In Chap. 1 we discussed that, although the four geologically relevant halogen elements have comparative chemical characteristics, these characteristics are different enough to separate the elements into different geological environments. That results in fluorine enriched in magmatic environments, while iodine is enriched in organic material such as in seaweed. Under natural circumstances it also is clear that iodate is more stable than iodide. The fact that fluorides of the alkaline earth metals are not very soluble in water results in low fluoride concentrations in seawater. Overall these effects result in iodine and fluorine that are decoupled from chlorine and bromine in most geological environments. Chlorine and bromine on the other hand appear to be largely coupled in geological environments. Although chlorine and bromine do appear in a large array of (mass/ionic) ratios in geological material, invariably they are always found together, a characteristic that has been studied regularly in the past (e.g. Boeke 1908, Behne 1953, Valyashko 1956, Yoshida et al. 1971, Alcala and Custodio 2008, Wang et al. 2020), because it supplies very important information on the provenance of the samples from which chlorine and bromine are studied.