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Definition: pegmatite from The Hutchinson Unabridged Encyclopedia with Atlas and Weather Guide

Extremely coarse-grained igneous rock of any composition found in veins; pegmatites are usually associated with large granite masses.


Summary Article: Pegmatite
from Rock and Gem

PEGMATITE IS THE NAME given to very coarse-grained, igneous rocks. Most have the same major constituents – quartz and feldspar – as granites or syenites, but intermediate, mafic, and ultramafic pegmatites are sometimes found. They usually form as sheets, pods, lenses, or cigar-shaped bodies in large plutonic bodies with the same principal minerals. Although crystals can be huge – yards long – the average size is 3–4in (8–10cm). The large crystals are due to the large amount of water in the magma rather than to slow cooling. Crystals can occur in hollows in the pegmatite, or the entire pegmatite may be completely filled. Some pegmatites may consist of a single or a predominant mineral, and take their names from that mineral. Quartz and the silica-rich feldspars are the major constituents of granitic and syenitic pegmatites. These types of pegmatites are particularly important because they are the chief commercial source of feldspar and sheet mica, and also a major source of gemstones. Tourmaline, aquamarine, emerald, rock crystal, smoky quartz, rose quartz, topaz, moonstone, and garnet are all pegmatite minerals. Pegmatites are also the source of other important economic minerals, which contain beryllium, lithium, tin, titanium, molybdenum, tungsten, tantalum, niobium, and other rare elements.

  • Properties
  • Rock type Felsic, plutonic, igneous
  • Major minerals Quartz, feldspar, mica
  • Minor minerals Tourmaline, topaz
  • Color Light
  • Texture Very coarse

Tungsten-tipped

Mined from pegmatites, tungsten adds strength to many everyday objects.

Thin section

The crystals in this pegmatite from Scourie, Scotland, are up to 3ft (1m) in size.

Mining tungsten

Pegmatites are one of the sources of important minerals. These minerals are, in turn, the ores of less common but vitally important metals. Tungsten is one of these. It goes into the manufacture of items as exotic as a rocket nozzle (see Uses of tungsten), or as mundane as a set of darts or the tip of a ballpoint pen. The pegmatite minerals wolframite (in the form of hübnerite and ferberite) and scheelite are its major mineral sources. China has about 50 per cent of the world’s reserves, and in Russia deposits are located in the northern Caucasus and around Lake Baikal.

Himalaya mine

The Himalaya Mine in California has pegmatite veins that produce stunning tourmalines.

© 2008 Dorling Kindersley Limited

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