Geological and Mineralogical Analysis of Phosphorites in the Jebel Dhyr Syncline, Eastern Algerian Atlas

The sustainability of modern human civilization depends heavily on the availability and management of Earth’s natural resources (Kallel et al. 2017; Besser et al. 2021; Ncibi et al. 2021; Bagwan et al. 2023). Nearly every industry requires minerals as essential inputs for their operations (Zeqiri et al. 2019; Fredj et al. 2020; Kerbati et al. 2020; Zerzour et al. 2020, 2021). Valuable chemical elements and useful minerals can be found in rocks, soils, and water (Nekkoub et al. 2020; Benmarce et al. 2021; Orabi et al. 2023; Sankar et al. 2023). Phosphorite is a vital resource that is essential for agriculture, as it is a primary source of phosphorus for fertilizers (Gadri et al. 2015; Rais et al. 2017; Dahoua et al. 2018; Anis et al. 2019; Barra et al. 2019; Zhang et al. 2019).

Phosphorus (P) is an indispensable component of all living organisms, including plants and animals (Crosby, Bailey 2012). Due to their significance as agricultural fertilizers, phosphate minerals are widely utilized, thereby making their exploration, exploitation, and processing highly valuable (Barra et al. 2019; Boulemia et al. 2021). Sedimentary phosphates, which possess high concentrations of phosphorus, represent the most crucial mineable phosphate ores globally (Yi et al. 2013; Buccione et al. 2021). Phosphorite mineralization can be found in strata spanning from the Precambrian to the Holocene, but it appears to be particularly abundant from the late Cretaceous to the Eocene (Notholt 2005). The vast Tethyan phosphogenic province, which stretches from Morocco in the west to Iraq and Turkey in the east, is particularly noteworthy, as it is estimated to hold approximately 70% of the world’s phosphorite reserves (Abed et al. 2005; Garnit et al. 2017; Hamed et al. 2017, 2018; El Bamiki et al. 2021; Zhang et al. 2021).

Phosphorite is a marine sedimentary non-detrital deposit containing at least 18 wt% of P₂O₅ (Gál et al. 2020). The authigenic phosphatic particles in sedimentary rocks are present as concretions (pellets, nodules and crusts), composed mainly of apatite group minerals typically in cryptocrystalline masses (grain sizes < 1 μm) referred to as ‘collophane’. They are hosted in calcareous or siliceous groundmass, commonly with brownish, yellowish or whitish colour (Boulemia et al. 2015).

The classification of the apatite group minerals is based on the presence of chloride, fluoride, carbonate, hydroxyl, and oxygen anions. The most abundant pure stoichiometric apatite are fluorapatite Ca₅(PO₄)₃F, hydroxylapatite Ca₅(PO₄)₃(OH) and chlorapatite Ca₅(PO₄)₃Cl (Fleet, Liu 2005) however ‘carbonate-apatite’ Ca₅(PO₄,CO₃)₃(OH,F) is the main phosphate mineral (Sengul et al. 2006). Generally, the hexagonal crystal size of this apatite is about a few microns (Dar et al. 2017). Silica, clayey and calcareous materials constitute the gangue matrix for phosphates of sedimentary origin. The most sedimentary calcareous phosphate rocks have significant amounts of carbonates, and are considered as carbonate-apatite or francolite (Jaballi et al. 2019; Abou El-Anwar, Abd El Rahim 2022). To date, not a lot of studies on phosphorites have been undertaken in the transboundary Tebessa El Gasrine collapsed basin. We enumerate the bio-stratigraphic study of Flandrin (1948), the petro-mineralogical contribution of Boulemia et al. (2015), and the geochemical analyses of Kechiched et al. (2020) and Ferhaoui et al. (2022), which advances our contribution as the first research that deals with the qualitative analysis of the Jebel Dhyr syncline phosphorites. The questions asked are: Are the phosphorites of the Tebessa-Casserine collapsed basin mineralogically monotonous? Are they of the same family as those of North Africa? To answer these questions, our research focuses on the mineralogical characterization of the phosphate rocks of the Jebel Dhyr syncline. The outcome of the present work permits better elucidation of the mineralogy of the phosphorite rocks in order to recognize its detailed composition (bulk rock and phosphatic allochem grains) and contribute to deducing the diagenetic history and the depositional environment.

In order to examine the geological context and gather representative samples of phosphatic rocks in the study area, a fieldwork expedition was carried out. A total of 22 samples were carefully collected from Paleocene-Eocene phosphatic outcrops situated along the southern, eastern, and western escarpments of the Jebel Dhyr syncline. This sampling strategy aimed to ensure a diverse representation of phosphatic formations within the study area, enabling a thorough analysis of their petrographical, mineralogical, and geochemical properties. The collected samples serve as valuable specimens to advance our understanding of the phosphatic rocks in this region (Fig. 3).

Figure 3.

Petrographic studies were performed using a polarizing optical microscope of the Leica DMLP type. Thin and polished sections were prepared to facilitate detailed petrographic analysis.

Geochemical analysis of the collected samples was conducted to determine the major elements using Perkin Elmer flame atomic absorption spectrometry. The samples were initially dissolved by melting with strontium metaborate and then further dissolved in 2% nitric acid. Fluorine content was measured using a potentiometer after mobilizing this element with concentrated hydrochloric acid. A Metrohm instrument equipped with a specific fluorine electrode was utilized for this purpose. Loss on ignition was determined through calcination at 1000°C.

The used protocol for XRD analysis for the mineralogical identification of phosphorites involves following key steps. Initially, friable phosphatic sediments are processed to extract phosphatic allochem grains, including pellets, coprolites, intraclasts, and shark teeth, using an optimized screening method after simple disintegration. The disintegrated sediments are then subjected to sieve analysis using metallic sieves following the ASTM standard (ASTM D6913). After careful sorting under binoculars, the phosphatic grains, such as pellets, coprolites, granules, and fish teeth, are isolated and crushed to a particle size of 0.3 mm.

Next, X-ray diffraction measurements of the natural samples, in the form of multiphase powder, are recorded using an X’Pert Pro PANalytical diffractometer operating at 45 kV and 40 mA with Cu Ka radiation. The samples are scanned in the 2θ range from 5 to 75° (2θ) with a step size of 0.017°/s per step.

For the identification and quantification of different crystalline phases, a standard acquisition approach is employed, with parameters set at a step size of 0.02° and a time/number of scans of 20s. The resulting X-ray data are then interpreted using the “HighScore Plus” software. This conventional XRD protocol provides valuable information about the mineralogical composition of phosphorite samples, aiding in the accurate identification and characterization of the crystalline phases present.

The geological cross-sections conducted along the outcrops of Jebel Dhyr provide insightful information about the composition and structure of the phosphorite deposits in the region. These sections reveal the presence of sub-tabular phosphorite layers, which are relatively friable and have a thickness of several decimetres. These phosphorite layers alternate with layers of limestone, marl-limestone, and flint limestone located at the uppermost section. The observed alternation of different rock formations signifies the complex sedimentary history and depositional environment of the Paleocene-Eocene era in this area. These findings contribute to a better understanding of the geological context and distribution of phosphorites within the Jebel Dhyr region (Fig. 2).

The collected samples are mainly distributed over sandy phosphorite, calcareous phosphorite, and phosphatic mudstone rocks. On the other hand, the phosphatic allochems are represented by the following particles (Fig. 4): coprolites of various shapes, pellets of arenite sizes, fish teeth completely phosphate of varying shapes, and intraclasts (granules), which are hard to break.

Figure 4.

The petrographic examination revealed that the texture of the phosphate samples predominantly represented the grainstone facies (Fig. 5). Within this facies, all samples exhibited a high percentage of allochems compared to the matrix.

Figure 5.

The dominant phosphatic particles in this facies were pellets, which lacked concentric structures and were commonly referred to as pseudo-ooliths (Fig. 2). These pellets varied in colour from brownish white to brown and even almost black, depending on the organic matter content. They displayed a structure-less, sub- to well-rounded shape, often oval, and were relatively uniform in size, with diameters ranging from 200 to 250 μm. The particles exhibited polished and smooth surfaces due to remobilization from the original depositional environment (Chabou-Mostefaioui 1987; Ben Hassen et al. 2011). The phosphatic ooids within the pellets sometimes contained multiple cores, and these cores were also coated ooids. Some pellets showed a relict structure of organic material and consisted of apatite, commonly referred to as “collophane” (Boulemia et al. 2021). Apart from the presence of pellets, the samples also contained coprolites, which are fossilized excrement particles. These particles, originating from fecal matter, exhibited a diverse array of shapes and sizes, often surpassing 250 μm and occasionally even extending to a few millimetres. Their colours varied, spanning from a pale yellowish hue to a deep, darker shade (Boulemia et al. 2015).

The phosphorites also contained intraclasts, which were characterized by angular to irregular shapes and had internal microstructures (up to 1 cm in diameter). Skeletal fragments, including bones and teeth, were observed within the phosphorites. These biogenic phosphates consisted of well-preserved triangular teeth derived from sharks. Petrographically, the internal part of the phosphate particles consisted of microcrystalline translucent apatite or noncrystalline (amorphous) material (Fig. 5). Some skeletal fragments were replaced by microcrystalline quartz, indicating silicification. Silicification was also observed in certain phosphate particles within the cherty phosphate horizon. The cement within the phosphorites was calcareous (sparite) and occasionally siliceous, resulting from diagenetic calcitization and partial silicification (Abed, Al Agha 1989; Dar et al. 2015).

The major element compositions of 14 phosphorite samples are presented in Table 1. The phosphate beds at Jebel Dhyr are generally compared to the Kouif phosphorite horizons in the northern-eastern parts of the Tebessa region (Boulemia et al. 2015). The P₂O₅ content exhibited a narrow range, varying from 20.60 wt% to 25.80 wt%. This limited range of P₂O₅ content in the studied samples is primarily attributed to the dominance of the apatite group. On the other hand, the beds exhibited higher CaO content, ranging from 32.00 wt% to 35.10 wt%. Phosphorites are predominantly composed of apatite minerals, which consist of phosphorus (P) and calcium (Ca). Additionally, a significant amount of calcium is present in the form of carbonates such as calcite and dolomite (Boulemia et al. 2021). The CO₂ content ranged from 10.7 wt% to 12.60 wt%. McConnell (1973) suggested that the carbonate anionic group partly substitutes for the PO₄ group in apatite. In the studied samples, CO₂ is mainly found in the carbonate and phosphate (francolite) facies.

The MgO content in the samples varied from 1.80 wt% to 3.00 wt%. The lower MgO content suggests limited dolomitization and indicates the formation of phosphatic grains in low MgO seawater (Baioumy 2007). The silica phase (SiO₂) exhibited remarkable contents, ranging from 9.2 wt% to 10.3 wt%. The high silica content in the chemical analysis is attributed to the presence of biogenic quartz and partial or complete silicification (chert) of certain layers (Boulemia et al. 2021). The concentrations of K₂O, Na₂O, Al₂O₃, and Fe₂O₃ were relatively low. This can be attributed to minor occurrences of clay minerals, which do not possess a phosphatic phase.

The SO₃ content was relatively high, ranging from 10.10 wt% to 12.40 wt%. Sulphur may occur in the apatite structure as a normal constituent of the phosphate lattice or in low amounts as gypsum, as detected by X-ray diffraction. The (PO₄) group in apatite can also be partially replaced by SO₄ (Stowasser 1975). The fluorine content ranged from 2.60 wt% to 6.20 wt%. Fluorine is present in the apatite lattice and can occur freely or in association with (OH) and O ions (McConnell 1973). Apparently, phosphorites of Jebel Dhyr syncline are more sulphurized and fluorinated than those of El Kouif locality (Boulemia et al. 2021).

The mineralogical analysis conducted reveals significant insights into the composition of the studied samples. The analysis indicates the presence of several mineral phases within the samples, shedding light on the diverse geological processes and environments that have influenced their formation. The identified minerals include prominent groups such as phosphates, sulphates, sulphides, tectosilicates, phyllosilicates, and native elements like graphite. These findings demonstrate the complex nature of the mineral assemblages found within the phosphorites of Jebel Dhyr. Furthermore, the results of the mineralogical analysis provide a foundation for further research into the origin, evolution, and potential applications of these mineral resources.

4.1.

Bulk rock composition

The representative samples of phosphate from phosphatic layers were analysed using XRD. This showed that samples are mainly composed of apatite group (hydroxylapatite, francolite, fluorapatite, and dahllite), associated with carbonates (calcite, magnesium calcite, dolomite, and ankerite), silica (quartz and cristobalite - tridymite), sulphides (pyrite-cinnabar-covellite) and gypsum.

Hydroxylapatite Ca₅(PO₄)₃(OH): The XRD of bulk rock phosphorite has shown the almost systematic presence of hydroxylapatite. The main lines of the mineral occurring in the studied diffractograms correspond to the lattice planes or most intense peaks as follows: (211), (300), (112), (202), (102), (210), (130), (222), (100), (312), (213), (200), (231), (140), (402), (004), (502) and (002). The predominant mineral occurs either single or associated with fluorapatite, francolite, and dahllite. Other non-phosphate minerals, gangue constituents’ carbonates, silica, sulphates, and sulphides are coexisting.

Francolite (Ca,Mg,Sr,Na)₁₀(PO₄,SO₄,CO₃)₆F_2-3 is a carbonate-rich fluorapatite and has substitutions occurring in all sites (i.e., Ca, PO₄ and F sites; Jarvis et al. 1994). It appears at the phosphorite layers and is associated mainly with hydroxylapatite. Apparently, it features less quantitative dominance in our field compared to those of North Africa.

Fluorapatite Ca₅(PO₄)₃F: This mineral, associated with hydroxylapatite, is apparently mainly characterized by the wealthy bioclasts levels (tooth and bone fragments).

Dahllite Ca₅(PO₄,CO₃)₃(OH,O): This mineral of calcium-phosphate presents sodium carbonate crystallographic parameters (a and c) relatively higher than those of other apatites (Table 2). The dahllite is rarely found in the XRD records associated with hydroxylapatite and quartz. Substitutions that affect the sedimentary apatite are frequent and directly influence the symmetry of the crystal lattice which result in changes to the parameters a and c. The decrease in the crystallographic parameter of the apatite may be due to the substitutions F-OH, O-2OH, Ca-Na, and PO₄-CO₃.

Table 2.

Crystallographic parameters of the crystalline unit cell of apatite group in phosphorites from the Jebel Dhyr syncline, Eastern Algerian Atlas.

Mineral	a or b (Å)	C (Å)	c/a	Vol. of unit cell	Chemical formula
fluorapatite	9.371	6.885	0.734	523.12	Ca5(PO4)3F
hydroxylapatite	9.352	6.882	0.735	521.26	Ca5(PO4)3(OH)
francolite	9.340	6.880	0.736	519.77	(Ca,Mg,Sr,Na)10(PO4,SO4,CO3)6F2-3
dahllite	9.450	6.885	0.725	532.09	Ca5(PO4,CO3)3(OH,O)

Carbonate sediments are composed mainly of calcite, ankerite, dolomite, and magnesium calcite. The calcite is characterized by its main line (104) at a distance of 3.036 Å. Sometimes the main peak (2Θ 29.395°) shows a slight shift towards higher angles (Fig. 6, Table 3). This displacement results from the incorporation of magnesium and/or iron in the calcite structure. Indeed, the magnesium calcite is marked to 2Θ 29.500°, dolomite to 30.815°, and ankerite to 30.918° (Table 3). Associations calcite-ankerite, calcite-dolomite, and magnesium calcite are recorded in the same samples that provide information on metasomatic phenomena — dolomitization. It is important to highlight that cherty layers are characterized by the presence of dolomite and magnesium calcite, which are often associated with tridymite. Furthermore, as was encountered rarely, a sodium aluminium carbonate hydroxide “dawsonite” NaAl(CO₃)(OH)₂, is associated with the zeolite and albite.

Figure 6.

Table 3.

X-ray diffraction parameters of carbonate group in phosphorites from Djebel Dhyr syncline, Eastern Algerian Atlas.

Mineral	hkl	d (Å)	2Θ [deg]	Geochemical formula
calcite	104	3.0360	29.395	CaCO3
magnesian calcite	104	3.0254	29.500	(CaMg)CO3
dolomite	104	2.8893	30.815	CaMg(CO3)2
ankerite	104	2.8890	30.918	Ca(Fe,Mg)
				(CO3)2

Quartz α is identified by its main line 3.33 Å at 2Θ 26.73°. It is often associated only with apatite (hydroxylapatite, francolite, fluorapatite, and dahllite) and sometimes in the presence of ankerite. It is probably of a detrital origin. However other varieties of silice (cristobalite-tridymite) met in high levels in cherty levels (interbed the dolomitic phosphates layers) accompanied dolomite and magnesium calcite. It is probably of a biochemical origin (Fig. 6).

Gypsum is recognized by its main diffraction peaks at 2Θ values of 11.59°, 20.74° and 29.16°. It is encountered in diffractograms, and observed at a single phosphatized sample, fluorine-sodium sulphate Na₃Ca₂(SO₄)₃F associated with graphite (Fig. 7, right). Presumably, the gypsum is of secondary type. This can result, in an arid climate, precipitation of rain water enriched with sulphates when runoff from overlying formations. Especially, the evaporite formations, which are one of the traits that characterize this region of diapirs of Algerian-Tunisian border.

Figure 7.

Sulphides: pyrite-cinnabar-covellite apparently, with low amounts (in traces) are detected on the X-ray records of different samples of the same vertical profile by well individualized peaks (Fig. 7, left). In fact, due to the presence of sulphate (SO₃) ions in the water, the pyrite could be precipitated at the time of phosphatization.

Zeolites (alumina silicates hydrated minerals): occasionally observed on the diffractograms are: cowlesite (CaAl₂Si₃O₁₀·6H₂O), yugawaralite (CaAl₂Si₆O₁₆·4H₂O), and offretite (KCaMg(Si₁₃Al₅) O₃₆·15H₂O). Indeed, these zeolites have seen associated with palygorskite and albite in the presence of apatite, (Fig. 8).

Figure 8.

Clays; palygorskite (Mg,Al)₂Si₄O₁₀(OH)·4(H₂O) was the sole encountered phyllo-silicates. It was recognized by its main line position (peak) with d (10.12 Å) at 2Θ 8.73° on diffractograms. The other clay types are absent which requires a priori oriented thin section, heated and glycolic, (Fig. 8, left).

Feldspar: albite (Na,Ca)(Si,Al)₄O₈, is detected and occasionally associated with yugawaralite and dawsonite. This alumino-sodic mineral shows its maximum intensity at 2Θ 29.7°. Its magmatic origin seems more logical, especially in the presence of zeolites (Fig. 8, right).

Graphite: This carbon hexagonal system is recognized by its main peak (002) at 2Θ 26.6°. Its presence is rarely associated with apatite fluoride and hydroxylapatite. Its existence indicates a confined environment rich in organic matter.

The existence of minerals such as zeolites, palygorskite, dawsonite and sodium-fluorinated sulphate in the study area is due to the enrichment of the depositional environment in sodium, aluminum, magnesium, potassium and silica. The origin of this type of minerals in the Tebessa-El Kasserine collapsed basin is very controversial, but the most likely is the volcanic origin as interpreted by Sassi (1974) in the Gafsa mining basin.

Table 4 provides a summary of mineral phases identified by light microscopy, X-ray diffraction studies and macroscopics observations, in phosphorites from Jebel Dhyr syncline.

Table 4.

Recapitulation of mineral phases identified by light microscopy, X-ray diffraction and macroscopics observations, in phosphorites from the Jebel Dhyr syncline, Eastern Algerian Atlas.

Mineral groups encountered	Sampling type
	Bulk rock	Phosphatic allochems grains
apatite (hydroxylapatite, fluorapatite, francolite, dahllite*)	++	++
Carbonates (calcite, dolomite, magnesian calcite*, ankerite*, dawsonite*)	++	--
Sulfates (gypsum)	+	--
Sulfides (pyrite*-cinnabar*-covellite*)	+	--
Tectosilicates (quartz)	++	+
Tectosilicates (feldspar, zeolite*)	+	--
Clays (palygorskite*)	++	--
Native element (graphite)	+	--

AAAA* : Mineral detected only by XRD

++: abundant

+: present

--: absent

4.2.

Phosphatic allochem grains

The analyzed samples from the phosphorite layer consisted of representative allochems, including teeth, coproliths, granules, and pellets. XRD analysis revealed that these specimens were predominantly composed of apatites, with the exception of a few phosphatic pellets. The following are the obtained results:

Bioclasts are represented by fossil fish teeth (Salesians) of variable sizes and shapes and which are in the majority completely phosphated. XRD performed on samples of teeth from phosphates layers of Jebel Dhyr has overwhelmingly demonstrated the systematic presence of fluorapatite, Ca₅(PO₄)₃F.

Intraclasts and granules: They are hard to crack, beige or gray with a black patina and similar to large gravels and are not characterized by any typical shape. Diffractometry on granules (greater than 10 mm) of different shapes and sizes indicated that the latter are represented by the hydroxylapatite associated with fluorapatite or francolite (Fig. 9).

Figure 9.

Coprolits: particles greater than 2 mm, fossil’s of fecal origin, recognized widely of various shapes (Fig. 10, right). Coprolites are found in the state of fragments, thus supporting the hypothesis of the reworking at the origin of the accumulations of phosphorites. XRD analysis performed on isolated forms showed that coprolits consist of hydroxylapatite and fluorapatite, sometimes associated with dahllite.

Figure 10.

Phosphatic pellets: These arenite-and lutite-sized particles are spheric and/or ovoid in shape. These pellets carry organic matter at various rates, which is evident in their color ranging from light yellow to very dark or even opaque brown. The XRD analysis carried out on the powder of the (0.1-0.125 mm) sand fraction separated from the friable phosphorites revealed a mineralogical composition of hydroxylapatite and fluorapatite, sometimes associated with dahllite, in the presence of quartz (Fig. 10, left). Indeed, the presence of other non-phosphated mineral phases such as quartz was observed. This could be explained by the existence of non-apatitic minerals serving as nuclei for these ooids (type of pellets) with apatitic cortex.

The phosphorites investigated in the Jebel Dhyr syncline exhibit a homogeneous mineral composition, primarily comprising hydroxylapatite, fluorapatite, francolite, and dahllite. These apatite minerals display similar diffractometric characteristics, as shown by the hkl and d values in Table 5, with minor variations. The results suggest that the spatiotemporal distribution has minimal influence on the diffractometric properties. Regardless of sample location, X-ray analysis consistently reveals a remarkable consistency in the positions of the primary apatite diffraction lines, with only slight deviations within a few tenths of a degree.

Table 5.

Comparative X-ray diffraction parameters (d_hkl) of apatite group from phosphorites deposits in North Africa.

(hkl) (Miller index)	dhklof peaks (Å)
	J. Jebs, Tunisia (Beji-Sassi 1984)	Foum Selja,Tunisia (Chaabani 1978)	Morocco (El Haddi 2014)	Dhyr, Algeria (this study)
100	8.164	8.15	8.074	8.099
101	5.281	5.29	5.250	5.244
200	4.052	4.04	4.041	4.049
111	3.873	3.87	3.862	3.867
002	3.456	3.45	3.444	3.441
102	3.183	3.17	3.170	3.167
120	3.062	3.05	3.051	3.061
211	2.792	2.79	2.792	2.796
112	---	2.78	2.771	2.771
300	2.695	2.69	2.693	2.699
202	2.627	2.62	2.622	2.622
301	2.507	2.51	2.510	2.513
122	2.284	2.28	2.282’	2.287
310	2.240	2.24	2.240	2.240
131	2.130	2.13	2.131	2.135
113	2.062	2.07	2.061	2.059
203	1.997	---	1.996	1.995
222	1.931	1.93	1.930	1.933
132	1.879	1.87	1.880	1.880
123	1.836	1.84	1.836	---
231	1.788	1.77	1.790	1.790
410	1.762	1.76	1.765	1.766
303	1.741	1.73	1.743	---
004	1.722	1.71	1.723	1.720
322	---	---	1.632	1.634

The phosphorites of the Jebel Dhyr syncline are commonly interbedded with limestones, shales, and occasionally cherts. Petrographic examination of the phosphorites reveals similarities with the phosphates of the El Kouif locality, albeit with minor differences in the abundance of phosphatic particles and the nature of the orthochems. In fact, pellets, coprolites, bioclasts (including bone and teeth) and phosphatized intraclasts are relatively more present in Dhyr phosphorites than those of El Kouif locality. Moreover, the clayey and siliceous matrix is more predominant than calcareous (calcitic and dolomitic) cement in Dhyr phosphorites. Furthermore, the content of organic matter is with the same distribution within the various phosphate beds.

The high concentrations of P₂O₅ and CaO in the phosphorites indicate a greater abundance of apatite constituents. The sulphur content is associated with francolite and secondary gypsum formation. Additionally, significant amounts of calcium are present in carbonates such as calcite and dolomite. The low concentrations of K₂O, Na₂O, Al₂O₃, and Fe₂O₃ oxides may be attributed to minor occurrences of clay minerals.

The Paleocene-Eocene phosphorites samples of the Jebel Dhyr syncline were analyzed using the XRD technique to identify the multiphase structure and crystallographic parameters. The analysis of the bulk rock pattern, enriched in phosphate grains, enabled the identification of the phosphatic phase represented by the apatite group, which includes hydroxylapatite (Ca₅(PO₄)₃OH) as the most widespread species, as well as dominant francolite. The presence of francolite indicates the significant presence of carbonates in the phosphatic rocks. Other phosphatic components, such as fluorapatite, Ca₅(PO₄)₃F and dahllite, are mainly found in levels rich in bioclasts. Substitutions affecting sedimentary apatite are common and directly impact the crystal lattice symmetry. These apatitic components are primarily associated with carbonates (calcite-dolomite-ankerite) and/or silica, which form variable exogenous gangue. Associations such as calcite-ankerite, calcite-dolomite-calcite, and magnesium calcite were observed in the same samples, providing information on metasomatic phenomena such as dolomitization and dedolomitization.

At the cherty passages, opal-CT associated with dolomite is the main mineral. The enrichment in opal suggests high silica content in the depositional environment during the formation of these layers, possibly due to contributions from organisms with siliceous tests (radiolarians, diatoms, etc.).

Additionally, non-phosphate phases, including evaporated minerals (gypsum) and sulphide minerals (pyrite, cinnabar, and covellite), are observed in low representation in some samples. The presence of iron-sulphur and copper-sulphur minerals in the phosphatic rocks indicates highly reducing conditions during their formation. The presence of graphite (carbon) indicates anoxic conditions associated with phosphorite genesis.

The existence of minerals such as zeolites, palygorskite, dawsonite, and sod-fluorinated sulphate in the Tebessa-El Kasserine collapsed basin suggests environmental enrichment of sodium, aluminum, magnesium, potassium, and silica. Under favourable physicochemical conditions, these elements can combine to form these types of minerals. The origin of these chemical elements is likely associated with the alteration of igneous rocks, specifically ophites, found in the Tebessa region.

Allochems, which are various phosphatic components or grains separated from the phosphorites of the Jebel Dhyr syncline, include phosphorite pellets, phosphatized fossil teeth, coproliths, and granules. These constituents are dispersed within the clay matrix of these deposits. XRD analysis conducted on phosphatic allochems powder revealed a mineralogical composition consisting of hydroxylapatite with francolite or fluorapatite, sometimes associated with dahllite. Pelletal phosphorites are particularly common, and some are occasionally associated with quartz, suggesting the existence of non-apatitic cores serving as nuclei for these pellets with an apatitic cortex. This mineralogical composition, as highlighted by XRD analysis, is recorded for the first time in the phosphorites of the Jebel Dhyr syncline. These results demonstrate certain consistency regardless of the spatiotemporal distribution of the samples. In sum, with great mineralogical similarities, the phosphorites on Jebel Dhyr syncline belong to the same family as one of the Algerian deposits