Answer:
A pegmatite is an exceptionally coarse-grained intrusive
igneous rock with large interlocking crystals (usually > one
inch), typically forming as masses in dikes and veins,
especially along the margins of batholiths. A granitic
pegmatite has the mineralogy of a granite and abnormally
large grains, whereas a gabbroic pegmatite has the
mineralogy of a gabbro and very large grains. Pegmatites
represent the last and most hydrous magma to crystallize,
and sometimes contain rare minerals enriched in the elements
boron, lithium, uranium, and rare earths.
