Reddish-Brown Zircons of Sri Lanka: A Detailed Study and Development of a New Technique for Yellow to Golden Yellow Colour Enhancement

Abstract

Sri Lanka is famous for various types of gem minerals. Among these minerals, gem-quality zircon is found in both primary and secondary deposits. The main objective of the research was to study the physical, chemical, and spectroscopic features of reddish-brown zircons (RBZ) of Sri Lanka and to develop a methodology of heat treatment for RBZ of Sri Lanka for colour enhancement. Five randomly selected RBZ samples were selected for this study. All samples were translucent, highly fractured, sub-adamantine, and with euhedral to subhedral crystal form. Samples were analysed with EDXRF, UV-Vis Spectrophotometer, and FTIR methods. The UV-Vis absorption spectrum indicates the cause of colour is due to structural defect colour center by radiation damage from radioactive elements such as U and Th. FTIR spectroscopy indicates that the RBZ of Sri Lanka has gone through a metamictization process. Further FWHM of Raman spectroscopy confirms that RBZ of Sri Lanka belongs to the Intermediate-type. This is also confirmed by encourages embedding into jewellery. Being the most abundant chemical substitution is Hf, while all the RBZ samples contained a low amount of REE and radioactive elements. Additionally, the ratio of Th: U was consistent with a magmatic origin. Hence, our results confirm that heat treatment under either oxidizing or reducing conditions in an electric furnace produces a yellow to golden yellow colour at around 500-600 °C with a soaking period of fewer than 90 minutes.

Full text

Journal of Geological Society of Sri Lanka Vol. 22-2 (2021), 39-45
39
REDDISH-BROWN ZIRCONS OF SRI LANKA: A DETAILED
STUDY AND DEVELOPMENT OF A NEW TECHNIQUE FOR
YELLOW TO GOLDEN YELLOW COLOUR ENHANCEMENT
M.N.M. RIFKHAN1,2*, R.M.N.P.K. JAYASINGHE1,2, T.S. DHARMARATNE1,
ROHANA CHANDRAJITH3, M.A.S.P.K. MALAVIARACHCHI2,3
1Gem and Jewellery Research and Training Institute, Kaduwela, Sri Lanka
2Postgraduate Institute of Science, University of Peradeniya, Sri Lanka
3Department of Geology, Faculty of Science, University of Peradeniya, Sri Lanka
* Corresponding author email: rifkhannayeem@gmail.com
(Received 15st November 2021; Accepted 27th December 2021)
ABSTRACT
Sri Lanka is famous for various types of gem minerals. Among these minerals, gem-quality zircon is found
in both primary and secondary deposits. The main objective of the research was to study the physical,
chemical, and spectroscopic features of reddish-brown zircons (RBZ) of Sri Lanka and to develop a
methodology of heat treatment for RBZ of Sri Lanka for colour enhancement. Five randomly selected RBZ
samples were selected for this study. All samples were translucent, highly fractured, sub-adamantine, and
with euhedral to subhedral crystal form. Samples were analysed with EDXRF, UV-Vis Spectrophotometer,
and FTIR methods. The UV-Vis absorption spectrum indicates the cause of colour is due to structural defect
colour center by radiation damage from radioactive elements such as U and Th. FTIR spectroscopy
indicates that the RBZ of Sri Lanka has gone through a metamictization process. Further FWHM of Raman
spectroscopy confirms that RBZ of Sri Lanka belongs to the Intermediate-type. This is also confirmed by
encourages embedding into jewellery. Being the most abundant chemical substitution is Hf, while all the
RBZ samples contained a low amount of REE and radioactive elements. Additionally, the ratio of Th: U
was consistent with a magmatic origin. Hence, our results confirm that heat treatment under either oxidizing
or reducing conditions in an electric furnace produces a yellow to golden yellow colour at around 500- 600
°C with a soaking period of fewer than 90 minutes.
Keywords: Zircon, Colour, Radioactive, Metamictization, Heat treatment
1. INTRODUCTION
Sri Lanka is famous for various types of gem
minerals. Among these gem minerals, zircon
is found in both primary and secondary
deposits. Zircon is found in a range of
colours varying from colourless, yellow, red,
orange, brown, green, and blue (Webster,
1994). Among different colour varieties of
zircons, colourless, golden brown (yellow),
and sky-blue colours are the most important
in jewellery and show the best advantage;
the adamantine lustre of zircon and has a
higher demand compared to reddish-brown
zircons (Abewardana & Malaviarachchi,
2018). In nature, the greyish brown and
reddish-browns were the most common
colours (Schumann, 1977). Nevertheless,
colour changes can be made on zircon
through the heat treatment process to acquire
certain desired colours (Rossman, 2011).
A common empirical formula of zircon
(ZrSiO4) shows some of the range of
substitution in (Zr1–y, REEy)(SiO4)1–
x(OH)4x–y (Vương & Hương, 2015).
Zircon also shares a habit with the uranium
mineral coffinite U(SiO₄)1−x(OH)₄ₓ and
thorium mineral thorite, (Th, U) SiO4. The
radiation from radioactive elements can
affect the specific gravity, unit cell
dimension, and optical properties of zircon
(Holland & Gottfried, 1955). The trace and
the specific gravity of the stones. And more importantly, they were not detectably radioactive which

Journal of Geological Society of Sri Lanka Vol. 22-2 (2021), 39-45
40
rare earth element compositions and
radiation damage contribute to the color of
zircon. For instance, red zircon has
radiation-induced color centres in which
Nb4+ substitutes for Zr4+. Blue zircon is
attributed to the presence of U4+ (Vương &
Hương, 2015).
The zircons are classified into three types
based on their level of radiation damage.
They are High-type, Intermediate-type, and
Low-type. High-type is highly crystalline
structures. The structure is completely
destroyed to the level of amorphous state in
Low-type zircon, whereas the Intermediate-
type consists of nearly regular lattices (Yada
et al., 1981).
Reddish-brown zircons (RBZ) are common
in gem gravels of Sri Lanka. Although,
traditional heat treatment methods are
employed for colour development of RBZ,
yet such techniques have few disadvantages.
back to its original hue with time and the
treatments are time and energy-consuming.
Therefore, developing low-cost and more
effective treatment techniques to improve
the quality of low-grade zircon to a more
demanding colour is one of the important
considerations in the gem and jewellery
industry. Hence, the main aim of this work
is to investigate the physical, chemical, and
spectroscopic features of RBZ of Sri Lanka
and to develop a feasible methodology to
enhance RBZ through heat treatment
processes.
2. MATERIALS AND METHODS
RBZs with purplish tint were collected from
Kolonna, one of the main gem fields in Sri
Lanka for the heat treatment. In this area,
zircons are found mainly in primary
was type D according to the GIA sample
collection manual(Vertriest et al., 2019).
Randomly selected samples were heated
with an electric muffle furnace (Therm-Craft
electric chamber furnace) at different
temperatures, soaking time, and reducing
agents to find the optimum condition. The
optimum condition was selected by the least
temperature and soaking period for the finest
colour change. Several trials were
performed to distinguish the exact
temperature to produce yellow to golden
yellow zircons from RBZ. Trials were
performed at a temperature between 120 °C
to 800 °C with different soaking times with
both reducing and oxidizing conditions.
Some raw and treated samples were then
spectroscopic investigations after soaking in
3. RESULTS AND DISCUSSION
Fig. 1: Tetragonal prisms with distorted
zircon
More importantly, the colour can reverse
The raw untreated samples collected from
-brown,mostly translucent. Most stones were
highly fractured with euhedral to subhedral
form (Fig. 1)
subjected to physical, chemical, and
aqua regia for 24 hours. Samples were
analyzed by EDXRF (Rigaku NEXCG),
UV-Vis Spectrophotometer (Jasco V-760),
(GemmoFTIR). In addition, specific
320+) respectively.
and FTIR
Raman spectrophotometer
(GemmoRaman),
gravity and the radioactivity of samples
were determined by hydrostatic balance
(RADWAG) and Geiger counter (GMC-
Kolonna region were mainly dark reddish
deposits. The sample collection procedure
termination habit of Kolonna reddish brown

Journal of Geological Society of Sri Lanka Vol. 22-2 (2021), 39-45
41
3.1 Physical, Chemical, and Spectroscopic Features
According to hydrostatic specific gravity
(SG) measurements, most of the raw RBZ
ranged in between 4.10 to 4.60 confirming
that Sri Lanka RBZ belongs to the
Intermediate-type (Table 1). Radioactivity
of the samples was between 15 - 40 cpm.
Since the count is less than 50 cpm, stones
are a safe mineral and hence can be used in
jewellery.
Table 1 Table of Zircon Type with Specific
Gravity
The mass percentage of elements of RBZ is
shown in Table 2. The most abundant
chemical substitution was Hf, which varied
from 1.03% to 1.33 %. The total rare earth
element contents were 0.023%- 0.466%
while the Th and U were varying from 0.071
to 0.1603 %. The Th: U ratio was greater
than 0.2 for all samples, which is typical for
zircon of magmatic origin (Huong et al.,
2017).
The typical dominant peaks and bands of the
Raman spectrum at 1008, 975, 437, 392,
355, 225, 214, and 202 cm-1 were observed
in raw samples (Fig. 2). The bands in the 450
– 350 cm–1 range and around 1000 cm–1 is
due to internal (intra-tetrahedral) vibrations
of SiO4 tetrahedral. Special attention was
paid to the band of 230 - 200 cm-1 which
represents the intense external lattice (inter-
tetrahedral) vibrations (Nasdalal, 1995). The
three peaks in the band range have
broadened and shifted to form a single broad
band at 200 cm-1, confirming the medium to
a high degree of metamictization. But the
samples were not completely metamict since
the 230 – 200 cm-1 band is not completely
absent.
Further, the degree of metamictization can
be studied with the vibrational peak at 1008
cm-1 of the Raman spectrum. The High-type
zircon shows a full width at half maximum
(FWHM) value of less than 5 cm-1 for this
peak, whereas this has a greater than 30 cm–
1 value for Low-type. The Intermediate-type
has to be between 5 to 30 cm-1 (Huong et al.,
2017).
The FWHM values of Vietnamese and
Ratanakiri RBZ are in the range of 2–3 cm-1
(Wittwer et al., 2013) showing that they are
well crystallized whereas the calculated
value of Sri Lankan RBZ was 20cm-1. This
further confirms the Sri Lankan RBZ has
been subjected to a considerable amount of
metamictization. Accordingly, the Sri
Lankan RBZ can be categorized as an
Intermediate-type. Typical strong bands of
FTIR spectrum at 2334, 2501, 2761, 2856,
2918, and 3196 cm-1 were found in raw RBZ
from Sri Lanka (Fig. 3). Some were strong
while others were not. A vague absorption
band related to Si-O at 1400 - 200 cm-1 is
also found.
Table 2: Table of RBZ samples with few elemental mass percentages
Zn R A
Zn R B
Zn R C
Zn R D
Average
Zr
74.8
57.1
64.4
73.5
67.45
Si
21.7
31.3
27.5
22.8
26.15
Hf
1.33
1.15
1.03
1.18
1.1425
U
0.0697
0.129
0.0595
0.0707
0.087
Th
0.0199
0.0313
0.0182
0.0250
0.02485
Tb
0.0109
0.186
0.0039
0.197
0.096725
Specific Gravity
Zircon Type
<4.1
Low
4.1-4.6
Intermediate
>4.6
High

Journal of Geological Society of Sri Lanka Vol. 22-2 (2021), 39-45
42
Fig. 2: Raman spectrum of reddish-brown zircon of Sri Lanka
Fig. 3: FTIR spectrum of reddish-brown zircon of Sri Lanka
The band at 3800 and 3400 cm-1 which
corresponds to hydrous components were
observed. A sharp single band has been
observed in this region which also confirms
the significant degree of metamictization.
This band is very strong compared to the
Vietnamese RBZ spectrum.
UV-Vis-NIR absorption spectra for the
untreated rough RBZ samples are shown in
Fig. 4. The spectra of all five reddish-brown
categories showed an increase in absorption
toward the UV region. This gives the brown
component of the colour (Vương & Hương,
2015). The broad absorption band in the UV
region with an absorption "tail" extending
into the visible region can be considered as a
result of colour center, which is the cause of
the formation of the above colour.

Journal of Geological Society of Sri Lanka Vol. 22-2 (2021), 39-45
43
The radiation from radioactive elements
such as U and Th may have caused the defect
in the crystal structure. This can be
recognized by the shoulder at around 600 nm
of the absorption spectrum (Vương &
Hương, 2015). Whereas this shoulder is
observed at around 500 nm in both
Vietnamese and Cambodian RBZs (Zeug et
al., 2018). This can be also considered as a
factor of identification of the crystalline
zircon from metamictized zircon.
Fig. 4: Visible spectrum of reddish-brown
zircon of Sri Lanka
A sharp peak at 652 nm and 689 nm were
observed in untreated rough RBZ samples
which attribute the trace amount of uranium
as U4+ (Vương & Hương, 2015). Meantime
the peak of 652 nm is attributed to the Tb3+
ion (Laithummanoon & Wongkokua, 2013).
Similar absorption patterns have been
reported for reddish brown and brownish red
zircon from other localities, including
Ratanakiri (Cambodia), Central Highlands
of Vietnam, and Muling (China)(Huong et
al., 2017; Chen et al., 2011).
3.2 Development of possible Heat
Treatment conditions for Reddish
Brown Zircons of Sri Lanka
Several heat-treatment trials were performed
between 120 °C to 800 °C. Around 500 °C -
600 °C with less than 90 minutes soaking
period with either oxidizing or reducing
condition is known to be the optimum
condition to produce yellow and golden
yellow colour for RBZ of Sri Lanka.
More than 90% of RBZ of Sri Lanka
becomes lighter coloured with heat
treatment under oxidizing or reducing
conditions. Fig. 5 shows the RBZ before heat
treatment (left corner) and a series of heat-
treated stones between golden yellow to
yellow colours. The intensity of the yellow
saturation depends on the red component of
the initial stone. The treated stones were
stable and did not revert to their original
appearance when exposed to light.
Fig. 5: Reddish brown zircon before heat treatment and range of colours produced after heat
treatment

Journal of Geological Society of Sri Lanka Vol. 22-2 (2021), 39-45
44
Fig. 6: Visible spectrum of reddish-brown zircon and heat-treated yellow zircon
3.3 Unheated Reddish-Brown Zircon
Vs. Heat-treated Yellow Zircon
UV-Vis spectroscopy of heat-treated yellow
to golden yellow zircons showed a quick
drop in absorption around 400nm whereas
unheated RBZ has a smooth flow in
spectrum with a tail (Fig. 6). This may be
due to the absence of brown components in
the heat-treated yellow to golden yellow
zircon. But the peaks at 652 nm and 689 nm
which account for U remain the same in both
unheated and heated samples.
A quick drop is observed in yellow to golden
yellow zircon while a smooth drop is
4. CONCLUSIONS
consists of low to good gem quality reddish-
chemical substitution was Hf, while all the
RBZ samples contained a low amount of
REE and radioactive elements.
Also, the ratio of Th: U was consistent with
a magmatic origin. Raman and FTIR
spectroscopy indicate that the RBZ of Sri
Lanka is of the Intermediate-type which has
gone through a metamictization process.
This is also confirmed by the specific gravity
of the stones. And more importantly, they
were not detectably radioactive which
encourages embedding into jewellery. The
UV-Vis absorption spectrum indicates the
cause of colour is due to structural defect by
radiation damage from radioactive elements
such as U and Th. Heat treatment under
either oxidizing or reducing conditions on an
electric furnace produces a yellow to golden
yellow colour at around 500 °C - 600 °C with
a soaking period of less than 90 minutes. The
difference in colour of RBZ and heat-treated
yellow to golden zircon were distinguished
using the UV-Vis spectrum by a smooth and
quick drop respectively. Hence, these results
help to understand the internal structure of
zircon and have contributed towards the
development of a precise heat treatment
methodology adding extra value to the RBZ
of Sri Lanka.
observed in raw RBZ which accounts for the
colour change of the stone. There are no
significant changes observed in the level
The primary deposit of the Kolonna region
brown zircons (RBZ). The most abundant
of metamictization even after heat treatment.

Journal of Geological Society of Sri Lanka Vol. 22-2 (2021), 39-45
45
ACKNOWLEDGMENTS
The financial assistance from the National
Science Foundation under the technical
grant no.TG/2016/Tech-D/05 is
acknowledged. The principal author extends
Research and Training Institute for
providing field & laboratory facilities.
REFERENCES
Abewardana, U.G.A.M.P. and Malaviarachchi,
S.P.K. (2018). Change of colour & clarity of
Sri Lankan zircon by heat treatment.
Proceedings of the 34th Technical Session of
Geological Society of Sri Lanka, 15, pp 6.
Chen, T., Ai, H., Yang, M., Zheng, S.and Liu, Y.
(2011) Brownish red zircon from Muling,
China. Gems and Gemology, 47(1): 36–41.
Holland, H. D., & Gottfried, D. (1955). The
effect of nuclear radiation on the structure of
zircon. Acta Crystallographica, 8(6): 291–
300.
Huong, L., Vuong, B., Thuyet, N., Khoi, N.,
Satitkune, S., Wanthanachaisaeng, B.,
Hofmeister, W., Häger, T., and
Hauzenberger, C. (2017). Geology,
Gemmological Properties and Preliminary
Heat Treatment of Gem-Quality Zircon from
the Central Highlands of Vietnam. Journal of
Gemmology, 35.
Laithummanoon, T., and Wongkokua, W.
(2013). Effect of heat treatment on color of
natural zircon. The Journal of KMUTNB,
23(2): 261–267.
degree of metamictization in zircon: a Raman
spectroscopic study. European Journal of
Mineralogy, 7(3), 471–478.
Rossman, R.G. (2011). The Chinese red feldspar
controversy: chronology of research through
July 2009. Gems and Gemology, 47: 16–30.
Schumann, W. (1977). Gemstones of the World.
Sterling Publishing Co.Inc, New York.
Vertriest, W., Palke, A. C., and Renfro, N. D.
(2019). Field gemology: Building a research
collection and understanding the
development of gem deposits. Gems and
Gemology, 55(4): 490–511.
Premilinary study in the cause of color in
zircon from Krông Năng Mining Area in Đắk
Lắk province. VNU Journal of Science: Earth
and Environmental Sciences, 31(3): 60–66.
Webster, R. revised by P. G. R. (1994). GEMS;
Identifaction. Butterworth-Heinemann. 873
pp.
Wittwer, A., Nasdala, L., Wanthenachaisaeng,
B., Bunnag, N., Skoda, R., Balmer, W. A.,
Giester, G., and Zeug, M. (2013).
Mineralogical characterisation of gem zircon
from Ratanakiri, Cambodia. Conference on
Raman and Luminescence Spectroscopy in
the Earth Sciences, 2013, 115–116.
Yada, K., Tanji, T., and Sunagawa, I. (1981).
Application of lattice imagery to radiation
damage investigation in natural zircon.
Physics and Chemistry of Minerals, 7(1): 47–
52.
Zeug, M., Nasdala, L., Wanthanachaisaeng, B.,
Balmer, W. A., Corfu, F., and Wildner, M.
(2018). Blue Zircon from Ratanakiri,
Cambodia. Journal of Gemmology, 36: 112-
132.
Vương, B. T. S., and Hương, L. T. T. (2015).
Nasdala, L., Irmer, G., and Wolf, D. (1995a). The
Their Sources, Descriptions and
sincere gratitude to the Gem and Jewellery