Rare-earth elements, thermal history, and the colour of natural fluorites

Abstract

The contents of La and of all 13 naturally occurring rare-earth elements (REEs) in samples of natural fluorites were determined by neutron activation analysis. The samples were chosen to test the relation of REE content to colour. Total REE contents of the samples ranged from 3.46 to 97.2 ppm; the range of the least abundant REE, Lu, was 0.031–0.57 ppm and the range of the most abundant REE, Ce, was 0.94–33.3 ppm. No relation was found between the absolute amounts nor the enrichment or depletion of a particular REE and colour. The absorption spectra from 400 to 700 nm were determined by photo-acoustic spectroscopy using a few milligrams of powdered fluorite sample. Optical absorption spectra were obtained for all samples including those that were too small or too opaque for transmission or reflectance methods. The absorption maxima obtained are similar to those reported by others for samples of similar colours. It is concluded that ionizing radiations from incorporated radioactive elements produce the divalent REE ions that account initially for purple colour. From the description given in the literature on the thermoluminescence of fluorites it is then shown that the thermal history can account for the ultimate colour. However, other factors, such as the presence of water vapour, oxygen, or hydrogen; variation in the growth rate of crystals; colloidal particles; and exposure to light, can contribute to the colour.