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 portion of the same 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 up to 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.

yellow diamond, 47K; A technological product of irradiation and heating.
orange diamond, 53K; A technological product of irradiation and heating.
blue diamond, 29K; A technological product of irradiation. Although they look similar to natural blue diamonds, the color is from an entirely different origin.
green diamond, 68K; A technological product of irradiation.

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 are 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.

Quartz

A number of colored quartz varieties owe their color to either natural or laboratory irradiation.

Amethyst from the state of Rio Grande do Sul, Brazil, is the result of natural irradiation of Fe³⁺ in the quartz to Fe⁴⁺. The amethst color will fade in time when exposed to sunlight.
Ametrine
Lemon yellow quartz
Pink quartz from the state of Maine, USA. A small amount of coupled substitution of aluminum and phosphorous for silicon followed by natural irradiation is believed to cause the color. It is found only at a small number of localities such as this specimen from Brazil
Smoky quartz: An irradiated quartz crystal cluster from Arkansas. Irradiation removes an elctron from an oxygen ion associated with an aluminum 3+ ion substituting for silicon 4+.

Spodumene (variety kunzite)

Green kunzite from the Oceanview Mine, Pala, California. When freshly mined, the kunzite can be green due to natural 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.

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last update: 1-Aug-2016