Colors from ionizing radiation
All of the examples of colored minerals on this page owe their color to
the effects of ionizing radiation. The changes can come from oxidation
of cations (Mn2+ to Mn3+), trapped electrons
(f-centers
and related centers), molecular clusters often with unpaired electrons,
or, as is often the case, from unknown causes
Beryl
Much beryl is heated to remove the golden to green
shades that result from radiation inoder to turn the crystal into blue
aquamarine.
- Here is a blue aquamarine on the
left from the Tenente Ananias area, Rio Grande do Norte, Brazil, next
to a crystal that was irradiated with gamma rays to turn it green.
- Naturally colored (from natural irradiation) golden beryl from Volodarsk-Volynskii,
Ukraine, next to a crystal that has been heated to turn it blue. The
blue color is from Fe2+ whereas the golden-yellow color is
from Fe3+ that forms from irradiation.
Calcite
Radiation is associated with blue and amber colors of calcite.
- Amber Calcite from the Tri-state district, USA, with amber color from natural irradiation next to a colorless calcite cleavage rhomb.
- Blue calcite, 38K;
Natural radiation interacts
with sheared calcite to produce blue colors.
- An interesting experiment
is to break a colorless calcite crystal into chips upto 3 mm in size.
When some of the are chips are exposed to ionizing
radiation (such as gamma-rays) they turn amber colored. If some more of
the same chips are put into hydrolyic press and squeezed (One can use a
KBr pellet press such as are used in chemisty laboratories and
pressurize the die to the same pressure used to prepare
KBr pellets), they will remain colorless. If they are subsequently
exposed to ionizing radiation, they will turn blue.
Diamonds
Naturally occuring green diamonds are colored by natural radiation. An
often proposed model is that the radiation dislodges a carbon atom from
the diamond structure. The resulting
color center is known as the GR1 center. Many colored diamonds are also
produced by laboratory irradiation. The following examples are
representative.
Fluorite
The great diversity of colors of fluorite is mostly due to natural
irradiation.
Rare-earth elements in fluorite interact with radiation to produce a
variety
of colors.
- Purple fluorite;
I turned the sample on the left purple by irradiating a piece of
colorless
Mexican fluorite with gamma rays from 137Cs.
- Blue fluorite; The
sample in the middle is its natural color when mined in Espirito Santo,
Brazil. The two samples on the left and right ar the result of
gamma-irradiation.
Halite
- Blue halite from Germany
is the result of exposure to natural
radiation. Initially, if halite (common salt) is exposed to gamma
radiation, it turns amber because of
F-centers.
They are mostly electrons trapped at sites of missing Cl-
ions.
In time the electrons migrate to Na+ ions and reduce it to
Na
metal. Atoms of Na metal, in turn, migrate to form colloidal
sized
aggregrates of sodium metal. They are the cause of the blue color.
Spodumene (variety kunzite)
- Green
kunzite
from the Oceanview Mine, Pala, California. When freshly mined, the
kunzite can be green due to natuiral irradiation which oxidizes a
fraction of the manganese content to Mn4+. When exposed
to sunlight for a few hours, the kunzite turns pink as the manganese is
reduced to Mn3+ by the electrons freed from
electron traps due to the energy of sunlight.
Topaz
Naturally occuring brown topaz is often a product of natural radiation.
The color is unstable and fades in light in a matter of hours to days.
- brown
topaz, 45K; Topaz as mined from Thomas Mountain, Utah, is brown due
to radiation-induced color
centers. After several hours in the sun, it turn colorless as the color
centers
are bleached away by the light.
- blue
topaz, 57K; Here is an example
of topaz from Brazil in its natural colorless state. After it is
irradiated,
in this case with gamma rays, it may turn brown. If the brown material
is heated it may turn blue.
- Essentially all of the blue topaz of commerce now available is
irradiated to turn it blue.
Gamma
rays, high-energy electrons, and nuclear reactors are used to irradiate
topaz.
Tourmaline
Much of the pink, manganese-containing, tourmaline in nature owes its
color to natural ionizing radiation. Laboratory irradiation can
essentially
duplicate the color of natural tourmaline in the appropriate samples.
- bi-colored
tourmaline, 47K;
Here is an example of a bi-colored crystal I made by gamma-ray
irradiating
a crystal from Afghanistan which was initially half green and half
colorless.
Back to the list of colors
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last update: 12-Apr-2011
