Researchers from the University of Turku, Finland explain why the natural mineral hackmanite can change color when exposed to UV rays. As this process can occur repeatedly and without exhausting the material, hackmanite could form the basis of new LED and UV monitoring techniques.
Hackmanite has been under study at the University of Turku for almost a decade now. The mineral is easy to synthesize and has excellent durability. Together with its reaction to UV radiation, these properties make it a very attractive material for researchers looking to leverage its use for applications ranging from consumer electronics to medical devices.
What sets it apart is that hackmanite is one of only three known minerals that can change color from white to purple when exposed to UV light: this process is known as photochromism. Unlike the other two, however, the hackmanite change is reversible, relatively durable, and does not wear down the mineral in any way.
The exact reason for this color change and how, however, has so far remained unknown. The new paper worked with the three natural color-changing minerals – hackmanite, tugtupite and scapolite – to find the answer.
Reliable color change
Although the three minerals studied for this article are all naturally occurring inorganic compounds, there are a number of organic compounds that can also change color reversibly when exposed to UV light. These hydrocarbons, however, can only endure the process a few times before their molecular structure undergoes complete degradation. This is because any noticeable change in the color of a substance is created by significant changes in its structure, and repeatedly experiencing these changes damages the hydrocarbon molecule.
“In this research, we discovered for the first time that there is actually a structural change involved in the color change process. When the color changes, the sodium atoms in the structure move relatively far from their location. habitual and then come back. It can be called structural breathing and it does not destroy the structure even if it is repeated a large number of times,” says Professor Mika Lastusaari from the Department of Chemistry at the University of Turku, Finland, co – author of the article.
According to the results, the three inorganic minerals can apparently survive this process indefinitely. Their ability to do so stems from their three-dimensional chemical structure. This is similar to that of zeolites, a class of minerals used to produce detergents, drying agents and air purifiers, as their cage-like structure allows them to capture and release different particles. Zeolite-based detergents, for example, remove magnesium and calcium atoms from water by binding them inside the pores of their cage-like molecules.
“In these color-changing minerals, all of the processes associated with color change occur inside the pores of the zeolite cage where the sodium and chlorine atoms reside. That is, the cage-like structure allows for atomic movement within the cage while keeping the cage itself 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 article.
Additionally, the team explains that the rate at which these minerals can change color depends on how far the sodium atoms inside their structures have to travel. This information is particularly valuable for medical applications, as we now know how to more precisely control the color-changing properties of a particular structure.
This is the first time we have had a model of how color-changing minerals work, the team explained. The team is currently exploring different applications for hackmanite, such as replacing LEDs and other bulbs or using it for X-ray imaging. Another interesting possibility is the development of radiation detectors and detection tools. measure based on hackmanite; these would be deployed on the International Space Station and other manned space missions to allow crew members to measure the radiation absorption of different materials.
“The color strength of hackmanite depends on the amount of UV radiation it is exposed to, which means that the material can be used, for example, to determine the UV index of solar radiation. The hackmanite that will be 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 application for measuring UV radiation that can be used by everyone,” explains Sami Vuori.
The article “Structural origin of effective photochromism in natural minerals” was published in the review PNAS.