Megacrysts

Megacrysts are unusually large crystals that occur within igneous rocks, typically much larger than the surrounding groundmass or coexisting crystals. There is no universally fixed size, but megacrysts are generally described as crystals on the order of centimeters to tens of centimeters in diameter, or larger, embedded in a finer-grained matrix.

Megacrysts occur in a variety of rock types. Feldspar megacrysts are common in granitoids and related rocks, where large potassium or plagioclase crystals contrast with a finer groundmass. Olivine megacrysts are characteristic of some kimberlites and other ultramafic rocks, while pyroxene or quartz megacrysts can occur in certain felsic or hybrid magmas. In many cases, the megacrysts are euhedral and show little to no reaction with the surrounding melt, making them visually distinctive in hand sample or thin section.

Origin and interpretation are diverse. Megacrysts may be xenocrysts—crystals derived from older crust or mantle rock that were entrained by ascending magma. They can also form as early-formed crystals within the same melt, or as cumulate crystals that separated from the remaining liquid during fractional crystallization. In some instances, megacrysts reflect slow crystallization in a low-temperature zone or crystal accumulation within a magma chamber.

Megacrysts provide clues to magmatic history, including magma genesis, crustal assimilation, and crystallization dynamics. Their study helps distinguish between processes such as xenolith entrainment, long residence in magma reservoirs, and rapid emplacement of crystalline-rich magmas.