| A pegmatitic texture is one in which the mineral grains are exceptionally large. The largest ones are, by convention, more than about 3 cm long. This texture is found in intrusive rocks.
The extra large size does not mean that they cooled extra slowly. Instead, the large crystals of a pegmatite formed in a magma that was extra rich in dissolved water. The water allowed the necessary elements to diffuse extra fast to the sites of crystallization. Pegmatites often form in veins that opened up at the end of the crystallization of a large intrusive mass. Most igneous minerals exclude water from their structure, so the last magma to cool often ends up enriched in water and other incompatible elements. Most of the large igneous minerals that are seen in geology labs are from pegmatites.
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