A scientific contingent from the University of Turku has determined that specific minerals, namely hackmanite, tugtupite, and scapolite, undergo a chromatic transformation from their inherent white hue to shades of purple, pink, and blue, respectively, when subjected to ultraviolet (UV) illumination.
Hackmanite exhibits a change to purple under UV irradiation, with the coloration reverting to white within a few minutes when exposed to ambient white light. The image is credited to Mika Lastusaari.
“While inorganic natural substances are capable of color alteration, certain organic compounds, specifically hydrocarbons, can also reversibly modify their color upon exposure to radiation,” remarked Professor Mika Lastusaari, an investigator within the Department of Chemistry at the University of Turku.
“However, these particular hydrocarbons possess a limited capacity for color change, typically undergoing degradation after only a few such transitions.”
“This limitation arises because the phenomenon of color change necessitates a profound alteration of the molecular structure, and persistent repetition of this structural shift ultimately leads to molecular disintegration.”
Through a synergistic application of experimental techniques and computational modeling, Professor Lastusaari and his associates conducted an in-depth examination of the properties characterizing three distinct aluminosilicate minerals: hackmanite, tugtupite, and scapolite.
“Our research has yielded the novel discovery that a structural reconfiguration is intrinsically involved in the chromic transformation process,” Professor Lastusaari stated.
“During the color change, sodium atoms within the crystal lattice migrate considerably from their typical positions before eventually returning to their original sites.”
“This dynamic process can be conceptualized as a form of structural respiration, and it does not compromise the integrity of the structure, even when subjected to numerous cycles.”
“Within these color-shifting minerals, all activities associated with the chromatic alteration are confined to the internal cavities of the zeolitic framework, where sodium and chlorine atoms are situated,” he elaborated.
“Consequently, the cage-like architecture facilitates intracellular atomic displacement while preserving the overall structural stability of the cage itself.”
“This inherent characteristic is precisely why these minerals can undergo color changes and revert to their original coloration virtually infinitely,” added Sami Vuori, a doctoral candidate concurrently pursuing studies in the Department of Chemistry at the University of Turku.
The research team observed that scapolite exhibits a significantly accelerated rate of color change compared to hackmanite, while tugtupite’s transformations occur at a markedly slower pace.
“Our findings indicate a correlation between the velocity of the color alteration and the extent of displacement experienced by the sodium atoms,” noted Hannah Byron, a doctoral student affiliated with the Department of Chemistry at the University of Turku.
“These insights are paramount for the advancement of future material science, as we now possess a clear understanding of the requisite structural attributes for the controlled manipulation and customization of chromic properties.”
“The intensity of hackmanite’s coloration is directly proportional to the duration and strength of UV radiation exposure, suggesting its potential utility in quantifying solar radiation’s UV index, for instance,” Vuori explained.
“While hackmanite slated for testing aboard the space station will serve a similar purpose, this property also holds promise for a variety of terrestrial applications.”
“We have already developed, for example, a mobile application designed to measure UV radiation, making this capability accessible to the general public.”
The collective research efforts of the team are documented in a publication within the Proceedings of the National Academy of Sciences, accessible via the following link: work.
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Pauline Colinet et al. 2022. The structural origin of the efficient photochromism in natural minerals. PNAS 119 (23): e2202487119; doi: 10.1073/pnas.2202487119
