The study explains how certain minerals can change colors repeatedly

The study explains how certain minerals can change colors repeatedly

During the study of hackmanite, researchers found that it can change color when exposed to UV radiation repeatedly without wearing out. The results show that the cheap mineral, which is easy to synthesize, also has high durability and several uses.

Hackmanite changes its color from white to purple during UV irradiation and eventually returns to white if no UV is present. There are also some organic compounds that can change color due to exposure to radiation. However, these materials 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. However, some minerals retain their color-changing properties indefinitely.

By examining three naturally occurring minerals – hackmanite, tugtupit and skapolit – a new study found the answer to how they do this.

White decent turns blue during UV irradiation. The staining and return to white after removing the UV source takes only a few seconds. Scapolite is a fairly common mineral. The rare mineral hackmanit turns purple during UV irradiation, and the color fades back to white in a few minutes under normal white light. Tugtupite is a very rare mineral found in magmatic intrusions in southern Greenland. When exposed to UV radiation, it turns 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 durability of the effect are due to the strong three-dimensional cage-like zeolitic structure of these minerals. In a zeolite crystal structuregroups of atoms and molecules are arranged around a cavity, and atoms can move in and out of this central cavity without damaging 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. 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 the author Professor Mika Lastusaari from the Department of Chemistry.

“In these color-changing minerals, all the processes associated with the color change take place inside the pores of the zeolite cage where the sodium and chlorine atoms are located. That is, the cage-like structure allows atomic movement inside the cage while keeping the cage itself intact. color and return to its original color virtually indefinitely, “adds co-author Sami Vuori.

In the past, it has been known that skapolite changes color much faster than hackmanite, while tugtupit’s changes are much slower.

“Based on the results of this work, we found that the rate of color change correlates with the distance 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 control and tailored properties for color change,” says co-author Hannah Byron.

The Research group for intelligent materials, led by Lastusaari, has long conducted pioneering research on materials with light and color-related properties, especially on hackmanite. They are currently investigating many applications for hackmanite, such as possibly replacing LEDs and other light bulbs with the natural mineral and using it in X-rays.

One of the most interesting avenues that scientists 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 material during spaceflight.

“The strength of the 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 the sun’s radiation. The hackmanite to be tested on the space station is used in a similar way, but this property “We can also use it in everyday applications. For example, we have already developed a mobile phone application for measuring UV radiation that can be used by everyone,” says Vuori.

The paper “The structural origin of the effective photochromism in natural minerals“is published in Proceedings of the National Academy of Sciences (2022). Materials provided by University of Turku.

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