Igneous rocks consist essentially of silicate minerals which are unstable at the earth's surface where they gradually undergo hydrolysis. When the constitutive chemical elements of these silicate minerals pass from the crystal state to that of ions in water, the relative affinities among the elements change, some of them moving away from each other, others becoming attracted towards each other. This concept of ``convergence and divergence of elements'' was precious to Millot (1964), who considered it to be one of the keys to understanding the organization and evolution of the geologic materials at the surface of the earth. Some elements in solution have a mainly inorganic geochemical behavior and are quickly incorporated into clay minerals. Other elements are involved preferentially in biological activity and are associated with organisms and organic matter before becoming constituents of authigenic minerals in sediments. Apatite is one of these authigenic minerals which, because of its relatively low solubility, is commonly preserved in its primary state. As a consequence, it has a far greater potential to reflect the original geochemical characteristics of its sedimentary environment than common biogenic minerals such as carbonates and sulfates.
Lucas, J., & Prevot-Lucas, L. (1997). On the Genesis of Sedimentary Apatite and Phosphate-Rich Sediments. In Soils and Sediments (pp. 249–268). Springer Berlin Heidelberg. https://doi.org/10.1007/978-3-642-60525-3_12