Introduction
Calcium carbonate (CaCO3) is a widely used substance for various purposes. It is used as
fertilizer and a filler and pigment material in paper, plastics, rubbers, paints, and inks but also
in pharmaceutics, cosmetics, construction materials, and asphalts and as a nutritional
supplement in animal foods(Martinez,2002).Just like many minerals calcium carbonate is also
one of the substances that are mined. It is mined from limestone and it is processed to size for
use in different applications (Crapper, 2012)
There are three polymorphs calcium carbonate: vaterite, aragonite and calcite, what
differentiate them is their thermodynamically stability. All these polymorphs can occur at the
same time in some environment and conditions Aragonite and calcite are more
thermodynamically stable structures, and they most commonly occur in nature (Ratner,
2008).It is important to understand the chemistry of these because Vaterite has been shown to
transform to calcite and aragonite in aqueous solution. Experimental evidence has
demonstrated that vaterite can transform to aragonite in 60 minutes at 60°C and to calcite in 24
hours at room temperature (Frebida et al , 2021). Even though there is such abundant deposits
of limestone on the planet that the possibility of depletion is remote, the application of calcium
carbonate depend on the availability particular polymorph. Understanding the behaviour and
controlling the growth of the polymorphs of calcium carbonate plays a major role in the
synthesis of specific polymorph.in recent times vaterite has been found to be very useful in the
in the area of regenerative medicine, drug delivery and a broad range of personal care
products. It has been established above that vaterite is very much unstable so knowledge of the
composition of the synthesised crystals is important because it will be provide information that
can be useful for the manipulation of conditions necessary for the production of the desired
polymorph.
There are many factors that influence the precipitation of calcium carbonate polymorphs. The
most determining factors is the presence of foreign ions or molecules in the solution from which
the calcium carbonate precipitate (Frebida et al, 2021). The technique of choice for quantifying
these polymorphs of calcium carbonate is X-Ray Diffraction. diffraction occurs when light is scattered
by periodic array with long range order producing constructive interference at specific angels
(Banerjee, n.d.) .X-Ray Diffraction is a technique for analysing the atomic or molecular structure
of materials(esli, 2019). The scattering of light is proportional to the number of electrons around
the atom. Diffraction peak is attributed to the scattering from a specific set of parallel planes of atoms,
miller indices are used to identify the different planes of atoms( Banerjee,N.D)
Therefore, the main objective of this study is to quantify the three polymorphs of calcium
carbonate in two samples that were synthesised from a 0.01; & 0.02 M respectively of aqueous
solutions of each of Na2CO3 and CaCl2 at different temperature using XRD.
Experimential
Two precursor materials were used: Sample 5 0.01 M Na2CO3 and 0.02 M CaCl2 at PH OF
8.5 Sample 6 0,O1M Na2CO3, O.O2 PH 6 both sources are analytical grade (Sigma Aldrich ).
The synthesized CACO3 was prepared by mixing O.O1M Na2CO3 with 0.02 M CaCL2 at two
PH OF 8.5 and 6 respectively . The mixture was left stirring on a magnetic stirrer bar for
another 10 minutes before it was filtered with a 0.8 μm membrane filter, then washed
thoroughly with distilled water. The powder precipitate was dried in an oven, set at 90℃
overnight and then stored in a blue silica desiccator in darkness
X-ray powder diffraction measurements were carried out using a Bruker D8 Advance X-ray
diffractometer instrument using Cu Kα radiation. Ideally, the sample should be pulverised to a
particle size of less than 20 microns – this is achieved though micronising pulverised powders.
This is especially important for quantitative analysis .The powder should be loaded into one of
the plastic Bruker sample holders, with the sample identification and depth of at least 1 mm.
The surface of the packed powder was pressed and smoothed with a piece of flat glass. Care
was taken to ensure that the surface of the sample is flat and level with the top of the sample
holder The quantities were sufficient to make a compact sample with correct height and smooth
surface. Once the sample was prepared the sample holder was wiped to remove any nonessential sample that is present.
After the instrument was switched on the software was started .The commander module of the
Bruker AXS model was selected by click on the commander icon. Once the communication
between the software was established, Corundum was used for alignment of the goniometer.
The diffraction pattern was recorded for 2θ from 0° to 90° and a 2θ step scan of 0.050° was
used, counting for 1 s at every step. The total scan time for each sample was 28 min. The
voltage and current of the generator were set at 40 kV and 40 mA respectively
4 Results
1.200
RALATIVE INTENSITY
1.000
0.800
0.600
0.400
0.200
0.000
0
10
20
30
-0.200
40
50
2 THETA
60
70
80
90
100
HUNDREDS
Difractogram A for sample 5(0.01 M Na2CO3 and 0.02 M CaCl2 at PH 8.5)
1.2000
1.0000
relative intensity
0.8000
0.6000
0.4000
0.2000
0.0000
0
-0.2000
10
20
30
40
50
60
70
2 theta(in degrees)
Difractogram B for sample 6 (0.01 M Na2CO3 and 0.02 M CaCl2 at PH 6)
80
90
100
Difractogram C (Fig 1 is the difractograms for pure polymorphs of calcium carbonate)
Table of XRD intensities
TABLE 1
Polymorphs of Pure
calcium
polymorphs
carbonate
intensity
Vaterite(25.0
Calcite (29.5
Aragonite(45.9
Sample 5
Sample 6
0.583
no peak at the
expected
position
1.00
1.0078
no peak at the 0.3388
expected
position
Calculations for the individual polymorphs for sample 5
%aragonite=(I/Iºarogonite)/ (I/Iºvaterite)+ (I/Iºcalcite)+(I/Iºarogonite)
=0/(0.583+1+0)x100
=0%
%vaterite =(I/Iºvaterite )/ (I/Iºvaterite)+ (I/Iºcalcite)+(I/Iºarogonite
=0.583/ (0.583+1+0)X100
=36.8%
%calcite =(I/Iºcalcite )/ (I/Iºvaterite)+ (I/Iºcalcite)+(I/Iºarogonite
=1/(1+0.583+0)X100
= 63.2%
Calculations for the individual polymorphs for sample 6
%aragonite=(I/Iºarogonite)/ (I/Iºvaterite)+ (I/Iºcalcite)+(I/Iºarogonite)
=0.3388/(0.3388+1.0078+0)x100
=25.2%
%vaterite =(I/Iºvaterite )/ (I/Iºvaterite)+ (I/Iºcalcite)+(I/Iºarogonite)
=(0/(0+1.0078+0.3388)
=0%
%calcite =(I/Iºcalcite )/ (I/Iºvaterite)+ (I/Iºcalcite)+(I/Iºarogonite)
=1.0078/(0+1.0078+0.3388)
=74.8%
Table 2 concentration of calcium carbonate polymorphs
Polymorphs of Sample 5 (PH Sample
CaCO3
8.5)
6)
%Arogonite
0
25.2
%vaterite
36.8
0
%calcite
63.2
74.8
6(PH
Discussions
Crystals of calcium carbonate were synthesised using sodium carbonate and calcium chloride.
These two samples were prepared at room temperature and at different PH.
Table 2 show the results from quantification that was carried on XRD instrument.The result
for sample show that the synthesis did not produce arogonite but it is composed by 63.2% of
calcite and 36.2 % vaterite. Chen et al. Observed drastic decrease in super saturation during CaCO3
particle synthesis due to short induction times 10 at high pH solutions. For sample 5 it seems as the
high Ph of 10 it has the impact in the formation of Aragonite polymorph.Temperature is shown to have
impact in the formation of the polymorphs (Tawada, 1987). Suzuki (1991) “The transformed
polymorphs are vaterite and calcite at low temperature (14 to 30°C), and aragonite and calcite
at high temperature (60 to 80°C)”.Sample 5 results confirm this finding with the formation of
vaterite and calcite with the formation of Aragonite.
For sample 6 results are not consistent with what other researchers has found. Kojima(2019)
found that at PH of 5 to 10 it is possible to synthesize polymorphs of calcium carbonate which
contain vaterite but in this current investigation vaterite is not found. Further studies on other
thermodynamical properties is needed.