VISTAS IN GEOLOGICAL RESEARCH (ISBN: 81-900907-0-4)
Special Publication in Geology (13), January 2015, pp.71-85
13/2015 © Utkal University
Pyrochlore from Nepheline Syenite around
Karlakot, Nuapada District, Odisha
JANISAR M. SHEIKH
1,
*, SURESH C. PATEL
1
, DURYADHAN BEHERA
2
, N. PRABHAKAR
1
1
Department of Earth Sciences, I.I.T. Bombay, Powai, Mumbai 400076
2
Department of Earth Sciences, Sambalpur University, Burla, Odisha 768019
* Email: janisar.sk@iitb.ac.in
Abstract: Pyrochlore is interpreted as a primary liquidus phase in the nepheline syenite intrusion around
Karlakot in the Nuapada district of Odisha. The intrusion occurs within granite mylonite of the Bastar
Craton, close to its thrust contact with the Eastern Ghats Mobile Belt. Pyrochlore compositions in the
studied samples are almost similar with major oxide ranges of Nb
2
O
5
(60.53–62.67 wt%), CaO (17.60–
19.04 wt%), Na
2
O (5.91–6.98 wt%), Ta
2
O
5
(3.19–5.18 wt%) and TiO
2
(4.39–5.60 wt%). Among the minor
elements SrO is in the range of 0.30–0.82 wt% and Ce
2
O
3
between 0.17 and 1.18 wt%. The pyrochlores are
fluorine-rich with 1.99–2.87 wt% F. Crystallographically, the B position, which is a [6]-coordinated cation
site, is dominantly occupied by Nb with minor substitution by Ti. The A position, which is a [8]-
coordination cation site, contains Ca and Na as the major cations. Dominance of Ca over Na, combined
with negligible vacancy in the A-site, puts the pyrochlores into the calico-pyrochlore subgroup. Pyrochlore
was stabilized as a liquidus phase most likely by F and Ta during magmatic crystallization.
Keywords: Pyrochlore, Nepheline syenite, Alkaline rock, Khariar, Karlakot, Odisha
INTRODUCTION
Niobium and (tantalum) are rare, refractory metals with a wide range of applications in electronics,
nuclear plants and aircraft industries. Most minerals of Nb and Ta, and all economically important species of
these elements, belong to the class of oxide minerals (Cerny and Ercit, 1989). There are several groups of Nb-
and Ta-bearing oxide minerals, of which the two most common are the pyrochlore group and columbite-tantalite
group. The minerals of the pyrochlore group crystallize in the isometric system with a structure highly flexible
and permissive of substitutions involving cations of diverse radii and charges. This results in a variety of
compositionally different species within the group under the general formula A
2-m
B
2
O
6
(O,OH,F)
1-n
⋅pH
2
O and the
limits of m = 0–2, n = 0–1 and p = 0–2 (Hogarth, 1977, 1989; Atencio et al. 2010; Yaroshevskii and
Bagdasarov, 2008). The A position typically is a large [8]-coordination cation site which can host Na, Ca, Ag,
Mn, Sr, Ba, Fe
2+
, Pb
2+
, Sn
2+
, Sb
3+
, Bi
3+
, Y, REE, Sc, U and Th, but can also accommodate H
2
O. The B position
is a [6]-coordinated cation site, typically occupied by high field-strength elements although other elements can
also be present. This site may thus contain Nb, Ta, Ti, Sb
5+
, W, V
5+
, Sn
4+
, Zr, Hf, Fe
3+
, Mg, Al and Si. The most
common element in the A position are Na and Ca, in the B position Nb, and thus most pyrochlores have a
composition close to NaCaNb
2
O
6
(OH,F). The number of atoms in the B site is very constant, while there is
considerable variation in the A site because of possible vacancies in this site. These variations are controlled by
both initial stoichiometry (substitutions 2Na
+
↔ Ca
2+
+ , 2Ca
2+
↔ U
4+
+ , etc.) and secondary processes such
as cation leaching. Pyrochlores may contain up to 10–15 wt% H
2
O (Lumpkin, 1989). The dominant element in
the B site determines the root name: pyrochlore (Nb), microlite (Ta), roméite (Sb), betafite (Ti) and elsmoreite
(W).
Pyrochlore is principally found in three host rock categories: carbonatites, nepheline syenites, and
granitic pegmatites (Cerny and Ercit, 1989; Yaroshevskii and Bagdasarov, 2008; Mitchell, 2015). The
variegation in the pyrochlore composition is useful to trace geochemical processes in varied primary and