While investigating hackmanite, researchers found that it can change color upon exposure to UV radiation without repeatedly wearing out. The results show that the inexpensive mineral, which is easy to synthesize, also has high durability and multiple applications.
Hackmanite changes its color from white to purple under UV irradiation and eventually reverts back to white if no UV is present. There are also some organic compounds that can change color due to exposure to radiation. These materials, however, can only change color a few times before their molecular structure breaks down. This is because the color change involves a drastic change in the molecular structure, and undergoing this change repeatedly eventually breaks down the molecules. But certain minerals retain their color-changing properties indefinitely.
Investigating three naturally occurring minerals – hackmanite, tugtupite and scapolite – a new study found the answer to how they do this.
White scapolite turns blue under UV irradiation. The coloration and reversion back to white after the removal of the UV source take only a few seconds. Scapolite is a rather common mineral. The rare mineral hackmanite turns purple under UV irradiation, and the color fades back to white in a few minutes under regular white light. Tugtupite is a very rare mineral found in magmatic intrusions in South Greenland. When exposed to UV radiation it becomes pink for several hours.
According to a study by researchers from the Department of Chemistry at the University of Turku, Finland, the color-change and the effect’s durability is due to the strong three-dimensional cage-like zeolitic structure of these minerals. In a zeolitic crystal structure, groups of atoms and molecules are arranged around a cavity, and atoms can move in and out of this central cavity without the crystal.
“In this research, we found out for the first time that there is actually a structural change involved in the color change process, as well. When the color changes, sodium atoms in the structure move relatively far away from their usual places and then return back. This can be called ‘structural breathing,’ and it does not destroy the structure even if it is repeated a large number of times,” explains author Professor Mika Lastusaari from the Department of Chemistry.
“In these color-changing minerals, all processes associated with the color change occur inside the pores of the zeolitic cage where the sodium and chlorine atoms reside. That is, the cage-like structure allows atomic movement inside the cage while keeping the cage itself intact. This is why minerals can change color and revert back to their original color practically indefinitely,” co-author Sami Vuori adds.
Previously, it has been known that scapolite changes color much faster than hackmanite, whereas tugtupite’s changes are much slower.
“Based on the results of this work, we found out that the speed of the color change correlates with the distance that the sodium atoms move. These observations are important for future material development, because now we know what is required from the host structure to allow the control and tailoring of the color change properties,” says co-author Hannah Byron.
The Intelligent Materials Research Group, led by Lastusaari, has long conducted pioneering research on materials with light and color-related properties, especially on hackmanite. They are currently exploring numerous applications for hackmanite, such as possibly replacing LEDs and other light bulbs with the natural mineral and using it in X-ray imaging.
One of the most interesting avenues that the researchers are currently exploring is a hackmanite-based dosimeter and passive detectors for the International Space Station, intended to be used to measure the radiation dose uptake of materials during space flights.
“The strength of hackmanite’s color depends on how much UV radiation it is exposed to, which means that the material can be used, for example, to determine the UV index of Sun’s radiation. The hackmanite that will be tested on the space station will be used in a similar fashion, but this property can also be used in everyday applications.We have for example already developed a mobile phone application for measuring UV radiation that can be used by anyone,” so Vuori.
The paper “The structural origin of the efficient photochromism in natural minerals” is published in the Proceedings of the National Academy of Sciences (2022). Materials provided by the University of Turku.