5.2. Minerals in rock samples
Figure 1 shows the infrared spectra of the rock samples in the frequency range 400-4000 cm -1. The minerals such as quartz, orthoclase feldspar, microcline feldspar, calcite, kaolinite, montmorillonite, calcite, aragonite and Palygroskite are identified in rock samples and its wavenumbers are given in Table 1. In all the investigated samples, quartz, calcite and kaolinite are major minerals and others are minor minerals.
5.2.1. Quartz
Crystallinity nature is confirmed form the observed peak at 695 cm-1 for quartz. If the intensity of this peak increases, the crystallinity increases (Mullainathan and Nithiyanantham et al., 2016; Ramasamy et al., 2003). Quartz is the main silicate mineral and identified in all the rock samples. This quartz identified at peak 695 cm-1 due to Si-O symmetrical bending vibrations and 778 cm-1 due to Si-O symmetrical stretching vibrations. In addition to that, quartz found at peak 460 cm-1 Si-O asymmetrical bending vibrations. Some other peaks also found at 1615 cm-1, 1870 cm-1 for quartz due to weathered from metamorphic origin (Ramasamy et al 2009).
5.2.2. Feldspar
Feldspar is a one of the most abundant mineral group on Earth. This mineral group includes the orthoclase, microcline and plagioclase feldspars. In the present work, rock samples of AR8, AR9 and AR10 shows that presence of orthoclase feldspar at peak 435 cm-1 due to Si-O asymmetrical bending vibrations and at peak 539 cm-1 due to O-Si (Al)-O symmetrical bending vibrations (Laves, 1960; Ramasamy and Dheenathayalu, 2001; Müller et al. 2014; Papakosta et al., 2020). This band is due to the coupling between the O-Si-O bending vibration and the K-O stretching vibration (Theodosoglou et al., 2010; Matteson and Herron, 1993). Rock samples AR1 and AR2 are convey the presence of orthoclase feldspar at peak 1175 cm-1 due to Si-O stretching vibration whereas other samples show the absence of orthoclase. In addition to that, the presence of microclines is observed by the bands in the region between 1000 and 1100 cm-1. That is, microcline feldspar was recorded at peak 1055 cm-1 due to the Si(Al)-O stretching vibrations for the samples AR1, AR2, AR8, AR9 and AR10 and other samples shows absence of this mineral.
5.2.3. Clay minerals
Clay minerals are formed as a result of alteration of such primary minerals as feldspars, micas, chlorides, etc. (Khang, et al., 2016). Clay minerals of montmorillonite and kaolinite are identified in rock samples of Attirampakkam, Tamil Nadu. From the FT-IR spectrum, Si-O-Si and Al-O-Si bands of the mineral lattice around 935, 940 and 535 cm-1 and it clearly show the absorption bands of O-H stretching. There are two narrow and intensive absorption peaks are observed at 3695 and 3625 cm-1 and weak absorption bands are registered at peaks around 3650 and 3665 cm-1 for kaolinite due to O-H stretching. According to Russell (1987), if four peaks are observed in the region 3697–3620cm-1, the mineral is said to be ordered state. For collected rock samples, the four peaks are observed at 3625, 3650, 3665 and 3695 cm-1 which indicates that mineral is in ordered state. The clay mineral montmorillonite was identified at peaks 479, 1645 and 3440 cm-1 from only samples AR1-AR7. The peak 479 cm-1 appears due to AL-O-Si deformation and peak 1645 cm-1 is due to OH deformation of water. Including with these peaks, strong band 3440cm-1 also observed for montmorillonite due to H-O-H stretching of structural hydroxyl groups and water (Maina et al., 2015). Palygorskite are hydrous Mg silicate clay minerals with fibrous-like morphologies that typically occur as fine-grained, poorly crystalline masses. In the present study, this mineral was identified in the samples AR3-AR7 at peak 515 cm-1 due to Mg (2) octahedral deformation (McKeown et al., 2002).
5.2.4. Carbonate minerals
Carbonate is a key component influencing both chemical and physical properties of samples; it has been accurately estimated by using FTIR spectroscopy (Tinti et al., 2015 Bruckman, et al., 2013; Grinard, et al., 2012). Using obtained FT-IR spectrum, carbonate minerals dolomite and aragonite are identified by peaks at 1445 and 1085 cm-1 respectively. It seems that dolomite peaks appear at shorter wavelengths of spectrum and invariably present in all the samples due to a strong band between 1430 and 1500 cm-1due to the CO32- stretching vibration whereas mineral aragonite was found only AR3-AR7.
5.3 Crystallinity index of Quartz:
In the present stud, quartz is the major mineral present in all the rock samples, hence the crystallinity index of quartz has been calculated. On the other side, crystallinity of quartz will give a clear indication on the crystalline forms of other minerals because quartz is the mineral, which crystallizes last (Ramasamy and Suresh, 2009). Crystalline samples (natural and synthetic) quartz have the highest values of the index of crystallinity. Minimal variation in crystallinity index shows the sustained processes of crystallization and formation of a committed and well-ordered crystal. A spectrum of x-ray diffraction also reflects the presence of crystallinity (Korovkin et al., 2016). The crystallinity index is calculated using the formula
Where I778 is the intensity of absorption band around 778cm-1 due to the vibrations in tetrahedral site symmetry and I695 is the intensity of the absorption band around 695cm-1 due to the vibrations in octahedral site symmetry. The calculated crystallinity index of quartz is given in Table 3. If the crystallinity index values are greater than 1.00, it is said to be disordered whereas if the crystallinity index values are below 0.75, then it is considered to be ordered crystalline quartz (Ramasamy et al., 2004). The intermediate crystalline quartz is represented by values in between 0.75 to 1.00 (Ramasamy et al., 2004). As seen from table 3, crystallinity index of quartz for all the rock samples are greater than 1.00. This indicates that mineral quartz is disordered in rock samples and variation of crystallinity index of quartz in rock samples are given in Fig 3.
5.4 X-ray diffraction analysis
The recorded XRD spectra of rock samples are shown in Fig 4. The derived peaks and corresponding minerals are given in table 2. Using 2-theta (degree) and d-spacing (Å) of XRD spectrum, various minerals are identified from JCPDS, 2000. The minerals such as quartz, kaolinite, calcite, dolomite, aragonite, hematite and rutile are identified in rock samples. The XRD analysis shows the crystalline and non-crystalline nature of minerals in the samples. That is, above mentioned minerals are confirmed by XRD indicates the crystalline form whereas non-crystalline form was identified for some minerals which are identified through FT-IR only (Suresh et al., 2011).