1 Introduction

The beach sand mineral industry in India owes its growth to the discovery of monazite, a complex phosphate of rare earth elements [1]. Monazite occurs in the beach placer deposit along with other heavy minerals such as ilmenite, rutile, leucoxene, zircon, sillimanite, and garnet. The monazite is separated as a by-product of titanium minerals. The monazite, being the commercial ore of rare earth elements (REE), draws the attention of researchers interest all over the world [2]. The most important REE-bearing phosphate minerals like monazite and xenotime mostly occur in different magmatic and metamorphic rocks as an accessory or even rock-forming mineral and hence, be upgraded to form mineral deposits such as in some carbonatites [3, 4]. The occurrence of monazite in beach sands causes radionuclide enrichment along the coast [5]. In Chavara–Neendakara area of Kerala state, India, the thoron exhalation rate was reported around 4–12 times higher than that of normal soil surfaces [6]. In Brazil, monazite-rich sands show a significant rise in thorium decay chain activities compared to other parts of the world [7]. The gamma spectral analysis shows that the presence of monazite and microcline feldspar are the primary source of 232Th and 40K in the intertidal sand samples of Tamilnadu coast [8].

In India, monazite is the only commercial source of rare earth elements. The group consists of yttrium and the 15 lanthanide elements viz., lanthanum, cerium, praseodymium, neodymium, etc. The uses of REE range greatly from the least sophisticated product such as a pair of sunglasses to highly advanced products such as nuclear reactors. A systematic study on the distribution and separation of monazite in various locations along the south-west coastal area of India and the geochemistry of monazite, its major, minor and trace elements, etc. are interesting fields of research, and these are mainly addressed in the present study. The presence of radio elements and rare earth elements were scientifically studied using advanced characterization techniques for the beach sands of Andhra Pradesh, eastern coast of India [9], beach black sand at north of Nile Delta, Egypt [10], southeastern US coastal plain [11]. Shrivastava et al. (2018) use HPGe semiconductor detector for studying the uranium content in Indian monazite samples [12]. Dating of beach monazites collected from Alleppey in Kerala clearly reveals the age of tectono-thermal history of Trivandrum block [13].

Grain size studies of beach sediments provide information on the intrinsic properties of sediments, and their depositional environment [14]. Several researchers, Chaudhari et al.(1981), Rajmanickam and Gujar (1993), Rao et al. (2005), Angusamy and Rajamanickam (2006), Gandhi et al. (2008), Ramathan et al.(2009), Gayathri et al. (2017), and Sajimol et al. (2017) carried out systematic granulometric, and heavy mineral studies of the east and west coasts of India [15,16,17,18,19,20,21,22]. Outside India also, many studies were conducted on mineralogy, chemistry, and provenance of beach minerals in South China Sea [23], Africa [24], Sri Lanka [25], Egypt [2], and Somaliland coastal areas [10]. Investigation on texture and mineralogy of beach sediments play a prime role in deriving the depositional history of beach regions [18]. Mineral processing is also an important area where immense research has been carried out for improving the efficiency in recovering the mineral in beach sands [26,27,28]. Since the monazite is magnetic and non-conducting in nature, a combination of magnetic, electrostatic, and gravity separation units can be successfully used for separating monazite mineral from beach sands [29]. Recovery of strategic minerals was successfully carried from red sediments of Badlands, Andhra Pradesh [30], beach sands of Thiruvananthapuram district [29], placer deposits of North Korea [31], etc.

The objective of the present study is to enlighten the chemistry and distribution of monazite mineral along the coast of Neendakara–Kayamkulam belt in Kerala, south-west coast of India. An attempt is made to investigate the texture and mineralogy of beach sediments followed by the recovery of monazite mineral for detailed structural and chemical characterization.

2 Study area

The study area, Chavara deposit, is a major beach placer deposit along the south-west coast of India. It occurs along the coastal area in Karunagapally Taluk of Kollam district, Kerala, comprising 22.5 km length extending from Neendakara (8°56′03″ N and 76° 32′34″ E) in the south to Kayamkulam (9° 8′24″ N and 76° 27′36″ E) in the north, and width ranging from 200 to 300 m. The study area of the south-western part of the Indian subcontinent is a significant portion of the Indian shield. The region comprises largely Precambrian crystallines such as charnockites, khondalites, and migmatitic gneisses. The beach sands occur in Neendakara–Kayamkulam belt (Fig. 1) is one of the best qualities in the world containing high-grade TiO2 in ilmenite. Along the belt, the ilmenites mostly exist, followed by other important minerals like monazite, sillimanite, zircon, rutile, etc. The existence of all these minerals along the coastal tract totally depends on provenance rocks, climate, coastal geomorphology, and drainage pattern. The wave action and currents act as the primary factor deciding the size of the grain, and also result in relative sorting of these heavy minerals [32].

Fig. 1
figure 1

Map of study area

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Granulite facies rocks of south Kerala comprising khondalites and migmatites are assigned as the provenance rocks of the beach deposits [33, 34]. The various lithological units are clearly shown in the geology map of the study area (Fig. 2). The supply and enrichment of heavies on the beach face are related to wave processes [35]. The sand grains originated by the weathering action in the Western Ghats were carried down along the rivers to the sea and deposited along the coast. The high-grade metasediments of khondalite belt are responsible for high-Ti ilmenite [36]. The garnet and pyribole-free heavy mineral suits occurred along the coastal plain of Kollam are mainly derived from the denudation of Neogene sedimentary deposits formed during the rising sea level phases occurred in late quaternary periods [37]. The erosion of the heavy mineral-rich beach and the winnowing of the more easily entrained quartz sediments results in high concentration of heavy minerals [35].

Fig. 2
figure 2

Geology of the study area

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3 Materials and methods

Detailed mineralogical analysis of raw sand collected from each location along the coastal strip has to be done to grade the deposit based on monazite content. The coast of Neendakara–Kayakulam belt is 22.5 km long and 200–300 m wide. The sample collection was done in three steps, (1) five beach sand samples (MS1–MS5) were collected to a depth of 100 cm from five sampling stations at an interval of ~ 4 km, (2) forty raw sand samples (RS1–RS40) were collected to a depth of 100 cm from the grids (grid size = 200 m × 500 m) along the coastal strip, and (3) ten soil samples (SS1–SS10) collected to a depth of 100 cm within 2 km buffer zone from the shoreline. All samples were subjected to detailed mineralogical analysis, but only the five beach samples (MS1 to MS5) were taken for textural analysis and recovery of monazite mineral. A representative composite sample of 100 g was taken for analysis after mixing the bulk sample thoroughly, the samples from all the segments were collected in polythene sample bags and labelled correctly in the same way and kept ready for analysis.

Mineralogical analysis to determine the per cent content has been done as per the procedure developed by the Atomic Minerals Directorate (AMD), Govt. of India, and the flow chart is shown (Fig. 3). At first, the samples were washed thoroughly with water to remove organic matter, slime, etc. and then with dil. HCl (1:1 by volume) to dissolve shells and shell fragments. The preprocessed sample is then sieved in a Ro-tap sieve shaker in different sieves, and the percentage of each sieve fraction is calculated. If magnetite is present in the samples, then, it was removed by using a hand magnet. The magnetite-free fractions were separated into magnetic and non-magnetic fractions with the help of a laboratory electromagnet. Ilmenite, garnet, and traces of monazite present were separated at a magnetic susceptibility around 0.3–0.4 A. The non-magnetic portion contains rutile, zircon, monazite, leucoxene, and sillimanite along with quartz. The quartz was separated by a heavy media separation method using bromoform, which has a specific gravity of 2.89 [38]. The fractions are analyzed using a Leitz binocular-polarized microscope. The weight percentage of each mineral was computed from the total number of grains counted for each mineral [39]. Spatial distribution of total heavy minerals (THM) and monazite in the study area was also mapped using inverse distance weighted (IDW) interpolation technique available in the spatial analyst extension of ArcGIS software. GIS-based surface interpolation techniques incorporated in the spatial analyst tool of ArcGIS software produces surface maps showing spatial variability and continuity of data [40, 41]. The grain size parameters like graphic mean (M), inclusive graphic standard deviation (SD), inclusive graphic skewness (SK), and graphic kurtosis (KU) were determined using the software package. The various graphic and moment measures were calculated with the formulae of [39]. The calculated statistical values were interpreted for Sahu’s (1964) linear discriminate functions of Y1 (Aeolian: Beach), Y2 (Beach: Shallow agitated water), for analyzing the relationship of energy and the fluidity factors with different processes modes of sediment deposition and transportation [42].

Fig. 3
figure 3

Flow chart showing the heavy mineral analysis of beach sands

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The Y1 and Y2 were calculated from textural parameters using the Eqs. (1) and (2).

$$Y_{1} \left( {\text{Aeolian/Beach}} \right)\, = \, - 3.5688 \;{\text{M}}\, + \,3.7016\;{\text{SD}}{-}2.0766\;{\text{SK}}\, + \,3.1135\;{\text{KU}}$$
(1)

If the value of Y1 is > − 2.7411, the environment is ‘Beach’ and if Y1 < − 2.7411, then it corresponds to ‘Aeolian’ environment.

$$Y_{2} \left( {\text{Beach/shallow agitated water}} \right) = 15.6534\;{\text{M}}\, + \,65.7091\;{\text{SD}}\, + \,18. \, 1071\;{\text{SK}}\, + \,18.5043\;{\text{KU}}$$
(2)

For Y2 < 63.3650, the environment is ‘Beach’,and in case of Y2 > 63.3650, the environment is ‘Shallow marine’.

Where M is graphic mean, SD is inclusive graphic standard deviation, SK is inclusive graphic skewness, and KU is graphic kurtosis.

A combination of unit operations including magnetic, electrostatic, and gravity separation techniques was applied to the beach sediments collected from the five locations (MS1–MS5) for recovering the monazite mineral [29]. The selection of separation techniques depends upon the physical properties of monazite mineral such as magnetic susceptibility, electrical conductivity, and specific gravity [43]. The raw beach samples were sieved for removing larger pieces of stones, shells, etc. and fed into Mineral Technologies short spiral concentrator (laboratory model) for recovering heavy mineral concentrate [44]. The magnetic minerals such as ilmenite and garnet were removed using Carpco rare earth drum magnetic separator (laboratory model). The paramagnetic mineral such as ilmenite was selectively separated using high intensity dry magnetic separators [45,46,47]. The zircon and sillimanite were removed as non-conducting minerals using Orekinetics Corona Stat high tension separator (laboratory model) [48]. The non-magnetic rutile is removed by using Reading’s induced roll magnetic separator (IRMS) (laboratory model), and non-conducting leucoxene is removed using Mineral Technologies electrostatic plate separator (laboratory model). The feed samples for the electrostatic separation are dried and heated to 110–180 °C for removing the moisture [43, 49]. Any changes in ambient conditions may cause poor separation of minerals [50]. Finally, the panning is done to recover monazite grains from ilmenite having low specific gravity. The S.G. Frantz isodynamic separator recovers high-grade monazite by removing all other impurities. The samples recovered at different stages during the process were analyzed under the microscope for identifying the minerals of interest. Flow sheet showing the recovery of monazite is shown in Fig. 4.

Fig. 4
figure 4

Flow sheet showing the recovery of monazite from the beach sands

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The X-ray diffraction analysis of monazite mineral was conducted using PANalytical X’Pert Pro diffractometer, and results were processed using X’Pert High Score Plus software (PANalytical). Pan Analytical Epsilon 3 ED-XRF instrument with Omnion software (elemental range is Mg-U) is used for analyzing the bulk geochemistry of the recovered monazite and soil samples (SS1–SS10). The surface morphology of monazite grains was analyzed using scanning electron microscope (SEM) of JEOL make model JSM5600 LV. Energy dispersive X-ray spectroscopy (EDS) elemental analysis was also carried out using a Silicon Drift Detector − X-MaxN attached to the SEM of Carl Zeiss make EVO18 model.

4 Results and discussion

Detailed investigation on texture, distribution, composition, and rare earth chemistry of monazite mineral along the coastal sediments of Neendakara–Kayamkulam belt in Kerala, SW coast of India was carried out using systematic and scientific procedures.

4.1 Textural studies

Detailed and systematic studies on marine sediments made a breakthrough in deriving the relation between textural parameters and transportation process of sediments [19, 39, 51]. The textural parameters like mean, sorting, skewness, and kurtosis help to understand the nature of sediment deposition [18]. The descriptive statistics of textural parameters for five samples (MS1–MS5) are shown in Table 1. All the locations contain fine-grained sediments (size range = 2.6–2.7phi) with sorting ranges from well sorted to moderately well sorted (range = 0.44–0.59 phi). Skewness reached coarse skewed, except for MS4, where it shows nearly symmetrical. The kurtosis for MS4 is platykurtic, while others show mesokurtic. Large variations in the values of kurtosis show the flow characteristics of the medium of deposition, and also it indicates the existence of a high energy environment which results in the sorting of sediments [52, 53]. The location MS4 shows the dominance of finer size of sediments with platykurtic nature and highlights the maturity of sand, which is mainly due to the aggregation of sediment particle size by compaction. The variation in sorting values occurs due to the continuous deposition of finer or coarser-sized sediments in varying proportions [20].

Table 1 Statistical data on textural parameters of samples
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The bivariant plots are generated based on the relation between the textural parameters and fluid-flow mechanisms of sediment movement [54]. The Fig. 5a shows the relationship between mean grain size and sorting, Fig. 5b shows the relationship between sorting and skewness and Fig. 5d shows the relationship between skewness and kurtosis. In Fig. 5a, clustering can be seen in both fine-sized well-sorted and moderately well-sorted regions, which indicates the best sorting of sediments. Since the mean size and sorting are hydraulically controlled, the best sorting can be only seen in sedimentary environments having fine-sized grains [55]. Only one well-sorted fine-sized region falls under near symmetrical range with negative skewness and platykurtic range. In contrast, all other regions fall under coarse skewed and mesokurtic range. Folk (1966) showed that a small proportion of coarser grains leads to a slightly negatively skewed sedimentary environment [56]. Rajaganapathi et al. (2013) showed that for a predominant population of medium sands, subordinate coarser grains leads to negative skewness and fine leads to positive skewness [14]. Thus, it can be concluded that the beach sediments of the study area mainly consist of fine-sized grains as the predominant population with subordinates of medium grains, which leads to negative skewness ranges from meso to platykurtic.

Fig. 5
figure 5

Bivariant plots and LDF Curves. a Mean versus sorting b Sorting versus skewness c Skewness versus kurtosis d Y1 versus Y2

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The process and environment of deposition of sediments have an excellent correlation with the variations in the energy of waves and fluidity factors. Sahu (1964) uses a statistical method of linear discriminate function (LDF) analysis for interpreting these variations [42]. The results of the LDF calculation are shown in Table 2. The Fig. 5d shows the relationship between Y1 and Y2. With reference to the values of Y1 and Y2, all samples fall in beach shallow marine agitated water except MS4, which falls in Aeolian shallow marine agitated water. The different modes of deposition of sediments can be easily found out using CM pattern [57, 58]. The CM pattern is obtained by plotting the micron values of the parameters C (one percentile of the grain size distribution) against M (the median), as shown in Fig. 6a. The CM pattern has been divided into different segments, namely NO (rolling), OPQ (bottom suspension androlling), QR (graded suspension no rolling), RS (uniformsuspension), and S (pelagic suspension). All the samples of study area fall in bottom suspension rolling. On comparing with the TCD diagram shown in Fig. 6b, the samples fall in the tractive current environment due to their interaction with wave actions. During monsoon, waves are characterized at higher heights and shorter periods with the direction being more westerly but the premonsoon and postmonsoon waves are dominated towards southerly direction at lower heights and higher periods [59].

Table 2 Linear discriminate function (LDF) values
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Fig. 6
figure 6

CM and TCD diagrams. a CM diagram b TCD diagram

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4.2 Heavy mineral analysis

High energy waves are required for the transportation and concentration of heavy minerals compared to light minerals due to their high specific gravity [14]. The type of weathering, mode and distance of transportation, and the nature of the depositional environment are closely related to the occurrence of heavy minerals [60]. The total heavy mineral weight percentage derived from the bromoform analysis helps to evaluate the mineral resource of the study area and also depicts a clear picture of the nature of the source from which these strategic minerals are derived.

Tables 3, 4 and Fig. 7 show the results of heavy mineral analysis carried out for beach sediments (MS1–MS5), soil samples (SS1–SS10), and grid samples (RS1–RS40) collected from the study area. The total heavy mineral weight percentage (THM %) is maximum for the sample location MS4 (79.5%), followed by MS3 (68.2%), MS5 (55%), MS2 (34.4%), and MS1 (18.1%) in the decreasing order of abundance. The percentage content of heavy minerals is high in areas from Karithura to Vellanathuruthu. The results of grain counting shows that ilmenite is dominant with a maximum of about 58.2%, followed by sillimanite (10.8%), zircon (5.8%), rutile(4.2%), monazite(0.9%), leucoxene (0.6%), and garnet (0.2%) in the decreasing order of abundance. Others, such as pyroxenes and amphiboles, were also present to a maximum of 0.6%. The concentration of ilmenite mineral varies with the locations in the order of MS4 > MS3 > MS5 > MS2 > MS1 and that of monazite mineral is MS3 > MS4 > MS5 > MS2 > MS1. The high concentrations of minerals, especially that of ilmenite and monazite, are seen in locations MS3 to MS5. The concentration of heavy minerals in the samples varies from 13 to 84.4%.

Table 3 Heavy mineral count percentages of the beach samples (MS1–MS5)
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Table 4 Heavy mineral count percentages of the soil samples (SS1–SS10)
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Fig. 7
figure 7

Graph showing the results of heavy mineral analysis carried out for grid samples (RS1–RS40)

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The results of mineralogical analysis (Fig. 7) for the bulk sediment show that ilmenite is the most abundant heavy mineral in all the 40 samples studied. The content of ilmenite ranges from 6.7 (RS10) to 60.5 wt% (RS 40) with an average value of 35.6 wt%. This is followed by sillimanite (averageis 9.0, value ranges from 2.6 (RS8) to 13.5 (RS20), zircon (average is 5.2, value ranges from 1.7 (RS8) to 8.6 (RS 20), rutile (average is 3.0, value ranges from 0.9 (RS8) to 4.4 (RS 20), monazite (average is 0.6, value ranges from 0.1 (RS5) to 1.2 (RS 28), and garnet (averageis 0.2, value ranges from 0.1 to 0.4) in the decreasing order of abundance. Similarly, the total heavy minerals (THM) ranges from 14.5 (RS 10) to 85.1 (RS 40), with an average value of 54.5 wt%. Among the total 40 stations, RS 40 shows the highest content of THM.

The THM % in sediment samples varies from 17.3% (SS 4) to 98.9% (SS 7), and the major constituent mineral, ilmenite, varies from 5.4% (SS 8) to 77.6% (SS 7) followed by sillimanite [varies from 2.8% (SS 7) to 5.8 (SS 10)], zircon [varies from to 0.8% (SS 9) to 10.9% (SS 7)], rutile, monazite, leucoxene, and garnet. The sample SS7 is located very near to the coastline at Alappad, whereas sample SS4 is located about 3 km away from the coastline, which clearly depicts the deposition of heavy minerals, including monazite, at a distance of 3 km from the coastline.

The heavy mineral distribution map, as shown in Fig. 8a, interpolated using the results of the heavy mineral analysis, also conveys the same. Maximum concentration is noticed around the Alappad region, and it gradually decreases in Kovilthottam and Neendakara regions. Figure 8b shows the distribution of monazite mineral. The concentration of monazite is also maximum around the Alappad and gradually decreases towards the south side of Neendakara. While comparing with other regions, the concentration of heavy minerals was maximum around the coastal side as well as the inland area of the Alappad region.

Fig. 8
figure 8

Map showing the distribution of THM (a) and Monazite (b)

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From the study, it is found that a gradual decrease in heavy mineral concentration is found from the north towards the south of the study area and is due to the fact the central towards the southern part of the study area (Kovilthottam) is either a mined-out area or active mining area which shows comparatively less content because of the mining process. Maximum concentration is noticed around the Alappad (RS 22-24)–Azheekkal (RS 35–40) region, and it gradually decreases in Kovilthottam (RS 12–13)–Neendakara(RS 1) regions. As per the exploration study by AMD, up to a depth of 7.5 m in the study area indicated an average total heavy mineral (THM) content of 45 wt%, with ilmenite—35 wt%, sillimanite—7, rutile—2.5, zircon—2.5, and monazite—0.5 wt% [61]. However, in the present study, up to 1 m depth shows an average HM content of 54.5 wt%, and this indicates that the grade of deposit decreases with increasing depth. The studies by Krishnan et al. (2001) revealed that the high-grade placer deposits are seen from the surface up to a depth of 1.52 m, with THM value varies from 73 to 91% [32]. The first report available on the mineral analysis of Neendakara–Kayamkulam placer deposit was that of the analysis conducted during 1935–39 [62], which shows that the heavy mineral content in the beach sand from north to south of study area during the 1930s were high with similar content. On comparing these with the present study for seven locations (i.e., Alappad, Cheriazheekkal, Pandarathuruth, Ponmana, Karithura, Parimanam, and Neendakara) covering the study area revealed that the content of ilmenite was high in the past, and it got reduced to low abundance recently. However, other minerals like rutile, zircon, sillimanite, and monazite show fluctuating trend, and their high content in some region is due to the dumping of leftovers (tailings) of earlier mining activities which take only ilmenite and monazite.

4.3 Recovery of monazite

Table 5 shows the recovery of monazite mineral from five sample locations (MS1–MS5). The data indicate that the samples MS3 and MS4 collected from Vellanathuruthe and Alappad shows a maximum yield of monazite mineral of about 0.574% and 0.500%. The least recovery of monazite is from MS1 collected around the Neendakara. Altogether, 1.37% of monazite was successfully recovered from 45 kg of raw sand collected from the study area. The successful separation of minerals was achieved based on the difference in specific gravity, electrostatic, and magnetic properties [43]. The XRD peak obtained for the recovered mineral is shown in Fig. 9. The XRD analysis of recovered mineral shows significant diffraction peaks of LaPO4, CePO4, NdPO4, PmPO4, GdPO4, and P2O5, and thereby it confirms that of monazite mineral [63].

Table 5 Recovery of monazite from the beach sediments
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Fig. 9
figure 9

XRD of monazite mineral

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4.4 Geochemistry of monazite

The chemical composition of the monazite mineral determined using ED-XRF is given in Table 6. The major chemical constituents of monazite mineral like Ce2O3, La2O3, and P2O5 corresponds to 26.658, 13.421, and 23.649%, respectively. The radioactive content includes U and ThO2, which corresponds to 0.572% and 10.491%. The major constituents present in the mineral are in order of Ce2O3 > P2O5 > La2O3 > Nd2O3 > ThO2 > TiO2 > SiO2 > Pr2O3 > CaO > Al2O3 > Sm2O3 > CdO > U > Fe2O3 > Y2O3 > PbO > MgO. ZnO, IrO2, Gd2O3, and Re are also seen in ppm-level. The values show a strong agreement with the reported values using other geochemical characterization techniques [64]. In the present work, monazite grains were separated from the mineral assemblage using physical separation methods, and hence, the concentrate obtained is 99.9% monazite. The 0.1% may contain other magnetic minerals like ilmenite and leucoxene, since the monazite shows feebly magnetic property and may entrap some other grains also in the concentrate, which is the reason for the contamination. The presence of TiO2, FeO, CdO, ZnO, Ir, and Re shows the contamination of very less amount of ilmenite and rutile minerals. The silicates show the presence of silliminate and kyanite. These XRF values may help to analyze the possible contamination occurred to monazite minerals. In the same manner, the bulk chemical composition using XRF helps to analyze the contaminants in Malaysian monazite [65], Egyptian monazite [66], etc.

Table 6 Chemical composition of monazite using ED-XRF
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On analyzing the monazite from Chavara and Manavalakurichi using ICP-AES and ICP-MS, Jeya et al. 2008 proposed that La content varies from 9.73 to 12.04%, Ce varies from 23.23 to 27.68%, and that of ThO2 and U3O8 is 10.50% and 0.04% [64]. The total REE contents (TREE) ranges from 56.6 to 48.4% in which the light lanthanides (LREE) is 55.6 to 46.8%, and the heavy lanthanides (HREE) is 1.55–0.908%. The reported values of Th, U, Ce, and other rare earths obtained from ED-XRF show good agreement with the results of Jeya et al. (2008) [64]. On comparing with other states like Bhimunipatnam–Konada coastin Andhra Pradesh, India, Bangaku et al. (2016) reported that ThO2 content varies from 3.78 to 13.39%, the REE ranges from 43.47 to 67.78% in which the LREE varies from 42.90 to 64.08%, while HREE varies from 0.57 to 3.70% [67]. It was proved that monazite chemical composition can be used as proxy of provenance and source rock type [68, 69]. The variation in rare earth, Th, and U content is due to the presence of rocks act as provenance for heavy minerals, particularly monazite mineral. It was reported that the thorium shows a considerable variation in charnockites when compared with Khondalites of Eastern Ghats [70, 71].

The monazite mineral along with zircon usually seen as an accessory mineral in the granitic, meta-aluminous to para-aluminous granitoid rocks and in peliticschists and gneisses and rarely in sedimentary rocks [72]. The geochronological studies such as207Pb/206Pb (radiogenic) confirmed that monazite from Chavara region in Kolam, Kerala, vary from 462 to 790 Ma, demonstrating the evidences of Pan-African event. The Sm–Nd (TDM) model ages clearly shows their provenances with protolith ages which varies from 2074 to 2242 Ma. On comparing with monazite mineral collected from Kanyakumari coast in India, the combined EPMA and Sm–Nd isotope studies show tectono-thermal evolution of provenance and crustal accretionary processes [73]. The geochronlogical studies carried out on monazite and zircon confirms the provenance beach placer minerals as hinterland granulites belongs to Southern Granulite Segment (Trivandrum Block). Perumalsamy (2015) found the εNd values of monazites from the southern Kerala about 550 Ma (− 14.6 to − 28.1) which is similar to charnockites and gneisses of the Western Ghats [73]. The TDM ages (from1884 to 2718 Ma) of the beach placer monazites is also similar to these rocks. Thus, the studies confirm that provenance of monazite mineral is Paleoproterozoic—late Neo archaean charnockites and gneisses that evolved from mantle source regions during Neo archaean.

4.5 SEM–EDS analysis

The micromorphology of monazite mineral was studied under the scanning electron microscope (SEM), and its chemistry was analyzed using EDS. Fig. 10a–h shows the SEM images of monazite mineral. The grains are subhedral and tabular in shape (Fig. 9a–e). The monazite grains are elongated to subrounded in shape [74]. Physically attributed fractured features are also observed on the surface of the grains (Fig. 9g). Irregular pits, linear and irregular features formed by impact effects, are also present on the surface (Fig. 9f and h). These morphological features clearly reveal the various mechanical and chemical processes undergone by monazite mineral, especially the impact structures represents the high energy conditions and long transportation experienced by the monazite grains. Mallik (1986) suggests that these morphological features are indicators of polycyclic nature [75].

Fig. 10
figure 10

SEM images of monazite mineral

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SEM–EDS provides a semi-quantitative analytical approach for establishing the existence of La, Ce, Th, and U in the monazite mineral [76]. The EDS analysis mainly shows the presence of Ce, La, Nd, Th, and U. The rare earths like Ce, La, and Nd present around 21.42, 9.61, and 7.87% by weight, respectively. The radioactive elements like Th and U present around 7.42 and 0.71% by weight. The values of Th. U, Ce, and other rare earth results show a strong correlation with the results of Jeya et al. (2008) [64] and the ED-XRF results. The results of SEM–EDS were summarized in Table 7 and Fig. 11.

Table 7 SEM–EDS results of monazite
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Fig. 11
figure 11

SEM-EDS results

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The monazite mineral grains are mounted on a metal stub using a sticky carbon disc for SEM–EDS analysis. The presence of carbon tapes may give the values of C [65]. The value 8.21 corresponds to C not Al. The presence of Al or Al2O3 in monazite cannot be determined using EDS. The EDS measurements are carried out for single monazite grains, and the higher values of rare earths and radioactive elements in EDS results confirm that it is monazite. The columns (a) and (b) in Table 7 shows the EDS analysis of two monazite grains which are clearly shown in Fig. 11.

4.6 Bulk geochemistry of soil samples

The ED-XRF analysis gives the chemical composition as major oxides and trace elements, which helps to identify the presence of heavy minerals, mainly the monazite mineral in soil samples. The ED-XRF technique provides a rapid multi-elemental analysis of geological samples, especially the major, trace, radioactive, and rare earth elements in soil and beach sediments [77, 78]. These results paved the qualitative confirmation of minerals present in the soil [79]. The results are shown in Table 8. The major oxides present in the order of SiO2 > TiO2 > Al2O3 > CaO > ZrO2 > P2O5 > MgO > K2O, and trace elements are in the order of Cd > V>Mn > Hf > Nb > Th > Nd > Cr > Zn > Eu > Pb > Sr > In > Co > U>Ni > Rb > Ga > Yb > Br > Se > Ir > Ta > As > Re. The high SiO2 content in the soil indicates the presence of light minerals like quartz. The presence of TiO2 and FeO3 suggest the presence of ilmenite, rutile, leucoxene, etc., and the Th, U, and REE suggest the presence of monazite mineral. The sample SS7 with ilmenite content of 77.6% corresponds to 52.98% TiO2 and 27.88% Fe2O3. Monazite is maximum for SS3 with 1.4%. The sample SS3 corresponds to maximum Th content of 853.68 ppm, and the nearby location SS4 corresponds to maximum U content of 63.1 ppm. The high silica content of the samples such as SS2, SS4, SS8, and SS9 results in high SiO2 values of around 70%. The soil geochemistry shows good matching with the results of the heavy mineral analysis carried out on soil samples (given in Table 4).

Table 8 Bulk geochemistry of soil samples using ED-XRF (BDL-below detection level)
Full size table

5 Conclusion

The combination of ED-XRF and SEM–EDS is an effective method for determining the chemical composition and surface morphological characteristics of monazite mineral. Using these techniques, major chemical constituents of the monazite mineral in the beach sands of Neendakara–Kayamkulam belt, Kerala, India, were determined. The SEM provides high-resolution images that precisely show the weathering mechanism, both mechanical and chemical, that occurred in monazite mineral. The texture and mineralogy of beach sediments give a clear picture of the distribution of strategic heavy minerals along the coast, which can guide for potential target exploration. Effective recovery of high-grade monazite mineral can be achieved by a judicious combination of magnetic, electrostatic, and gravity separation techniques. Evaluation of economically important monazite mineral using advanced chemical characterization techniques provides solid information to policymakers and scientific community for better utilization and exploration of mineral deposits.