Iodine in karst-type phosphorites from the Lahn region,Germany

The concentration of iodine in 31 Tertiary continental phosphorites from the Lahn region in Germany ranges from about 0.01% to 0.18%, the mean value of 0.044% being quite compatible with that of typical commercial-grade iodine deposits. Microscopic and X-ray diffractometric studies manifested three main mineral phases in these phosphorites viz. francolite, clay minerals (kaolinite, illite), and ferric oxides (goethite, hematite). These mineral phases were investigated for iodine distribution, mode of its fixation and possible source. Sample heating at 600°C evinced no appreciable organic matter in the sample and further showed that the iodine was non-volatile in character. Regression analysis revealed a good correlation between I and Al (Ti), practically no correlation with Fe and negative correlation with P₂O₅. It was thus deduced that the iodine content is inorganic in nature and bound principally and probably internally to Al-bearing minerals. An iodine content of about 150 ppm is believed to be associated with the apatite phase of the phosphorite.Devonian volcanic rocks are assumed to be the ultimate source of both phosphorus and iodine. Those elements were released during a Tertiary weathering period, their accumulation being controlled by chemical environment and karst topography of a Devonian reef limestone.