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
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 and an electron takes its place. The resulting color center is known as the GR1 center. Many colored diamonds are also produced by laboratory irradiation. The following examples are representative.
Calcite
Radiation is associated with blue and amber colors of calcite.
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.
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.
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Purple fluorite, 45K;
I turned the sample on the left purple by irradiating a piece of colorless
Mexican fluorite with gamma rays from 137Cs.
Halite
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Blue halite 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.
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.
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