Study of calcium oxalate monohydrate of kidney stones
by X-ray diffraction
M. T. D. Orlando, L. Kuplich, D. O. de Souza, H. Belich, J. B. Depianti, C. G. P. Orlando,
E. F. Medeiros, and P. C. M. da Cruz
Universidade Federal do Espírito Santo (UFES), Av. Fernando Ferrari 514, 29075-910 Vitória, Espírito
L. G. Martinez
Instituto de Pesquisas Energéticas e Nucleares (IPEN), Campus Universidade de São Paulo (USP),
05508-900 São Paulo, São Paulo, Brasil
H. P. S. Corrêa
Universidade Federal de Mato Grosso do Sul (UFMS), 79070-900 Campo Grande, Mato Grosso do Sul,
Escola de Artes, Ciências e Humanidades (EACH), Universidade de São Paulo (USP), Avenue Arlindo
Bettio 1000, 03828-000 São Paulo, São Paulo, Brasil
?Received 16 May 2007; accepted 28 February 2008?
X-ray powder diffraction was used to study the phase composition of human renal calculi. The
stones were collected from 56 donors in Vitória, Espírito Santo state, southeastern Brazil. An XRD
phase quantification revealed that 61% of the studied renal stones were composed exclusively of
calcium oxalate ?34% formed only by calcium oxalate monohydrate ?COM? and 27% presents both
monohydrate and dihydratate calcium oxalate?. The 39% multi-composed calculi have various other
phases such as uric acid and calcium phosphate. Rietveld refinement of XRD data of one apparent
monophasic ?COM? renal calculus revealed the presence of a small amount of hydroxyapatite. The
presence of this second phase and the morphology of the stone ?ellipsoidal? indicated that this
calculus can be classified as non-papillary type and its nucleation process developed in closed
kidney cavities. In order to show some advantages of the X-ray powder diffraction technique, a
study of the phase transformation of monohydrate calcium oxalate into calcium carbonate ?CaCO3?
was carried out by annealing of a monophasic COM calculi at 200, 300, and 400 °C for 48 h in a
N2gas atmosphere. The results of the XRD for the heat treated samples is in good agreement with
the thermogravimetric analysis found in the literature and shows that X-ray powder diffraction can
be used as a suitable technique to study the composition and phase diagram of renal calculi. © 2008
International Centre for Diffraction Data. ?DOI: 10.1154/1.2903738?
Key words: kidney stones, calcium oxalate, calcium oxalate monohydrated, X-ray diffraction
Urolithiasis has been described as the third most com-
mon affliction of human urinary tract, estimated to occur in
approximately 5% of the population of any industrial coun-
try. In addition, statistics have shown a recurrence rate of
2.5%, affecting mainly adults 20 to 60 years of age ?Asokan
et al., 2004?. The renal calculi are formed as a result of a
biological maladjustment of the urine and are often hetero-
geneous containing mainly oxalate, phosphate, and uric acid
crystals ?Koide et al., 1986; Grases et al., 1998?.
Although urinary calculi can be easily removed in most
cases, it is not possible to prevent their recurrence even after
the stone’s removal, either by invasive or non-invasive clini-
cal or surgical procedures ?Coe et al., 1992; Singh et al.,
1999?. The nucleation process of the crystal is an important
factor during the initial stage of the calculi genesis. Once the
heterogeneous nucleation starts, the stone continues to grow
because of the deposition of either original or different com-
position substances on the original substrate. The potential
risk of recurrence, with consequent degradation of the renal
function along the expected lifetime ?especially in children?,
justifies an investigation of urinary calculi composition,
nucleation, and its associated pathologies.
The formation of renal calculi based on calcium oxalate
crystals has been widely reported since 1985, but researchers
have paid little attention to its nucleation process ?Koide
et al., 1986?. According to schematically kidney morphology
proposed by Söhnel and Grases ?1995?, calcium oxalate
monohydrate calculi ?CaC2O4·H2O or COM? can be classi-
fied in two groups: ?I? COM papillary calculi and ?II? non-
papillary COM calculi ?Grases et al., 1998?.
The first group ?I?, COM papillary calculi, is associated
with COM renal stones, which appear attached to papillae.
This group presents a conical morphology and is formed by
a core, or stone nucleus, situated in the urolith interior and a
shell in its outer part grown out from the core ?Söhnel and
Grases, 1995?. Grases et al. ?1998? point out the existence of
four different types of papillary calculi core upon which the
crystal growth occurs. Furthermore, the same authors remark
that the fixation process of the nucleus on the papilla has
been overlooked in previous studies.
S59S59 Powder Diffraction Suppl. 23 ?2?, June 2008 0885-7156/2008/23?2?/S59/6/$23.00 © 2008 JCPDS-ICDD
The second group ?II?, non-papillary COM calculi, pre-
sents typically ellipsoidal morphology, which is clearly dif-
ferent from the papillary calculi ?type I?. It grows in renal
closed cavities and can be broadly classified into two main
groups: ?II-a? and ?II-b?. The II-a type renal calculi contain
no core and their inner structures resemble the random pat-
terns exhibited by sedimentary rocks. In this type of kidney
stone, the material is distributed irregularly on the inner part
and may occasionally contain small spheres of hydroxyapa-
tite. On the other hand, the II-b calculi contain a core that is
mainly organic matter that functions as a seed for the devel-
opment of the stone body. Most of the bodies of this type of
stone are constituted by columnar COM crystals emerging
from the core and, because of the absence of an attachment
to the epithelium, are found in renal cavities. Moreover, El-
liot ?1973? points out that these stones can be initiated by a
small papillary stone detached accidentally from the papilla
and trapped in the kidney cavities.
There are many useful techniques that can be used to
obtain the urinary calculi composition, including chemical
analysis ?Hodgkinson, 1971; Westbury and Omenogor,
1970?, infrared spectroscopy ?Bellanato et al., 1973;
Takasaki, 1971?, X-ray powder diffraction ?Gibson, 1974;
Lonsdale et al., 1968?, electron microscopy ?Spector et al.,
1978?, and atomic absorption spectroscopy. XRD analysis,
used to study the composition of calculi, provides the iden-
tification of its phases and their type and quantities, allowing
for the discovery of the origin of the calculi.
Renal calculi with calcium oxalates are represented by
the general formula CaC2O4·xH2O, where x is the number of
bonded-water molecules, which can vary from 1 to 2. It can
be formed on crystalline seed particles of organic or inor-
ganic compounds that work as a nucleating substrate. There-
fore, the H2O molecule might be bound or free, depending
on if the H2O molecule belongs to the crystal structure or the
organic compound among them. Some of the characteriza-
tion techniques commonly used are not suitable to give the
structural information about the H2O molecule. For instance,
chemical, infrared spectroscopy, and thermogravimetric
analysis ?TGA? cannot define by themselves whether the
H2O molecule is structural or interstitial. On the other hand,
the X-ray powder diffraction technique ?Gibson, 1974; Lons-
dale et al., 1968? associated with a Rietveld refinement of the
data can define the position of the H2O molecule and, as a
consequence, can help to determine the CaC2O4·xH2O phase
present in renal calculi. Therefore, the analysis of the crys-
talline phases and the determination of the morphological
characteristics of renal calculi are important tools for the
etiological diagnosis of this disease. Another advantage of
the X-ray powder diffraction technique is that the powder
can be characterized without a surgical procedure by analyz-
ing the fragmented crystals collected from the urine, which
?ECSWL?. Our research group has already reported a pre-
liminary XRD data analysis on urinary calculi fragments col-
lected from 25 donors submitted to ECSWL procedures in
Vitória, Brazil ?Azevedo, 2002?. In that work, a semi-
quantitative analysis of the calculi composition revealed that
they consisted of a combination of calcium oxalate monohy-
drate, calcium oxalate dihydrate, uric acid, and calcium
phosphate. Among the 25 stones, 36% had calcium oxalate
monohydrate type ?CaC2O4·H2O? as the main phase and
60% exhibited calcium oxalate dihydrate ?CaC2O4·2H2O? as
the main component.
Renal calculi of 56 donors from the inhabitants of
Vitória and surrounding areas were collected for this study. It
is important to note that all kidney stones in this work were
expelled naturally or obtained by surgical procedures ?Ku-
First, all collected calculi were dried in a dissector filled
with nitrogen gas at room temperature for 2 weeks. Second,
the dry calculi were photographed and classified according to
the criteria proposed by Grases et al. ?1998?. Figure 1 repre-
sents a typical calculus fragment. After this procedure, the
calculi were crushed in an agate mortar in order to obtain a
fine mesh powder ?57 ?m in size?.
TABLE I. Composition of calculi.
Calculi phaseN ?%?
TABLE II. Composition of calculi by gender.
Calculi phase Male ?%?
Figure 1. ?Color online? Calculus #30 ?II-a morphology? from a local resi-
dent in Vitória, Brazil ?measurement in cm?.
S60S60 Powder Diffr. Suppl., Vol. 23, No. 2, June 2008Orlando et al.
B. X-ray diffraction and refinement
X-ray diffraction patterns of the samples were measured
in order to obtain the phase composition of the renal calculi.
The XRD measurements were performed with a Rigaku
Multiflex laboratory diffractometer, using a Cu K? sealed
tube working at 40 kV, 30 mA, a scintillation counter, and a
diffracted beam graphite monochromator. The diffraction
patterns were recorded in the step scan mode at 0.02 steps
and at a measurement rate of 2 s/step. The divergence, re-
ceiving, and scatter slits used were
spectively. The diffraction patterns were registered within the
angle range from 4 to 120°2?. Some of the samples had their
diffraction data collected using the X-ray Powder Diffraction
?D10B–XPD? beamline ?Ferreira et al., 2006? of the National
Synchrotron Light Laboratory ?LNLS?, Campinas, Brazil.
The wavelength used was 1.19197?3? Å and selected by a
double-bounce Si?111? monochromator, with the first crystal
water cooled and the second one bent for sagittal focusing
?Giles et al., 2003? using an Na?Tl?I scintillation counter.
In both cases, the instrumental parameters were obtained
from the refinement of standards samples of LaB6and Al2O3
?NIST, 2000, 2005?. The Powder Diffraction File database
was used for phase identification. The identified phases were
calcium oxalate monohydrate ?COM? ?PDF 20-0231?, cal-
cium carbonate ?PDF 05-0586?, calcium oxalate dihydrate
?COD? ?PDF 17-0541?, and uric acid ?PDF 28-2016? ?ICDD,
2005?. The Rietveld refinement of the X-ray diffraction data
was performed using the GSAS and EXPGUI package ?Larson
and Von Dreele, 2000; Toby, 2001?.
2°, 0.3 mm, and
C. Thermal treatment
A partial phase diagram study of CaC2O4·H2O was car-
ried out, submitting the calculi to heat treatments performed
with a computer controlled furnace under an inert nitrogen
?99.9% N2? atmosphere controlled by a mechanical flux
meter, in order to remove the bonded H2O molecule. The
study was carried out on two calculi containing only the
calcium oxalate monohydrate phases ?labeled here as #45
and #25?, which were divided in four equal weight samples.
Three samples of the same stone were submitted to heat
treatments for 48 h at 200, 300, and 400 °C, whereas the
fourth sample was held at room temperature ?N2atmosphere?
in order to compare with the treated samples. After each
thermal treatment, the sample was submitted for an X-ray
diffraction measurement under a nitrogen atmosphere.
This process allows for the determination of the true
phase transformation, but TGA can only determine the tem-
perature at which the loss-of-mass occurs, regardless if it is
attributable to interstitial water or bonded H2O molecule.
III. RESULTS AND DISCUSSION
A. X-ray diffraction and Rietveld analysis
The quantitative analysis was done using the integrated
area method both for the laboratory and synchrotron diffrac-
tion data ?Table I?. The gender phase composition of the 56
calculi is presented in Table II. As it can be seen in Table I,
TABLE IV. Results of Rietveld refinement and the reliability factors for Sample #30.
Unit cell dimensions
CaC2O4·H2O ?Calcium carbonate monohydrate?
a=6.2945?1? Å, b=14.5961?3? Å,
c=10.1177?2? Å, ?=109.46?0?
a=9.479?5? Å, b=18.883?9? Å,
c=6.8888?9? Å, ?=120.45?3? Å
TABLE III. Results of Rietveld refinement and the reliability factors for Sample #25.
Unit cell dimensions
CaC2O4·H2O ?Calcium carbonate monohydrate?
a=6.2952?2? Å, b=14.5951?4? Å, c=10.1185?3? Å,
S61 S61 Powder Diffr. Suppl., Vol. 23, No. 2, June 2008Study of calcium oxalate monohydrate of kidney stones by X-ray diffraction
the results are in good agreement with the results reported by
Ansari et al. ?2005?, in which calcium oxalate monohydrate
is the calculi major phase.
In order to get an improvement on the calculi phase
analysis, a Rietveld refinement of two apparently monopha-
sic COM calculi ?labeled here as #25 and #30? was per-
formed. Table III presents the main results of the refinements
for Sample #25, with the goodness-of-fit.
Thompson-Cox-Hastings pseudo-Voigt modified profile
function in the Rietveld refinements was used, and the start-
ing crystal structure data for the refinements were taken from
the ICSD database: 30-782 for CaC2O4·H2O ?COM?, 34-457
for hydroxyapatite, and 30-783 for CaC2O4·2H2O ?COD?
?FIZ and NIST, 2006?. The Rietveld analysis of Sample #25
showed that the sample is monophasic, i.e., CaC2O4·H2O
?COM?. On the other hand, the Rietveld refinement of the
calculus #30 indicated the presence of 15.1% weight fraction
of hydroxyapatite and 84.9% weight fraction of COM ?Table
IV?. The ?2, R?F?2, wRp, and Rp indicate the quality of the
fit for the whole set of powder diffraction data, regardless of
the number of phases. Figures 2 and 3 show the Rietveld
B. Phase transition
By means of the heat treatments followed by XRD
analysis, presented in Figures 4 and 5, it was possible to
verify the temperature dependence of the phase transforma-
tion of CaC2O4·H2O ?COM? into CaC2O4?calcium oxalate-
anhydrous–CO? and CaCO3?calcium carbonate–CCa? or, in
other words, to establish a partial phase diagram of this
transformation. It was investigated at which temperature the
bonded H2O molecules were removed from CaC2O4·H2O
?COM? with the subsequent phase transition to CaC2O4?CO?
In Figures 4 and 5, the phase evolutions of the renal
calculi #45 and #25, respectively, as a function of the anneal-
ing temperature treatment is shown. The intensity of the peak
at 3.67 Å in Figure 4 shows how the amount of anhydrous
calcium oxalate CaC2O4?CO? increased steadily from 25 °C
?as collected? up to 200 °C. The broadening of the peak at
?6 Å can also be observed, which can be associated to the
crystallite size reduction of the CaC2O4·H2O phase, caused
Figure 2. Final Rietveld refinement ?line? of Sample #25, data experimental
?circle?, and difference between both.
Figure 3. Final Rietveld refinement ?line? of Sample #30, data experimental
?circle?, and difference between both. The first phase is CaC2O4·H2O
?COM? and the second is Ca4.87HO12.56P3?hydroxiapatite?.
Figure 4. Thermal treatment of Sample #45: ?a? as collected, ?b? 200 °C, ?c?
300 °C, and ?d? 400 °C. It was detected as CaC2O4·H2O ?COM? with a
consequent phase transition to CaC2O4?CO? and CaCO3?CCa?.
Figure 5. Thermal treatment of Sample #25: ?a? as collected, ?b? 300 °C,
and ?c? 400 °C. It was detected as CaC2O4·H2O ?COM? with a consequent
phase transition to CaC2O4?CO? and CaCO3?CCa?.
S62S62Powder Diffr. Suppl., Vol. 23, No. 2, June 2008 Orlando et al.
by the removal of the bonded H2O, with subsequent trans-
formation into CaC2O4. The XRD features of the latter treat-
ment ?400 °C? reveal that CaC2O4?CO? was transformed to
The crystalline-to-amorphous phase transition has not
been observed to occur, as can be seen following the peak
?3.7 Å. The test of repeatability shown in Figure 5, carried
out on calculus #45, also illustrates the same behavior. These
results show that the bonded H2O molecule leaves the
CaC2O4·H2O below 200 °C. This behavior is in good agree-
ment with Kaloustian et al. ?2002, 2003? studies using ther-
mogravimetric analysis, which indicate that the bonded H2O
molecule, in calcium oxalate urinary calculi, was removed at
about 180 °C. Moreover, observing the thermal evolution of
the ?2.96 Å peak ?COM?, the calcium oxalate ?CO? was
completely transformed into calcium carbonate ?CCa? after
the heat treatment at 400 °C for 48 h, which is confirmed by
the non-existence of the CaC2O4?CO? peak at 3.67 Å after
the heat treatment at 400 °C. These results are also in good
agreement with thermogravimetric analysis results reported
by Kaloustian et al. ?2002, 2003?, which shows the phase
CaC2O4·H2O at temperatures of 450 to 550 °C. These find-
ings also show that X-ray powder diffraction can be used as
a suitable technique to study the composition and phase equi-
librium of renal calculi.
X-ray powder diffraction was used as a suitable tech-
nique to study the phase composition of renal calculi ex-
pelled naturally or obtained by surgical procedure. The
stones were collected from 56 donors in Vitória, Espírito
Santo state, southeastern Brazil. The XRD phase quantifica-
tion revealed that 61% of the renal stones were composed
exclusively by calcium oxalate, being 34% formed only by
calcium oxalate monohydrate ?COM?, whereas the remaining
27% presents both monohydrate and dihydratate calcium ox-
alate. The 39% non-exclusive calcium oxalate calculi pre-
sented other phases like uric acid and calcium phosphate.
Two calcium oxalate renal calculi samples were analyzed by
Rietveld refinement with the results confirming that these
samples presented CaC2O4·H2O as the main phase. How-
ever, for one of the samples ?#30?, the refinement has shown
the presence of 15 vol % hydroxyapatite. Considering this
result and the Grases et al. ?1998? model, Sample #30 has
grown upon a hydroxyapatite core. In addition, Sample #30
may be classified as non-papillary type II-a calculus.
In order to show the versatility of the XRD technique,
we have also studied the phase transformations from mono-
hydrate calcium oxalate ?CaC2O4·H2O? to anhydrous cal-
cium oxalate ?CaC2O4? and to calcium carbonate ?CaCO3?,
when COM calculi were heat treated for 48 h at 200, 300,
and 400 °C in a N2gas atmosphere. Using only XRPD we
have confirmed that there exists a phase transition CaC2O4
→CaCO3+CO after an annealing at 400 °C for 48 h. The
results, obtained by X-ray powder diffraction technique, are
in good agreement with those from a TGA reported in the
literature, and indicate that X-ray powder diffraction can be
used as a suitable technique to study the composition and
phase diagram of renal calculi.
The authors thank the Brazilian national scientific agen-
cies CNPq ?Projects CNPq 504578/2004-9 and CNPq
471536/2004-0? and CAPES for financial support. We also
thank Companhia Vale do Rio Doce ?CVRD? and Compan-
hia Siderúgica de Tubarão ?CST?, and gratefully acknowl-
edge the LNLS ?Project XPD-4742?.
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