The power of imperfections : the sources of colour and luminescence of natural cryolite from Ivittuut (SW Greenland)

This study is dedicated to understanding the sources of colour and luminescence of natural cryolite, and to constraining its defect structures. Little research has been carried out on cryolite for the last few decades. There are no published luminescence data, hypotheses on the colour do not draw upon modern research, and publications on the defect structures of cryolite are scarce. Filling this knowledge gap is especially important at a time where cryolite-structure derived materials are explored for their potential in dosimetry and optics. The luminescence band shapes, lifetimes, thermal behaviour and compositional studies all confirm that intrinsic structural defects, rather than impurity ions, serve as colour and luminescence centres. To constrain the intrinsic defects available for the cryolite structure, the mineral is irradiated with swift heavy ions (SHI) and compared with untreated material. Thermally treated samples, which lose colour by 500 °C, are also analysed. Cryolithionite and chiolite, two other aluminofluorides, are used to constrain the position of the defects in the structure. Dark blue, pale blue and green luminescence are emitted by all cryolite samples; some samples also show low intensity orange luminescence. The blue luminescence centre are interpreted as self-trapped exciton (STE) on the AlF6 octahedron, while green and orange luminescence result from an 𝐹-centre located on a fluorine vacancy. This interpretation is aided by their interaction with SHI irradiation and UV (enhancing the green and diminishing the blue) and high temperature (‘resetting’ all cryolites to the white cryolite defect state). Purple cryolite is interpreted to contain 𝐹-centres and possibly their aggregates, which contribute to the colouration. Two absorption centres specific for purple variety are identified and connected with related luminescence centres. Black cryolite absorbs the same wavelengths as white and purple variety, but shows much higher absorbance levels. This is interpreted as high concentration of defect centres, reminiscent of smoky quartz, which leads to the luminescence quenching. The hypothesis of organic matter colouration is rejected after thorough examination.