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JOURNAL OF RARE EARTHS 25 (2007) 637 - 642
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Ce(m) Salt Induced Changes of Superficial Micmstructm in Fmshwater P e d s
Shi Weilin (%&&), Jin Yefei (&PtX), M a Xiying (44&%)*
(School of Life Sciences, Shmxing University, Shmxing 312000, China)
Received 21 April 2006; revised 1 June 2006
Abstmt: The transition of the superficial microstructure of freshwater pearls induced by Ce was investigited by means
of scanning electron microscopy. The pearls were cultured in freshwater containing 0 (control group), 0.5, 1, and
1.5 mg * L - of a Ce additive. X-ray photoelectron spectroscopy (XPS) measurement showed that the concentration of
Ce absorbed in the superficial microstructure of pearls was positively correlated to the additive Ce. At the same time, the
surface microstructure of pearls changed greatly with Ce concentration, the shape of the blocks changed from spindly to
perfect regular hexagonal sheets and finally to round discs. The glossiness of the pearls changed correspondingly with the
microstructure, pearls possessing the regular hexagonal blocks having the highest glossiness. Therefore, the REE Ce exerted
a significant influence on the microstructure and glossiness of freshwater pearls. An appropriate quantity of Ce may improve the glossiness of pearls.
Key wolds: pearls; microstructure; aragonite blocks; glossiness; rare earth elements
CLC number: 4178.51; 4246; P579; P579 Document code: A Article ID: 1002 -0721(2007)05 -0637 -06
Pearls are widely used in adornments, such as
pearl necklaces, bracelets, and eardrops. In China, the
yield of freshwater pearls exceeds 106 kg each year,
over 90% of the global production, but the amount
sold constitutes only 8% of the global market. With
low or poor glossiness, a large number of pearls fail to
meet aesthetic requirements. Glossiness is a perceived
optical beauty of elegmce and soft luster closely related to the reflection, refraction, and interference of
light at the multi-layer complex surface of pearls. Owing to these complicated effects, glossiness has become the main reference standard for pearls, and their
grading is largely dependent on it. To improve the
glossiness of pearls, further study of their growth
mechanism must be done with a view to refrne their
microstructure.
Recently, rare earth elements (REEs) have been
widely used in the breeding of freshwater livestock,
such as shrimp, abalone, Spirulina, and so on. For example, Yang et al"]. reported that the hatching rate of
grass carp increased by 13% -25% when REEs were
added to feeds. Song's group found that the fraction
of surviving Cyprinus carpi0 improved by 13. 1%
and the yield increased by 11.1% when a special REE
additive was introduced into feeds'*'. Also, in research concerning both freshwater a l g o l o g ~ [ ~and
'~]
f i ~ h ' ~ REEs
, ~ ] , have been found to have an enhancing
effect on the growth rate and quality. However, the
effects of REEs on the glossiness and surface microstructure of freshwater pearls have been less studied.
In this study, the influence of the REE Ce on the surface microstructure and glossiness of freshwater pearls
was investigated. The pearls were cultured in freshwater containing either 0 (control group), 0.5, 1, or 1.5
* Companding authot(E-mai1: maxy@zscas.edu.cn)
Foundation item: Project supported by the Chinese Ministry of Education (205065)
Biopphy: Shi Weilin (1965-) , Male, Doctor, Associate professor; Majoring in environment science
Copyright 02007, by Editorial Committee of Journal of the Chinese Rare Earths Society. Published by Elsevier B.V. All rights reserved.
JOURNAL OF RARE EARTHS, VoL25, No.5, Oct. 2007
638
mg * L - of a Ce additive. The changes in the superficial microstructure and the glossiness of the pearls
with the concentration of Ce in the aqueous culturing
medium were analyzed by means of SEM images and
X-ray diffraction spectra. Lastly, the growth mechanism of pearls, and the correlation between their surface glossiness and microstructure were considered.
1 Materials andMethods
1.1 Prepamtion of Ce(NO,), additive solution
Stable isotope Ce was selected for the experiments as a representative of light REEs. A stock solution of 0.600 mg ml-’ Ce was prepared by dissolving cerium oxide (Ce,O,; 0.3 111 g) (purchased from
the Rare Earth New Materials Company of Beijing) in
aqueous nitric acid (HNO,; 2 ml), which was then diluted with water to provide a 1.O mg * L - Ce additive solution.
-
’
12 Test peari bivalves
-
Healthy freshwater pearl bivalves, 1 2 years
old, 7.0 8.0 cm long and weighing 20 30 g, were
purchased from Zhuji, Zhejiang Province. These were
randomly divided into two sets for parallel experiments, each set comprising four test groups, and each
group consisting of 12 bivalves. One group cultured in
freshwater with 0 mg L - ’ Ce served as a control
group and was also used to obtain background values.
The remaining served as REE test groups, cultured in
freshwater containing Ce(NO,), at levels of 0.5, 1.0,
and 1.5 mg * L - I , referred to as low, middle, and high
Ce groups, respectively. To simulate open water,
100 p.1 vessels containing 40 p.1 of freshwater served
as culturing vessels, in which the pH was maintained
7. 8, the concentration of Ca” was 20
at 7 . 5
mg L - I , the dissolved oxygen content was 7.5 8.2
mg * L‘ ’, and the temperature was in the range 18
22 “c. The bivalves were fed with home-made feed
once a day; the water was changed every 15 d. Additionally, algae, other microelements, and environmental
factors were kept identical. After 45 d of culturing six
bivalves were randomly selected from each group, and
were dissected in an ice-box to extract their pearls.
The pearls were immediately rinsed with deionized
water and carehlly dried with clean filter paper. After
weighing these were finally stored at 5 “c prior to
the examination of X-ray photoelectron spectroscopy
(XPS), X-ray diffraction analysis, and their microstructure observation by field emission scanning electron microscopy (FESEM , Holland).
-
-
-
-
-
2 Results and Discussion
The average weight of pearls is (3.60 iO.61),
(3.88 *0.29), (3.94 *0.51), and (6.94 t0.92) mg for
0,0.5, 1.0, and 1.5 rng L-’(control, low, middle and
high) rare earth Ce group, respectively. Clearly, the
average weight increases with the increase of Ce concentration in the culturing water. Particularly, for the
high group, it is almost two times than that for the
control group. It indicates that Ce may possess large
activities and catalysis, accordingly accelerating the
growth rate of pearls.
To c o n f m the concentration of Ce in the superficial microstructure of pearls, XPS measurement was
applied to the pearls. The corresponding results are
shown in Fig. 1. For the low Ce group, only a small
peak appears at 882 eV, attributed to the typical XPS
peaks of Ce 3d,,. For the middle and high Ce groups,
there is another peak center at 900.56 eV appearing
besides the peak of Ce 3d,,, corresponding to the
characteristic peak of Ce 3d,,; its intensity is lower
than that of Ce 3d,.
Obviously, the intensity of Ce in the superficial
microstructure of pearls increases with the concentration of Ce in the freshwater. Especially, when compared with the low group, it enhanced almost three
times for the high group. Moreover, there is a shoulder peak at 895 eV; this shows that the peak of Ce
3d,, is split into two peaks. The high intensity of Ce
3d,, shows considerable Ce3’ existing in the superficial microstructure of pearls of high Ce group, which
is consistent with the tendency of the average weight
of pearls.
A typical SEM image of the superficial microstructure of pearls sampled from the control group is
shown in Fig. 2. Several spindly blocks on the top
surface are arrayed towards the top-left of the image,
and some newly nucleated small spindly slices aligned
.
8o
t
(1) 0.5 m g C ’
(2) 1.0 m g L ’
0
870
880
890
900
910
920
Binding energy/cV
Fig. 1 XPS spectrum of pearls culturing in fresh water containing 0.5 (l), 1.O (2), and 1.5 (3) mg * L - rare earth Ce
’
in the same direction are scattered over the large
blocks. The blocks are all essentially of the same
form; the ratio of the long and short axes is about 3:l.
From the gpps and ridges between the spindly
blocks, it is evident that the subsurface is also composed of spindly blocks. However, the blocks do not
lie in a plane; some are overlapped, and as a result,
the pearls are not characterized by a layer structure.
The pearls were further analyzed by X-ray diffraction
analysis; the spectrum is shown in Fig. 3. Strong reflections are seen at 26.3", 27.2", 33.05", 36.2", 38",
43.1 O , 46" and 52.7", consistent with the characteristic diffraction peaks of vaterite crystals shown in the
lower panel, based on the JCPDS standard card, suggesting that the pearls are composed of vaterite crystals. It has been doc~mented'~'
that freshwater pearls
are usually formed from porcelaneous layers and pearl
Fig. 2
Typical SEM image of the surface microstructure of
pearls sampled from the control group (several spindly
crystal blocks on the top surface are arrayed towards
the top-left of the image, showing that the pearls are
made from spindly blocks)
microlayers. The former usually have low luster because they are largely composed of vaterites, which
are generally fibrous in nature. On the other hand, the
latter are often highly nacreous as a result of the ordering of aragonite blocks in a layered structure. Therefore, the pearls from the control group, composed of
the spindly vaterite porcelaneous layers, have low
glossiness.
The superficial microstructure of pearls sampled
from the 0.5 mg L - ' Ce processing group is shown
in Fig. 4. Clearly, there is a distribution of iwo kinds
of blocks, round wafers towards the top of the image,
and spindly blocks at the bottom. Moreover, unlike
the pearls from the control group, the pearls from the
low Ce group possess an unambiguously layered
structure. These changes show that a phase transition
in the microstructure of the pearls was induced by the
presence of Ce.
Fig. 5 shows superficial SEM images obtained
when the Ce concentration was increased to 1 . 0
mg L - ' . Several regular hexagonal blocks can be
clearly seen in Fig. 5(a); their size is largely uniform
at around 4 pm and their distribution is approximately homogeneous, without any overlap or aggregation.
Each block, even the smallest, has a perfect regular
hexagonal shape, free from breakages or fragments.
Moreover, the subsurface is a smooth plane as a result of a highly ordered and compact packing of the
hexagonal blocks, devoid of any gyps or ridges.
Fig. 5(b) shows a profile of the pearls, depicting
three microlayers: a top surface, a subsurface, and a
deeper plane. Each microlayer is made up of a compact arrangement of the regular hexagonal blocks , so
Vaterite
80
I
27.28"
40
10
20
30
40
50
60
70
80
204")
Fig, 3 X-ray diffraction spectrum of pearls sampled from the
control group (strong reflections appearing at 26.3", 27.
2", 33.05",362", 38", 43.1", 46", and 52.7" are consistent with the main peaks of the vaterite phase, sug
gesting that the pearls are composed of vaterite crystals)
Fig. 4
Typical SEM image of the pearls sampled from the 0.5
mg * L - ' Ce group (two kinds of blocks, round wafers
and spindly blocks, are seen to coexist in this image, indicating that a phase transition in the microstructure of
the pearls was induced by the action of the lowest Ce
concentration)
JOURNAL OF RARE E4RTH.9, VoL25, NOS, Oct. 2007
640
that each has the same thickness as the blocks. In addition, the blocks on the top surface, especially those
at the interface edge, are smaller than those of the
subsurface, indicating that they are growing. It is thus
confirmed that the pearls are built up of microlayers,
and that the microlayas are composed of regular hexagonal blocks, without any dislocations or faults. The
pearls appeared to be brighter and larger than those
obtained from the control group.
The corresponding X-ray diffraction pattern is
shown in Fig. 5(c), together with the characteristic
diffraction peaks of aragonite crystals. Strong reflections located at 25.08", 27. 31", 32.85", 44.28", and
50.63" clearly match the main peaks of aragonite
crystals, but differ greatly from the peaks of vaterite
shown in Fig. 2, suggesting that the pearls are made
from the regular hexagonal aragonite blocks.
A typical surface image of pearls grown in the
1.5 mg * L - Ce group is shown in Fig. 6. Several
round discs of diameter 3 km are seen to be scattered
over the top surface, without any overlap, breakages,
or fragments. Similarly, the subsurface also comprises
a compact arrangement of the round discs, and the
pearls are characterized by a planar layer structure.
According to the researches of the superficial microstructure of various degrees' pearls by means of
SEM and AFM"-91, the glossiness, roughness, and
color are strongly related to the superficial structure
of pearls, and the pearl grade is positively correlated
with the assembled degree of impact and order of the
aragonite blocks and aragonite microlayer. The high
quality pearls are generally formed by aragonite
blocks in terms of microlayer by microlayer with few
dislocations, while the plain pearls are composed of
vaterite blocks with large defects and dislocations[6 1 0 , I I I
Aragonite blocks usually adopt such shapes as
pseudo-hexagonal, round wafer, and irregular polygon,
among which the regular hexagon is the idea cell to
give rise to high quality pearls. The growth of Ce
pearls in this experiment can be described in terms of
three distinct stages , namely nucleation , normal
Fig. 5
Typical SEM b a g s of pearls sampled from the RE test
group with a Ce concentration of 1.0 mg L - ' in the aqueous culturing medium (a) Several regdar hexagonal
blocks are seen on the top surface, without any overlap
or aggtejgtion. Each block has a perfect regular hexagmal
structure; (b) A profile of the RE pearls, confirmingthat
the pearls are built up from microlayers composed of
ordered hexagonal blocks in a layer by layer manner,
without any dislocations or faults, each microlayer corresponding to the thickness of the blocks; (c) The corresponding X-ray diffraction spectrum. Strong reflections
from the pearls located at 25.08", 2731", 32.85", 44.28"
and 50.63' are in accordance with the main peaks of the
aragonite phase, showing that the pearls are made of reg
ular hexagonal aragonite blocks
Fig. 6
Typical SEM image of pearls sampled from the 1.5
mg L ' I Ce group (several round arapnite wafers are
seen on the top surface, without any overlap or we@ion)
-
Shi W L et aL
Ce(m) Sd! Induced Changes of Sqw@id Microstructure in Freshwater Pemls
growth, and layered growth. During the first stage,
small mosaic slices are nucleated, which are the fundamental seed cells deposited by calcium carbonate
crystals under the precise control of orgpnic material.
During the second stage, these slices undergo monotonic growth and turn into larger aragonite blocks.
During the third stage, when the blocks grow large enough, they are compacted in an ordered manner in
two dimensions, thus leading to a microlayer. Finally,
pearls are built-up from these microlayers, layer by
layer .
According to the XPS measurement in Fig. 1, the
Ce quantity in pearls is quickly improved with the Ce
concentration increasing in the freshwater, which
makes the structure pearls have taken great change.
The superficial microstructure of pearls from the experimental groups changes from one of vaterite crystals to one of aragonite crystals, and the shape of the
blocks changes from spindly to perfect regular hexagonal sheets and finally to round discs, suggesting that
the microstructure of pearls is strongly influenced by
the Ce concentration. The glossiness changes accordingly with the microstructure, the pearls with the regular hexagonal structure displaying the highest glossiness. Since the growth conditions, such as the water
temperature, the pH, and the feeds, were kept constant in the test groups, apart from the Ce concentration. It can be concluded that the microstructure of
pearls is greatly modulated by the presence of Ce. In
other words, Ce has great influence on the microstructure and glossiness of freshwater pearls. The addition
of an appropriate quantity of Ce may thus improve
the glossiness and the growth rate of pearls.
To the best knowledge, RE elements are not essential trace elements for nourishment nor are they
constituents of pearls or bivalve shells, but they play
important roles in the deposition of pearls. First, RE
elements have a function of luminescence and chromogenesis, and this can enhance the glossiness of pearls
by improving the degree of saturation of color and the
refractive index. Second, if some elements necessary
for the growth of pearls, such as Ca, Mg, Zn, Se, etc.,
are lacking in the culturing water, RE elements can
substitute them. Finally, RE elements are often referred to as '' super-calcium'"'*]. AS already well
known, calcium ions play key roles in maintaining the
n o d function of cells, muscle isotonic contraction,
information transfer, nerve center driving and bone
marrow formation in organisms.
Since RE ions have similar ionic radii and properties as calcium ions, they can also hlfill these roles.
Moreover, they are usually trivalent ions with a high-
641
er ionic potential than that of bivalent calcium ions.
Hence, they cannot only occupy the positions usually
occupied by calcium, but can also displace bound calcium ions. It has been reported that RE elements have
; it is
osteogenic action and mineral ~rystallization"~~
reasonable that RE elements give rise to great changes
in the microstructure of pearls. Thus, RE elements
will accelerate the growth of pearls, and enhance the
glossiness of the pearls by enhancing the activity of
enzymes and prompting secretions in the freshwater
bivalves if appropriate quantity RE is added to the
aqueous culturing medium.
3 Conclusion
The effects of the Ce on the superficial microscopic structure of pearls were studied by SEM. With
increasing Ce concentration, the superficial microstructure of pearls experienced great changes. First,
the constituent component of the pearls changed from
vaterite to aragonite blocks, while the shape of the
blocks changed from spindly to perfect regular hexagonal and finally to round wafers. Second, the pearls
showed an increasing tendency to form layered structures. Finally, the glossiness of the pearls also
changed with the microstructure, the pearls with the
regular hexagonal structure having the highest glossiness. Thus, the addition of an appropriate quantity of
an RE elements can refine the surface microstructure
and greatly enhance the luster of pearls.
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