What is a pegmatite in geology ?

Pegmatite : definition

A pegmatite is an igneous rock of granitic composition occurring in the form of veins or clusters occupying the periphery or the immediate proximity of granitoid massifs, characterized by the large size of its minerals (several centimeters to several meters). Pegmatite crystals hold most of the mineral world's size records : the largest known crystals of microcline, biotite, muscovite, spodumene, beryl, etc... have been extracted from them.

The term pegmatite is however incomplete and must be accompanied by a qualifier : granitic pegmatite, syenitic pegmatite, etc...

Pegmatites form between 370°C and 600°C and represent the end of crystallization of granitic magmas. Their essential minerals are those of granite : quartzfeldspars and micas, to which are frequently added rare minerals, called pneumatolytic (beryl, lepidolite, topaz, cassiterite, rare earth phosphates, tourmaline, etc...) resulting from the concentration in the last fluids of volatile and incompatible chemical elements (B, Be, Li, Zr, Nb, Ta, La, Ce...).

The particular pegmatites, devoid of quartz and associated with nepheline syenites and ultrabasites are called "pegmatoids".
Orthoclase and topaz from Pakistani pegmatites
Aquamarine and muscovite from Pakistani pegmatites
Tourmaline from Pakistani pegmatites

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