14560 Phys. Chem. Chem. Phys., 2010, 12, 14560–14566 This journal is c the Owner Societies 2010

Stability and transformation mechanism of weddellite nanocrystals

studied by X-ray diffraction and infrared spectroscopy

Claudia Conti,*a Luigi Brambilla,b Chiara Colombo,a David Dellasega,c

G. Diego Gatta,dMarco Realini

aand Giuseppe Zerbi

b

Received 19th May 2010, Accepted 13th September 2010

DOI: 10.1039/c0cp00624f

This study is focused on the stability of weddellite, the dihydrate phase of calcium oxalate

[CaC2O4�(2 + x)H2O], mainly detected in kidney stones and in oxalate films found on the

surfaces of several ancient monuments. Its occurrence is a critical issue since, at environmental

conditions, weddellite is unstable and quickly changes into whewellite, the monohydrate phase of

calcium oxalate (CaC2O4�H2O). New single crystal X-ray diffraction experiments have been

carried out, which confirm the structural model of weddellite previously published. Synthesised

nanocrystals of weddellite have been kept under different hygrometric conditions in order to

study, by X-ray powder diffraction, the influence of humidity on their stability. Moreover, the

mechanism of transformation of weddellite nanocrystals has been investigated by infrared

spectroscopy using D2O as a structural probe.

Introduction

Calcium oxalate crystallizes in nature in three different forms:

(1) calcium oxalate monohydrate, or whewellite (CaC2O4�H2O),

which represents the most common form; (2) calcium oxalate

dihydrate, or weddellite [CaC2O4�(2 + x)H2O] which is

significantly less frequent than whewellite; (3) calcium

oxalate trihydrate, caoxite, rarely observed.1 Whewellite and

weddellite are found naturally in plant tissues, in sediments

and also in kidney stones. Calcium oxalate, calcium phosphate,

uric acid, ammonium hydrogen urate and magnesium

ammonium phosphate are the main components of kidney

stones, with different distribution depending on the population

groups examined. Nowadays, calcium oxalate is the main

component, in the form of whewellite and/or weddellite, of

more than 70% of all kidney stones in Western countries.2,3

Calcium oxalates are extensively studied also in the field of

Cultural Heritage, because oxalate films have been found on

the surfaces of several ancient monuments.4 They consist of

whewellite, weddellite and other minor components as

gypsum, calcite, silicates and some accessory minerals.4 Their

colours range from pale pink to yellow, to ochre or brown and

they have been detected both on natural stone materials

as marble, calcarenite, sandstone, granite and on artificial

materials as plaster or terracotta.5 The undiscussed peculiarity

of these films is their chemical stability, proven by their

persistence over the centuries on surfaces exposed to

atmospheric conditions, in spite of the strong increase of

atmospheric acidity in all urban sites in the last century. The

stability of calcium oxalate is easily explained by its water

solubility; for instance, whewellite has a very low solubility

(0.05 mmole l�1 at pH between 5 and 11) still at acid pH

(0.37 mmole l�1 at pH 2.5).6 In the field of conservation

science calcium oxalate films are a hot issue and some

problems are still unsolved. Indeed, whewellite is stable at

environmental conditions, while weddellite, at the same

conditions, quickly changes into whewellite. Some questions

arise: why have whewellite and weddellite been often detected

within the same film? Which conditions can govern the

formation of whewellite and weddellite, together or separately?

Moreover, one of the most controversial issues is to explain the

transformation of one species into the other.7

This work aims at contributing to the solution of some of

these problems. In particular the research is devoted to explore

(i) the stability of suitably synthesised nanocrystals of

weddellite in different experimental conditions and (ii) the

transformation mechanism of weddellite into whewellite.

Experiments have been carried out using powder X-Ray

Diffraction (XRD), Fourier Transform Infrared Spectroscopy

(FTIR) and Scanning Electron Microscopy (SEM). Moreover,

a crystallographic study by single-crystal X-ray diffractometry

has been carried out.

In this work we have considered the influence of humidity

on the stability of calcium oxalates. The effect of temperature

on the stability of calcium oxalates has been already exten-

sively studied by many authors,8–16 thus in this work it is not

treated; however, all data suggest that, at environmental

temperature, the only calcium oxalate actually stable is

whewellite. Regarding the assignment of the absorption bands

of the two phases we refer to the wide bibliography.8,9,17–19

Previous studies

Experiments have previously been carried out in order to

investigate the stability of weddellite kept under different

a Istituto per la Conservazione e la Valorizzazione dei Beni Culturali,CNR, via Cozzi 53, 20125 Milano, Italy.E-mail: c.conti@icvbc.cnr.it

bDip. Chimica, Materiali e Ingegneria Chimica ‘‘Giulio Natta’’,Politecnico di Milano, Piazza Leonardo da Vinci 32,20133 Milano, Italy

cDip. Energia, Politecnico di Milano, via Ponzio 34/3,20133 Milano, Italy

dDip. Scienze della Terra, Universita degli Studi di Milano,via Botticelli 23, 20133 Milano, Italy

PAPER www.rsc.org/pccp | Physical Chemistry Chemical Physics

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hygrometric conditions; indeed, conflictual results exist for

this matter: (a) Lamprecht et al.20 suggested that weddellite

changes into whewellite when humidity drops. Then, some

authors considered this study to explain the presence of

weddellite in oxalate films on monument surfaces.21–23

Wadsten and Moberg,24 studying the crystallization of

calcium oxalates on the surface of lichens, suggested that the

occurrence of whewellite and weddellite depends on the

environmental humidity. (b) On the contrary, other experi-

mental studies proved the fast transformation of weddellite

into whewellite in high humidity conditions and its stability in

dry conditions.25,26 In particular, Manganelli Del Fa et al.26

investigated the transformation which occurs in weddellite

crystals when subjected to three different values of humidity

(0%, 50% and >95%). Similar experimental findings were

reported by more recent studies, carried out, for instance, on

oxalate films on Pentelic marble from the Parthenon,27 on

biodeteriorated granite monuments28 and also on kidney

stones.29

The frequent occurrence of the two minerals led scientists to

investigate their crystallographic structures.2,7,30–34 On the

basis of these studies, whewellite has a monocline P21/c

(C2h) space group with a zigzag chain structure consisting of

calcium and oxalate ions. These chains are linked by oxalate

ions and hydrogen bonds of the intralayered water molecules.

As a result, the chains form chain-oxalate sheet structures

parallel to (010). In tetragonal I4/m (C4h) weddellite, the

calcium coordination polyhedron is a distorted square anti-

prism; six oxygens belong to four oxalic groups and two to

water molecules. With respect to whewellite, in weddellite

calcium is coordinated to one less oxalic group and one more

water molecule. Each polyhedron of calcium in weddellite

structure is linked to two adjacent polyhedra making chains

running along the c-axis. The link between these chains is

achieved by means of ribbons oxalate–water–oxalate, lying in

planes parallel to (100). The repetition around the four-fold

axis of the calcium chains and the oxalate–water ribbons

generates channels in which ‘‘zeolitic’’ water occurs with

partial site occupancy. Therefore, in whewellite all water

molecules are strongly linked to the crystalline structure, while

in weddellite there is a portion of water (‘‘zeolitic’’) that is

quite mobile in the channels.

Experimental methods

Single crystal X-ray diffraction experiments: kidney stones of

10 patients have been provided by Genoa Hospital in order to

obtain single crystals of whewellite and weddellite. Stereo-

microscopic examination of stones has revealed the presence

of different morphologies (Fig. 1); the stones were first

examined by powder X-ray diffraction and then divided into

three groups based on their whewellite/weddellite relative

ratios: stones of only whewellite, stones of only weddellite

and stones of a mixture of the two forms. The first two groups

of samples have been considered and 20 single crystals have

been selected under an optical microscope.

Study of the stability of the phases: weddellite has been

synthesised in our laboratory. It is difficult to obtain weddellite

in the pure form and several authors reported the conditions

of their synthesis.10,25,35–37 In the present study, synthetic

weddellite has been prepared by slowly mixing stoichiometric

amounts of sodium oxalate (Sigma Aldrich) and calcium

chloride (Sigma Aldrich) at 0 1C; indeed, it has been observed

that at 0 1C weddellite is the only calcium oxalate phase

formed. The resulting material was filtered and washed with

deionized water to remove the sodium chloride. A nano-

crystalline material is thus obtained. The nanocrystals of

weddellite have been kept under different hygrometric

conditions to evaluate their stability over the time. Four

different conditions have been chosen: (1) environmental

conditions, (2) standing in dry air, (3) immersion in water

and (4) immersion in ethanol (EtOH).

Clean dry air for stability experiments was obtained using

molecular sieves.

Study of the transformation process of the phases: after the

synthesis of weddellite in H2O, portions of powder have been

dried and immersed in deuterated water (D2O—isotopic

purity Z 99.96%) for different times.

Analytical techniques

(1) Single crystal X-ray diffractometry (XRD) was used to

study the crystalline structure: diffraction data have been

collected at 298 K using an Xcalibur Oxford Diffraction

diffractometer equipped with a CCD detector and graphite

monochromated MoKa-radiation, operating at 50 kV and

40 mA. Intensity data have been collected up to 2ymax =

701 using a combination of o and j scans, in order to

maximize the reciprocal space coverage and redundancy, with

a fixed exposure time per frame (15 s per frame) and a

crystal–detector distance of 80 mm.

(2) A Panalytical X’Pert PRO X-ray powder diffractometer

(XRPD) was used to study the transformation of synthesized

weddellite at different conditions. The instrument is equipped

with an X’Celetator detector PW3015/20 and diffraction

patterns have been collected from 51 to 601 2y, scan speed

0.211 s�1, with a CuKa-radiation source, working conditions

40 kV and 40 mA. Powdered samples have been spread on an

amorphous silicon holder and then analysed.

(3) Infrared spectra were collected in double transmission

with an FTIR spectrophotometer Nicolet Nexus, equipped

Fig. 1 Kidney stone of whewellite.

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with a microscope Olympus Continuum (4000 to 400 cm�1)

with MCT detector cooled by liquid nitrogen. The resolutions

used were 4 or 2 cm�1 and the analyzed area is 100 mm� 100 mm.

The analyses have been carried out on thin films of powders

deposed on metallic mirror to study the role of water

molecules in the transformation mechanism of weddellite.

(4) A Zeiss Supra 40 Field Emission Scanning Electron

Microscope (SEM) was used in order to observe and measure

the synthesized weddellite crystals. The accelerating voltage

was 5 kV and the specimen chamber was maintained in high

vacuum modality.

Experimental results

Single-crystal X-ray diffraction

The single-crystal diffraction patterns confirmed the tetra-

gonal lattice of weddellite and the monoclinic lattice of

whewellite, as previously reported.33 The reflection conditions

were found to be consistent with the space group P21/c for

whewellite and I4/m for weddellite, respectively.

Unfortunately, only one of these crystals, consisting of

weddellite, proved to be suitable for a structural refinement,

since it appeared free of defects and twinning; indeed about

94% of its diffraction peaks have been indexed. For the

structure refinement the SHELX-97 software was used, starting

from the atomic coordinates of Tazzoli and Domeneghetti.33 At

the end of refinement, the agreement factor was R1(F) =

0.0678 (with F = structure factor) based on 628 unique

reflections with Fo > 4s(Fo) (with Fo = observed structure

factor and s = estimated standard deviation) and 49 refined

parameters. A careful inspection of the difference-Fourier map

of the electron density showed that no significant residual

occurs at the atomic position predicted for the W(30) site by

Tazzoli and Domeneghetti, partially occupied by zeolitic water

(17%, 1.4e�).33 Atomic positions and thermal displacement

parameters are reported in Table 1 and the obtained crystal

structure of weddellite is shown in Fig. 2. The structure

refinement leads to a chemical formula of the weddellite

crystal: CaC2O4�2.27H2O.

Monitoring of the transformation of weddellite nanocrystals in

different hygrometric conditions

The synthesised crystals have been studied by FTIR (Fig. 3),

XRD (Fig. 4), and SEM analyses (Fig. 5). The only phase

detected is weddellite, showing a distribution of crystal size

ranging from 70 nm to 120 nm. Their edges are rounded or

angular and crystal aggregates of 300–400 nm in size have been

found (Fig. 5).

The transformations of the powder under different

conditions (Table 2) have been monitored by powder XRD.

Further details are given in Table 2 and discussed below.

Environmental conditions. Powder XRD analyses have been

carried out every day on the same portion of the powder.

In Fig. 6 the most significant XRD patterns have been

reported.

The reported XRD data are not quantitative, but they show

the decrease of weddellite (14.321 2y) and the increase of

whewellite (14.921 2y) with time; after 44 days weddellite has

disappeared and all the synthesized crystals have transformed

into whewellite. Between 2 hours and 10 days, weddellite

does not show a continuous transformation; the peaks of

whewellite appear after 10 days (induction time).

Standing in dry air. XRD patterns show that no transforma-

tion occurs also after 100 days under dry conditions (Fig. 7).

These results show that when no external water molecules are

available, no transformation of unstable weddellite to stable

whewellite occurs.

Table 1 Refined positional, thermal displacement parameters (A2) and molecular bond distances (A) in weddellite structure. The anisotropicdisplacement factor exponent takes the form: �2p2[(ha*)2U11 +� � �+ 2hka*b*U12]. Estimated standard deviations in parentheses

Site X Y z Site occupancy U11 U22 U33 U23 U13 U12

Ca 0.19917(8) 0.30114(8) 0 1 0.0344(6) 0.0340(6) 0.0245(5) 0 0 0.0011(4)C 0.4465(3) 0.2423(3) 0.1043(5) 1 0.030(2) 0.036(2) 0.027(2) �0.001(1) �0.001(1) 0.002(1)O1 0.3572(2) 0.2463(2) 0.1826(3) 1 0.032(1) 0.049(2) 0.028(1) 0.001(1) 0.001(1) 0.005(1)O2 0.2362(3) 0.4628(2) 0.1798(4) 1 0.100(3) 0.031(2) 0.025(1) 0.004(1) �0.005(1) �0.009(2)W1 0.1503(4) 0.1143(4) 0 1 0.096(4) 0.032(2) 0.037(2) 0 0 �0.016(2)W2 0.0186(4) 0.3818(4) 0 1 0.049(3) 0.066(3) 0.053(3) 0 0 0.015(2)W3 0 0 0.701(3) 0.55(4) 0.134(7) 0.134(7) 0.26(2) 0 0 0W3� � �W3 2.966(5)W1� � �W3 3.209(5)

Fig. 2 Crystal structure of weddellite viewed down [001] based on the

single-crystal structure refinement of this study. Thermal ellipsoid

probability level: 50%. Dotted lines represent the H-bonds between

W3 and W1 sites.

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Immersion in H2O and in EtOH. XRD patterns (Fig. 8)

show that, when immersed in H2O, weddellite transforms

quickly and completely into whewellite. After 2 hours in water,

weddellite is completely transformed, whereas after the same

time in ethanol weddellite is still the only phase detected;

in EtOH the complete transformation is obtained only after

100 days.

The comparison between the behaviour of weddellite in

H2O and in EtOH highlights the strong influence of water

on the transformation of weddellite; the presence of external

H2O molecules favours the transformation. Indeed, in ethanol

only after 14 days evidence of the presence of whewellite

occurred (Fig. 9).

In order to obtain further information on the transforma-

tion of weddellite in liquid H2O, a single-crystal of weddellite

(approximately 300 mm � 200 mm � 150 mm), sampled from

kidney stones, has been immersed in liquid H2O. The crystal

was preserved for 25 days, then a further check after 60 days

showed that the crystal had transformed into a fine powder of

whewellite.

The transformation mechanism of weddellite into whewellite

As mentioned above, external water molecules play a funda-

mental role in the transformation dynamics of weddellite into

whewellite. Therefore, we thought that the substitution of H2O

with D2O molecules (O–H and O–D stretching vibration

frequencies are significantly different) could be a suitable

way to follow by FTIR spectra the behaviour of water

molecules when transformation occurs.

In Fig. 10, FTIR spectra of weddellite immersed in D2O are

shown. After 1 hour, weddellite does not show any change; the

spectrum is comparable to that of weddellite collected

immediately after the synthesis in H2O. After 22 and 31 hours,

in the OD stretching range (2200–2700 cm�1) some new bands

appear; unexpectedly these bands are not associated to

deuterated weddellite, but to deuterated whewellite. Indeed,

the strong difference between the infrared spectra of weddellite

and whewellite arises in the pattern of OH stretching

vibrations (3600–3200 cm�1), as clearly shown in Fig. 3,

because weddellite shows only a broad band, while whewellite

has five well distinct bands.

In the spectrum collected after 100 hours of immersion, in

the OH stretching region the broad band of weddellite

completely disappears, while in the OD stretching region the

Fig. 3 FTIR spectrum of synthesised weddellite (a) compared with the FTIR spectrum of whewellite Aldrich (b). In the pattern of OH stretching

vibrations (3600–3200 cm�1) weddellite shows only a broad band, while whewellite has five well distinct bands.

Fig. 4 XRD pattern between 101 and 301 2y of synthesised weddellite

with lattice plane distances (d). Miller indices calculated from ICSD

(1997).

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five bands of whewellite are observed. Thus, weddellite

does not accept D2O; D2O is accepted only when whewellite

is formed.

The same sample which gave the spectrum ‘‘d’’ of Fig. 10

(weddellite after 100 hours in D2O) has been exposed to the

laboratory environment and kept on the sample holder of the

FTIR interferometer. After 2 hours, the infrared spectrum

showed the leaking of deuterated water and the increase of

hydrogenated water (Fig. 11). This result confirms the strong

selectivity of calcium oxalates towards H2O.

Fig. 5 SEM images of synthesised weddellite.

Table 2 Induction and complete transformation times of weddellitenanocrystals at different conditions

Experimental conditionsApproximateinduction time

Completetransformation

Laboratory atmosphere 10 days 45 daysStanding in dry air — —Immersion in H2O Few minutes 2 hoursImmersion in ethanol 14 days 100 days

Fig. 6 Laboratory environment—time transformation of weddellite

into whewellite monitored by XRD (101–221 2y) after 2 hours (a),

10 days (b), 44 days (c) from the synthesis.

Fig. 7 Dried air—XRD pattern (101–221 2y) with lattice plane

distances (d) of weddellite after 100 days. Miller indices calculated

from ICSD (1997).

Fig. 8 XRD patterns of synthesised nanocrystals (101–221 2y) after2 hours in H2O (a) and in EtOH (b).

Fig. 9 Immersion in EtOH—time transformation of weddellite into

whewellite monitored by XRD (101–221 2y) after 14 days (a), 50 days

(b) and 100 days (c) from the synthesis.

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Conclusions

The crystalline structures of whewellite and weddellite

suggested by Tazzoli and Domeneghetti33 have been con-

firmed by our new XRD experiments.

Our experiments show that the stability of weddellite is

favoured by dry conditions; these results do not explain the

finding of weddellite in calcium oxalate films, since on the

monument surfaces dry conditions are never obtained and are

not constant with time.

Fig. 10 Immersion in D2O—time transformation of weddellite monitored by FTIR in the OH (3600–3200 cm�1) and OD (2700–2200 cm�1)

stretching regions, after 1 hour (a), 22 hours (b), 31 hours (c) and 100 hours (d) from the synthesis.

Fig. 11 Laboratory environment—time transformation of deuterated whewellite monitored by FTIR after 2 hours. The transformation mainly

occurs in the OH (3600–3200 cm�1) and OD (2700–2200 cm�1) stretching regions.

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A change in the configuration of the H2O channel sites

(i.e. site occupancy and bonding scheme) appears to act as the

driving force of the transformation of weddellite into

whewellite. The kinetics of the transformation of synthesised

weddellite in water is somehow quick and after two hours no

evidence of the presence of weddellite is found. On the

contrary, in dry air, when no external molecules of H2O

are available, no transformation occurs at least within the

time-range investigated.

In liquid EtOH and D2O the transformation occurs much

more slowly. We cannot exclude, in these transformations, the

action of the H2O impurities necessarily left in liquid D2O

or EtOH.

Before the crystallization of deuterated whewellite, infrared

spectra in the OD region did not show an intermediate

crystalline phase resulting from a mixture of hydrogenated

and deuterated weddellite species. Thus the experimental

results suggest that the transformation of weddellite does not

follow a continuous process. It appears, therefore, that crystals

transform directly from hydrogenated weddellite to deuterated

whewellite. This finding suggests that the mechanism of

transformation of weddellite to whewellite cannot be

governed by a pure displacive phase transition. In addition,

the experiments showed that whewellite has a strong

selectivity for H2O, because after 2 hours external hydro-

genated water replaced all the deuterated water of the

crystalline structure.

We suggest that the instability of weddellite is due to two

main reasons: (1) the large size of its channel, of approximately

4.7 A in diameter (with ‘‘free diameter ofB2.1 A’’) and (2) the

presence of ‘‘zeolitic water’’ at the extra-framework W3

site (Fig. 2, Table 1), which is strongly under-bonded, with

partial site occupancy (B60%) and with a significantly

high static or dynamic disorder (as shown by its high thermal

displacement parameter). The W3 molecule is weakly bonded

to the W1 sites via H-bonds (with W3� � �W1 of about 3.2 A).

All these factors influence the stability of the crystalline

structure of weddellite. Indeed, the site occupancy of the

‘‘zeolitic water’’ can change if a partial dehydration occurs

or, in contrast, if an over-hydration occurs through

selective sorption of extra H2O molecules from the

surrounding aqueous medium. In both cases, the ephemeral

equilibrium of the weddellite structure does not hold out

to the variations of the site occupancy of the ‘‘zeolitic water’’,

therefore the breakdown of the crystalline structure of

weddellite occurs. In addition, the high symmetry of weddellite

implies low degree of freedom and hinders the structural

re-arrangement due to a potential variation of the channel

content. However, despite the novel results obtained in this

multi-methodological study, the transformation mecha-

nisms from weddellite to whewellite are not entirely under-

stood and additional experiments are needed focusing also on

the influence of crystal sizes on the mechanisms of

transformation.

Acknowledgements

We thank Giorgio Fustella for his help in weddellite synthesis

and fruitful discussions.

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