pegmaattiset

Pegmaattiset refers to pegmatitic rocks, a category of extremely coarse-grained igneous rocks that form from late-stage crystallization of granitic magmas. They are characterized by crystals that are unusually large, typically exceeding 2 centimeters in size, and by a texture in which minerals are intergrown in a mosaic of very large grains. Pegmaattiset bodies commonly occur as dikes, veins, or irregular intrusions cutting through surrounding rocks.

The mineral composition of pegmaattiset rocks is dominated by quartz and alkali feldspar, with plagioclase often present in lesser amounts. Micas such as biotite and muscovite are common starters, and a range of accessory minerals can occur. Among these, lithium-bearing minerals (for example spodumene), beryl, tourmaline, lepidolite, cassiterite, and niobium-timony minerals are notable. The precise assemblage depends on the specific pegmatite and its host terrane, but the hallmark is a very coarse, interlocking crystal network often enriched in volatile-rich, incompatible elements.

Texture and structure are key characteristics. Pegmaattiset rocks exhibit holocrystalline, coarse-grained textures with crystals that can be centimeter- to decimeter-scale. They often show zoning, with quartz-alkali feldspar cores and mineral-rich margins or pockets containing accessorial minerals. Pegmatites occur in various settings, typically within or at the margins of granitic intrusions and as tabular bodies that cut through surrounding lithologies.

Formation is linked to late-stage differentiation of granitic magmas and the concentration of volatiles and incompatible elements in residual melts. Very slow cooling and fluid-rich conditions promote the growth of oversized crystals and the concentration of rare minerals. Pegmaattiset are economically important as sources of lithium, beryllium, tantalum, niobium, and other rare-element minerals, and they remain a focal point for exploration in many crustal terranes.