**7.1.3. Biogenic apatites**

The biogenic (endogenous) mineral deposits form in surface environments as the transforma‐ tion of primary organic aggregates or as a result of biochemical processes. Since the organ‐ ism produces many of the same substances that form inorganically in rocks, the biogenic minerals are not minerals in the conventional sense1 . Biogenic minerals originate from living organisms or with their assistance (**Table 2**). These compounds are crystallized within living organisms as a result of cell activity and are surrounded by organic matter. Classical exam‐ ples are the bones of vertebrates. The bones and teeth consist of fine fibers or platy crystals (**Section 10.9.2**) of a mineral closely related to carbonate-hydroxylapatite (**Section 4.6**). These crystals are suspended in organic collagen. The crystals of apatite, which often do not exceed 10 nm in length, comprise up to 70% of weight of dried bone. The proteins make up the remain‐ ing 30% [48].

<sup>15</sup> Term mafic is the abbreviation of names of elements **ma**gnesium and iron (Latin word **f**errum) [45].

<sup>16</sup> Term felsic is the abbreviator of names of minerals **fel**dspar and **si**lica [45].


**Table 2.** Mineralogical composition of solid plant and animal tissues [48].

**3. Chemistry and mineralogy**: the rocks comprising more than 90 vol.% of ferromagnesi‐ an minerals, such as olivine, pyroxene, amphibole and biotite, are called **ultramafic** (**ultrabasic**) **rocks**. The rocks composed from essentially one or more ferromagnesian

nantly light-colored minerals, such as quartz, feldspar, feldspathoid and muscovite.

Igneous rocks are formed by the solidification of silicate melt from high temperatures. Since the sequence of crystallization follows the liquidus-solidus phase relationships, the minerals of low content will normally crystallize the least, but diorite and granodiorite melts may have enough phosphorus present for the FAP phase field to intersect the liquidus and to allow early formation of fluorapatite. Later-crystallizing phases should form in the interstices between early-crystallizing phases of alkali-rich igneous rocks and should form an immiscible phos‐ phate-rich liquid phase, which leads to large late-stage segregations of FAP, some of which

Where the content of phosphorus is very low, phosphorus may remain in the fluid phase, and apatite will form during the time at which the rock re-reacts with this fluid. This reaction is termed as pneumatolitic, and formed crystals will be small and often euhedral (with crystal facets). They may be included inside preexisting mineral grains. This situation is often encountered in granites and other related siliceous igneous rocks. The concentration of apatite minerals in igneous rocks is rarely sufficient to yield the source for mining the deposits for the

The biogenic (endogenous) mineral deposits form in surface environments as the transforma‐ tion of primary organic aggregates or as a result of biochemical processes. Since the organ‐ ism produces many of the same substances that form inorganically in rocks, the biogenic

organisms or with their assistance (**Table 2**). These compounds are crystallized within living organisms as a result of cell activity and are surrounded by organic matter. Classical exam‐ ples are the bones of vertebrates. The bones and teeth consist of fine fibers or platy crystals (**Section 10.9.2**) of a mineral closely related to carbonate-hydroxylapatite (**Section 4.6**). These crystals are suspended in organic collagen. The crystals of apatite, which often do not exceed 10 nm in length, comprise up to 70% of weight of dried bone. The proteins make up the remain‐

15 Term mafic is the abbreviation of names of elements **ma**gnesium and iron (Latin word **f**errum) [45].

An acidic rock contains > 60% SiO2, whereas a basic rock is characterized by silica content ranging from 44 to 52% of SiO2. Many of ultramafic rocks are ultrabasic with the content of silica < 44%, but such ultramafic rocks as pyroxenites and amphibolites are not

minerals are present in approximately equal amounts. **Felsic rocks** <sup>16</sup>

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

(**basic**) **rocks**. In **mafelsic rocks**, the mafic and felsic

. Biogenic minerals originate from living

contain predomi‐

minerals are termed as **mafic** <sup>15</sup>

ultrabasic, but they are rather basic [21].

minerals are not minerals in the conventional sense1

16 Term felsic is the abbreviator of names of minerals **fel**dspar and **si**lica [45].

are associated with magnetite [47].

phosphorus content [47].

**7.1.3. Biogenic apatites**

ing 30% [48].

In addition to the occurrence within the bones and teeth of vertebrates, mineral-organic aggregates are also found in mollusk shells, solid tissues of foraminifera corals, trilobites and other arthropods, echinoderms, some algae, etc. Some other biogenic processes involve bacteria. Large deposits of native sulfur, manganese oxides and hydroxides and iron are attributed to bacterial activity. Bacterial activity is also involved in the weathering processes of sulfide oxidation and transformation of kaolinite into bauxites [48].

Biogenic apatite is one of the most promising authigenic phases in this respect, as it is present in most siliciclastic deposits and is strongly enriched in a large suite of trace elements. Biogenic (fish teeth and bones) and diagenetic apatites are essential repositories of sedimen‐ tary phosphorus. They occasionally form huge deposits, as in West Africa, which are active‐ ly mined to provide agricultural fertilizers. Some of these deposits, found in particular in the Late Precambrian of China, after chemical precipitates seem to be associated with the episodes of global glaciation. In low-temperature waters, phosphates form numerous complexes. The concentration of phosphorus in sea and river water is limited by very low solubility of apatite. Phosphate radicals often attach to the surface of iron oxyhydroxide colloids when they precipitate in estuaries [49].
