**7.1.1. Sedimentary phosphate rocks**

Most marine sediments and rocks contain less than 0.3% of P2O5. However, periodically through geological time, phosphorites (with the content of P2O5 of 5% or greater) formed on the seafloor in response to specialized oceanic conditions and accumulated in sufficient concentrations to produce major deposits of regional extent4 [26].

Marine phosphate formation and deposition represent the periods of low rates of sedimenta‐ tion in combination with large supplies of nutrients. Phosphorus is then concentrated by various mechanisms, possibly bacterial (refer to discussion of **Fig. 7**), at either the sedimentwater interface or within interstitial pore waters. This process leads to primary formation and growth of phosphate grains, which remain where they were formed or are transported as clastic particles within the environment offormation. During subsequent periods oftime, some primary phosphate grains may be physically reworked into another sediment unit in re‐ sponse to either changing or different environmental processes [26].

**Fig. 2.** The distribution of the world's phosphate resources [20].

<sup>4</sup> Most of the world's phosphate production comes from marine phosphorites [27].

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

Most marine sediments and rocks contain less than 0.3% of P2O5. However, periodically through geological time, phosphorites (with the content of P2O5 of 5% or greater) formed on the seafloor in response to specialized oceanic conditions and accumulated in sufficient

Marine phosphate formation and deposition represent the periods of low rates of sedimenta‐ tion in combination with large supplies of nutrients. Phosphorus is then concentrated by various mechanisms, possibly bacterial (refer to discussion of **Fig. 7**), at either the sedimentwater interface or within interstitial pore waters. This process leads to primary formation and growth of phosphate grains, which remain where they were formed or are transported as clastic particles within the environment offormation. During subsequent periods oftime, some primary phosphate grains may be physically reworked into another sediment unit in re‐

[26].

apatite mainly in igneous rocks [25].

**7.1.1. Sedimentary phosphate rocks**

concentrations to produce major deposits of regional extent4

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

sponse to either changing or different environmental processes [26].

**Fig. 2.** The distribution of the world's phosphate resources [20].

Most of the world's phosphate production comes from marine phosphorites [27].

4

**Fig. 3.** Stratigraphic distribution of phosphorites based on 1982 production data (a) and distribution of major Protero‐ zoic-Cambrian phosphorites (b) [28].

The stratigraphic distribution5 [29] of phosphorites [28] is shown in **Fig. 3**(**a**). The major discoveries in Proterozoic and Cambrian rocks were not made until late 1930s, initially in the USSR (Kazakhstan), Poland, Korea, China and northern Vietnam. Early discoveries were made in the course of regional geological mapping and exploration for metal deposits, but later deposits were found mainly using more direct exploration techniques. It is possible that the greatest global phosphogenic episode in geological history took place in Late Proterozoic and Cambrian. While pelletal phosphorites are common in most Cambrian deposits, the Protero‐ zoic age phosphorites contain mudstone (**Fig. 3**(**b**), microphosphorite) and stromatolitic phosphorite6 [28],[30].

Some phosphates were formed during all major sea-level transgressions during 67 million years of Cenozoic history; however, some periods were more important than others with respect to producing large volumes of phosphorites and preserving them in the geologic column. During the Paleocene and Eocene, several major episodes of phosphogenesis occurred within the major episodes of phosphogenesis in the major east-west ocean, which included Tethys [31],7 producing extensive amounts of phosphorites throughout the Middle East, Mediterranean and northern South American regions. By the Neogene, this circum-global ocean had been destroyed by the plate tectonic processes, and the north-south Pacific and

<sup>5</sup> Since the Earth is stratified, in a broad sense, all rocks and classes of rocks (sedimentary, igneous and metamorphic) fall within the scope of stratigraphy and stratigraphic classification. Rocks can be classified according to lithology, fossil content, magnetic polarity, electrical properties, seismic response, chemical or mineralogical composition, etc. Rocks can be also classified according to time of their origin or environment of genesis. Rock bodies can be classified into many different categories of stratigraphic classification, including lithostratigraphic units (1), biostratigraphic units (2), chronostratigraphic units (3), unconformity-bounded units (4) and magnetostratigraphic units (5). Please see work [29] for further details.

<sup>6</sup> Phosphate occurs as concentrated in stromatolite columns, laminar algal (stromatolitic) phosphorite, reworked fragments of stromatolites forming silicified conglomeratic or brecciated phosphorite, massive-bedded phosphorite with sandy and clayey laminate and disseminate pellets and nodules in dolomite [30].

<sup>7</sup> The Tethys Ocean divided the continental masses into northern and southern groups. It merged at both ends with the Panthalassa or proto-Pacific Ocean. The remnants of Tethys are now located within Alpine mountain belts from the Caribbean in the west to recent collision zone between Australia and Eurasia in the east and within still-growing Central Atlantic Ocean between Africa and North America [31].

Atlantic oceans dominated global circulation patterns. Upper Cenozoic phosphogenesis (**Table 1**) occurred along the north-south ocean-ways, which now contain modern continen‐ tal margins. On the basis of the extent of known phosphate deposits, the Miocene was by far the most important episode of phosphate formation in Upper Cenozoic [26].


*ma* – million years ago, *ta* – thousand years

**Table 1.** Formation of phosphorites during Upper Cenozoic phosphogenesis [26].

Phosphorites and phosphatic sediments are known on the floor of the Pacific, Indian and Atlantic oceans. They occur in a number of inshore areas (the shelves and upper part of the continental slopes) and in pelagic zones, chiefly on seamounts. Most of the shelf phosphor‐ ites are localized in four very large oceanic phosphorite provinces [32],[26],[33]:


Sedimentary rocks with the content of 18 – 20 wt.% of P2O5 are termed as phosphorites. The main phosphate mineral in phosphorites is carbonate-fluorapatite (CAF, francolite): Ca10−a−b −cNaaMgb(PO4)6−x(CO3)x−y−z(CO3,F)y(SO4)zF2, where *x* = *y* + *a* + 2*c* and *c* denotes the number of Ca vacancies, present as grain or mud. Most phosphorites are of marine origin [34],[35],[36]. Phosphorites on the sea floor occur in two types of environments on [37]:


Atlantic oceans dominated global circulation patterns. Upper Cenozoic phosphogenesis (**Table 1**) occurred along the north-south ocean-ways, which now contain modern continen‐ tal margins. On the basis of the extent of known phosphate deposits, the Miocene was by far

the most important episode of phosphate formation in Upper Cenozoic [26].

**Quaternary** 258 *ma* to present

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

**Neogene** 23.03 – 2.58 *ma*

**Table 1.** Formation of phosphorites during Upper Cenozoic phosphogenesis [26].

**Phanerozoic** (541 *ma* to present) **Cenozoic** (66 *ma* to present)

*ma* – million years ago, *ta* – thousand years

and East Australian shelf.

**Geological period Age Duration**

**Holocene**

**Pleistocene** 2.58 – 0.0117 *ma*

**Pliocene** 5.333 – 2.58 *ma*

**Miocene** 23.03 – 5.333 *ma*

**Paleogene** 66 – 23.03 *ma*

Phosphorites and phosphatic sediments are known on the floor of the Pacific, Indian and Atlantic oceans. They occur in a number of inshore areas (the shelves and upper part of the continental slopes) and in pelagic zones, chiefly on seamounts. Most of the shelf phosphor‐

**a. East Atlantic**: Portugal, northwest Africa throughout South Africa and Agulhas Bank;

**b. West Atlantic**: North Carolina throughout Florida, Cuba, Venezuela and Argentina;

**c. East Pacific**: California throughout Baja California, Mexico and Peru throughout Chile;

**d. West Pacific**: Sakalin Island, Sea of Japan, Indonesia, Chathman Rise east of New Zealand

Sedimentary rocks with the content of 18 – 20 wt.% of P2O5 are termed as phosphorites. The main phosphate mineral in phosphorites is carbonate-fluorapatite (CAF, francolite): Ca10−a−b −cNaaMgb(PO4)6−x(CO3)x−y−z(CO3,F)y(SO4)zF2, where *x* = *y* + *a* + 2*c* and *c* denotes the number of Ca vacancies, present as grain or mud. Most phosphorites are of marine origin [34],[35],[36].

**ii.** Submerged mountains in association with calcareous and volcanogenic rocks.

ites are localized in four very large oceanic phosphorite provinces [32],[26],[33]:

Phosphorites on the sea floor occur in two types of environments on [37]:

**i.** Continental margins in association with terrigenous;

0.0117 *ma* to present

<3 *ma* 10 *ta*

5 – 4 *ma* 1 *ma*

19 – 13 *ma* 6 *ma*

29 – 25 *ma* 4 *ma*

**Oligocene** 33.9 – 23.03 *ma*

**Fig. 4.** The surface currents in an idealized ocean, showing the areas of ascending nutrient-rich water (a) and the distri‐ bution of upwelling water and related phenomena in modern oceans (b) [27].

Phosphorites consist mainly of phosphate cement enveloping small grains of phosphatic and non-phosphatic materials. Phosphate in the cavities of foraminifera is purer than that enveloping the grains [37].

The largest phosphorite-bearing regions are situated along the west coasts of Africa and America, at the east coast of the USA, off New Zealand and in the central part of the north‐ ern Pacific. The phosphatic matter of phosphorites consists of carbonate-fluorapatite and is intermixed with variable amounts of terrigenous, biogenic and diagenetic non-phosphatic components, which are the cause of a wide range of fluctuations in their chemical composi‐ tions. The age of sea-floor phosphorites varies from Cretaceous to Recent. Recent phosphor‐ ites are localized in the south west of Africa and at Peru-Chile shelves, which are the areas influenced by strong upwelling of nutrient-rich waters (**Fig. 4**(**a**)), resulting in high biologi‐ cal productivity, intensive biogenic sedimentation and diagenetic redistribution of geochem‐ ically active, mobile, organic-derived phosphorus in sediments. This phosphorus is accreted in the form of initially soft and friable nodules undergoing gradual lithification [27],[37].

Pronounced climatic, biological and geologic effects accompany upwelling, especially where it is produced by the divergence in coastal areas (**Fig. 4**(**b**)). The presence of cold waters along the coasts produces the coastal fogs and humid-air deserts, such as those of northern Chile and southwest Africa. The nutrient-rich waters that lie alongside these deserts are the lushest gardens of the sea, as the upwelling cold waters there support tremendous quantities of organisms. Most of large accumulations of guano (**Section 7.2.2**) are formed by the seafowl colonies feeding in these waters, and it is the extremely dry climate created by upwelling that makes the preservation of guano possible [27].

Sedimentary deposits usually contain varieties of carbonate-fluorapatite called francolite (described in **Section 2.6**). Francolite is defined as apatite that contains significant amount of CO2 with less than 1% of fluorine. Apatite associated with igneous source rocks may be of primary magmatic, hydrothermal or secondary origin. Primary apatite from igneous sources may be of fluorapatite, hydroxylapatite or chlorapatite varieties. Pure apatites from igneous deposits contain slightly over 42% of P2O5 [23].

Since a wide range of very different particles and processes offormation complicates the simple classification of phosphorites, there is not any unified phosphorite classification8 [18],[36],[38]. The proposed classification schemes for phosphorites describe their constituent particles, such as pelletal phosphorite. Nonetheless, the descriptor pelletal indicates nothing more than rounded phosphate particles of any origin. A widely recognized distinction in phosphorites is based on the grain size and holds specifically among phosphorites where the phosphate particles are of sand- or coarse silt-size and those that are of clay- and fine silt-size [28]. The phosphorites can be classified as follows [37]:

	- **a. Ferruginized** with glazed surface. The cement of ferruginized phosphorites is much richer in finely dispersed goethite than that of non-ferruginized phosphorites. Furthermore, ferruginized phosphorites do not contain the fragments of macrofauna.
	- **b. Non-ferruginized** with rough surface. In non-ferruginized phosphorites, the cement is micrite-collophane and its color varies form yellow (collophane, described in **Section 2.6**) to gray (micrite9 [19]). The chambers of foraminifera are filled with phosphate-carbonate cement, less often with glauconite or goethite (FeO(OH) [33]).

Some nodules of phosphatized limestone are coated with a discontinuous layer of secondary phosphate with the thickness up to 1 cm.

**2. Conglomeratic phosphorites** consist of pebbles of phosphatized limestone (up to 50% of the rock) held together by the cement similar in composition to the phosphatized glauconite-quartz sandstones described above. In many samples of this type, two orthree conglomerate layers are clearly visible, differing in size of pebbles and in content of glauconite. The bedding planes separating the layers with denser or less dense packing of grains are also distinguished in the cement. The surface of these planes is glazed and brown due to higher content of iron hydroxides and organic matter. Upon impact, the rock breaks along the planes. In addition, irregular microerosion surfaces are observed in

<sup>8</sup> The nature and origin of phosphorites have been a matter of much speculation since they were first discovered more than 150 years ago [38], and there is not any commonly accepted nomenclature [39].

<sup>9</sup> Micrite (the abbreviation for microcrystalline calcite) is characterized by crypto- to microcrystalline crystal texture. As the synonym of micrite, the names as lime mud, lime ooze, lime mudstone, calcimudstone and calcilutite are also used. The original definition sets a grain-size limit to < 4 μm, but current terminology distinguishes between minimicrite (<1 μm), micrite I (1 – 4 μm) and micrite II (4 – 30 μm). Furthermore, primary micrites (orthomicrites and nannoagorites), secondary micrites and pseudomicrites are recognized. Orthomicrites consist of subhedral polygonal calcite grains meeting at the interfaces. Nannoagorites are composed of calcareous pelagic biota. Secondary and pseudo micrites result from the diagenetic processes [19].

the conglomeratic phosphorites, which run across the grains of glauconite, shells and bedding planes [33].

The investigation of the microstructure of phosphorites by electron microscopy enables to recognize the following varieties [33]:


CO2 with less than 1% of fluorine. Apatite associated with igneous source rocks may be of primary magmatic, hydrothermal or secondary origin. Primary apatite from igneous sources may be of fluorapatite, hydroxylapatite or chlorapatite varieties. Pure apatites from igneous

Since a wide range of very different particles and processes offormation complicates the simple

The proposed classification schemes for phosphorites describe their constituent particles, such as pelletal phosphorite. Nonetheless, the descriptor pelletal indicates nothing more than rounded phosphate particles of any origin. A widely recognized distinction in phosphorites is based on the grain size and holds specifically among phosphorites where the phosphate particles are of sand- or coarse silt-size and those that are of clay- and fine silt-size [28]. The

**1. Non-conglomeratic** (also termed as **nodular**) phosphorites: consist of phosphatized limestones and gluconate-quartz sandstones. Two varieties of nodular phosphorites can

**a. Ferruginized** with glazed surface. The cement of ferruginized phosphorites is much richer in finely dispersed goethite than that of non-ferruginized phosphorites. Furthermore, ferruginized phosphorites do not contain the fragments of macrofauna.

**b. Non-ferruginized** with rough surface. In non-ferruginized phosphorites, the cement is micrite-collophane and its color varies form yellow (collophane, described in

Some nodules of phosphatized limestone are coated with a discontinuous layer of

**2. Conglomeratic phosphorites** consist of pebbles of phosphatized limestone (up to 50% of the rock) held together by the cement similar in composition to the phosphatized glauconite-quartz sandstones described above. In many samples of this type, two orthree conglomerate layers are clearly visible, differing in size of pebbles and in content of glauconite. The bedding planes separating the layers with denser or less dense packing of grains are also distinguished in the cement. The surface of these planes is glazed and brown due to higher content of iron hydroxides and organic matter. Upon impact, the rock breaks along the planes. In addition, irregular microerosion surfaces are observed in

<sup>8</sup> The nature and origin of phosphorites have been a matter of much speculation since they were first discovered more

<sup>9</sup> Micrite (the abbreviation for microcrystalline calcite) is characterized by crypto- to microcrystalline crystal texture. As the synonym of micrite, the names as lime mud, lime ooze, lime mudstone, calcimudstone and calcilutite are also used. The original definition sets a grain-size limit to < 4 μm, but current terminology distinguishes between minimicrite (<1 μm), micrite I (1 – 4 μm) and micrite II (4 – 30 μm). Furthermore, primary micrites (orthomicrites and nannoagorites), secondary micrites and pseudomicrites are recognized. Orthomicrites consist of subhedral polygonal calcite grains meeting at the interfaces. Nannoagorites are composed of calcareous pelagic biota. Secondary and pseudo micrites result

phosphate-carbonate cement, less often with glauconite or goethite (FeO(OH) [33]).

[19]). The chambers of foraminifera are filled with

[18],[36],[38].

classification of phosphorites, there is not any unified phosphorite classification8

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

deposits contain slightly over 42% of P2O5 [23].

phosphorites can be classified as follows [37]:

**Section 2.6**) to gray (micrite9

secondary phosphate with the thickness up to 1 cm.

than 150 years ago [38], and there is not any commonly accepted nomenclature [39].

be recognized:

from the diagenetic processes [19].


With regard to their texture and petrographic character, phosphorites can be classified according to the predominant size of the phosphorite component into four types [40]:


Although the types are named on the structural basis, the phosphate grains do not always have the dimensions given above. At the same time, the classified types fairly differ in many features, such as the association with various geological formations, the phosphate mineralo‐ gy and the stratigraphic sequence, thus being of lithologic character [40].

Most attempts to classify the phosphorite rocks adopt and modify the classification scheme for carbonates [36]. In 1962, DUNHAM [41] published the classification scheme for limestone. This scheme for carbonate rocks was modified for non-genetic classification of phosphorites (**Fig. 5**) [28].


**Fig. 5.** The classification scheme of carbonate rocks modified for phosphorites [28].

The macroscopic classification scheme for phosphate sediments suggested by RIGGS [42] is shown in **Fig. 6**(**a**). The ancient deposits are better characterized via the scheme in **Fig. 6**(**b**), which was proposed by KASTERN and GARRISON [43]. In this model, three types of phosphates were recognized [39],[43]:


CFAP cements in P- and D-phosphates are often replaced microbial structures, but our data do not reveal whether this microbial involvement was passive or active. F-phosphates are most common in deeper water, outer-shelf/upper-slope sites, whereas D- and P-phosphates tend to predominate at shallower shelf sites more subjected to episodic high-energy conditions, especially during the low stands of sea level. This concept reveals the paleoenvironmental and time relationships of various phosphate sediments [39],[43].

**Fig. 6.** The classification scheme of phosphorites proposed by RIGGS (a) and GARRISON and KASTNER [ 43 ].

Phosphorites can be formed in nature **authigenically** or **diagenically**. In authigenesis, phosphorite forms as a result of the reaction of soluble phosphate with calcium ions forming corresponding insoluble phosphate compound. The role of microbes in these processes may be one or more of the following [44]:


**Fig. 5.** The classification scheme of carbonate rocks modified for phosphorites [28].

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

were recognized [39],[43]:

The macroscopic classification scheme for phosphate sediments suggested by RIGGS [42] is shown in **Fig. 6**(**a**). The ancient deposits are better characterized via the scheme in **Fig. 6**(**b**), which was proposed by KASTERN and GARRISON [43]. In this model, three types of phosphates

**i.** F-phosphates are friable, light-colored micronodules and peloids of carbonate-

**ii.** Phosphatic sands, termed as P-phosphates, consist of phosphatic peloids, coated

**iii.** Dark and dense phosphates, herein called D-phosphates, are the most abundant.

CFAP cements in P- and D-phosphates are often replaced microbial structures, but our data do not reveal whether this microbial involvement was passive or active. F-phosphates are most common in deeper water, outer-shelf/upper-slope sites, whereas D- and P-phosphates tend to predominate at shallower shelf sites more subjected to episodic high-energy conditions, especially during the low stands of sea level. This concept reveals the paleoenvironmental and

diatom muds deposited within the oxygen-minimum zone.

sands occur in thin layers and burrowed beds up to 2 m thick.

time relationships of various phosphate sediments [39],[43].

fluorapatite (CFAP); they were formed by the precipitation of CFAP in laminated

grains and fish debris, often having an admixture of fine siliciclastic grains. These

They occur as nodules, gravels and hard grounds. These phosphates were formed through complicated cycles of CFAP precipitation during early diagenesis, erosion and exhumation and reburial and rephosphatization processes associated with changing energy conditions, which may reflect the effects of changes in the sea level.

> **c.** Generating or maintaining the pH and redox conditions, which favor the precipitation of phosphate.

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

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,

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

contribute to the phosphorite formation. Locally elevated, excreted orthophosphate becomes available for the precipitation as phosphorite by reacting with seawater calcium [44].

Authigenic phosphorite formation at some eastern continental margins, where upwelling, if occurred at all, was a weak and intermittent process that may have been formed more directly as a result of intracellular bacterial phosphate accumulation, which became transformed into carbonate-fluorapatite upon the death of cells accumulated in sediments in areas where the sedimentation rate was very low [44].

The model of diagenetic formation of phosphorite generally assumes the exchange of phosphate for carbonate in accretions that have the form of calcite and aragonite. The role of bacteria in this process is to mobilize phosphate by mineralizing detrital organic matter. The demonstration of this process in marine and freshwater environment under laboratory conditions leads to the hypothesis that the diagenesis of calcite to form apatite explains the origin of some deposits in the North Atlantic. The phosphorite deposits of Baja California and in the core of eastern Pacific Ocean seem to have formed as a result of partial diagenesis [44].
