Radiation damage of variscite in historic crafts: Solarization, decolouration, structural changes and spectra from ionoluminescence

Abstract

X-ray diffraction measurements, during halogen lamp illumination to simulate sunlight, (TXRD) show a phase transition from variscite (AlPO4·H2O) Messbach to variscite Lucin and a loss of the dark green colour. The differential-thermal and thermo-gravimetric (DTA-TG) analyses and thermoluminescence (TL) peaks all depict this first-order phase transition which takes place under sunlight. From the water vaporization temperature up to circa 650 °C, a second-order phase transition progressively occurs from variscite to berlinite (AlPO4) by loss of a second unit of water with hydrogen bonded to the lattice. The ion beam luminescence (IBL) spectra of the Zamora variscite display a spectral band from 500 to 570 nm attributed to [UO2]2+ in phosphates, and another spectral band from 670 to 740 nm is linked with Cr(VI)3+ defects situated in octahedral Al(VI)3+ positions. In the hydrous variscite lattice, the Al¿O and P¿O chemical bonds are mainly covalent; with the degree of covalency of the P¿O chemical bond significantly larger than of Al¿O. This open structure of variscite, which has a crystal field of reduced strength, involves small shifts of the absorption bands which intensify the blue¿green transmission producing the characteristic emerald colour of the dark green variscite of Zamora. These data provide a valuable basis for detection of solarization damage in historic crafts with inlaid variscite in the Museo del Prado (Madrid, Spain)