In 1945, the world suddenly became aware of the awesome power of atomic
energy when the element _uranium_ was used to produce some of the first
atomic bombs. Uranium does not occur alone in nature but is found
combined with other elements in a number of minerals.
All of the uranium minerals are radioactive. The uranium they contain is
gradually breaking down and changing into a series of 13 other elements,
called _daughter_ elements. Each daughter element breaks down and
changes into the next daughter element of the series. While breaking
down, these elements give off particles and rays of energy.
This energy or _radioactivity_ is made up of what are called alpha
particles, beta particles, and gamma rays. You cannot see, hear, taste,
smell, or feel them. The alpha and beta particles are weak and do not
travel far. The gamma rays, however, can travel farther and can pass
through seemingly solid material. Scientists have found that these rays
can move through about 1 foot of rock, 2½ feet of water, and several
hundred feet of air.
Prospectors searching for uranium minerals carry instruments that are
able to detect this radioactivity. The uranium itself gives off only
alpha particles, but some of its daughter elements give off gamma rays.
These daughter elements are normally found with the uranium, and it is
their strong gamma rays that the instruments are most apt to detect.
[Illustration: A Geiger counter is used to detect radioactivity.]
One of the instruments used is the _Geiger counter_. It indicates
radioactivity by means of a meter, a flashing light, or a clicking
sound, which can be heard through earphones. Another instrument for
detecting radioactivity is the _scintillation counter_. It is more
sensitive than the Geiger counter and it can detect radioactivity from a
greater distance. The scintillation counter can be used from an
automobile or an airplane, but the Geiger counter must be quite close to
the source of radioactivity to be of use.
Various uranium minerals have been found, mostly in small amounts, in a
number of places in Texas. Some of these minerals, such as uraninite or
pitchblende, are heavy and dark colored. Others, including carnotite,
tyuyamunite, autunite, and uranophane, are a shade of yellow or green.
They are quite soft. Deposits of the light-colored uranium minerals have
been mined from two areas of Texas. One of these areas is in Garza
County on the Texas High Plains, and the other is in Karnes and Live Oak
counties in the Gulf Coastal Plain.
One of the light-colored uranium minerals, _carnotite_, is a
potassium-uranium vanadate, which has a bright canary-yellow or
lemon-yellow color. This mineral is transparent to translucent and has
an earthy or a pearly luster. Carnotite usually is found as crusts and
as powdery masses. It is quite soft and can be scratched with a
fingernail.
Carnotite, along with tyuyamunite, autunite, and several other soft,
yellowish or greenish uranium minerals, is found in the Texas Gulf
Coastal Plain. These minerals occur in the Jackson, Catahoula, and
Oakville strata (which are Tertiary in age) in an area extending from
Gonzales County to the Rio Grande (in parts of the area indicated by no.
2 and no. 3 on the geologic map, pp. 4-5). The largest deposits in this
district have been found in the Karnes County area.
The Gulf Coastal Plain uranium minerals occur mostly with sandstones and
clays in a sequence of strata that contains volcanic ash. It is believed
that small scattered amounts of uranium compounds that were present in
the volcanic ash sediments were dissolved by seeping underground water.
These waters then moved into the sandstones and clays where they
deposited the uranium as carnotite and as other uranium minerals.
Another uranium mineral, _uranophane_ (calcium-uranium silicate), also
occurs in Texas. Uranophane has a yellow to yellow-orange color and a
pearly to greasy luster. When rubbed across a streak plate, it leaves a
light yellow to a light yellow-orange streak. It is soft enough to be
scratched by a copper penny. Uranophane has been found in extrusive
igneous rocks in northwestern Presidio County in west Texas.
A dark-colored uranium mineral, _pitchblende_, is a variety of the
mineral _uraninite_, uranium dioxide. Pitchblende does not occur with a
crystal shape but rather as rounded and irregular-shaped masses. It is
brownish black, greenish black, or black. If you rub it across a streak
plate, pitchblende leaves a brownish-black streak. This mineral is heavy
(it has a specific gravity of 6.5 to 8.5) and hard (a pocket knife will
not scratch it, although a steel file will). Pitchblende has a
submetallic luster and looks dull, greasy, or like pitch or tar.
Small amounts of pitchblende have been found at several places in Texas.
One of these localities is a few miles west of Burnet in Burnet County
in central Texas. Here, the pitchblende occurs in Precambrian igneous
rocks that are associated with gneiss. In south Texas, some fine,
scattered particles of pitchblende have been found about 325 feet below
the surface in Tertiary (Pliocene) sediments that cover the Palangana
salt dome in Duval County. No pitchblende is mined in Texas.
Uranophane. _See_ Uranium Minerals.
Vitrophyre. _See_ Obsidian and Vitrophyre.