Crystal-chemical characterisation and spectroscopy of fluorcarletonite and carletonite

The minerals of carletonite group, fluorcarletonite, KNa (sub 4) Ca (sub 4) [Si (sub 8) O (sub 18) ](CO (sub 3) ) (sub 4) (F,OH).H (sub 2) O and carletonite, Na (sub 4) Ca (sub 4) [Si (sub 8) O (sub 18) ](CO (sub 3) ) (sub 4) (OH,F).H (sub 2) O, were investigated using a multi-method approach. A detailed comparative chemical study of the minerals was carried out using electron probe microanalysis and Fourier transform infrared spectroscopy. Using X-ray techniques and the results obtained, geometrical and distortion characteristics of the mineral structures are calculated and the successful crystal-structure refinement of these two natural compounds are given. Using spectroscopic and luminescence methods and ab initio calculations, it is shown that hole defects (CO (sub 3) ) (super -) are responsible for the colouration of the samples studied. Luminescence due to 5d-4f transition in Ce (super 3+) ions is observed in both investigated compounds. Moreover, luminescence attributed to intrinsic luminescence, corresponding to the decay of electronic excitations of (CO (sub 3) ) (super 2-) complexes in the carletonite sample, is registered for the first time in phyllosilicates. An analysis of the optical absorption spectra and g-tensor values suggests that (CO (sub 3) ) (super -) defects in the crystal structure are localised in the C1 positions. Identification of these specific properties for these sheet silicates, with a two-dimensional infinite tetrahedral polymerisation, indicates that carletonites could be prospective materials for novel phosphors and luminophores.