Variability in composition of apatite associated with carbonatites

Abstract

Apatite, one of the most abundant non-silicate minerals, is practically the only ore mineral in phosphatic mines over the world. The simplified structural formula of fluorapatite (the commonest type) is Ca1O (P0(4))6 F2. These elements may be substituted by others, in all crystallochemical sites, in apatite from all sorts of geological environments, even from organic and synthetic origins. Many of the natural and inorganic examples, mainly associated with carbonatites, are discussed here. The occupation ofthe monovalent site gives the official names to the species: fluorapatite, hydroxylapatite and chlorapatite. Among the substituents for calcium, can be mentioned rare earth elements and others of environmental importance, such as cadmium, which may be incorporated into fertilizers, leading to contamination problems. There are two possible positions for calcium substituents; several recent studies explore this aspect, evincing interacting factors, controlling the Ca1 or Ca2 site preference of each substituent cation, depending on their characteristics. ln the substitution for phosphate anions, carbonate is the most important anion, and its entry in apatite produces structural changes and requires mechanisms to keep the charge equilibrium neutral, -such as coupled substitutions or creation of vacancies in the structure. This question has not yet been solved, and has been argued for decades; it is considered fundamental to the understanding of solubility, reactivity, clistalinity, and themal stability.