Pegmatites are a variety of granite with the largest crystals in the granitic family of rocks, usually larger than a half inch (1.25 cm). Extremely large crystals have been found in pegmatites, including some over 30 feet in diameter (10 m). Pegmatites formed below the Earth’s surface at the top or along the outer edges of magma chambers, or in smaller intrusions leading out of the magma chambers. This rock developed during the final stage of crystallization within magma chambers, so it was the last rock to solidify. Like granite, most minerals in pegmatites include quartz, feldspar, and mica. Rare minerals are also found in pegmatites such as beryl (emerald and aquamarine), tourmaline, topaz, fluorite, apatite, corundum (ruby), tin, tungsten, and others. The photo above is a pegmatite boulder with large emerald crystals.
The Crabtree Pegmatite photos shown above are from North Carolina. They have black schorl tourmaline crystals, pinkish-reddish garnet crystals, medium-gray quartz, and whitish-gray feldspar. This deposit from the Devonian period is around 400 million years old.
The above Brazilian pegmatite specimen has large mica and feldspar crystals.
This Colorado pegmatite has large feldspar and quartz crystals (1.4 billion years old).
The photos above show that mineral crystals in pegmatites may not be uniform in size as they are in granite. Pegmatites usually have different zones of crystallization with a variety of mineral sizes and assemblages.
The two photos above show pegmatites from Lake Superior beach. Both of these specimens are intermediate between granite and pegmatite.
This cross-polarized microscopic image of a Norway pegmatite shows large mica and feldspar crystals. Field of view = .13 inches wide (3.3 mm).
Figure Cites (in order of appearance above):
–James St. John, File:Emeralds in pegmatitic granite 8 (37992559234).jpg – Wikimedia Commons.
–James St. John, https://commons.wikimedia.org/wiki/File:Garnets_and_tourmaline_in_pegmatitic_ granite_(Crabtree_Pegmatite,_Devonian;_Crabtree_Mountain,_Mitchell_County,_North_Carolina,_ USA)_1_(25144467358).jpg.
–James St. John, https://commons.wikimedia.org/wiki/File:Garnets_and_tourmaline_in_pegmatitic_ granite_(Crabtree_Pegmatite,_Devonian;_Crabtree_Mountain,_Mitchell_County,_North_Carolina,_ USA)_1_(25144467358).jpg.
–Géry Parent, https://commons.wikimedia.org/wiki/File:Mica,_feldspar.jpg.
–James St. John, https://commons.wikimedia.org/wiki/File:Graphic_granite_(runite)_(Eight_Mile_Park_ Pegmatite_District,_
Mesoproterozoic,_1.430_to_1.474_Ga;_roadcut_along_3A_Road,_north_of_Royal_Gorge_Bridge,_west_of_Canon_City,_south-Central_Colorado,_USA)_10_(48751697192).jpg.
–Karen Brzys photograph.
–Karen Brzys photograph.
–Karen Brzys photograph.
–Karen Brzys photograph.
–Jonas Börje Lundin, https://commons.wikimedia.org/wiki/File:H.ellglimmer_in_Pegmatit.jpg.
Very interesting, Thank You
Hi Karen, many thanks for all of the great and useful information you extend to us in your posts. This really sets one’s mind to wonder and really appreciate all that our great home (Earth) has gone through. Keep ’em coming. Thanks again,
Jennie
Kinde MI