Microspheres from the plume of the Kilauea volcano, Hawaii, display previously unrecognized structural and compositional features indicating that the spheres are concentrically differentiated and at least partially crystalline. The surfaces of some spheres display evenly spaced hexagonal, sub-micrometer grains whose compositions suggest that they may be spinels. Distinctive prismatic and dendritic structures on the surfaces of the microspheres are remarkably consistent through the population and indicate that the spheres have crystalline outer shells. Indications of chemical etching are present, probably from exposure to HF in the volcanic plume. The structures and compositions of the microspheres suggest that they differentiated and then solidified during rapid cooling, as they passed through the gradients of temperature, chemical composition, and redox conditions established in the plume as air diluted the gaseous species evolved from magma.

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