**Abstract**

Apatite is the most abundant phosphate mineral which include more than 95% of al phosphorus in the Earth´s crust. The seventh chapter of this book provides brief description of sedimentary and igneous phosphate rocks and introduces basic ideas for characterization and classification of phosphate rocks. The chapter continues with description of biogenic apatites, description of phosphate rocks deposits and introdu‐ ces other sources of phosphorus. Furthermore, geological role of apatite, cycle of phosphorus, weathering of apatite, fission track analysis and extraterrestrial apatites were described. The last section is dedicated to structure and properties important of non-apatitic phosphate minerals, such as atuanite, crandallite, lazulite, millisite, monazite, tobernite, xenotime etc.

**Keywords:** Apatite, Phosphate Rock, Biogenic Apatites, Fission Track, Extraterrestrial Epatite, Non-Apatitic Phosphate Minerals

Apatite [Ca5(PO4,CO3)3(OH,F,Cl)] is the most abundant phosphate mineral, which accounts for more than 95% of all phosphorus in the Earth's crust and is found as an accessory mineral in most rock types1 on the Earth's surface, primarily because it is stable in a wide variety of geological conditions and over a range of different geological processes [1],[2],[3],[4],[5],[6],[7],[8]. According to the list of symbols for rock- and ore-forming minerals, the abbreviated symbol used for apatite is **Ap** [9].

However, exploitable deposits of apatite are mainly found in igneous rocks and also in sedimentary and metamorphic rocks. The former comprises the stratiform phosphorite deposits in shelf-type shale-carbonate sequences, which contain high phosphorus ores of microcrystal‐

<sup>1</sup> A rock may be best defined as any mineral or aggregate of minerals that forms an essential part of the Earth [10]. Apatite is found in all classes of rock: igneous, metamorphic and sedimentary [2]. Also, much of phosphorus in coal is present in the form of apatite [7]. Phosphorus is the 10th most abundant element on Earth, with an average crustal abundance of 0.1% [8].

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line CO2-rich fluorapatite (francolite) and cryptocrystalline collophane. The igneous deposits comprise fluorapatite ores,whichmostly accommodate 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], [12].

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

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

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


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

with P2O5. In some low-grade phosphate rock mines, the content of Fe and Al is even higher than that of P2O5, and it is usual that the P2O5 content in phosphate rock is less than the Si and Ca content. In addition, some elements that are very rare in the Earth's crust are found to be relatively abundant (**Fig. 1**) in phosphate rocks [17].
