Geoderma 136 (2006) 300 – 309 www.elsevier.com/locate/geoderma
Mineralogical study of some arsenic contaminated soils of West Bengal, India A.K. Ghosh a , D. Sarkar a , P. Bhattacharyya b,⁎, U.K. Maurya a , D.C. Nayak a b
a National Bureau of Soil Survey and Land Use Planning (ICAR), Block-DK, Sector-II, Salt Lake City, Kolkata-700091, West Bengal, India West Bengal State Council of Science and Technology, North block, 4th floor, Bikash Bhawan, Salt Lake City, Kolkata-700091, West Bengal, India
Received 30 August 2004; received in revised form 16 March 2006; accepted 28 March 2006 Available online 12 May 2006
Abstract Five arsenic affected soil profiles, one each from Ghentugachhi and Gotera village of Chakdah block, Gayeshpur, Kalyani block, Nadia district; Ramnagar village of Baruipur block and Sonarpur mouza of Sonarpur block of South 24 Parganas district of West Bengal, India covering the soils of Typic Haplustepts, Typic Endoaquepts, Vertic Haplustepts and Aquic Haplustepts respectively have been studied for their detail mineralogical, chemical composition and also to know the level of arsenic contamination and their probable source. These soils are very deep, developed on Alluvium and are under rice, vegetable and guava cultivation. Sand mineralogy data of these soils are dominated by opaque and limonite, whereas silt is mainly constituted of micas followed by kaolinite and chlorite. The clay mineralogy of the soils indicates that smectite is in higher proportion in Ghentugachhi and Gotera soils followed by mica, kaolinite, chlorite and vermiculite and in the other soils, mica is the dominant clay mineral. In some of these soils, the interstratified mixed layer minerals are also present. The mineralogy class for the soils of Ghentugachhi, Gayeshpur, Baruipur is ‘mixed’ and for Gotera and Sonarpur it is ‘illitic’. Differential thermal analysis (DTA) of clay fraction of surface soil of the profiles displayed the presence of the minerals like magnetite, marcasite, arsenopyrite, illite, montmorillonite and quartz. The source of arsenic in these soils are probably due to the presence of arsenic bearing minerals, marcasite and arsenopyrite, and also it may occur as adsorbtion on iron hydroxide coated sand grains and clay minerals. Arsenic concentration is very high in the surface horizon of pedon 2 (20.2 mg kg− 1) and it shows a decreasing trend down the subsurface horizon. The higher concentration of arsenic in the surface horizon may be due to higher abundance of opaque minerals, limonite, and marcasite. © 2006 Elsevier B.V. All rights reserved. Keywords: Arsenic; Soil; Mineralogy; XRD; DTA
1. Introduction Arsenic pollution in groundwater in India and Bangladesh is considered to be the largest contamination problem in the world. In the eastern state of West Bengal (India), groundwater resources are quite rich and a major part of the groundwater is used for drinking, ⁎ Corresponding author. Tel.: +91 332 661 8751. E-mail address:
[email protected] (P. Bhattacharyya). 0016-7061/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.geoderma.2006.03.044
agricultural and industrial purposes. With the evergrowing population due to rapid urbanization, industrial and agricultural expansion, groundwater is often being used extensively and erratically, causing a deleterious effect on water quality. In some areas of Bangladesh and West Bengal (India), the concentration of arsenic (As) in groundwater exceeds the guideline concentration set internationally and nationally at 10–50 mg As/l. Presently, in West Bengal 5 million people in 978 villages from eight districts and southern part of Kolkata
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(Ghosh et al., 2004a,b) are drinking As contaminated water, having As concentration above 0.01 mg As/ l (WHO, 1996). Such groundwater is also used for irrigation and this makes it possible for As to enter the human food chain through locally grown food crops and vegetables. Adak et al. (2002) showed that the potato contents appreciable amount of As in its tuber, stem and leaf which are grown in the soils having As level of 1.50 mg kg− 1 and irrigated by arsenic contaminated ground water in Nadia district, West Bengal, India. The similar findings are also reported by Bhattacharyya et al. (2003) in rice growing soils under submerged condition (As content in soil was 2.37 mg kg− 1). Ghosh et al. (2004) showed that the low levels of As (3.84–6.15 mg kg− 1) decreased the microbial biomass and their activities. There is also a chance of a concomitant biomagnifications of arsenic as it moves up the food chain. Soil is a principal sink of As in the environment and as most of the arsenical residues have low solubility and low volatility, they generally accumulate in the topsoil layers (Woolson et al., 1973). Mineralogy plays an important role to understand the genesis, physical and chemical properties of the soils. The amount and the nature of clay, silt and sand minerals mainly control As adsorption in soils. The study of primary minerals in sand fractions are important to know the amount and nature of weatherable minerals, nature of inclusion, shape of the grains, nature of parent materials and stages of soil transformations during weathering processes (Bullock et al., 1985; Venugopal, 1992; Lekha et al., 1998; Niranjane et al., 2001). The important minerals that control the arsenic absorption capacity of the soils include Fe and Al oxides (Jacabs et al., 1970; Livesey and Huang, 1981; Fuller et al., 1993). Many workers (Ghosh and De, 1995; Saha et al., 1997; Acharyya et al., 1999, 2000) studied the toxicity in the ground water of West Bengal by taking core sample sediment collected by drilling up to depth of 250 m and analysed the samples for heavy minerals in the sediment particularly belonging to the upper delta plain of Meander belt. These authors indicated that arsenic rich pyrite or other arsenic minerals are rare or absent in the sediments. They also concluded that arsenic appears to occur adsorbed on iron hydroxide coated sand grains and clay minerals and transported in soluble form and coprecipitated with or is scavenged by Fe (III) and Mn (IV) in the sediment. With this background information, an attempt has been made to report on physical, chemical and mineralogical characteristics of the soils and to know the probable soil components that are responsible for As retention/source in the soils of West Bengal.
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2. Materials and methods 2.1. Study area The study area lies in the Indo-Gangetic alluvial plain covering Gotera village (latitude 23°0′37ʺ and longitude 88°34′30ʺ), Ghentugachhi village (latitude 23°1′19ʺ and longitude 88°33′37ʺ) in Chakdah Block, Gayeshpur (latitude 22°56′35ʺ and longitude 88°29′44ʺ) in Kalyani block, District-Nadia; Ramnagar village (latitude 22°25′ 15ʺN and longitude 88°27′30ʺE) in Baruipur block, District-24 Parganas (S) and in Sonarpur, District-24 Parganas (S). 2.2. Soil surveys Detailed soil survey of the above stated blocks were carried out on a 1:4000 scale and five soil profiles were studied in details in different arsenic contaminated zones and soil samples were collected horizon wise. Soil samples were air dried and ground in a mortar pestle. The morphological properties of the profiles are described as per standard terminology of the USDA soil survey manual (Soil Survey Staff, 1998). All the important physical and chemical properties were determined according to the methods outlined by Jackson (1979). 2.3. Mineralogical analysis: sand mineralogy Particle size analyses were carried out following the international pipette method. Sand (2000–50 μm), silt (50–2 μm) and total clay (b2 μm) fractions were separated from the samples after dispersion according to the size segregation procedure of Jackson (1979). Total sand was passed through sieve and different size fractions were separated following the standard procedure (Day, 1965). Fine and very fine sand of the selected horizons were separated and fractionated into heavy and light mineral fractions using bromoform (specific gravity 2.85). These mineral fractions were then mounted on slides with Canada Balsam and mineral species were identified using Laborlux-12 petrographic microscope for qualitative estimation (Cady, 1965). For semi-quantitative estimate nearly 275 grains were counted in all the slides and their percentage abundance was calculated. 2.4. Clay and silt mineralogy Oriented clay (b 2 μm) and silt fractions (50–2 μm) were subjected to X-ray diffraction (XRD) analyses after saturating the samples with Ca and solvated with ethylene glycol, K saturated and heated to 25, 110, 300 and
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Table 1 Site and morphological characteristics of the soils of Ghentugachhi, Gotera, Gayeshpur of Nadia district, Baruipur and Sonarpur of South 24 Parganas district in West Bengal Depth (m)
Horizon
Matrix colour (moist)
Structure
Nodules
Land use
Pedon 1 : Ghentugachhi, Vertic Haplustepts 0–0.20 Ap 0.20–0.46 Bw1 0.46–0.82 Bw2 0.82–1.27 Bw3 1.27–1.50 Bw4
10YR3/2 10YR3/2 2.5Y4/2 2.5Y4/0 2.5Y5/2
m m2sbk m3sbk m2sbk m3sbk
– – vf,f vf,f –
Paddy
Pedon 2 : Gotera, Typic Endoaquepts 0–0.20 Ap 0.20–0.43 Bw1 0.43–0.64 Bw2 0.64–1.02 2C1
2.5Y4/2 2.5Y5/2 2.5Y5/2 2.5Y5/2
m m2sbk m2sbk sg
vf,f vf,f vf,f –
Paddy
Pedon 3 : Gayeshpur, Typic Haplustepts 0–0.14 Ap 0.14–0.35 Bw1 0.35–0.54 Bw2 0.54–0.75 Bw3 0.75–0.92 C1 0.92–1.21 C2
2.5Y4/4 2.5Y4/3 2.5Y4/3 2.5Y5/4 2.5Y5/4 2.5Y5/4
m1sbk m2sbk m2sbk m1sbk sg sg
vf,f f,m f,m vf,m – –
Paddy, vegetables
Pedon 4 : Baruipur, Typic Haplustepts 0–0.18 Ap 0.18–0.38 Bw1 0.38–0.64 Bw2 0.64–0.86 Bw3 0.86–1.10 Bw4 1.10–1.35 Bw5
10YR5/2 10YR5/2 10YR5/2 10YR5/3 10YR6/3 10YR5/3
f1sbk m2sbk m2sbk m1sbk m1sbk m1sbk
– – – – – –
Guava orchard
Pedon 5 : Sonarpur, Typic Haplustepts 0–0.14 Ap 0.14–0.46 Bw1 0.46–0.69 Bw2 0.69–1.04 Bw3 1.04–1.35 Bw4
10YR5/2 10YR5/4 10YR5/4 10YR4/3 10YR5/3
f1sbk m1sbk m2sbk m2sbk f1sbk
– – m,c m,c m,c
Paddy, vegetables
m1sbk — medium weak sub angular blocky; m2sbk — medium moderate sub angular blocky; m3sbk — medium strong sub angular blocky; sg — single grain; f1sbk — fine weak sub angular blocky; nodules: vf — very fine; f — fine; m — medium, (abundance): f — few, c — common, m — many.
550 °C with a Philips diffractometer using a Ni filtered CoKα radiation at a scanning speed of 2°2θ/min. The identification of minerals in clay and silt fractions was done following the criteria laid down by Wilson (1987) and semi-quantitative estimates of the clay minerals were done based on the principles outlined by Gjems (1967). 2.5. Differential thermal analysis (DTA) Thermogravimetry is an analytical technique concerned with the measurement of weight changes in samples on heating. Moisture is lost by many clay minerals on heating and these changes are of course ac-
companied by a corresponding loss in weights commonly evident by Differential thermal analysis (DTA). This technique is most commonly employed for qualitative analysis of clay minerals. It supplements the information obtained by optical microscopy and XRD studies. In the present investigation, this technique has proved useful in revealing more specific information, which is not adequately available by other methods. In this study DTA of clay fraction of surface soil samples were conducted with a 100 mg sample to a maximum of 1000 °C at a heating rate of 10 °C per minute with the help of Differential Thermal Analyser (Model: Netzsch Geratabau D-8672, Germany). Burnt Al2O3 of 100 mg was used as a reference material under conditions
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A.K. Ghosh et al. / Geoderma 136 (2006) 300–309 Table 2 Physical and chemical characteristics of arsenic contaminated soils of West Bengal Depth (cm)
Particle size
Tex. class
pH
O.C (g kg− 1)
CEC (cmol (p+) kg− 1)
Fed (g kg− 1)
Feo (g kg− 1)
Pedon 1 : Ghentugachhi, Vertic Haplustepts 0–20 1.3 42.7 56 20–46 0.4 39.8 59.8 46–82 1.7 48.5 49.8 82–127 1 40.2 58.8 127–150 1 40.6 58.4
sic sic sic sic sic
7.5 7.6 7.9 7.8 7.6
16 8 5 5 4
34.6 35 26.5 35.9 29.6
5 8.1 9.6 4.4 4.6
4.5 3.2 3 2.4 2.7
1.3 0.9 0.9 1 0.3
Pedon 2 : Gotera, Typic Endoaquepts 0–20 41.8 41.5 20–43 24.8 39.7 43–64 24.8 50.1 64–102 82.3 11.6
16.7 35.5 25.1 6.1
1 cl l ls
7.5 7.5 7.7 7.6
12 4 3 2
13.8 22.4 18.7 4.5
0.5 3.7 2.1 2.6
4.9 3.4 3.7 1.8
20.2 1.3 1.4 1.5
Pedon 3 : Gayeshpur, Typic Haplustepts 0–14 26.3 55 14–35 16.7 54.9 35–54 19.3 52.4 54–75 55 24.1 75–92 86.3 5.4 92–121 88.1 1.8
18.7 28.4 28.3 20.9 8.3 10.1
sil sicl sicl scl ls ls
7.1 7.6 7.5 7.4 7.3 7.6
7 5 3 2 1 1
12.5 16.6 18.8 11.9 3.7 3.2
1.8 4.4 5 4.2 2.5 1.9
4.6 4.9 4 3 1.6 1.8
1.2 1 0.9 0.8 0.6 0.1
Pedon 4 : Baruipur, Typic Haplustepts 0–18 26.7 59.2 18–38 15.2 63.1 38–64 18.4 55.9 64–86 48.1 36.2 86–110 63.6 26.7 110–135 6.8 69.5
14.1 21.7 25.7 15.7 9.7 23.7
sil sil sil l sl sil
6.5 6.7 6.8 7.8 8.6 8.3
4.5 2.7 2.6 1.1 0.5 0.5
8.3 12.2 14.9 9.7 5.6 13.1
2.1 1.8 2.5 2.1 2.3 3.1
5.2 4.3 3.8 1.9 2.5 1.7
3.6 2.3 2.4 2.2 2.4 2.4
Pedon 5 : Sonarpur, Typic Haplustepts 0–14 11 77.3 14–46 5.2 67.1 46–69 4.1 72.2 69–104 6 71.3 104–135 18.7 66.6
11.7 27.7 23.7 22.7 14.7
sil sicl sil sil sil
7.2 7.3 8.3 8.2 8.1
8 6 4 4 3
6.8 17.7 11.5 10.5 7.9
3.4 2 2.8 3.1 2.7
4.3 4.8 3.5 2.8 2.3
4.5 2.3 2.5 2.4 2.1
Sand (%)
Silt (%)
Clay (%)
As (mg kg− 1)
sic — silty clay, sicl — silty clay loam, l — loam, sil — silt loam, sl — sandy loam, ls — loamy sand.
3. Results and discussion
pedon 1. Lithological discontinuity was observed in the soils of pedon 3 (Gayeshpur) and pedon 2 (Gotera). The site and morphological characteristics of soils are described in Table 1.
3.1. Site and morphological characteristics
3.2. Physical and chemical characteristics
The soils that occur on very gently sloping plains (1– 3%) are mostly deep, moderately to imperfectly drained and they are subject to occasional flooding during rainy season. The dominant hue is 2.5Y, whereas the Baruipur and Sonarpur soils show a hue of 10YR, which may be due to the presence of abundant Fe–Mn nodules. The lower chroma in all soils indicates the aquic characteristics associated with seasonal wetting. The soil textures are medium for pedons 2, 3, 4 and 5 whereas it is fine for
Physical and chemical properties of the soils are presented in Table 2. The soils of the alluvial flood plain (pedons 2, 3 and 4) contained more sand and less clay, whereas more stable landform (pedons 1 and 5) contained more clay (N 35%) and less sand. This reversal in the distribution of sand and clay with depth in the alluvial and flood plain may be due to preferential deposition of coarser sediments brought down by the Hugli river and its tributaries. The clay content
detailed by Smykatz - Kloss (1974) and the minerals were identified accordingly.
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decreases with depth likewise cation exchange capacity. The soils under study are neutral to mildly alkaline in reaction (pH 6.5 to 8.6) and the pH value increases with depth. Soils of Gotera and Ghentugachhi (pedons 1 and 2) have high organic carbon content in the surface horizon and low to medium in the subsurface horizon whereas the soils of Gayeshpur, Baruipur and Sonarpur have medium organic carbon content in their surface horizon and low in the subsurface horizon. Arsenic concentration is higher in surface soils than subsurface soils. Arsenic absorption in surface soils is highly correlated with Feo (ammonium oxalate extractable Fe) and it decreases downwards the profile (Ghosh et al., 2002). As in the studied profile Feo is high, therefore, arsenic
content in the surface soils is relatively higher than the subsurface soils. 3.3. Differential thermal analysis (DTA) Differential thermal analysis of the clay fraction of the surface soils of pedons 1, 2, 3, 4 and 5 from Ghentugachhi, Gotera, Gayeshpur, Baruipur and Sonarpur respectively have been studied to know the structural changes of minerals during thermal treatment. The characteristics of DTA curve are shown in Fig. 1 and the results of the analysis are given in Tables 3 and 4. The following characteristic properties have been observed from the figures. The curve between 75 and 235 °C shows
Fig. 1. DTA curves of the studied pedons.
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305
Table 3 Minerals identified by DTA studies (temperature,T in °C) Mineral
Standard value
Observed value
Endothermic reactions (ΔT)
Montmorillonite Illite Magnetite (FeO·Fe2O3) Quartz (SiO2) Arsenopyrite (FeAsS) Marcasite (FeS2) Vermiculite
Exothermic reactions (ΔT)
Endothermic reactions (ΔT) Dehydration (H20)
Dehydration (OH) and decomposition
155–235 75–140 –
690–695 527 –
– – 350
520–578 –
947 426–926 300–450 480–700 – 490 and 540
– –
580–585 –
– 540
–
–
415, 480
–
–
450–465
90–125
503–826
851
115–155
–
850
Dehydration (H20)
Dehydration (OH) and decomposition
145–220 100–142 –
725 560–740 450–590
– –
Exothermic reactions (ΔT)
two separate endothermic effects due to the dehydration of the adsorbed water and the loss of interlayer water. The shape of this endothermic deflection is well rounded and the differences of the width at half height of dehydration peaks seem to be caused by a different amount of water being adsorbed in different soils. An exothermic curve between 235 and 400 °C with the tip of the peak at 350 °C indicates the oxidation of magnetite. Further heating a characteristic exothermic curve of marcasite appears in the temperature range of 420 to 480 °C, with the tip of peak at 450, 455 and 465 °C. An endothermic curve appears in the temperature range of 510–550 °C with the weak tip at about 527 °C which corresponds to the decomposition of the mineral and the elimination of hydroxyl groups (OH−) as water which has been a part of octahedral layer in Ghentugachhi and Sonarpur soils indicating the presence of illite. An exothermic curve between 515 and 555 °C in the soils of Baruipur with tip at 540 °C indicates the presence of arsenopyrite. An endothermic curve between 525 and 620 °C and sharp peak at 580 and 585 °C indicates the inversion temperature of well order quartz crystals. This shows that the quartz crystals present in the soils has been derived from an igneous source. The presence of smectite (montmorillonite) in Gotera soils is ascertained by its temperature of decomposition peak at 695 °C indicating that smectite is dioctahedral type. The broad rounded DTA deflection between 750 and 950 °C with weak peak at 850 °C indicates the presence of trioctahedral vermiculite in all soils. This exothermic peak also reflects the recrystallisation of a new structure (a kind of mullite).
Sonarpur has been carried out and the results are presented in Table 5. Representative X-ray diffractograms of pedon 1 has been shown in Fig. 2. Smectite expands to 17.2 Å on solvation with ethylene glycol but on Ksaturation and heating at 25 °C, the peak collapses to 10.1 Å, indicating that clay has high charge smectite and slight expansion on glycolation. The gradual decrease in the intensity of 7.2 Å kaolin peak with the reinforcement of 10.1 Å peak on K-saturation and subsequent heating suggest that these kaolins are to some extent interstratified with smectite (Sm/K) in most of the studied profiles. Similar interstratification of smectite with kaolinite has also been reported in some benchmark soils of West Bengal (Sarkar et al., 1999) and in red soils of southern India (Pal et al., 1989). Randomly interstratified kaolinite–smectite yields a reflection between 7.2 and 8.2 Å in the glycolated state resulting from the combined effect of 001 kaolinite and 002 smectite in all horizons of pedon 1 (Ghentugachhi), pedon 2 (Gotera) and pedon 3 (Gayeshpur) whereas in pedon 5 (Sonarpur), it is
Table 4 Minerals identified by DTA techniques in surface horizons of different pedons Pedon No.
Location
Minerals
1
Ghentugachhi
2
Gotera
3
Gayeshpur
3.4. Mineralogy of clay and silt fractions
4
Baruipur
X-ray analysis of clay fraction of restricted horizons of Ghentugachhi, Gotera, Gayeshpur, Baruipur and
5
Sonarpur
Magnetite, marcasite, montmorillonite, vermiculite, illite and quartz Magnetite, marcasite, montmorillonite, vermiculite and quartz Magnetite, marcasite, vermiculite and quartz Magnetite, marcasite, arsenopyrite, vermiculite and quartz Magnetite, marcasite, illite, vermiculite and quartz
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Table 5 Semi-quantitative mineralogical analysis of clay and silt fractions of Ghentugachhi, Gotera and Gayeshpur soils of Nadia Dist., West Bengal and Baruipur and Sonarpur soils of South 24 Parganas of West Bengal Depth (cm)
Mineral abundance in clay fraction (%)
Mineral abundance in silt fraction (%)
Intensity ratio of peak height of 001//002 mica
Smectite Mica Kaolinite Chlorite Vermiculite Mixed Quartz layer
Mica Kaolinite Chlorite Vermiculite Quartz Feldspar
silt
clay
Pedon 1 0–20 20–46 82–127
: Ghentugachhi, Vertic Haplustepts 36 26 14 12 9 35 29 12 8 13 38 28 14 7 11
– – –
4 2 2
57 59 64
16 15 17
17 15 13
1 2 2
6 5 5
4 5 6
1.84 2.00 2.01
1.71 1.62 1.52
Pedon 2 0–20 20–43 64–102
: Gotera, Typic Endoaquepts 29 54 10 4 38 49 12 – 34 53 9 –
– – –
– – 7
1 2 2
70 60 69
13 14 12
10 10 6
– – –
4 4 3
3 3 4
2.09 2.03 2.53
1.77 1.97 2.17
Pedon 3 0–14 14–35 54–75
: Gayeshpur, Typic Haplustepts 20 42 24 12 34 34 22 11 30 40 22 8
– – –
– – –
2 2 –
64 60 60
19 22 17
8 9 8
– – –
5 4 7
5 5 9
2.23 2.05 2.11
2.18 1.51 2.15
Pedon 4 0–18 38–64 86–110
: Baruipur, Typic Haplustepts 15 55 14 16 25 54 13 2 35 32 15 7
– 6 5
– – –
– 1 6
63 64 53
16 12 25
8 8 3
– – –
6 7 8
8 8 12
2.56 2.53 2.45
2.93 2.75 2.61
Pedon 5 : Sonarpur, Typic Haplustepts 0–14 15 58 18 6 46–69 14 57 18 9 104–135 12 64 15 7
3 – –
– – –
1 2 2
64 58 63
13 17 16
8 6 5
– – –
8 10 8
7 9 8
2.46 2.44 2.39
2.25 2.24 2.18
restricted to surface and subsurface horizons respectively. Vermiculite (14 Å) is present in each horizon of Gotera and Ghentugachhi soils whereas in Gayeshpur, Baruipur and Sonarpur soils, it is present in trace amount and in some restricted horizons only. Mica peak at 10 Å reflection is dominantly asymmetrical towards low angle side with prominent higher orders at 5.0 and 3.33 Å. The 002 reflections of mica is usually about one third of the intensity of 10 Å reflections in most of the profiles indicating that the mica is in a dioctahedral nature. The presence of 12 Å mixed layer mineral is found in some layers of the studied soils. Semi-quantitative analysis of the clay fraction (Table 5) of Ghentugachhi and Gotera soils indicates that smectite and mica are in higher proportion followed by kaolinite, vermiculite and chlorite whereas quartz is present in subordinate amount. Baruipur and Sonarpur soils are constituted dominantly of mica followed by smectite, kaolinite and chlorite; quartz and vermiculite. Smectite shows an increasing trend from surface to subsurface in most pedon 1 whereas in pedon 5 it
shows a decreasing trend. Mica shows an inverse relationship with smectite. This clearly indicates that during weathering and soil forming processes, mica is transforming to smectite. The ratio of 001/002 reflections of mica in all profiles is higher than unity indicating that mica consists of more biotite than muscovite and this biotite mica has been supposed to weather to smectite. Mineralogical analysis of clay fraction of soils under study indicates that in the pedons (3 and 4), the abundance of individual clay minerals is below 50% of the total clay in the series control section (Table 5) and according to the Keys to Soil taxonomy (Soil Survey Staff, 1998), these soils (Ghentugachhi, Gayeshpur, Baruipur) qualify for ‘mixed’ mineralogy class whereas soils of Gotera and Sonarpur qualify for illitic mineralogy class. The mineralogy of silt fraction indicates that mica is the dominant (53–70%) followed by kaolinite (12–25%), chlorite (5–17%), traces of vermiculite (1–2%) and feldspar (3–12%).
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Fig. 2. Representative X-ray diffractogram of clay fraction of pedon 1. m — mica, k — kaolinite, sm — smectite, ch — chlorite, Q — quartz, Ca — calcium saturated, Ca–Eg — calcium saturated and ethylene glycol solvated, k25 °C, k110 °C, k300 °C, k550 °C — potassium saturated and heated to k25 °C, k110 °C, k300 °C, k550 °C respectively.
3.5. Sand mineralogy 3.5.1. Light mineral assemblages Quartz are the most dominant minerals in these soils ranging from 55% to 78% and are polycrystalline in nature (Table 6). Inclusion of iron oxide minerals can be observed on the surfaces of these minerals. Plagioclase and potash feldspar are maximum in abundance and their concentration varies from 7% to 25%. The former is characterized by lamellar twining where as the latter shows a crosshatched twining. Plagioclase feldspar shows the stinging of iron oxide on its surfaces. It also shows the alteration to clay minerals along the cleavage plain. Muscovite flakes are mostly altered and their abundance varies from 9% to 17 %. 3.5.2. Heavy mineral assemblages It is evident from Table 6 that the opaque mineral constitutes about 27% to 43% of the total heavy minerals that includes mainly hematite, magnetite, and ilmenite. Limonites are yellow to brownish yellow in colour formed by the alteration of iron minerals mixed
with clay and other impurities are next in abundance. Tourmaline and hornblende are mostly euhedral in shape. Iron oxide inclusions can be observed on the grain surfaces of these minerals. Altered biotite flakes and pyroxene grains with ferruginous coating is common in most of these soils. Thus opaque mineral is the dominant followed by limonite, pyroxene and biotite in the heavy minerals of sand fractions in these soils. The presence of small amount of prismatic zircon, euhedral tourmaline and rutile in these soils suggest that these soils are less weathered due to recent to sub recent alluvium parent material. Presence of small amount of Kyanite and sillimanite indicate metamorphic source provenance. Presence of euhedral zircon crystal and opaque inclusion indicate that the soils developed in these areas are in situ. Prismatic crystals of zircon indicate that the soils have not undergone long distance of transportation. From the foregoing observations on heavy mineral assemblages it is concluded that the soils has high amount of opaque and limonite mineral assemblages. The presence of potash feldspar, plagioclase feldspar and biotite
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Table 6 Semi-quantitative mineralogical analysis of fine sand fraction of soils of Ghentugachhi, Gotera, Gayeshpur of Nadia Dist., Baruipur and Sonarpur of 24 Parganas (S) Dist. of West Bengal Depth (cm)
Mineral abundance in heavy sand fractions of soils (percent by counts)
Lm
Op
Hb
St
Ph
Mineral abundance in light sand fraction (%)
Ky
Sil
A
Gt
T
E
R
Z
Px
B
Qtz
Fld
M
Pedon 1 : Ghentugachhi, Vertic Haplustepts 0–20 20 28 7 tr 6 20–46 18 34 6 1 5 82–127 15 32 8 2 4
3 3 2
tr tr tr
tr 2 3
tr tr tr
3 7 6
4 3 4
tr 2 tr
tr 1 tr
15 7 14
13 11 10
76 71 68
7 18 20
17 11 12
Pedon 2 : Gotera, Typic Endoaquepts 0–20 27 43 6 tr 20–43 16 27 3 tr 43–64 24 30 7 2 64–102 18 35 8 tr
2 2 3 4
tr 7 tr 2
tr tr tr 2
3 2 tr tr
1 1 tr 1
5 6 7 5
tr tr 2 tr
tr tr tr tr
tr tr tr tr
9 25 16 17
4 11 9 8
73 70 59 67
11 19 25 20
16 11 16 13
Pedon 3 : Gayeshpur, Typic Haplustepts 0–14 20 30 15 2 14–35 22 30 9 tr
3 7
1 1
tr tr
2 2
tr 1
4 5
2 3
tr tr
tr tr
14 12
7 8
69 62
19 19
12 12
Pedon 4 : Baruipur, Typic Haplustepts 0–18 23 31 5 2 86–110 20 34 8 tr
5 tr
2 3
2 3
3 5
tr tr
6 3
4 2
tr tr
2 tr
7 10
9 12
72 58
15 25
13 17
Pedon 5 : Sonarpur, Typic Haplustepts 0–14 20 30 7 2 14–46 23 37 7 tr 69–104 26 30 9 tr
7 5 3
2 2 2
tr 1 1
tr 1 1
2 2 1
3 5 3
tr 2 2
2 tr tr
tr 1 2
9 8 13
15 6 7
78 69 66
9 17 25
13 14 9
Op—Opaque, E—Epidote, Lm—Limonite, R—Rutile, Hb—Hornblende, Z—Zircon, St—Staurolite, Ph—Phlogopite, Ky—Kyanite, B—Biotite, Sil—Sillimanite, Qtz.—Quartz, Gt—Garnet, Fld—Feldspar, T—Tourmaline, M—Muscovite, A—Apatite, Px—Pyroxene, tr—trace amount.
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