Spherulites

Spherulites are roughly spherical or ellipsoidal aggregates of crystalline material that form within a surrounding matrix, often appearing as radiating crystals from a common center. They occur in both geological systems and polymer science, where the term describes distinct but related textures produced by crystallization processes.

In geology and volcanology, spherulites form when a melt rapidly cools or is introduced into a glassy or partly crystalline matrix, allowing crystals to nucleate and grow outward in a radial pattern. This creates a spongy, starburst texture embedded in volcanic glass or fine igneous rocks. Common spherulite minerals include feldspar, quartz, or silica polymorphs; their sizes range from tens of micrometers to several millimeters. Spherulitic textures can influence the appearance and physical properties of rocks, including opacity and fracture behavior.

In polymer science, spherulites arise during the crystallization of semi-crystalline polymers from the melt or solution. Crystals form as thin lamellae that radiate from a nucleation site, producing a spherical aggregate with complex internal lamellar structure. Under polarized light, polymer spherulites often display a Maltese cross pattern, reflecting birefringence. Typical polymers that form spherulites include polyethylene, polypropylene, and polyamides. The final size and morphology of polymer spherulites depend on temperature history, cooling rate, and additives, and they can affect mechanical properties, optical clarity, and barrier performance in films and molded parts.

Observation methods commonly include polarized light microscopy for texture and birefringence, and electron or X-ray techniques for substructure. Spherulites thus provide important clues about crystallization dynamics in both natural and synthetic systems.