spherulite

A spherulite is a microscopic, spherical aggregate of radiating acicular or fibrous crystals. They are commonly found in silicate rocks, particularly in volcanic and metamorphic environments. Spherulites form through the rapid crystallization of a liquid melt, where the mineral crystals grow outward from a central point in a radial pattern. The size of spherulites can vary significantly, from a few micrometers to several centimeters in diameter.

The most common minerals found in spherulites are quartz and feldspar. In obsidian, a volcanic glass, spherulites are often visible as small, dark, or iridescent spots. Their formation is favored by conditions of rapid cooling and a high degree of supersaturation, which promotes nucleation and growth of radiating crystals.

Spherulites can be identified in thin sections of rocks under a petrographic microscope. Their characteristic radiating extinction pattern under polarized light is a key diagnostic feature. The presence and abundance of spherulites can provide valuable information about the thermal history and crystallization processes of the rock. In some cases, the size and distribution of spherulites can influence the physical properties of the rock, such as its strength and porosity. They are also observed in some synthetic materials, like polymers, where they represent a similar growth mechanism from a molten state.