eruptibility of magmatic suspensions.

Cover image from Topic Editor Gianluca Iezzi.

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