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Pegmatite body size is on the order of magnitude of one to a few hundred meters. Compared to typical igneous rocks they are rather inhomogeneous and may show zones with different mineral assemblages. Crystal size and mineral assemblages are usually oriented parallel to the wall rock or even concentric for pegmatite lenses. [10]
This is typical of pegmatites. Pegmatites are most commonly formed as coarse-grained igneous rocks of granitic composition, containing large clasts of gemstones such as amazonite, garnet, and topaz. Phaneritic (phaner = visible) textures are typical of intrusive igneous rocks, these rocks crystallized slowly below Earth's surface.
Macrocrystalline or phaneritic texture is common in intrusive igneous rocks that cooled slowly enough for crystal growth. Pegmatites are noted for their large crystal size. The texture is also commonly found in late-diagenetic dolomite, recrystallized limestone, and some types of anhydrite. [2]
The Tin Mountain pegmatite is roughly L-shaped, with the longer leg trending N7’W and the other trending N75’W. [2] It is approximately 200 m long and is 30 m wide at the maximum. [3] It is a vertically differentiated zoned pegmatite, and consists of five main zones: the wall zone, three intermediate zones, and a core.
Phenakite crystal from the Noumas II Pegmatite, South Africa (Size: 1.2 × 0.5 × 0.4 cm) Crystal structure of phenakite viewed along the c axis. Phenakite is found in high-temperature pegmatite veins and in mica-schists associated with quartz, chrysoberyl, apatite and topaz.
Igneous rock, with gray groundmass and white phenocrysts marked. Orthoclase phenocrysts within a finer-grained matrix of a granite porphyry. The matrix or groundmass of a rock is the finer-grained mass of material in which larger grains, crystals, or clasts are embedded.
Orthoclase crystal twinning from the Organ Mountains in New Mexico. Orthoclase is a common constituent of most granites and other felsic igneous rocks and often forms huge crystals and masses in pegmatite. Typically, the pure potassium endmember of orthoclase forms a solid solution with albite, the sodium endmember (NaAlSi 3 O 8), of plagioclase.
Large crystals of mica used for various applications are typically mined from granitic pegmatites. [6] The largest documented single crystal of mica was found in Lacey Mine, Ontario, Canada; it measured 10 m × 4.3 m × 4.3 m (33 ft × 14 ft × 14 ft) and weighed about 330 tonnes (320 long tons; 360 short tons). [18]