pegmatites
 Pegmatites are very coarse-grained igneous rocks in which crystal grains are larger than 20 mm, usually larger than 50 mm. Most pegmatites are composed of quartz, feldspars, and micas (granitic composition). More rarely, intermediate and mafic pegmatites contain amphiboles, Ca-plagioclase feldspar, pyroxenes and other minerals in recrystallized zones and apophyses associated with large layered intrusions. Feldspars within pegmatites may display exaggerated and perfect twinning, exsolution lamellae. When affected by hydrous crystallisation, macroscale graphic texture is known, with feldspar and quartz intergrown. Perthite feldspar within pegmatite often shows gigantic perthitic texture visible to the naked eye. Three theoretical explanations for pegmatite formation: ◙ metamorphic – pegmatite fluids created by devolatilisation (dewatering) of metamorphic rocks, particularly felsic gneiss. ◙ magmatic – pegmatites are often found in the aureoles of granites, often closely matching the compositions of the nearby granites – such pegmatites could represent exsolved granitic material that has crystallised in the country rocks. ◙ metasomatic – some pegmatites could be explained by the hydrothermal action of hot fluids upon a rock mass (producing bulk chemical and textural alteration). Pegmatitic growth mechanisms probably involve a combination of: ◙ Low rates of crystal nucleation coupled with high diffusivity, forcing growth of a few large crystals instead of many smaller crystals. ◙ High vapor and water pressure, assisting the enhancement of conditions of diffusivity. ◙ High concentrations of fluxing elements such as B and Li that lower the temperature of magmatic solidification. ◙ Low thermal gradients coupled with a high wall rock temperature, explaining the preponderance for pegmatite to occur only within greenschist metamorphic terranes |
| links: rocks: pegmatite with large mica crystals, pegmatite, 2, granitic pegmatite with quartz, feldspar, and large black crystals of tourmaline; formations: replacement pegmatites (r,l) amongst dipping colliform layers, Ivnarmiut Island, Skaergaard intrusion, Greenland, and pre-detachment dikes (crosscutting autolith block) with irregular layers near the margins becoming pegmatitic toward its interior, and mafic pegmatite replacement bodies, 2, and unusual small mafic pegmatite on Uttental Plateau that grades from mafic to felsic, base to top in the Homestead area, and close-up and close-up of 2 of these pegmatite replacement bodies that are frequently associated with the anorthositic replacement features probably related to the availability of water and its control on the residual liquid phase relations (white and gray, plagioclase; black, pyroxene and oxides; rusty patches, olivine), mafic pegmatite with interior that has abundant fine-grained gray granophyre and its contact with gabbro, and close-up of small mafic pegmatite on the Uttental Plateau ; pegmatite in granite boulder, pegmatite with irregular contacts with a fine-grained granite, webpages: Japanese pegmatite localities; locations: map |
