**7.1. Characterization and classification of phosphate rocks**

line CO2-rich fluorapatite (francolite) and cryptocrystalline collophane. The igneous deposits comprise fluorapatite ores,whichmostlyaccommodate carbonatites andothertypes of alkaline intrusions. The magmatic ores are generally of lower grade but give higher-quality beneficia‐ tion products with low contents of unwanted contaminants (Cd, Pb, As, U, Th, Mg and Al) [11],

The beneficiation products of apatite ores as a commodity are traded as phosphate rock. It is the only significant global resource of phosphorus used dominantly in the manufacturing of nitrogen-phosphorus-potassium (NPK) fertilizers for food-crop nutrition and in the produc‐ tion of animal feed supplements. Only 10 – 15% of the world's production of phosphate rock has other applications (e.g. pharmaceuticals, ceramics, textiles and explosives) and repre‐ sents an important alternative source of rare-earth elements (REE) [12],[13]. The REE con‐ tents in apatites are useful in paleoceanographic studies to identify the seawater masses and

The composition of phosphate rocks varies from one deposit to another. Therefore, phos‐ phate rocks from different sources may be expected to behave differently in beneficiation and acidulation processes. Phosphate rocks are primarily composed of the apatite group in association with a wide assortment of accessory minerals, mainly fluorides, carbonates, clays,

Si, Ca, Fe and Al are the most common companion elements in phosphate rocks, with the median abundances of 53.3 wt.%, 30.0 wt.%, 13.6 wt.% and 8.0 wt.%, respectively, compared

circulation patterns or to quantify the redox state of the ocean [14].

336 Apatites and their Synthetic Analogues - Synthesis, Structure, Properties and Applications

quartz, silicates and metal oxides [13],[15],[16].

**Fig. 1.** The average distribution of trace elements in phosphate rock [17].

[12].

When rock contains phosphate components between 5 and 50% (by volume), then it is phosphatic, and the name of the main lithology is used as a suffix (phosphatic limestone, phosphatic claystone, etc.). In addition, the dominant textural form of the phosphate compo‐ nents in a phosphorite can be used in defining the rock name (e.g. peloidal2 ) (phosphorite, coprolitic phosphorite, etc.) [10],[18].

There are two main kinds of phosphate rocks deposits in the world [10],[20],[21],[22],[17],[23]:


Depending on their origin (igneous or sedimentary), phosphate rocks have widely varying mineralogical, textural and chemical characteristics [23]. The locations of the major phos‐ phate rocks deposit and producers are shown in **Fig. 2** [20],[24].

Sedimentary phosphate deposits are exploited to produce about 80% of the total world production of phosphate rocks. Igneous phosphate deposits are often associated with carbonatites3 and/or alkalic (silica-deficient) intrusions. Igneous phosphate rock concentrates are produced from the deposits mainly exploited inRussia, theRepublic of SouthAfrica, Brazil, Finland and Zimbabwe. Igneous phosphate ores are often low in grade (less than 5% P2O5) but can be upgraded to high-grade products (from about 35% to over 40% P2O5) [22],[23].

The atom ratio of P:N = 1:15(16) in the oceans is not greatly different from that found in living organisms. The availability of soluble phosphate from weathering of apatite-containing rocks may initially has been the rate-determining factor in early live development. In most ecolog‐

<sup>2</sup> Peloid is a comprehensive descriptive term for polygenetic grains composed of micro- and polycrystalline carbonate. The term was proposed in order to replace the widely used name "pellet," which for many authors had become a synonym for pelletal coprolites (fossilized faces). Peloids differ from ooids and oncoids by the absence of centrosymmetric or radial internal structures [19].

<sup>3</sup> Igneous rocks (intrusive or extrusive) that contain carbonates in the amount higher than 50%.

ical systems, the phosphate content is the limiting factor for growth. Nearly all igneous rocks contain some phosphate, even if it is only ~0.1% (0.2% P2O5 on average in lithosphere), with nearly all of it in the form of apatite. Sedimentary rocks generally contain rather less (~0.1% P2O5 on average). Sedimentary phosphorite is believed to originate from widely dispersed apatite mainly in igneous rocks [25].
