Journal of Minerals & Materials Characterization & Engineering, Vol. 8, No.3, pp 223-228, 2009 Printed in the USA. All rights reserved
Characterization of Sulphides Patches from the Neyveli Lignite Deposit
D.S. Rao*
, R. Nagendra
, E. Rasool Mohideen
and B.R. Nayak
NML-MC, CSIR Madras Complex, Taramani, Chennai-600 113
Present address, Mineralogy Department, IMMT, Bhubaneswar-751 013
Centre for Geoscience and Engineering, Anna University, Chennai-600 025
Dept. of Civil Engineering, Crescent Engineering College, Chennai – 600 048
MNP Division, National Metallurgical Laboratory, Jamshedpur – 831007
*Corresponding Author:
The mineralogy and mineral chemistry of the hard, compact and heavy sulphide patches, found
along with lignites of Neyveli, Tamilnadu, India was investigated by optical and electron probe
microanalysis. These sulphide boulders consist mostly of marcasite, pyrite and rarely pyrrhotite.
Presence of gold in rare and trace quantities from the marcasite of these sulphide patches is
reported here for the first time. If this gold content evaluated properly and recovered
economically and in an environmental friendly way it adds up to the revenue of the mining
company as well as stop the environmental degradation. If marcasite is left as waste, then it may
add up to the environment as acid mine drainage.
Key words: Marcasite, Lignite, Neyveli, Mineral chemistry
The main Neyveli lignite field is situated between 11
13’ –11
38’ N Latitude and 79
East Longitude. It extends over a distance of 44 kilometers in N-S direction and 12 kilometers in
E-W direction. This lignite deposit forms a part of the Cuddalore Formation of Late Miocene age
and are associated with argillaceous and ferruginous sandstone and beds of clay. Neyveli Lignite
Corporation Limited is engaged in mining of lignite from its two mines (Mine I and Mine II)
which in turn used for thermal power generation in its two captive power stations. Open cast
mining is being adopted in Neyveli for exploration of lignite. Lignite being a lower ranking coal
224 D. S. Rao, R. Nagendra, E. Rasool Mohideen, and B.R. Nayak Vol.8, No.3
is characterized by high moisture as well as volatile matter and lower heating values in
comparison to high ranking coals. Above all this Neyveli liginite also varies widely in its
sulphide sulphur content. Sulphur from these sulphides creates multifold problems during
combustion in boilers leading to slagging, fauling, corrosion and of course environmental
pollution. It is generally assumed that the sulphur could be present in coal as pyritic sulphur or
sulphate sulphur or organic sulphur (Banerjee et al., 1997) all of which behave in different
manner during combustion in the boilers/power plants.
In the Neyveli lignite mines lignite is contaminated with sulphides. These sulphides are mostly
pale bronze yellow to greenish yellow in colour, heavy as well as non-magnetic in nature;
hardness varies from 6 to 7 and with a compressive strength of around 82 N/mm
(Kompella and
Raghuthaman, 2002). The sulphides present in these mines as boulders, lenticular masses,
nodules, veins, platty, wedge shaped lenticular masses and massive concretionary forms with
size range of 2-3 cm to around 20-25 cm. The percentage of these sulphide boulders in the run of
mine sample is around 1.97% (Kompella and Raghuthaman, 2002). These sulphides differ in
size, shape and quantities from place to place in the mine, sporadic occurrence in nature and
mostly confined to the Mine II. These sulphides cause extensive damage to the mining
machineries as well as milling systems as these have high hardness as well as compressive
strength and when admitted into the furnance reduces ash fusion temperature and cause slag
formation which in turn affects boiler efficiency.
2.1. Microscopic Studies
Microscopic studies on the sulphide samples indicated that the sample consists of mostly
marcasite, rarely pyrite and pyrrhotite and quartz as the major silicate only. The size of the
sulphide grains are highly variable and present as veins, disseminations, irregular and subangular
grains showing yellow to yellowish white colour and high reflectivity. Well developed pyrite
crystals are rarely observed. Pyrrhotite is present in these sulphids and observed rarely (Fig.1).
Quartz is in the size range of 20-300 microns. The quartz grain mostly present as subhedral, sub-
angular and highly irregular shapes. Many of these quartz grains show undulose extinction and
are fractured indicating effect of stress and strain. Occasionally the silicates contain pyrite as
inclusions and vice-versa.
2.2. XRD Studies
X-ray diffraction (Fig.2) studies of the sulphide patches indicate that these are marcasite and
quartz only.
Vol.8, No.3 Characterization of Sulphides Patches from the Neyveli Lignite Deposit 225
Fig.1. Marcasite enclosing various shapes and sizes of silicates (S) and a grain of pyrrhotite
(shown by arrow mark).
XRD of the marcasite sample
226 D. S. Rao, R. Nagendra, E. Rasool Mohideen, and B.R. Nayak Vol.8, No.3
2.3. Chemistry of These Sulphides
The typical bulk chemical composition of these sulphide boulders is given in Table 1 (Kompella
and Raghuthaman, 2002). From the bulk chemical composition it can be inferred that the
sulphide boulders are nothing but iron sulphide which could be pyrite and/or marcasite as
mentioned by Kompella and Rahuthaman, (2002).
Table 1. Bulk chemical composition of the sulphides from Neyveli lignite
Compounds In wt.%
0.6 to 26
Fe (total) 30 to 80
CaO 0.5 to 1.5
MgO 1.0 to 2.0
O 0.1 to 2
2 to 10
For electron probe micro-analysis the sulphide sample was mounted in epoxy resin (cold
mounting) and polished using conventional methods for electron microscopic studies. The
polished section of of the sulphide was then examined and analysed with a JEOL, Super Probe
JXA-8600 model electron microprobe operating with a current setting of 2x10
A and with
Standard Programme International (SPI) mineral standards, using on-line ZAF correction
procedures. The major and minor elemental composition analyses of the sulphide phase from the
Neyveli lignite deposit is reported in Table 2.
From the EPMA it can be inferred that the sulphide phase is nothing but marcasite and values
show of Fe (45.617 to 47.324%) and S (51.878 to 52.829%). Out of all thirteen marcasites grains
analyzed only seven grains show significant amounts (0.004 to 0.080%) of gold content while Cu
(0.003 to 0.018%), Pb (0.020 to 0.097%), Ni (0.008 to 0.034%), Zn (0.005 to 0.019%), Sb (
0.006 to 0.044%), Bi (0.008 to 0.382%), As (0.097 to 0.502%), Se (0.004 to 0.019%) and Te
(0.011 to 0.014%) present sporadically and in traces. Recently Nayak et al., (2008) recorded
occurrence of gold in the iron sulphide (pyrites/marcasite) associated with the high sulphur
bearing tertiary coals of Northeast India. They concluded that the gold is lattice bound forming
solid solutions within iron sulphides. The silicates present along with these Neyveli sulphide
patches/marcasite were also analysed and reported in Table 3. The analyses of these silicates
indicated that these are nothing but pure quartz.
Vol.8, No.3 Characterization of Sulphides Patches from the Neyveli Lignite Deposit 227
Table 2: EPMA analyses of marcasite from the Neyveli lignite deposit (wt.%)
“---“Not detected
Table 3: EPMA analyses of the silicates present within the sulphides (wt.%)
Compounds 1 2
100.156 99.921
FeO 0.153 0.137
CaO 0.001 0.023
O ----- 0.008
----- 0.009
0.005 0.009
NiO ----- 0.019
0.001 0.001
Total 100.317 100.129
“----- “ not detected
The variation of chemical composition in the present marcasite analysis may be attributed to its
origin in a lignite province. It can be inferred that these sulphide patches of Neyveli lignite
deposit mostly syngenetic with coal. The gold concentration in these sulphides may be by
biological activity and/or by inorganic adsorption process during recrystallization in a reducing
environment. 17 million tones of lignite are excavated annually from this deposit (Kompella and
1 2 3 4 5 6 7 8 9 10 11 12 13
S 52.829 52.002 51.878 52.249 52.162 52.124 52.180 52.061 52.404 52.475 52.824 52.740 52.813
Cu --- 0.014 --- 0.013 --- 0.016 --- --- --- 0.003 --- 0.018 0.003
Fe 45.960 45.802 45.794 45.617 46.153 46.301 46.866 47.275 46.975 46.832 47.199 47.324 47.184
Bi 0.382 --- 0.341 0.057 --- 0.008 --- 0.340 0.042 --- 0.102 --- 0.243
Ni --- 0.014 --- --- --- 0.008 --- 0.034 --- 0.008 --- --- ---
Pb 0.062 0.164 0.063 0.066 0.097 --- --- --- 0.020 --- 0.138 0.087 ---
Sb 0.006 --- --- 0.017 --- --- 0.006 0.011 0.044 0.022 --- 0.040 0.021
Zn --- 0.014 0.019 --- --- --- --- 0.011 --- --- 0.005 --- 0.005
Co 0.062 0.046 0.031 0.046 0.041 0.044 0.021 0.064 0.053 0.030 0.030 0.059 0.021
Au 0.017 --- --- --- 0.040 0.004 --- 0.030 0.004 --- --- 0.057 0.080
As 0.104 --- 0.127 --- --- 0.393 0.185 0.097 0.170 0.215 0.502 0.191 ---
Se 0.019 --- 0.006 --- 0.013 --- --- --- 0.013 0.012 --- 0.004 0.011
Te 0.014 0.011 --- 0.011 --- --- --- --- --- --- --- --- ---
Total 99.456 98.066 98.259 98.074 98.507 98.807 99.258 99.923 99.725 99.597 100.800 100.519 100.388
228 D. S. Rao, R. Nagendra, E. Rasool Mohideen, and B.R. Nayak Vol.8, No.3
Raghuthaman, 2002). Out of which around 325 tonnes of sulphide will be accumulated annually
at the rate of 1.97% of sulphide boulders from the lignite run of mine sample. In view of this, it
may be worth studying these sulphides for resources development of marcasite in general and
gold in particular. As it is known that most of the mining industries wastes contain useful
materials and if recovered economically and in an environmental friendly way it adds up to the
revenue of the mining company. In this contest, utilization of these sulphides, from the lignite
mine of Neyveli not only essential from resource conservation/utilization but also from
environmental point of view. Hence, if the sulphides patches are studied in detail then the
sulphides which are dumped as waste can be a source of gold.
The samples were collected by Dr. R. Nagendra and Dr.E. Rasool Mohideen. The authors (RN
and ERM) are thankful to the authorities of Neyveli Lignite Corporation for permission as well
as helps during collection of marcasite samples from Neyveli mine. The authors are also thankful
IIC, IIT, Roorkee for EPMA work.
Banerjee, D., Mandal, P.K., Nithyananthan, N., Thakur, J,S. and Hirani, M. (1997) Effect of
different forms of sulphur on coal combustion. R & D Journal (NTPC), V.3, No.1, pp.37-42.
Kompella, R.M. and Raghuthaman, S. (2002) Removal of marcasite from lignite. Proc. of the Int.
Sem. on Mineral Processing Technology (Eds. S.Subramanian, K.A..Natarajan, B.S.Rao and
T.R.R.Rao), 3-5 January 2002, V.2, pp.638-643.
Nayak, B., Chakravarty, S and Bhattacharyya, K.K. (2008) Invisible gold in the high sulphur
Tertiary coals of Northeast India. Current Science, V.95, No.9, pp.1334-1337.