Some pegmatites cut through [igneous] or [metamorphic rocks] in such a way that they resemble walls (called dikes). Others are found as veins, as flat masses, or as odd-shaped bodies in rocks. Many pegmatites occur in granites and contain [feldspar], [quartz], [mica], and other minerals, as [granite] does. Some pegmatites occur with other kinds of [igneous rocks] and contain the same minerals as these rocks. A few pegmatites contain rare and unusual minerals.
Many geologists believe that pegmatites form from hot [fluids] of [igneous] origin that are left after other [igneous rocks], such as [granite], have already formed. These left-over fluids contain large amounts of aluminum, potassium, silicon, sodium, and several other [elements]. While the granite or other rocks are still far underground, this material pushes up into them, and may even partly dissolve them. Then it slowly cools and hardens into [pegmatite]. It is believed that, later, more fluids move into cracks in some pegmatites. This new material adds other minerals to the pegmatites and alters some of those minerals already there.
Quartz-feldspar [pegmatite] from Burnet County, Texas.
Some of the pegmatites we now see at the surface in Texas are probably about a billion years old. They formed during [Precambrian] time and occur with other extremely old rocks. One well-known Texas [pegmatite] area is the [Mica] Mine district of west Texas. It is about 15 miles south of Van Horn in the Van Horn Mountains of Culberson and Hudspeth counties. Another pegmatite area is in the [Llano uplift] of central Texas. These central Texas pegmatites occur in Burnet, Gillespie, Llano, and Mason counties.
Large crystals and grains of [feldspar], [mica], and [quartz] are found in the pegmatites of both these areas. A small amount of mica has been mined from the west Texas pegmatites, and feldspar has been produced from the central Texas pegmatites.
An extremely rare and unusual [pegmatite] occurs in the [Llano uplift] area at Baringer Hill, which is west of Burnet in Llano County. This pegmatite was once on the bank of the Colorado River, but when Buchanan Dam was built, the area was flooded. The Baringer Hill pegmatite now lies beneath the water of Lake Buchanan. Many rare minerals, which contain beryllium, cerium, thorium, uranium, yttrium, zirconium, and a number of other [elements], occur in this pegmatite. Some of these minerals, such as those containing yttrium and zirconium, glow or incandesce when they are heated. During the early part of this century, before the area was flooded, several of the yttrium minerals were mined and used in making lamp mantles.
Pitchblende. See [Uranium Minerals].
Precious Opal. See [Opal].
Pulverulent Limestone. See [Limestone].