1. INTRODUCTION:

Natural zeolites are anhydrous aluminosilicates characterized by a three-dimensional (3D), porous structure. It is a complex framework formed by the sharing of edges between SiO₄ and AlO₄ tetrahedra, resulting in a 3D system of molecular channels and pores. This type of framework is characterised by the isomorphic substitution of Si⁴⁺with Al³⁺, resulting in the framework carrying a negative charge^1,2.

This charge is perfectly countered by the mobile extra-framework cations, e.g., Ca²⁺, Na⁺, and K⁺. The entrained cations and included H₂O in the pores are responsible for the unique properties of zeolites, such as high cation-exchange capacity, molecular sieving, and the reversibility of dehydration and rehydration, among others, which make them an important class of materials in various industrial and environmental applications^3,4. The name "zeolite" was given in 1756 by the Swedish Mineralogist Axel Fredrik Cronstedt, who discovered these minerals among others and observed that they tended to froth and swell when heated⁵. Although synthetic zeolites are well-known porous materials used for catalysis in the petrochemical industry, gas separation, and adsorption, natural zeolites are a cheaper and more biocompatible alternative^6-8. They are in ample supply worldwide. They are widely distributed across most of the globe, making them excellent candidates for various sustainable uses, including environmental remediation, agriculture, and water treatment. Natural zeolites are abundant and have attracted the attention of researchers seeking to address current sustainability problems related to dyes^9-12.

In Western India, the Deccan Traps are among the world's largest volcanic provinces, with extensive deposits of various natural zeolites. These minerals form through hydrothermal alteration of basaltic lavas, with mesoscopic distortion of the geochemical environment. The unique geological history of the Deccan Traps has led to the formation of a variety of zeolite species as veins or fillings within cavities in the basaltic host rock. Zeolites in the Deccan Traps have been well known to geologists for over 100 years; early papers recorded numerous species and noted their zonal distribution within lava flows.^13-15 The material studied here comprises six well-crystallised and representative members of the zeolite family from various localities within this prominent geological region. A fundamental understanding of the physicochemical properties of natural zeolites is essential for their potential applications in different industrial fields. That requires microscopic and chemical analysis, along with a range of advanced techniques^10,13,16. The most widely used technique is XRD, which provides valuable information on the mineral phase, crystal structure, and sample purity. The standard diffraction patterns are the fingerprints of individual zeolite species. SEM-EDX can provide information on sample morphology, crystal habits, and elemental composition. The combination of these two methods enables a more reliable characterization for the generalist in both fundamental mineralogy and industrial exploitation of the materials. Earlier investigations of zeolites from the Deccan Traps were mostly limited to their classification and distribution rather than systematic, contemporary characterization studies, which would help in better understanding their potential^17-20.

This work aims to satisfy the lack of a comprehensive structural–chemical analysis of specific natural zeolites from the Deccan Traps. The studies are carried out on six different samples, namely, Stilbite (Pune), Heulandite (Jalgaon), Mordenite (Sambhaji Nagar), Laumontite (Nashik), Mesolite (Pune), and Scolecite (Ahilyanagar). The chosen samples were selected based on their local abundance and crystallinity. The primary goal is to characterise these zeolites comprehensively using XRD and SEM-EDX. The results are not only valid for a comprehensive mineralogy of zeolites from this geologically interesting area, but also provide a solid basis for future investigations on the utilization of the investigated zeolites, such as heavy metal removal, soil conditioning, and catalysis^17,21-23. This work aims to highlight the natural resources of this type and promote their sustainable use as an alternative to synthetic polymers. The detailed characterization of these zeolites, which this study focuses on, should help drive further RandD toward their practical application, producing new products that contribute to developing more sustainable solutions addressing current global challenges, are cost-effective and energy-efficient, and ultimately reduce environmental impact^24,25.

2. MATERIALS AND METHODS:

2.1 Sample Collection and Preparation:

Natural zeolite samples were collected from six different locations within the Deccan Traps in the state of Maharashtra, India (Fig. 1). The minerals studied, along with their collection sites, are Mesolite (Pune), Scolecite (Ahmednagar), Laumontite (Nashik), Mordenite (Sambhaji Nagar), Heulandite (Jalgaon), and Stilbite (Pune). Samples were then visually examined and evaluated based on their characteristic macroscopic properties. This material was subsequently ground into a fine powder in a mortar and pestle. The powdered sample was sieved and homogenised to ensure its purity was maintained during analysis.

Fig. 1: Photographs of zeolite samples from the Deccan Traps.

2.2 XRD Analysis:

The mineralogical identification and the degree of crystallinity of all the zeolite powder samples were determined by XRD techniques. XRD patterns were recorded on a diffractometer using CuKα radiation source (λ = 1.5406 A˚) (40 kV, 30 mA). The XRD analysis yielded accurate crystallographic data (d-spacings and peak intensities) when compared with the reported standards for all zeolite species^26,27.

The average crystallite size of the samples can be estimated using the Scherrer equation 1:

D = βcosθ Kλ – – – – – – – – – – – – (1)

Where: D is the average crystallite size, K is the shape factor (typically 0.9), λ is the X-ray wavelength (1.5406 A˚ for CuKα), β is the full width at half maximum (FWHM) of the most intense peak, in radians, and θ is the Bragg angle of the most intense peak^28-30.

2.3 SEM-EDX Analysis:

Surface morphology and crystal habits of the zeolite samples were examined using SEM. Small pieces from each sample were mounted on Al stubs with double-sided carbon tape. The samples were then sputter-coated with a thin Au layer to facilitate charge dissipation during microscopic analysis. Different magnifications ranging from 1000x to 10,000x were used to analyze the morphological details of acicular, fibrous, prismatic, and tabular habits. Semi-quantitative element analyses of the samples were conducted using EDX attached to the SEM. Characteristic X-rays emitted from the sample surface, originating from the electron beam, were detected by the EDX detector. The main elements present in each zeolite, including Si, Al, Ca, Na, and K, were identified using the EDX spectrum. Elemental compositions in weight percentage (wt%) were then used to calculate the Si/Al ratio, a crucial property in zeolite characterization^31-33.

3. RESULTS AND DISCUSSION:

3.1 Mesolite (Na₂Ca₂(Si₉Al₆)O₃₀.8H₂O):

3.1.1 XRD studies: The XRD pattern of the Mesolite sample confirms the crystalline nature. The distinct, sharp peaks reflect the excellent crystallinity of the nano matrix (Fig. 2). The XRD pattern was compared with the standard JCPDS data for Mesolite. The positions and relative intensities of the main diffraction peaks in the sample are consistent with the standard data (26-1048), signifying the absence of impurities and the good quality of the pure Mesolite single phase³⁴.

Fig. 2: XRD pattern of Mesolite

The strong peaks appear at 2θ values of approximately 13.15°, 27.82°, 30.68°, 31.36°, and 43.54°, with the strongest at 31.36°. The two peaks also provide evidence for the isolation of the Mesolite mineral phase. The average particle size is 55.2 nm, determined from the major peak in the XRD pattern. This value suggests that the particles are on the nanoscale, which is consistent with the sharp, narrow peaks observed in the XRD pattern^34,35.

3.1.2 SEM-EDX analysis: The SEM images display prismatic and acicular crystals with a common radiated habit (Fig. 3a-d). This aligns with the typical habit of mesolite, a chain- or needle-shaped zeolite. The EDX spectrum (Fig. 3f) of the sample reveals its elemental composition. The spectrum shows the characteristic elements of the Mesolite chemical formula: Na, Ca, Al, and Si.

Fig. 3: SEM images and EDX analysis of Mesolite. The SEM morphology (a-d) and the elemental composition, as indicated by the EDX spectrum (f).

The results of the EDX analysis, based on the EDX data table, are as follows: Semi-quantitative values: Si, 21.70 wt.%, Al: 13.90 wt.%, Ca: 6.88 wt.%, Na: 3.95 wt.%, O: 52.19 wt.%. These data align with the expected composition of Mesolite. A key indicator for zeolites is the Si/Al ratio. From the chemical formula of Mesolite (Si₉Al₆), the Si/Al ratio is theoretically 9/6 = 1.5. The Si/Al ratio of the studied zeolites ranges from 1 to 3.92, and the calculated Mesolite value naturally falls within this range. The EDX measurement therefore confirms the expected stoichiometry and mineralogical identification. Here, the presence of Na and Ca as extra-framework cations indicates their role in charge-balancing the aluminosilicate framework, as further supported by the EDX results. This fundamental building block is crucial for determining the potential uses of the zeolite, such as ion exchange or catalysis, since both functions heavily rely on the Si/Al ratio and the compensating cations^1,2,11.

3.2 Scolecite (CaAl₂Si₃O₁₀.3H₂O):

3.2.1 XRD studies: The XRD pattern of Scolecite reveals the good crystallinity of the sample, as indicated by sharp and strong diffraction peaks (Fig. 4). The pattern was meticulously examined and compared to the reference data for Scolecite in the JCPDS (24-1064)³⁴.

Fig. 4: XRD pattern of Scolecite

Sharp peaks appear at 2θ values of 13.56⁰, 18.82⁰, 30.68⁰, 41.14⁰, 48.98⁰, 52.54⁰, 63.96^0, and 72.84⁰, with the most intense one located at about 48.98⁰. These peaks indicate the isolation of the Scolecite mineral phase. The lack of additional peaks suggests that the sample is pure and it contains no detectable polar crystalline impurities. Based on this estimation, the probable average crystallite size of the Scolecite sample is ~ 74 nm. This nanoscale particle size is consistent with the sharp, well-defined peaks observed in the XRD pattern, which are indicative of large, well-ordered crystallites³⁶.

3.2.2 SEM-EDX analysis: The SEM results show that the Scolecite is fibrous and tends to be acicular (needle-like) (Fig. 5a-d). The crystals are often found in radiating sprays or clusters, which is typical for the mineral. It is distinctive and visually unlike any other zeolite, making it a diagnostic characteristic. They display the delicate, slender, prismatic shape of individual crystals, which can form spectacular radiating groups. A notable physical property is the cleavage along the {110} plane, which is also visible in SEM images of broken crystals.

Fig. 5: SEM images and EDX analysis of Scolecite. The SEM morphology (a-d) and the elemental composition, as indicated by the EDX spectrum (f).

The elemental composition is determined using EDX. Fig. 5f shows the EDX spectrum of Scolecite, with the significant elements Ca, Al, and Si clearly identified. The Si/Al ratio should be 1.5, according to the theoretical formula CaAl₂Si₃O₁₀.3H₂O. If other trace elements, such as small amounts of Na or K, are present, this may indicate partial or complete substitution at various sites within the crystal lattice. However, the prominent peaks will still correspond to Si, Al, and Ca. The EDX results provide a semi-quantitative elemental composition that is compared with the designed weight percent: Si (21.48 wt.%), Al (13.75 wt.%), Ca (10.22 wt.%), and oxygen (53.01 wt.%), with oxygen contributing the most to the weight.

3.3 Laumontite (CaAl₂Si₄O₁₂.4H₂O):

3.3.1 XRD studies: The XRD pattern of the Laumontite sample is shown in Fig. 6, which confirms its crystallinity. The evident, intense diffraction peaks indicate the material's well-ordered, high crystallinity.

Fig. 6: XRD pattern of Laumontite

The XRD pattern was characterized and compared with JCPDS standard data for Laumontite. The characteristic peaks of Laumontite appear at 2θ angles of approximately 9.48⁰, 13.06⁰, 18.88⁰, 21.52⁰, 22.58⁰, 27.40⁰, 28.44⁰, 29.60⁰, 38.18⁰, and 42.80⁰. The strongest peak at around 9.8⁰ confirms that the mineral phase has been successfully isolated. The absence of any measurable charge on the base region(s) indicates that the sample is relatively pure. Based on this estimate, the probable average crystallite size of the Laumontite sample is ~53 nm. This nanoscale particle size is consistent with the sharp, well-defined peaks observed in the XRD pattern, which are indicative of large, well-ordered crystallites^37,38.

3.3.2 SEM-EDX analysis: The morphology of the Laumontite sample, as shown in SEM images (Fig. 7a-d), exhibits a prism crystal habit, which aligns with the mineral's crystallography. These crystals usually have a square or diamond-shaped cross-section with steeply inclined terminations.

Fig. 7: SEM images and EDX analysis of Laumontite. The SEM morphology (a-d) and the elemental composition, as indicated by the EDX spectrum (f).

The presence of the aluminosilicate network, composed of Si and Al, is evident in the EDX spectrum, which shows clear, strong peaks for both elements. A prominent peak for Ca is also observed, indicating that Ca is the dominant extra-framework cation. The absence of peaks for other major cations (Na, K) suggests a relatively pure, Ca-rich form of Laumontite. Weight percentages are provided based on the quantitative EDX data: Si, 21.49 wt%, Al: 11.47 wt.%, Ca: 8.52 wt.%, O: 58.52 wt.%. The chemical composition of Laumontite is CaAl₂Si₄O₁₂.4H₂O, and therefore the extrapolated Si/Al ratio is 4/2 = 2.0. This ratio can be confirmed by the experimental weight percent obtained through EDX analysis. This EDX result is consistent with the mineral's chemical formula, confirming its identification and providing crucial P data for various applications, such as ion exchange, where the Si/Al ratio is important.

3.4 Mordenite (Na₂(K₂, Ca)₄(H₂O)₂₈[Al₈Si₄₀O₉₆]):

3.4.1 XRD studies: The high purity and crystallinity of the Mordenite sample (Fig. 8) are evident from the XRD pattern. The pattern exhibits many sharp diffraction peaks, typical of an ordered system. The strongest peak appears at a 2θ position of about 26.4⁰, one of the Mordenite's identifying reflections. Other characteristic peaks are observed at 2θ values of 9.7⁰, 13.5⁰, 22.2⁰, and 27.8⁰, all of which correspond to the Mordenite crystal structure.

Fig. 8: XRD pattern of Mordenite

The XRD pattern matches well with the reference pattern of Mordenite (JCPDS card No. 19-0831). The absence of other peaks from other minerals or amorphous phases implies that the particles are high-purity Mordenite. This is a significant finding, as impurities can alter zeolite properties. The average crystallite size is 41 nm, estimated from the Scherrer equation for the most intense peak. The slender peak implies that the Mordenite crystallites are big. The peaks are sharp and fierce, indicating high crystallinity, which is essential for several industrial applications of mordenite, including catalysis and adsorption. This nanoscale particle size is consistent with the sharp, well-defined peaks observed in the XRD pattern, which are indicative of large, well-ordered crystallites^39-41.

3.4.2 SEM-EDX analysis: The SEM images (Fig. 9a-d) display the typical morphology, including fibrous and interconnecting material, which is further supported by the photos, showing a dense network of matted, hair-like fibers. Its morphology stems from its unique crystal structure, characterized by a high aspect ratio, which gives it a waxy needle or fiber form.

Fig. 9: SEM images and EDX analysis of Mordenite. The SEM morphology (a-d) and the elemental composition, as indicated by the EDX spectrum (f).

The EDX spectrum (Fig. 9f) shows significant peaks for Si, the framework components, and characterised peaks corresponding to the extra-framework cations, Na and Ca. K might be included in some of the Mordenite samples, based on the general formula. The semi-quantitative composition is Si: 28.60 wt%, Al: 6.84 wt%, Ca: 2.22 wt%, Na: 1.48 wt%, O: 60.85 wt%. The Si/Al ratio is a key property of zeolites. It is estimated to be approximately 3.86 from the EDX weight percentages (28.60 wt.% Si / 6.84 wt.% Al). The ideal chemical formula for Mordenite shows a perfect Si/Al ratio of 5 (40/8). The measured ratio of 3.86 is slightly below the ideal of 4.0 but is typical for natural zeolites, where some chemical variation and cation substitution are common. This high Si/Al ratio is typical for Mordenite and helps explain its high thermal and acid resistance, making it an important catalyst and adsorbent in industrial uses.

3.5 Heulandite ((Ca, Na, K, Sr)₅(Si₂₇Al₉)O₇₂.26H₂O):

3.5.1 XRD studies: The XRD pattern of the Heulandite sample (Fig. 10) confirms its mineralogical nature and high crystallinity. The diffraction peaks are sharp and well-defined, indicating a highly ordered crystal structure with fewer defects. The main characteristic peak of this zeolite occurs when its 20 value is about 9.90 for Cu radiation, which corresponds to the (200) interplanar spacing of the Heulandite crystal lattice. A few other strong reflections are observed at around 2θ angles of 22.40, 26.80, and 32.40, all of which match the known Heulandite diffraction pattern. The XRD pattern (Fig. 10) corresponds to JCPDS card No. 39-1383 for Heulandite-Ca. The close match between the measured peaks and the standard pattern confirms the phase purity of the sample^42,43.

Fig. 10: XRD pattern of Heulandite

No significant peaks from other gangue minerals or zeolite species were observed, indicating that the sample is highly pure. The average crystallite size, calculated using the Scherrer equation, is 53 nm, determined from the most intense peak in the XRD pattern. The high FWHMs of the peaks indicate that the Heulandite crystallites are large. The strong peak intensities further support the material's high crystallinity. This nanoscale particle size aligns with the sharp, well-defined peaks observed in the XRD pattern, which indicate large, well-ordered crystallites.

3.5.2 SEM-EDX analysis: SEM images of the prismatic and tabular morphology of the Heulandite sample (Fig. 11a-d). The crystals are sharply formed and have a blocky-to-tabular habit. In these images, we can see the characteristic form of Heulandite crystals: elongated orthorhombic tabular and frequently aggregate in groups oriented parallel. This morphology is characteristic of the mineral, and does not occur with fibrous zeolites such as Mordenite or Mesolite⁴².

Fig. 11: SEM images and EDX analysis of Heulandite. The SEM morphology (a-d) and the elemental composition, as indicated by the EDX spectrum (f).

The EDX spectrum (Fig. 11e-f) shows the main elements in the Heulandite formula: Si and Al as framework elements, along with Ca and Na as extra-framework cations. The semi-quantitative composition is derived from the EDX data table: O = 68.27 wt%, Si = 20.76 wt%, Al = 6.70 wt%, Ca = 2.97 wt%, Na = 1.29 wt%. K: 0.00 wt%. The Heulandite composition formula is somewhat complex; the typical Si/Al ratio is 27/9 = 3.0. This ratio is supported by the EDX results, which give an experimental weight percent (wt%) Si/Al ratio of about 3.10 (20.76 wt% Si / 6.70 wt% Al). The presence of Ca and Na in the EDX data aligns with the general formula for Heulandite, and it is well known that both cations are incorporated into the mineral's framework. The low Na content and absence of K suggest the analyzed sample is primarily a Ca-rich Heulandite. This chemical composition is crucial for understanding its ion-exchange capacity and thermal stability, which are essential for its potential industrial applications.

3.6 Stilbite (NaCa₄ (Si₂₇Al₉)O₇₂.28H₂O):

3.6.1 XRD studies: The XRD analysis confirms that stilbite is the main crystalline phase in the sample. The diffraction pattern (Fig. 12) shows several sharp, distinct peaks, indicating the material's high purity and crystallinity. The most intense peak occurs at approximately 2θ = 9.25⁰, a characteristic reflection for Stilbite. This peak, along with others at 2θ values of 17.60⁰, 22.20⁰, and 27.30⁰, matches the known diffraction pattern for Stilbite. The sample's diffraction pattern was compared to the JCPDS card No. 44-1479 for Stilbite-Ca⁴⁴. This comparison verifies the sample's phase purity, as no significant additional peaks from other minerals or amorphous phases were observed. Based on the provided XRD pattern, the most intense peak at approximately 2θ = 9.25⁰ (corresponding to θ = 4.6250) will be used for this calculation. The narrowness of the peaks visually suggests that the crystallites are relatively large and well-formed⁴⁵.

Fig. 12: XRD pattern of Stilbite

The XRD data successfully identify the sample as high-purity, crystalline Stilbite, with a diffraction pattern that matches established JCPDS standards. This confirms the mineralogical identification and complements the SEM-EDX results. Based on this estimation, the probable average crystallite size of the Stilbite sample is approximately 53 nm. This nanoscale particle size is consistent with the sharp, well-defined peaks observed in the XRD pattern, which are indicative of large, well-ordered crystallites^46,47-50.

3.6.2 SEM-EDX analysis: The SEM images display the morphology of Stilbite. The crystals are aggregates that radiate in flattened blades or tabular forms. This star-shaped, or fan-like, habit is typical of Stilbite and is clearly visible in the photos (Fig. 13a-d). The crystal habit generally appears as botryoidal, forming densely packed clusters with a radiating pattern⁵¹. This habit differs from the needle-like form of Mesolite and Scolecite, as well as the prismatic crystals observed in Laumontite and Heulandite.

Fig. 13: SEM images and EDX analysis of Stilbite. The SEM morphology (a-d) and the elemental composition, as indicated by the EDX spectrum (f).

The EDX spectrum (Fig. 13e-f) shows strong peaks for the framework elements Si and Al. Major peaks for Ca and Na are also present, indicating they are charge-balancing extra-framework cations. This composition is confirmed by the quantitative EDX data: O = 67.23 wt.%, Si = 15.39 wt.%, Al = 10.02 wt.%, Ca = 3.41 wt.%, Na = 3.95 wt.%. The theoretical Si/Al weight ratio for Stilbite is 27/9 = 3.0. The Si/Al weight percentage ratio of the sample is ~ 1.54 (15.39 wt.% Si / 10.02 wt.% Al). Although this is below perfect stoichiometry, it falls within the typical variation range for natural zeolite, where substitution and impurities affect the chemical composition. The presence of Al where Si was expected suggests that the stoichiometry is not entirely exact in this sample. The occurrence of both Na and Ca aligns with the mineral's chemical formula and confirms its identity⁵¹.

4. CONCLUSION:

The present work provides a mineralogical and geochemical analysis of six natural zeolite minerals: Mesolite, Scolecite, Laumontite, Mordenite, Heulandite, and Stilbite from the geologically important Deccan Trap of Maharashtra, India. XRD analysis reveals that all samples are pure and crystalline, with patterns that match the reported standard powder diffraction card. This suggests that these zeolites are of high quality and are likely to have a wide range of sought-after uses, as they do not require strict purification. These morphological and compositional features were elucidated using SEM-EDX, which also deepened our understanding. The variation in crystal habit as reflected in these samples, from fibrous (Mesolite and Scolecite) to prismatic (Laumontite and Heulandite) habit, not only characterizes the various geological environments of their formation, but also reflects variation in different functionalities of the samples, which are more significantly affected in terms of surface area and porosity. Major elements (Si, Al, Ca, Na, K) were identified, and Si/Al ratios ranging from 1 to 3.92 were estimated, a widely recognized parameter that affects their catalytic/adsorptive processes. We conclude that this study effectively characterises a range of natural zeolites from the Deccan Traps, introducing a baseline dataset for the mineralogical and geochemical character of this essential natural resource. The results have an impact that extends beyond the scientific description of zeolite formation in this exceptional geologic environment, and is also of importance for industrial applications. The high purity, well-defined morphology, and desired Si/Al ratios of these zeolite species render them excellent candidates for water purification, catalysts, and cements, providing an environmentally responsible and plentiful supply to meet significant industrial requirements.

5. AUTHOR CONTRIBUTIONS:

CRediT

Aditi Mookherjee:Conceptualization; Data curation; Formal analysis; Investigation; Methodology; Project administration; Resources; Software. Shweta Patil: Supervision; Validation; Visualization; Writing – original draft; Writing – review and editing

6. CONFLICTS OF INTEREST:

Authors declare no conflicts of interest.

7. DATA AVAILABILITY:

Data is provided within the manuscript

8. FUNDING DECLARATION:

No funding

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Received on 16.03.2026 Revised on 15.04.2026

Accepted on 12.05.2026 Published on 04.07.2026

Available online from July 30, 2026

Asian J. Research Chem.2026; 19(4):290-298.

DOI: 10.52711/0974-4150.2026.00045

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