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Journal of Minerals & Materials Characterization & Engineering, Vol. 10, No.5, pp.408-418, 2010
jmmce.org Printed in the USA. All rights reserved
Apatite Microstructure and Composition in
Manganese Formation of Eastern Ghats, Orissa, India
, B.K. Mohapatra
* and P. P. Singh
Institute of Minerals and Materials Technology, Bhubaneswar
Geology Department, Utkal University, Bhubaneswar
*Corresponding Author: firstname.lastname@example.org
Apatite grains in a stratabound manganese ore body from Eastern Ghats in Leliguma, Koraput
district south Orissa, India were studied under optical and electron microscope. Apatite shows
bimodal occurrence: one associated with granite/pegmatitic phase and other with manganese
phase, and exhibits different microstructure and composition. Apatite in pegmatitic association
(occurring as inclusion in quartz and feldspar) is small in size and occurs as subhedral grains
having irregular boundaries and poor in manganese content (MnO: 0.16%). Apatite associated
with manganese mineral phases (cryptomelane / romanechite) is mostly euhedral, relatively
larger in size, contains higher manganese value, and exhibits some peculiar features like
twinning, zoning, overgrowth, and occasionally contains inclusion of quartz and feldspar grains.
Apatite grains occurring as inclusions within quartz and feldspar are of hydrothermal origin and
formed along with pegmatitic minerals. Such apatite is almost devoid of manganese but
relatively enriched in Sr, F and LREE content. The apatite in manganese mineral association
formed during supergene process. The apatite of latter generation appears as idiomorphic
crystals; contains up to 6.85% of MnO in solid solution and shows relatively higher HREE
values. The higher HREE values may be due to its derivation from stratiform Mn-ore bodies and
associated granitised rock during remobilization, solution and precipitation of Mn –rich fluid
along structurally weak planes resulting in development of a stratabound ore body.
Key words: Apatite, Eastern Ghats rocks, Manganese ore
Apatite is one of the common accessory minerals found in crustal rocks and its presence and
distribution are often used to make a model on the geologic processes such as mantle melting,
hydrothermal processes etc. [1, 2]. Compositions of apatite at many places relate to the processes
involved. Apatite structure also accommodates some RE elements. It is often found as minor
minerals in manganese ore.
Vol.10, No.5 Apatite Microstructure and Composition 409
Naturally occurring apatite is represented by Ca
with many substitutions possible for
Ca, P and F. Minor replacement of Ca in natural apatite is mainly by Na, Sr, Mn and the REE.
Substitution of REE
in apatite is reported by some authors [3, 4]. The most common
substitution for P is by Si. REE distribution in a particular apatite depends not only on the apatite
structure, but also upon the chemical characteristics of the melt, rock or fluid reservoir from
which the apatite crystallized / formed . Further, apatite forming at different stages may have
a different REE concentration and distribution .
The Eastern Ghats Belt in south Orissa and Andhra Pradesh, India comprises a group of
regionally metamorphosed (granulite facies) rocks represented by quartzite, quartz-feldspar-
garnet-silimanite ± graphite gneiss (khondalite), granite gneiss and calc silicate rocks with
intercalation of manganese occurrence. The manganese ores of south Orissa are low-grade in
nature and have high phosphorus contents. Apatite is known to be the major contributor of
phosphorus [7, 8]. Occurrence of apatite is reported from several associations. Acharya et al 
and Bhattacharya et al  reported apatite in Mn-ores from Nishikhal region, Orissa and
Grabham-Garividi belt, AP of Eastern Ghats respectively.
Geological map on 1:50000 scale around Leliguma-Koka sector of Koraput district, south Orissa
(Toposheet no. 65 M/3) is shown in Fig. 1. The area forms an integral part of Eastern Ghats
complex comprising of calc-silicate, charnockite, khondalite, granite gneiss and Mn-ore. Some
of these country rocks, especially granite gneiss and khondalite are traversed by small scale
quartz and pegmatite veins. These rocks are metamorphosed to granulite facies.
The manganese ore in this area (Fig.1) generally shows two modes of occurrence: 1) As layer
body within granite gneiss extending in a NW-SE direction near Koka (grouped under stratiform
category). These types of ore bodies occur in tabular form and extend > 500mts in length and
>30mts in width. 2) As linear ore body cutting the granite gneiss in E-W directions near
Leliguma (grouped under stratabound category).
The apatites in this ore body are recorded in two associations: one with silicate minerals and
other with manganese minerals. No apatite is found to be associated with manganese mineral of
stratiform ore body. The present report highlights the microstructural and compositional
variation of apatite in both these association with a view to establish the genesis of apatite in the
stratabound category of manganese ore bodies.
Vol.10, No.5 Apatite Microstructure and Composition 410
Fig. 1: Location and geological map showing manganese ore bodies in Leliguma-Koka
sector, Koraput District, Orissa (survey of India Toposheet no. 65 M/3).
2. MATERIALS AND METHODS
Ten Mn-ore samples from stratabound ore deposit located at Leliguma, Koraput district, south
Orissa, India (Fig.1) were collected and prepared for study under optical (Leitz, orthoplan) and
electron microscope (JEOL). The identification of apatite and its microstructural peculiarities
were obtained through these studies.
It was followed by compositional analysis through Electron Probe Micro analyzer (Jeol, JXA-
8100). The Ca, P and other elemental analyses were carried out by wavelength dispersive
methods using appropriate apatite standard.
The rare earth element analyses of apatite grains were performed with the aid of EPMA using
accelerating voltage of 25kv, a beam current of 20 nA, beam diameter of 2.5µm and 20s counts.
Synthetic glass and international standards FeR2 (G.S., Canada) and G2 (USGS) were used as
standards for EPMA analysis.
Vol.10, No.5 Apatite Microstructure and Composition 411
3. RESULTS AND DISCUSSION
The characteristics of apatite grains in Mn-ores, occurring in two different associations viz.
silicate minerals (Ap1) and Mn-minerals (Ap2), were studied under optical and electron
microscopes. Results are discussed below:
3.1 Microstructure of Apatite
The manganese ore in the stratabound ore body of study area constitutes two major mineral
phases such as: manganese minerals and silicate minerals. Apatite is present in both these
minerals and display different habits (Table 1).
Fig. 2: Optical (A&B) and electron micrographs (C&D) of apatite in pegmatitic ( )
(A) Tiny grains of apatite within quartz x100
(B) Rounded to subrounded apatite grain within plagioclase feldspar x200
(C) An ovoidal grain of apatite in orthoclase
(D) Elongated apatite grain in plagioclase
Vol.10, No.5 Apatite Microstructure and Composition 412
Table 1: Characteristics of apatite in different association
Characteristics Pegmatitic association Mn –mineral association
Habit Ovoidal, tear drop, elongated Columnar, lenticular, tiny irregular,
Inclusion Apatite occur within quartz
Apatite contains quartz and
feldspar, cryptomelane as inclusions.
Other Characteristics Irregular contact with
Zoning, twinning and overgrowth.
Mostly as isolated grains Both as isolated grains and clusters
Fig. 3: Optical (B) and electron micrographs (A to E) in manganese association. (A)Tiny grains
of apatite within cryptomelane. (B)Cluster of apatite grains in different size within
cryptomelane x100. (C) Inclusion of quartz within apatite. (D)Inclusion of feldspar
within apatite. (E) Apatite showing compositional zoning. (F)A large apatite grain
containing small apatites of earlier generation.
Vol.10, No.5 Apatite Microstructure and Composition 413
Fig. 4 : Optical (A&D) and electron micrographs (B&C) of apatite in manganese
association showing different morphology
(A) Twined apatite grains in cryptomelane base (white) x200
(B) An ovoidal grain of apatite showing overgrowth
(C) Apatite showing lenticular structure
(D) A subrounded broken apatite grain (Ap) with lithiophorite encrustation (L) x200
In general, apatite looks colourless to pale pink under petrographic microscope, shows grey
interference colour and straight extinction. Apatite (Ap1), enclosed within quartz (Fig.2A),
orthoclase (Fig.2B) and plagioclase (Figs. 2C & D), is relatively less in modal percentage (2-3%)
compare to the one associated with manganese minerals (10%). The apatite in silicate association
is generally smaller in size (<100micron) and shows ovoidal, rounded to sub rounded or
elongated shape with corroded outlines (Fig.2).
Apatite (Ap2) associated with manganese mineral occurs as very tiny grains (Fig.3A), as
moderate size grains (20-50micron), in clusters or as euhedral crystals (>150 micron, Fig.3B).
Such apatite occasionally contains inclusions of quartz (Fig.3C) and feldspar (Fig.3D). Zoning in
some apatite is distinctly seen (Fig.3E). Some layers in zoned apatite are often found to be
replaced by cryptomelane. Occasionally, large apatite is observed to enclose small apatite grains
of earlier generation (Fig.3F). Twinned apatite crystals (Fig.4A), apatite showing overgrowth
(Fig.4B) and lenticular micro-structure (Fig.4C) are some of the features observed only in this
type of association. Thin encrustation of lithiophorite (Fig. 4D) is rarely seen over sub rounded
grain of apatite in the zone of lateritisation.
Vol.10, No.5 Apatite Microstructure and Composition 414
3.2 Compositional Characteristics of Apatite
The analytical data of Ap1 and Ap2 (Tables 2 & 3) reveal the following notable mineral-
chemical pattern. Apatite enclosed within quartz and feldspar minerals (Ap1) is almost devoid of
manganese and shows higher concentration of Sr and LREE. The broad composition of apatite
from this association is found out to be [Ca
Total LREE content in these apatite grains is higher than the other variety (Ap2, Table 3). La and
Pr are appreciably present in this apatite while Ap2 is almost free from them. Apatite associated
with manganese mineral (Ap2) contains between 1.89 and 6.85% MnO corresponding to the
be termed as manganoan
apatite or Mn-apatite. Total HREE content in Mn- apatite grains is higher than the Ap1 variety,
Table 2: EPMA results of apatite from two associations 1-5: Silicate Association;
6-10: Mn-association showing major compositional variation
Wt% 1 2 3 4 5 Avg. 6 7 8 9 10 Avg.
54.80 54.1 54.62
MnO 0.22 0.23 0.06 0.07 0.20 0.16 3.15 2.18 1.89 4.43 6.85 3.70
F 2.98 4.11 2.99 3.95 4.22 3.65 3.52 3.01 3.81 2.99 3.15 3.30
Sr 0.72 0.66 0.68 0.78 0.96 0.76 0.03 0.05 0.04 0.22 0.17 0.10
5.87 3.99 6.38 5.49 3.85 5.12 3.89 3.98 4.79 5.03 6.06 4.75
..……………………………..On the basis of 16 cations……………………………..……
Ca 10.39 10.45 10.54 10.55 10.49 10.20 10.27 10.35 9.74 9.49
Mn 0.03 0.03 0 0 0.02 0.40 0.32 0.27 0.67 1.05
Sr 0.08 0.08 0.08 0.09 0.11 0.03 0 0 0.02 0.02
Total 10.50 10.56 10.62 10.64 10.62 10.63 10.59 10.62 10.43 10.56
P 5.46 5.41 5.35 5.33 5.34 5.26 5.38 5.35 5.54 5.42
F 1.71 2.27 1.73 2.22 2.32 2.02 1.70 2.13 1.72 1.80
99.44 99.44 100 100 99.72 96.24 6.98 97.46 93.58 90.06
0.28 0.28 - - 0 .18 3.76 3.02 2.54 6.42 9.94
Vol.10, No.5 Apatite Microstructure and Composition 415
4. GENESIS OF APATITE
The stratabound ore bodies are generally of epigenetic origin and formed during supergene
processes due to remobilization and precipitation of Mn-rich fluid along structurally weak
planes. The manganese rich fluid in Liliguma stratabound ore body has in all probability
generated due to weathering of adjacent stratabound manganese ore and associated granitic and
The mode of occurrence, microstructure and mineral chemistry of apatite in Mn-ores of
Leliguma stratabound deposit reveal two different mode of origin. The apatite (Ap1) that occur
as inclusion within quartz and feldspar, and contain less than 1% MnO, is inferred to be of
hydrothermal origin, and developed during formation of pegmatitic minerals. Its irregular shape,
poor Mn-value and enriched Sr (Table-2) & LREE content (Table-3) support such interpretation.
Apatite crystallizing from a granitic melt tends to concentrate the LREEs or the middle rare-earth
elements relative to HREEs . Further, actual REE distribution in particular depends not only
on apatite structure, but also upon the REE distribution and chemical characteristics of melt or
fluid reservoir from which the apatite crystallized.
Table 3: EPMA analysis results (wt %) of apatite obtained from two different associations (1-5:
Hydrothermal; 6-10: Supergene) showing LREE & HREE distribution.
1 2 3 4 5 Avg. 6 7 8 9 10 Avg.
0.12 0.17 0.02 0.29 - 0.12 - - - - - -
0.09 - - 0.08 0.17 0.07 0.1 0.17 - 0.01 0.03 0.06
0.09 0.01 0.06 0.03 0.06 0.08 0.11 - - - 0.13 0.05
- 0.19 0.03 0.01 0.09 0.06 - 0.03 0.11 0.11 0.04 0.06
- 0.04 0.02 0.03 0.07 0.04 - 0.06 0.08 0.01 - 0.03
LREE 0.3 0.41 0.13 0.44 0.39 0.37 0.21 0.26 0.19 0.13 0.2 0.2
- 0.01 0.006
0.01 0.02 0.03 0.01 0.01
0.06 - - 0.11 0.17 0.07 - - 0.09 - - 0.02
0.01 0.03 - 0.02 0.01 0.01 - 0.02 - 0.07 0.1 0.03
0.02 - 0.21 0.18 0.02 0.09 0.29 0.43 - - 0.53 0.25
- 0.04 0.01 0.07 0.03 0.04 0.002
0.04 0.05 0.08 0.08 0.05
0.02 0.01 0.01 - 0.03 0.01 0.04 0.14 0.44 - 0.03 0.03
0.01 0.02 0.01 0.03 0.01
0.56 0.22 0.20 0.78 0.42
(-) not detected
Vol.10, No.5 Apatite Microstructure and Composition 416
Manganoan apatites (Ap2) in other association (with cryptomelane / romanechite) are formed
after Mn-Ca-P rich fluid emplacement along structurally weak planes of granitic rock. The
secondary growth of apatite is indicated by the presence of quartz and feldspar inclusions, its
composition (having 6.85% of MnO in solid solution Table-2), relatively low LREE and high
HREE content (Table-3).
Knutson et al  reported that zoning is developed due to repeatedly renewed growth or
rhythmic changes in the chemical composition between different zones, which points to variation
from time to time in the chemical concentration of elements in depositing solution. The
compositional variation between different layers in zoned apatite, brought out through line
scanning under EPMA (Fig. 5), is in agreement with Knutson  in the growth of this apatite in
the study area. Presence of Mn-layers in zoned apatite attests to their secondary development.
Quartz and feldspar inclusions within apatite probably indicate that apatite was grown over these
clasts from solution. Skeletal overgrowth is also marked by compositional discontinuity between
core and rims. Such features reveal to rhythmic growth of apatite. Small apatite grains enclosed
within larger grain (Fig.3F) also supports growth at different interval. Idiomorphic crystal habit
and lenticular structure are probably developed (Fig. 3C) during latter period.
Fig. 5: Line scan profile showing the compositional variation along a line (X-Y) shown
across the zoned apatite
Dual source of apatite is thus evidenced from their association, micromorphology, microstructure
and chemical composition. Subhedral apatite (Ap1) is of hydrothermal origin and formed along
with quartz and feldspar in the granite/pegmatite (country rock). The euhedral Manganoan
apatites (Ap2) are formed during supergene process and developed in a latter period.
Higher HREE in latter apatite is related to their availability in the mineral rich fluid from which
apatite formed. The REE chemistry of bulk Leliguma manganese ore shows relatively higher
HREE value than adjacent stratiform Mn-ores located at Koka. The higher HREE value in the
bulk Mn-ore may thus be attributed to apatite (Ap2)
Vol.10, No.5 Apatite Microstructure and Composition 417
From the foregoing discussions the following conclusions are made:
1. Apatite in stratabound Mn-ores of Eastern Ghats Group of rocks at Koraput dist., Orissa,
India shows bimodal distribution, viz i) enclosed within silicate minerals (Ap1) and ii) the
other associated with manganese minerals (Ap2).
2. Ap1 grains are subhedral, poor in MnO but have relatively enriched Sr and LREE contents.
In contrast, Ap2 are euhedral crystals and show typical features like twinning, zoning,
overgrowth and lenticular structure. Ap2 is rich in manganese content (MnO: 0.89 to 6.85%)
and may be termed as manganoan apatite. HRE elements in these apatite crystals are
relatively higher compared to other type (Ap1).
3. Apatite (Ap1) enclosed within quartz and feldspar are formed along with other granitic/
pegmatitic minerals. Higher LREE and Sr contents indicate the source solution to be
hydrothermal in nature. Euhedral apatite (Ap2) associated with manganese mineral phase is
developed at a latter stage, during supergene process. These are sometime grown over clasts
of different composition (quartz, feldspar and apatite grains).
4. Contrasting microstructure and composition of apatite in stratabound manganese ore bodies
indicate their dual source: one being hydrothermal formed along with granitic/pegmatitic
minerals (country rock) and other of secondary, supergene origin formed along with
stratabound ore body.
The authors are thankful to Prof. B.K. Mishra, Director, Institute of Minerals and Materials
Technology, Bhubaneswar, Orissa, India for his kind permission to publish this paper.
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