Nucleation and Growth Pathway of Magma Crystallisation: A Non-Classical Two-Stage, Four-Step Mechanism

Abstract

Although non-classical crystallization pathways have been extensively studied in low-temperature systems (e.g. biomineralization), directly observing the dynamics of nucleation and growth under high-temperature and high-pressure magmatic conditions remains a major challenge. Using naturally step-quenched rock samples and high-resolution transmission electron microscopy, we obtained a series of nanoscale ‘snapshots’ of final state inversion crystallization process. Our results reveal that structural units in the melt first assemble into amorphous pre-nucleation clusters. Upon reaching a critical size (~3.5 nm in diameter), these clusters undergo a phase transition into crystal nuclei with a well-defined lattice. Subsequently, structural units migrate toward the crystal nuclei, forming a growth boundary layer at the crystal-melt interface. Within this layer, structural units assemble into growth units, which are integrated into the crystal lattice through a cooperative mechanism, driving crystal growth. These findings uncover a non-classical two-stage, four-step nucleation-growth pathway. Under this mechanism, densely distributed and nearly synchronously formed pre-nucleation clusters and crystal nuclei compete for limited nutrients and growth space, ultimately generating polycrystalline aggregates composed of numerous tiny crystals. Direct observation of crystal nucleation during magma crystallization demonstrates that the uniformly distributed phenocrysts in volcanic rocks and the randomly oriented crystals in near-equigranular intrusive rocks (e.g. granites) result from homogeneous nucleation. Notably, despite the vast environmental differences between high-temperature magmatic systems and low-temperature aqueous solutions, both follow the same energy-optimized crystallization pathway at the atomic scale, which remains unchanged regardless of external conditions (e.g. temperature, pressure, composition, viscosity) that influence crystal growth rates and morphologies. This appears to be a naturally designed, energy-minimizing pathway: pre-nucleation cluster → crystal nucleus → growth boundary layer → polycrystal (or single crystal).