Researchers from the University of Turku, Finland, report why the natural mineral hackmanite can change color when exposed to UV radiation. Because this process can take place repeatedly and without wear on the material, hackmanite could form the basis for new LED and UV monitoring techniques.
Hackmanite has been studied at the University of Turku for nearly a decade now. The mineral is easy to synthesize and has excellent durability. Together with its response to UV radiation, these properties make it a very interesting material for researchers looking to leverage its use for applications ranging from consumer electronics to medicinal devices.
What stands out is that hackmanite is one of only three known minerals that can change color from white to purple when exposed to UV radiation: this process is known as photochromism. However, unlike the other two, the change in hackmanite is reversible, relatively long-lasting and does not wear out the mineral in any way.
Exactly why this color change takes place and how, however, has remained unknown until now. The new paper worked with all three natural color-shifting minerals — hackmanite, tugtupite and scapolite — to find the answer.
Reliable color change
While the three minerals studied for this paper are all inorganic natural compounds, there are quite a few organic compounds that can also reversibly change color when exposed to UV light. However, these hydrocarbons can only go through the process a few times before their molecular structure is completely broken down. This is due to the fact that any noticeable shift in the color of a substance is caused by significant changes in its structure, and undergoing these changes repeatedly damages the hydrocarbon molecule.
“In this study, we discovered for the first time that there is actually a structural change in the color change process. When the color changes, sodium atoms in the structure move relatively far away from their usual places and then return. This can be called structural respiration and it does not destroy the structure even if it is repeated a large number of times,” explains Professor Mika Lastusaari of the Department of Chemistry at the University of Turku, Finland, co-author of the paper.
According to the findings, the three inorganic minerals can seemingly survive this process indefinitely. Their ability to do this stems from their three-dimensional chemical structure. This is similar to that of zeolites, a class of minerals used to manufacture detergents, drying agents and air purifiers, because their cage-like structure allows them to trap and release various particles. For example, zeolite detergents remove magnesium and calcium atoms from water by binding them in the pores of their cage-like molecules.
“In these color-changing minerals, all the processes related to the color change take place in the pores of the zeolite cage where the sodium and chlorine atoms are located. That is, the cage-like structure allows for atomic movement within the cage while the cage itself remains intact. This is why minerals can change color and return to their original color practically indefinitely,” explains doctoral researcher Sami Vuori, co-author of the paper.
Furthermore, the team explains that the speed at which these minerals can change color depends on the distance the sodium atoms in their structures have to travel. This piece of information is especially valuable for medical applications, as we now know that we can more accurately control the color-changing properties of a given structure.
This is the first time we have a model of how color-changing minerals work, the team explains. The team is now exploring various uses for hackmanite, such as replacing LEDs and other light bulbs or using them for X-ray imaging. Another interesting possibility is the development of hackmanite-based radiation detectors and measuring instruments; these would be deployed on the International Space Station and other manned space missions to allow crew members to measure the radiation uptake of various materials.
“The strength of hackmanite’s color depends on the amount of UV radiation it is exposed to, allowing the material to be used, for example, to determine the UV index of solar radiation. The hackmanite being tested on the space station will be used in a similar way, but this property can also be used in everyday applications. For example, we have already developed a mobile phone application for measuring UV radiation that can be used by everyone,” explains Sami Vuori.
The article “The structural origin of the efficient photochromism in natural minerals” is published in the journal PNAS†