Composition and Potential Industrial Uses of Upper Cretaceous Carbonates of the Wadi Sir Limestone (WSL) and the Amman Silicified Limestone (ASL) Formations, North Jordan

Abstract

Upper Cretaceous carbonate rocks in Jordan are the main resources for construction and paint-related industrial applications. This study evaluates the elemental composition, mineralogy, and petrography of two main geological formations from two localities in northern Jordan (Hallabat, Turonian age, and Ajlun, Santonian–Campanian age) to shed light on their composition, depositional environments, and potential industrial end uses. The elemental composition of the Hallabat Wadi Sir Limestone (WSL) Formation indicates notable variability between the middle and upper parts of the WSL carbonates in the area, with higher CaO content in the middle part (mean 55 wt.%) and higher silica content observed in the upper part (mean 2 wt.%) compared with the middle part (mean 0.9 wt.%). Meanwhile, analysis of the elemental composition of the Ajlun Amman Silicified Limestone (ASL) Formation indicates that the CaO content is relatively higher in the upper part (mean 56 wt.%). In addition, the lower part is more influenced by detrital input when compared with the upper part of the studied section, in contrast to the Hallabat WSL Formation. Petrographic analysis demonstrates that the WSL and ASL samples are predominantly micritic limestone. The XRD results for the Hallabat WSL and Ajlun ASL show that the mineralogical composition is dominated by calcite (CaCO₃). Statistical and PCA analyses also confirm these variabilities between the two sites, indicating that all samples from both sites were deposited under variable hydrodynamic and environmental conditions that affected their physical and chemical composition. The results show that all studied samples are in the range of pure limestone and can be used for specific industrial applications in addition to their current uses, including those in the pottery and porcelain ware, soda ash and caustic soda, steel industry, sugar, and textile production industries, thus contributing to the economic resources in Jordan.

1. Introduction

Carbonate rocks are primarily made up of particles (biological and chemical) with more than 50% carbonate minerals embedded either in clasts or in cement [1]. Calcareous organisms commonly form carbonate rocks through the accumulation of their bioclasts; thus, these rocks are primarily deposited in areas that are favorable for biological activities, such as shallow marine shelf environments [2,3]. Carbonates have been used as raw materials in industrial, construction, and environmental applications for decades [4,5,6].

Carbonate rocks are heterogeneous in nature, both physically and chemically. The main mineral—calcium carbonate (CaCO₃)—is often found in association with other detrital minerals such as clays and quartz [7]. Depending on their origin, depositional environment conditions, and composition, these rocks are deposited as limestones, travertine, marble, chalk, tufa, and coquina [8,9]. Carbonate rocks have different crystalline polymorphs, of which the anhydrous calcite (rhombic), aragonite (needle-like), and vaterite (spherical) varieties are the most common [10]. Amongst these, calcite is the most stable form of carbonate under ambient conditions [2]. The magnesium/calcium (Mg/Ca) content results in the precipitation of either dolomite (high MgCO₃ content) or low-Mg calcite (<5% MgCO₃ content) [11].

In industry, lime, cement, building stone, and crushed stone production relies primarily on carbonate rocks. These rocks and their derived products are commonly used as fluxes, aggregates, fillers, soil conditioners, and in many other industrial applications [6,12]. However, specific physical or chemical characteristics are required to properly implement these stones in the context of certain industrial applications. These include compositions with CaCO₃ > 93%, SiO₂ < 3%, MgO < 1.2% (and higher for certain products), Fe₂O₃ < 1.5 (and lower for certain products), and/or S, P₂O₅, and alkaline salt contents as low as possible [13,14].

In Jordan, the use of natural stones for construction dates back to the Neolithic (ca. 10,000–4500 years BC) [15]. Several ancient uses of natural stones as a building material can be clearly observed in the city of Petra, which was carved into sandstone during the Nabatenean times (first century BC); the Roman City of Jerash (ca. 6000 years ago); and the Byzantine and Islamic heritage found across Jordan [16,17]. Due to its abundance in the geological record, Jordanian limestone is one of the most-used types of stone for building during these periods and even up to the present time.

The carbonate rocks in Jordan include both the Ajlun and Belqa Groups [18,19]. These primarily carbonate rocks were deposited under various hydrodynamic and environmental conditions due to sea level fluctuations and tectonic activity [19,20]. These deposits are widespread in Jordan and record the environmental variations in the Tethys Sea area over time [21]. Upper Cretaceous Limestone Formations—namely, the Turonian Wadi As Sir Limestone (WSL) Formation (Ajlun Group) and the Santonian–Campanian Amman Silicified Limestone (ASL) Formation (Belqa Group)—are the main sources of Jordanian limestone.

The WSL Formation outcrops in several sites across Jordan and forms the uppermost part of the Ajlun Group. This succession primarily consists of bedded massive limestone, hard dolomitic limestone with thin layers of marls and chert nodules, mainly in the middle and upper parts of the formation. The fossil assemblages include bivalves and gastropods [22]. The depositional environments vary from lagoonal conditions to open-shelf conditions and shallow marine environments. The thickness varies from ca. 125 to 150 m [18,23,24]. The ASL Formation is also found in various localities across Jordan. This formation comprises silicified limestones that alternate with chert, phosphatic chert, and limestone. The fossil assemblages in this formation include foraminifera, ammonites, gastropods, and bivalves [21,25]. The formation in northern Jordan was deposited in a marine shelf environment.

These limestones have a suitable composition for industrial uses and have been used as building stones for decades [14,26], as well as in various industrial applications such as paint and concrete production. Nonetheless, based on observations reported by several researchers (e.g., [26]), buildings built using such stones have shown deterioration over time due to weathering; therefore, a more precise examination of the composition of these limestones is required. This study, focusing on two main localities in northern Jordan (Al Hallabat and Ajlun; see Figure 1), provides new insights into the physical and chemical composition of the WSL and ASL carbonate rocks. The two study sites are currently used for limestone quarrying.

Limestones from two main localities—namely, Hallabat and Ajlun—were investigated. A total of 12 limestone samples were collected from two quarries in the two study sites and analyzed for their mineralogical (XRD), petrographic (microscopic), and elemental (XRF) compositions. The samples were collected from four distinctive beds (two beds in each quarry), representative of the WSL and ASL Formations. This paper provides an overview of the study sites, followed by the methods employed. Then, the results are discussed and put into a wider context for comparison between the two studied quarries and the regional extent. Finally, the composition and potential industrial uses of the carbonates are explored and defined.

2. Study Sites

The two study sites are shown in Figure 1. These sites were selected as they represent two localities in which limestone is currently being quarried and used for different industrial purposes.

2.1. Al Hallabat Quarry

Al Hallabat is located in northern Jordan at the borders between Mafraq and Zarqa governorates. This area is well known for limestone quarrying. The area encompasses geological formations spanning Cretaceous to recent (Holocene) sedimentary rocks. The WSL Formation is primarily found in the central and western parts of the study area, with a thickness of ca. 82 m. The formation consists of yellowish to grey limestone and dolomitic limestone in the lower part and yellowish to whitish grey marly limestone and chalky limestone in the middle part. The upper part of the formation is composed of pale grey micritic massive fossiliferous limestone. These characteristics suggest that the formation was deposited in a shallow, open marine environment [27].

A limestone quarry was selected for sampling (Figure 2). The exposed outcrop within the quarry site is ca. 15 m thick and composed of two main beds (Appendix A). The lower bed (the middle part of the WSL Formation) is primarily composed of hard yellowish limestone. The upper bed is generally grey fossiliferous limestone, as reported in [27].

2.2. Ajlun Quarry

Ajlun is located in northern Jordan, and the outcropping succession dates back to the Santonian–Campanian age. The ASL Formation is primarily found in the central and northern parts of the quarry, with a thickness of ca. 70 m. As reported in [25], it is distinguished by chert beds, silicified limestone, phosphatic chert, and limestone (Figure 3). The exposed section within the sampled quarry is composed of ca. 20 m of very hard reddish fossiliferous limestone at the bottom, presenting a lithological change towards the upper bed, which is composed of softer yellowish marly limestone (Appendix A).

3. Methodology

3.1. Fieldwork and Sampling

The fieldwork included several field trips for site selection and sample collection. Six carbonate rock samples were collected from each site, for a total of twelve samples. Sampling was carried out after cleaning the weathered surface, ensuring that fresh samples were collected. The main limestone beds (two from each site) were sampled. Three samples from each bed were collected at a lateral distance of 25 m (Figure 4 and Appendix A). The sampling process was based on the exposed outcrops in order to ensure the representation of lateral changes in the rock’s characteristics.

3.2. Geochemistry

At the laboratories of the Environment, Water, and Energy Research Center at Al al-Bayt University, the samples were crushed to the size of 50 µm using a Herzog Jaw Crusher BB 100/200 and a Manual Pulverizing Mill (HSM 100/HSM 250).

Sub-samples were taken for further mineralogical and elemental analyses. For the mineralogical analysis, the samples were prepared after grinding to be analyzed using X-ray diffraction (XRD). The samples were pressed into the XRD sample holder after cleaning the holder parts to avoid contamination. The samples were then pressed flat and leveled using a pressing block, and the excess sample was brushed off. A spinning sample stage was used to prevent preferred orientation effects. After that, the mineralogical analysis was performed using a PANalytical X’Pert Pro PW3040/60 X-ray diffractometer (XRD) for X-ray powder diffraction at room temperature with CuKα radiation, voltage set to 40 kV, current set to 20 mA, range 2θ 5–72°, 0.0170 degree step size, and 1.9050 s steps. The diffractogram interpretation was carried out using the X’Pert PRO-PHILIPS High Score software (v 5.2)

For the elemental composition, the samples were prepared and analyzed using XRF. The sample preparation consisted of grinding the sample to a particle size of fewer than 100 μm. Then, 8 g of the milled sample was weighed and mixed homogeneously with 2 g of a binder. The next step included assembling the pressing die, which was performed by placing the press housing onto the base plate and positioning the pressing plate onto the housing. The sample mixture was introduced into the assembly, and the second plate was placed on top. Using a TP 20p Herzog Manual Pellet Press, the sample was pressed to a force of 200 N. Finally, the pressed sample was carefully removed and placed in the X-ray fluorescence (XRF) sample holder. The XRF results were obtained using a Philips Magix pw2424 X-ray fluorescence (XRF) wavelength spectrometer. The spectrometer was operated at a voltage of 50 kV and a current of 40 mA, and the major elements of limestone were detected using the Super Q Manager (v 3) software.

3.3. Petrography

Thin sections (slides) were prepared for the 12 samples at the laboratories of the Faculty of Earth and Environmental Sciences at Al al-Bayt University for mineralogical characterization using polarizing microscopes (of Leica type) at the Hashemite University in Jordan, allowing for the determination of optical and morphological properties (i.e., microfacies, textures, and diagenetic features of carbonates) [28].

3.4. Statistical Analyses

The results were statistically analyzed using the Paleontological Statistics (PAST) software (v.4.17) [29]. Descriptive statistics were calculated alongside correlation analyses to gain more insight into the variability between the two study sites. In addition, linear Principal Component Analysis (PCA) was applied to identify the main parameters governing the covariance in the composition of the carbonate rocks and to determine whether the samples from different sites possessed distinctive characteristics.

4. Results and Discussion

4.1. Elemental Composition

The six samples collected from the Hallabat area were analyzed for their elemental composition (

Table 1. Elemental composition of the Hallabat area carbonate rocks (wt.%).

Sample No.	SiO2	TiO2	Al2O3	Fe2O3	MnO	MgO	CaO	Na2O	K2O	P2O5
H1.1	0.408	0.002	0.072	0.098	0.006	0.236	57.382	0.109	0.028	0.016
H2.1	0.617	0.003	0.057	0.098	0.005	0.207	55.625	0.077	0.027	0.018
H3.1	1.770	0.006	0.124	0.105	0.006	0.235	53.757	0.133	0.043	0.020
Mean	0.932	0.004	0.084	0.100	0.006	0.226	55.588	0.106	0.033	0.018
H1.2	1.357	0.015	0.216	0.084	0.007	0.238	55.025	0.100	0.041	0.020
H2.2	0.215	0.008	0.047	0.031	0.011	0.145	57.439	0.097	0.020	0.016
H3.2	4.498	0.018	0.374	0.076	0.005	0.307	49.089	0.200	0.074	0.023
Mean	2.023	0.014	0.212	0.064	0.008	0.230	53.851	0.132	0.045	0.020

). The results indicated variability between the middle and upper parts of the WSL carbonates in the area. Samples H1.1, 2.1, and 3.1 represent the middle part, while samples H1.2, 2.2, and 3.2 represent the upper part. Generally, higher silica content was observed in the upper part (mean 2.023 wt.%) compared with the middle part (mean 0.9 wt.%). This was also associated with higher percentages of detrital elements such as Ti and Al. This suggests that the two parts were probably deposited under different hydrodynamic conditions, with the detrital contribution being higher during the deposition of the upper part of the formation. Furthermore, the CaO content was higher for the middle part (mean 55 wt.%).

The correlation analysis (

Table 2. Correlation matrix for the elemental composition of the Hallabat area carbonate rocks.

	SiO2	TiO2	Al2O3	Fe2O3	MnO	MgO	CaO	Na2O	K2O	P2O5
SiO2	1.00
TiO2	0.76	1.00
Al2O3	0.94 *	0.90 *	1.00
Fe2O3	0.10	−0.30	0.04	1.00
MnO	−0.48	−0.01	−0.40	−0.86 *	1.00
MgO	0.85 *	0.53	0.84 *	0.52	−0.78	1.00
CaO	−0.99 *	−0.72	−0.91 *	−0.16	0.53	−0.83 *	1.00
Na2O	0.94 *	0.65	0.85 *	0.01	−0.35	0.78	−0.89 *	1.00
K2O	0.99 *	0.77	0.96 *	0.19	−0.53	0.90 *	−0.97 *	0.92 *	1.00
P2O5	0.93 *	0.78	0.91 *	0.24	−0.53	0.81 *	−0.96 *	0.77	0.94 *	1.00

* Significant at p < 0.05.

) indicated several statistically significant correlations. The silica content co-varied with detrital elements such as Ti and Al, and positive correlations (p < 0.05) were observed between SiO₂ and Mg, Na, K, and P, while significant negative correlations were observed between CaO and all elements except for MnO. It has been previously reported in [20] that the lower part of the WSL Formation in the Al-Tayyar area—located in close proximity to the Hallabat area—may have been deposited under low-energy hydrodynamic conditions based on its microfacies and fossil content, in agreement with lower detrital input and higher levels of in-basin deposition. In addition, the upper parts of the WSL Formation in the same area were reported to have been deposited during a period of high-energy hydrodynamics with current activity [20], in agreement with higher detrital input and basin disturbance.

Six samples collected from Ajlun were also analyzed for their elemental composition (

Table 3. Elemental composition of the Ajlun area carbonate rocks (wt.%).

Sample No.	SiO2	TiO2	Al2O3	Fe2O3	MnO	MgO	CaO	Na2O	K2O	P2O5
A1.1	1.04	0.027	0.334	0.21	0.01	0.435	54.072	0.357	0.046	0.006
A2.1	2.088	0.022	0.385	0.181	0.01	0.571	55.621	0.339	0.088	0.003
A3.1	2.658	0.165	0.726	0.389	0.01	0.345	53.856	0.308	0.078	0.006
Mean	1.929	0.071	0.482	0.260	0.010	0.450	54.516	0.335	0.071	0.005
A1.2	0.376	0.006	0.154	0.121	0.005	0.238	56.047	0.331	0.007	0.004
A2.2	0.377	0.014	0.145	0.093	0.006	0.288	55.884	0.320	0.031	0.004
A3.2	0.108	0.005	0.017	0.183	0.017	0.190	56.121	0.312	0.002	0.002
Mean	0.287	0.008	0.105	0.132	0.009	0.239	56.017	0.321	0.013	0.003

). The results show that, in contrast with the Hallabat WSL carbonate rocks, the lower part of the Ajlun ASL carbonates was more influenced by detrital input compared with the upper part. Higher silica content was recorded in the lower part (mean 2.023 wt.%) compared with the upper part (mean 0.3 wt.%). In addition, higher percentages of Ti, Al, and Fe were present in the lower part, suggesting that the two parts were probably deposited under variable hydrodynamic conditions, with the detrital contribution being higher during the deposition of the lower part of the formation. Furthermore, the upper part’s CaO content was relatively higher (mean 56 wt.%).

The correlation analysis (

Table 4. Correlation matrix for the elemental composition of the Ajlun area carbonate rocks.

	SiO2	TiO2	Al2O3	Fe2O3	MnO	MgO	CaO	Na2O	K2O	P2O5
SiO2	1.00
TiO2	0.79	1.00
Al2O3	0.95 *	0.90 *	1.00
Fe2O3	0.79	0.93 *	0.86 *	1.00
MnO	−0.02	0.02	−0.12	0.32	1.00
MgO	0.67	0.12	0.52	0.19	−0.11	1.00
CaO	−0.68	−0.75	−0.83 *	−0.80	−0.02	−0.39	1.00
Na2O	−0.03	−0.42	−0.06	−0.29	−0.25	0.60	−0.20	1.00
K2O	0.94 *	0.59	0.84 *	0.57	−0.08	0.85 *	−0.59	0.16	1.00
P2O5	0.48	0.63	0.70	0.55	−0.40	0.24	−0.89 *	0.27	0.40	1.00

* Significant at p < 0.05.

) indicated that the silica content was positively correlated with detrital elements such as Ti and Al (p < 0.05). Unlike the Hallabat samples, the SiO₂ content only showed a positive correlation (p < 0.05) with K and negative correlations with Mn, Ca, and Na, likely suggesting that these elements are more related to in-basin depositional processes than basin in-wash. The negative correlations between these elements and other detrital indicators, such as Ti, Al, and Fe, support this observation. On the other hand, the statistically significant negative correlations between CaO with all elements suggest that it may have a specific depositional behavior compared with the others. This agrees with previous research reporting on the environmental deposition of the ASL Formation in northern Jordan; for example, in [21], it was reported that, based on microfossil and geochemical analyses, the upper parts of the ASL Formation show less submarine weathering and detrital indicators compared to the lower parts, where higher submarine and continental weathering is recorded.

These differences in the elemental composition between the two study sites were further investigated using PCA analyses. The PCA plot (Figure 5) clearly shows the distinction between samples from the two study sites, namely, the Hallabat samples (1–6) and the Ajlun samples (7–12). The two eigenvectors (PC1 and PC2) explain 76% of the variance within the dataset. The data form two primary directional trends: a horizontal spread along PC1 (−10 to +20) and a vertical spread along PC2 (−6 to +4.5), resulting in two main clusters (as indicated by the grey ellipses). PC1 (47%) represents mixing between carbonate minerals (CaO and MgO) and siliciclastic elements (SiO₂, TiO₂, Al₂O₃), indicating variable depositional environments and suggests an environmental gradient from deep marine (−20) to terrestrial influence (+20). On the other hand, PC2 (29%) reflects diagenetic and authigenic processes.

Hallabat samples (1–6) present low PC1 values (mean = −5.6; −8 to −3) compared to the Ajlun samples’ (7–12) PC1 values (mean = +4.8; +2 to +8), suggesting carbonate-dominated compositions and siliciclastic influence, respectively. For the diagenetic trends, the PC2 values for Hallabat (mean = −1.2, −6 to +1) and Ajlun (mean = 0.5, −1 to +2) suggest a wider range of diagenetic alteration compared to a more consistent diagenetic signature, respectively. The centroid distance between the two groups (10.63) indicates differences in depositional history, confirming that the two groups originate from two different geological formations, namely, the WSL and ASL Formations. The Hallabat samples show variable CaO and MgO contents and siliciclastic content with variable Fe₂O₃ content. In terms of depositional history, the Hallabat samples suggest more carbonate facies with variable detrital input, while the Ajlun samples represent transitional facies with terrigenous input.

4.2. Petrography

The petrographic analysis (Figure 6 and Figure 7) indicated that the main component of the studied carbonates is microcrystalline calcite (micrite). Referring to widely known classifications of carbonates [30,31], the Hallabat limestone is composed of >10% mud-supported allochems (grains), where the allochem types are bioclasts (>25%) and intraclasts (<25%). Therefore, according to [30], the rock class is Biomicrite. These findings are similar to previous studies reporting the composition of the WSL Formation to include various fossil assemblages, micrite, and lithoclasts [20,22].

The Ajlun samples contain small lithoclasts of ancient carbonate grains—pellets that are composed of fine-grained carbonates without nuclei—in agreement with the PCA results, where PC 3 (11%) positively correlates with CaO and detrital indicators such as TiO₂, likely suggesting that minor amounts of carbonates were supplied through basin in-wash processes. The cracks developed by dissolution are filled with coarse-grained recrystallized silica. The Ajlun limestone is composed of <1% allochems with spray patches. According to Folk (1959) [30], the rock class is Dismicrite. These findings are in agreement with previous research into the composition of the WSL Formation [20,22].

4.3. Mineralogy

The XRD results for the Halabat and Ajlun samples (Figure 8 and Figure 9) revealed that the mineralogical composition is dominated by calcite (CaCO₃), as observed from the diffractograms according to the calcite 2-theta diffraction angle peaks. These observations are in agreement with the elemental composition of these samples, where CaO is dominant with minor amounts of other elements, and with the petrographic results, in which micrite and calcium carbonate were found to dominate. One sample (H3.2) showed a quartz (Qz) peak (26.66°), which was supported by its high content of SiO2 (4.5 wt.%) when compared with the other samples.

4.4. Synthesis

The results obtained for the samples from the two study sites indicate that each of the investigated lithological formations possesses distinct physical and chemical characteristics. The studied samples revealed that the carbonates in these formations were influenced by various environmental, hydrodynamic, and limnological conditions. In turn, these conditions affected the physical and chemical characteristics of the carbonates, rendering them suitable for various industrial uses. The following sections of this paper discuss the current uses of these carbonates and explore additional potential uses based on their specific compositions.

4.5. Limestone Classification

Based on the work of [32], the Ca/Mg and Mg/Ca ratios are indicators that can be used to identify the type of limestone (

Table 5. Limestone classification based on the chemical composition (after [32]).

Nomenclature	Ca/Mg Ratio	Mg/Ca Ratio
Dolomitic Limestone	1.41–12.3	0.08–0.18
Magnesian Limestone	12.3–39.0	0.03–0.08
Pure Limestone	39.0–100.0	0.00–0.03

).

The Ca and Mg contents for the Hallabat and Ajlun samples were calculated using the CaO and MgO percentages, respectively, according to the atomic masses of the different elements (

Table 6. Chemical indicators for limestone types from the Ajlun (A) and Hallabat (H) study sites.

Sample No.	MgO	CaO	Ca	Mg	Ca/Mg	Mg/Ca	Nomenclature
Ajlun ASL Limestone
A1.1	0.435	54.072	38.391	0.262	146.361	0.007	Pure Limestone
A2.1	0.571	55.621	39.491	0.344	114.695	0.009	Pure Limestone
A3.1	0.345	53.856	38.238	0.208	183.804	0.005	Pure Limestone
A1.2	0.238	56.047	39.793	0.144	277.279	0.004	Pure Limestone
A2.2	0.288	55.884	39.678	0.174	228.474	0.004	Pure Limestone
A3.2	0.190	56.121	39.846	0.115	347.787	0.003	Pure Limestone
Hallabat WSL Limestone
H1.1	0.236	57.382	40.741	0.142	286.289	0.003	Pure Limestone
H2.1	0.207	55.625	39.494	0.125	316.403	0.003	Pure Limestone
H3.1	0.235	53.757	38.167	0.142	269.345	0.004	Pure Limestone
H1.2	0.238	55.025	39.068	0.144	272.223	0.004	Pure Limestone
H2.2	0.145	57.439	40.782	0.087	466.423	0.002	Pure Limestone
H3.2	0.307	49.089	34.853	0.185	188.272	0.005	Pure Limestone

). The Ca/Mg ratios for the Ajlun samples showed a range from 114.7 to 347.8, while the Mg/Ca ratios ranged from 0.003 to 0.009. There was a clear distinction between the lower and upper parts, where the lower part samples indicated lower Ca/Mg ratios and higher Mg/Ca ratios. Higher Mg content in the lower part suggests a lower precipitation/evaporation (P/E) ratio during deposition compared with a higher P/E ratio in the upper part. For the Hallabat samples, the results indicated that the lower bed (middle part) of the formation was deposited under a relatively higher E/P ratio compared with the upper part, with Ca/Mg ratios ranging from 188.3 to 466.4 and Mg/Ca ratios ranging from 0.002 to 0.005.

Generally, all samples presented Ca/Mg and Mg/Ca ratios in the range of pure limestone, with no indication of dolomitization. This can likely be attributed to the relatively high P/E ratio during the deposition of the two formations, regardless of slight changes in the environmental conditions during the deposition of the different beds.

In addition to the Ca/Mg ratios, the CaCO₃ content can be used as an indicator of carbonate purity (see, e.g., [33]). In [34,35], a limestone purity classification based on the CaCO_3, CaO, MgO, SiO₂, and Fe₂O₃ contents was proposed, which has been extensively used for the assessment of limestone purity [36,37].

The limestone purity of the Hallabat and Ajlun samples was assessed using these values, indicating that the middle part of the WSL Formation in the Hallabat area is of very high purity, while the upper part showed medium purity limestone (

Table 7. Limestone purity classification (after [34,35]). H: Hallabat, A: Ajlun. 1: lower bed, 2: upper bed.

Purity Classification		CaCO3	CaO	MgO	SiO2	Fe2O3
Very High Purity		> 98.5	> 55.2	< 0.8	< 0.2	<0.05
High Purity		98.5–97.0	55.2–54.3	0.8–1.0	0.2–0.6	0.05- 0.1
Medium Purity		97.0–93.5	54.3–52.4	1.0–3.0	0.6–1.0	0.1 -1.0
Low Purity		93.5–85.0	52.4–47.6	> 3.0	1.0–2.0	> 1.0
Impure		< 85.0	< 47.6	> 3.0	> 2.0	> 1.0
	H1	99.21	55.59	0.23	0.93	0.10	Very High Purity
	H2	96.11	53.85	0.23	2.02	0.06	Medium Purity
	A1	97.30	54.52	0.45	1.93	0.26	High Purity
	A2	99.98	56.08	0.24	0.29	0.13	Very High Purity

). This is in agreement with the higher detrital content in the middle part observed in the elemental composition of the beds (Table 1), which may have affected the purity of these rocks. In Ajlun, the lower bed of the ASL Formation resulted in high purity compared with the very high-purity limestone in the upper bed.

4.6. Potential Industrial Uses for the WSL and ASL Limestones

Specific physical and chemical characteristics govern the industrial end uses of limestones [12,13,14]. Several authors have reported on chemical compositions that are suitable for various industries. Starting with the lowest calcium carbonate contents, [38] reported that limestone with CaCO₃ > 89.3%, CaO > 50%, SiO₂ < 2%, Al₂O₃ < 1.5% and MgO < 1% is suitable for use in the sugar industry. In [39,40], it was indicated that limestones with CaCO₃ > 91%, CaO > 51%, SiO₂ < 6%, Al₂O₃ < 1.3%, Fe₂O₃ < 1%, and MgO < 2% are suitable for use in the steel industry. On the other hand, [40,41] reported similar contents—namely, CaCO₃ > 92%, CaO > 51.55%, SiO₂ < 4.5%, and Fe₂O₃ < 0.1%—for the use of limestone in the adhesive and sealant industry, with MgO content <1.2% and <0.96% for these industries, respectively. For textile production, suitable carbonates should have a composition characterized by CaCO₃ > 94%, CaO > 52.64%, SiO₂ < 2.5%, Al₂O₃ < 2%, and MgO < 3% [40], while the soda ash and caustic soda industry requires a limestone composition of CaCO₃ > 94.6%, CaO > 53%, SiO₂ < 3%, and MgO < 1% [40,42]. Other industries such as those relating to paint and plastic fillers, pottery and porcelain ware, and bleaching powder require CaCO₃ > 96% and CaO > 53.76%, with varying amounts of the other oxides (Figure 10). Industries such as those relating to calcium carbide, ceramics, and food and pharmaceuticals require a CaCO₃ content > 97%, with different amounts of other oxides (Figure 10 and Figure 11). The highest content of CaCO₃ is required for the glassware industry (>98%), along with very low amounts of other oxides (Figure 10 and Figure 11) [39,42,43,44,45].

Therefore, the average chemical compositions for samples from the middle and upper parts of the WSL Formation in Hallabat and the lower and upper beds of the ASL in Ajlun were calculated in order to evaluate their potential industrial uses (Figure 10 and Figure 11). Generally, all investigated limestones were found to be suitable for use in the pottery and porcelain ware, soda ash and caustic soda, steel industry, sugar, and textile production industries. On the other hand, limestone from the Ajlun ASL upper bed was the only one suitable for use in the bleaching powder and calcium carbide industries.

The Hallabat limestones showed several potential industrial uses, varying from one bed to the other. The lower bed (WSL middle part) showed suitability for use in the adhesive and sealants, agriculture and animal feed, calcium carbide, pottery and porcelain ware, soda ash and caustic soda, steel industry, sugar, and textile production industries. Meanwhile, the upper bed (WSL upper part) showed potential for use in the adhesive and sealants, agriculture and animal feed, pottery and porcelain ware, soda ash and caustic soda, steel industry, sugar, and textile production industries.

The ASL Formation samples from Ajlun revealed that the lower bed is suitable for use in pottery and porcelain ware, soda ash and caustic soda, steel industry, sugar, and textile production industries. The upper bed, on the other hand, is appropriate for uses such as bleaching powder, calcium carbide, pottery and porcelain ware, soda and caustic soda, steel industry, sugar, and textile production (

Table 8. Comparison of Hallabat (H) and Ajlun (A) beds with specifications of limestone for different industrial uses.

Industrial End Uses	CaCO3				CaO				SiO2				Al2O3				Fe2O3				MgO
	H1	H2	A1	A2	H1	H2	A1	A2	H1	H2	A1	A2	H1	H2	A1	A2	H1	H2	A1	A2	H1	H2	A1	A2
Adhesive & sealants	√	√	√	√	√	√	√	√	√	√	√	√	√	√	√	√	√	√	×	×	√	√	√	√
Agriculture & animal feed	√	√	√	√	√	√	√	√	√	√	√	√	√	√	√	√	√	√	×	×	√	√	√	√
Bleaching powder	√	√	√	√	√	×	√	√	×	×	×	√	√	√	√	√	√	√	×	√	√	√	√	√
Calcium carbide	√	√	√	√	√	×	√	√	√	×	×	√	√	√	√	√	√	√	√	√	√	√	√	√
Ceramic	√	√	√	√	√	×	√	√	×	×	×	×	√	√	√	√	√	√	√	√	√	√	√	√
Filler	√	√	√	√	√	√	√	√	√	×	×	√	√	√	×	√	×	√	×	×	√	√	√	√
Food & pharmaceutical	√	√	√	√	√	×	√	√	×	×	×	×	√	√	√	√	√	√	×	×	√	√	×	√
Glassware	√	√	√	√	√	×	×	√	×	×	×	√	√	√	×	√	√	√	×	×	√	√	√	√
Paper	√	√	√	√	√	√	√	√	×	×	×	√	√	√	√	√	√	√	×	×	√	√	√	√
Pottery & Porcelain ware	√	√	√	√	√	√	√	√	√	√	√	√	√	√	√	√	√	√	√	√	√	√	√	√
Soda ash & caustic soda	√	√	√	√	√	√	√	√	√	√	√	√	√	√	√	√	√	√	√	√	√	√	√	√
Steel industry	√	√	√	√	√	√	√	√	√	√	√	√	√	√	√	√	√	√	√	√	√	√	√	√
Sugar	√	√	√	√	√	√	√	√	√	√	√	√	√	√	√	√	√	√	√	√	√	√	√	√
Textile production	√	√	√	√	√	√	√	√	√	√	√	√	√	√	√	√	√	√	√	√	√	√	√	√

).

4.7. Investment Opportunities

The Late Cretaceous carbonate rocks evaluated in this study are typical of comparable carbonate formations globally. These formations have similar depositional conditions and economic potential as Late Cretaceous carbonates from the Mediterranean, Middle East, and North Africa; for instance, Iraqi carbonates are primarily used for the cement industry in Kurdistan [46]. In Greece, carbonate rocks are suitable for the painting industry [47]. In Yemen, a study [48] has evaluated the industrial uses of Middle Eocene limestone deposits in Wadi Tanhalin based on geochemical assessment. The authors reported that these limestones are of high purity and suitable for industrial uses such as steel industry, paper, filler, pottery and porcelain ware, bleaching powder, soda ash and caustic soda, calcium carbide, sugar, textile production, adhesives and sealants, and agriculture and animal feed [48].

The outcomes of this study have implications beyond local applications, providing a framework for appraising similar resources globally.

The production of pure limestone has steadily increased in recent years due to increasing demand by industries for domestic use and exportation. According to [13], pure limestone in the Hallabat area is exposed on the surface with thicknesses ranging between 1–37 m, and the estimated reserve is about 69 million tons. At present, Jordanian pure limestone in different locations is being mined, produced, and exploited. Local companies are producing pure calcium carbonate at a production capacity of around 450,000 ton/year. Most of the production is used for white cement, carbonates, ground calcium carbonates (GCCs), paints, and magnesia industries, and half of this production is being exported. Thus, there is a promising opportunity for industrial investment in calcium carbonates through the development of paint, plastic, and polymer industries in Jordan and the wider region.

5. Conclusions

Limestones from two of these main localities, namely, Hallabat and Ajlun, were investigated. A total of 12 limestone samples were collected from two quarries at these study sites and analyzed for their mineralogical, petrographic, and elemental compositions. The samples were collected from four distinctive beds—two beds in each quarry—thus representing the WSL and ASL Formations.

• The Hallabat samples indicated that the different beds in the WSL and ASL Formations were deposited under various hydrodynamic conditions, which, in turn, affected their chemical and physical characteristics.
• The results showed that all samples were in the range of pure limestone. The middle part of the WSL Formation in the Hallabat Quarry is of very high purity (average CaO content of 55.59%), while the upper part showed medium purity limestone (average CaO content of 53.85%).
• In Ajlun, the lower bed of the ASL Formation resulted in high-purity limestone (average CaO content of 54.52%) compared with the very high-purity limestone of the upper bed (average CaO content of 56.08%). Generally, all investigated limestones contained minor amounts of other oxides.
• The results suggest that all limestones are suitable for industrial uses, such as those in the pottery and porcelain ware, soda ash and caustic soda, steel industry, sugar, and textile production industries. On the other hand, the Ajlun ASL upper bed is the only bed suitable for use in the bleaching powder and calcium carbide industries.

6. Recommendations

At present, these limestones are being used to produce cement or as raw materials for building blocks and coarse aggregates; nonetheless, the results of this study indicate that there is a wide range of industrial uses for these limestones. These industrial applications can be of importance in improving the economic resources in Jordan. More detailed investigations of these limestones are required, and more tests are needed to ensure their suitability for different industrial uses, particularly regarding the physical properties of the rocks.