**7.1.2. Igneous rocks**

The models of authigenic phosphorite genesis (**Fig. 7**) assume the occurrence of mineraliza‐ tion of organic phosphorus in biologically productive waters, such as at ocean margins, that

Here, detrital accumulations may be mineralized at the sediment-water interface and in interstitial pore waters, liberating phosphate, some of which may then interact chemically with calcium in seawater to form phosphorite grains. These grains may be subsequently redistrib‐ uted within the sediments units. The dissolution of fish debris (bones) is also considered an important source of phosphate in authigenic phosphorite genesis. The upwelling probably also plays an important role in many cases of authigenic formation of phosphorite. During non-upwelling period in winter, the phosphate-sequestering bacteria of oxidative genera *Pseudomonas* and *Acinetobacter* become dominant in the water column. Fermentative Vibrios and Enterobacteriaceae are dominant during upwelling in summer. It was suggested that *Pseudomonas* and *Acinetobacter*, which sequester phosphate as polyphosphate under aerobic conditions and hydrolyze polyphosphate under anaerobic conditions to obtain the energy of maintenance and to sequester volatile fatty acid from polyhydroxybutyrate formation,

is, at shallow depths on continental slopes, shelf areas or plateaus [44].

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

**Fig. 7.** The schematic presentation of formation of phosphorite in marine environment [44].

Apatites of igneous origin include hydrothermal veins and disseminated replacements, marginal differentiations near the boundaries of intrusions and pegmatites, but the largest deposits are intrusive masses or sheets associated with carbonatite, nepheline-syenite and other alkalic rocks [27]. Igneous rocks are classified on the basis of their [21],[45]:


Rocks that crystallize partly at depth and partly near the surface are called **hypabyssal** <sup>14</sup> (**subvolcanic**). The term **porphyry** is also related to hypabyssal rocks, which are charac‐ terized by one or more than one minerals present as phenocrysts in fine-grained ground‐ mass.

<sup>10</sup> The name for fully (100%) crystallized igneous rock [45].

<sup>11</sup> The matrix is defined as interstitial material between larger (skeletal) grains [46].

<sup>12</sup> The texture is characterized by many cavities (vesicles), which were formed by bubbles of volatile gasses during the decrease of pressure at extrusion of magma to the surface. Lava solidifies before bubbles of gases can escape to the atmosphere [46].

<sup>13</sup> The name for igneous rocks where the ratio of crystals to glassy phase is higher than 3:5. Rocks containing higher amount of glass are termed as hypohyaline or holohyaline [45].

<sup>14</sup> Denotes the intrusions of magma at shallow depths in the crust, often directly related to overlying volcanic edifices [45].

**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 minerals are termed as **mafic** 15 (**basic**) **rocks**. In **mafelsic rocks**, the mafic and felsic minerals are present in approximately equal amounts. **Felsic rocks** 16 contain predomi‐ nantly light-colored minerals, such as quartz, feldspar, feldspathoid and muscovite.

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 ultrabasic, but they are rather basic [21].

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 are associated with magnetite [47].

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 phosphorus content [47].
