Pegmatites occur in igneous rock areas, and most geologists consider
them intrusive igneous rocks. They are made up of crystals and
crystalline mineral grains that fit together—the grains are interlocked.
The crystals and grains in pegmatites are larger than those of
surrounding rocks, and some are huge, even larger than a man. However,
there is a wide range of grain sizes in pegmatite.
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.
[Illustration: 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.
Pumicite. _See_ Volcanic Ash.