Cite this: CrystEngComm, 2013, 15,6526

Crystallization of monohydrocalcite in a silica-richalkaline solution3

Received 11th April 2013,Accepted 11th June 2013

DOI: 10.1039/c3ce40624e

www.rsc.org/crystengcomm

Gan Zhang, Jose Manuel Delgado-Lopez, Duane Choquesillo-Lazarte and JuanManuel Garcıa-Ruiz*

Monohydrocalcite was crystallized in a silica-rich alkaline solution at room temperature using the counter

diffusion method (CDM) in the absence of magnesium, by the reaction of calcium chloride and sodium

carbonate. Field emission scanning electron microscopy observations showed monohydrocalcite crystals

that were hundreds of micrometers in size exhibiting a unique multi-layered structure. X-ray diffraction

and Raman microspectroscopy characterizations demonstrated that monohydrocalcite remained in a

stable phase for a number of months. After monitoring the crystallization process by in situ Raman

microspectroscopy, it was found that monohydrocalcite was the initial phase, with no phase

transformation occurring during the crystal growth. This work demonstrates that silica plays a key role

in the formation and stabilization of the monohydrocalcite phase.

1. Introduction

Monohydrocalcite (CaCO3?H2O) is a metastable hydratedcrystalline calcium carbonate, which was first reported in1930 and is formed during the dehydration of ikaite.1 Its firstnatural occurrence was observed in marine sediments.2 Sincethen, monohydrocalcite has also been found in beach rocks atLake Fellmongery and Lake Butler in Australia,3 and in surfacesediments in Walker Lake in Nevada,4 Nam Co in Tibet,5 andLake Tsagan-Tyrm in the west Baikal region.6 Biogeneticmonohydrocalcite has also been found in some livingorganisms, such as in the otoliths of a tiger shark,7 and inthe bladder stone of a guinea pig.8 Monohydrocalcite has alsobeen found as an intermediate product of the carbonation of acalcium silicate C–S–H gel, a hydration product of Portlandcement.9 The first synthetic preparation of monohydrocalcitewas achieved by Brooks et al. in 1950 by the reaction ofcalcium chloride and sodium carbonate in the presence ofmagnesium,10 and further studies indicated that monohydro-calcite would transform to aragonite or calcite in a dry state orunder water at near ambient conditions within a few days.11

The generally accepted crystallographic structure of monohy-drocalcite was solved by Effenberger from single-crystaldiffraction in 1981,12 which indicated that monohydrocalcitebelonged to the hexagonal system of the P3121 space groupwith cell parameters a = 6.0931 Å and c = 7.5446 Å. Asuperstructure of monohydrocalcite in P31 with a = 10.5536 Å

and c = 7.5446 Å was recently determined on the basis of weaksuperlattice reflection.13

Monohydrocalcite is a rare mineral but has been foundmost frequently in alkaline lakes at a pH usually higher than 8and with a Mg/Ca ratio higher than 4.14 These naturalscenarios suggested that the magnesium ion is a key factorfor the monohydrocalcite formation, which was later con-firmed by several laboratory syntheses of monohydrocalcite.For example, Kralj and Brecevic synthesized monohydrocalcitein dissolution experiments from artificial seawater with a Mg/Ca ratio of 5.3.15 A similar procedure was reported by Dejehetand co-workers who performed crystallization experimentswith a Mg/Ca ratio from 0.1 to 3.48 and only obtainedmonohydrocalcite when the Mg/Ca ratio was higher than 1.16

The synthesis of monohydrocalcite by mixing experimentsinvolving CaCl2, MgCl2 and Na2CO3 solutions was achievedfrom solution with a Mg/Ca ratio of at least up to 1.17,18

Furthermore, Loste et al. synthesized calcium and magnesiumcarbonates from solutions of CaCl2, MgCl2?6H2O and NaHCO3

to explore the effect of the Mg/Ca ratio within the range of 0and 10. The results from these authors indicated thatmonohydrocalcite can form when the Mg/Ca ratio is higherthan 3 but never as a pure phase, as other carbonateformations such as calcite, magnesium calcite and aragonitealso appeared to form.19 There is one case where theprecipitation of monohydrocalcite has been reported in theabsence of magnesium from CaCl2, NaHCO3 and NH3

solutions, but it transformed to calcite in a few hours.20 Formost experimental studies reported until now, as a function oftime, to the best of our knowledge, synthetic monohydrocal-cite is an intermediate unstable phase formed in magnesiumrich solutions, after the transformation of amorphous calcium

Laboratorio de Estudios Cristalograficos, Instituto Andaluz de Ciencias de la Tierra

(CSIC-UGR), Avenida de las Palmeras Nu 4, E-18100 Armilla, Granada, Spain.

E-mail: jmgruiz@ugr.es; Fax: +34 958 552620; Tel: +34 958 230000

3 Electronic supplementary information (ESI) available. See DOI: 10.1039/c3ce40624e

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carbonate to calcite and/or aragonite.19,21 These resultsindicate that Mg acts as an inhibitor of the formation ofstable calcium carbonate, which leads to the preferentialformation of monohydrocalcite.

In this study we have found a new way to producemonohydrocalcite in a silica-rich alkaline solution at roomtemperature by a diffusion method with the reaction of Ca2+

and CO322 in the absence of magnesium. We demonstrate by

parallel experiments that silica plays a key role in theformation and stabilization of monohydrocalcite. The forma-tion and growth of the crystalline phases was followed by X-raydiffraction and in situ Raman microspectroscopy, which allowsus to confirm that monohydrocalcite was the initial phase atthe micrometric scale, remaining stable for the time-scale ofthe experiments (months).

2. Materials and methods

2.1. Materials

In our experimental procedure we used a sodium silicatesolution (Na2Si3O7, 1–10 v/v, reagent grade, replaced monthly),anhydrous calcium chloride (CaCl2, ¢93.0%), sodium carbo-nate (Na2CO3, ¢99.0%) and sodium hydroxide (NaOH, 98%)as purchased from Sigma-Aldrich. The hydrochloric acidstandard water solution [1 M] was prepared from Fluka(analytical reagent). The agarose gel (agarose D-5, high gelstrength) was from Hispanagar (Spain). All of them were usedwithout further purification. Purified water with an electricalconductivity of less than 1026 S m21 was used. All theexperiments were carried out at room temperature.

2.2. Crystallization method

The experiments were performed by the counter diffusion ofcarbonate contained in an alkaline silica gel and calcium froma calcium chloride solution located on top of the gel. Thecrystallization cell used in our study has been describedpreviously.22 Two flat glass plates (55 6 110 mm) wereseparated by a rubber frame (2–3 mm thick) in order to createan empty space between the plates. To avoid leakage, therubber frame was coated with vacuum grease and the glassplates were held by clips to ensure the tightness of the cell.Three needles were inserted through the top and bottom of therubber for the injection of the reacting solutions, one at thebottom for the silica gel precursor and two at the top for thecalcium chloride solution. Firstly, the liquid precursor (sol) ofthe silica gel was injected into the cell until filling about halfthe volume of the cell. Then the needle at the bottom wasremoved to ensure a closed system for gelling. The gelling timedepends on the pH. For the pH values of 10.5 ¡ 0.1 that areused in this study, the waiting time for gelling is about oneweek. To set the gel pH, 1.39 g sodium silicate solution wasmixed together with 9 mL sodium carbonate solution (0.05 M)by stirring for around 30 min. Then 3.5 mL HCl solution [1 M]was added and stirred for 30 s. The resulting silica gelprecursor was transported into the crystallization cell forgelling. When the silica gel was ready, the calcium chloridesolution (0.05 M) was injected into the top part of the cell to fill

the rest of the cell, and the needles were removed after theinjection. The little holes in the rubber were blocked byvacuum grease again to prevent further diffusion of atmo-spheric carbon dioxide into the cell during the crystallization.The diffusion of calcium chloride into the gel containing thecarbonate and the diffusion of the carbonate into the calciumchloride solution provokes a chemical gradient that evolves intime and space. The concentration of calcium and carbonatevaries across the cell providing different supersaturationvalues and different Ca/CO3 ratios at different locations ofthe experiment and for different time values. This is whycounter diffusion gel experiments yield different morphologiesand even different polymorphs in a single experiment (Fig. 1).

A series of experiments were performed, always keepingequal amounts of the calcium chloride and sodium carbonatesolution. Three concentrations of both calcium and carbonatewere selected, namely 0.01 M, 0.1 M and 0.2 M. The volume ofHCl to set the pH of the gel was kept at 3.5 mL [1 M] for allcases. In order to test the importance of the silica in theformation of monohydrocalcite, we performed experiments inwhich the silica gel was replaced by an agarose gel. For theseexperiments we used 0.05 M and 0.2 M sodium carbonate andcalcium chloride and 1% agarose gel. The agarose gel wasprepared by mixing 0.1 g of agarose with 10 mL sodiumcarbonate solution. The solution was then heated to 86 uC inorder to dissolve the agarose completely. After five minutes thesolution was cooled down to 60 uC in a water bath and a smallamount of sodium hydroxide solution (1 M) was added to thesolution to set the pH to 10.5. The hot sol was injected into thecrystallization cell and it was cooled down to room tempera-ture to form the gel.

2.3. Characterization techniques

The crystals formed were observed in situ by optical micro-scopy using a Nikon AZ100 multi-purpose zoom. Fieldemission scanning electron microscopy (FESEM) observationsof the carbon-coated crystals were carried out with an AURIGA(Carl Zeiss SMT) system. For these observations the crystalswere removed from the crystallization cells and washed with

Fig. 1 A scheme showing the crystallization cell. The silica gel precursor wasprepared by mixing Na2Si3O7 (1–10 v/v), Na2CO3 (0.05 M) and HCl (1 M). After 7days at room temperature the silica is completely gelled. The crystallization isstarted simply by the injection of a CaCl2 solution (0.05 M). The crystallizationceased after 3–4 weeks and provided a morphological gradient in both theliquid zone and the gel zone, which can be easily observed by opticalmicroscopy. (Scale bar: 200 mm.)

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ultrapure water several times and then dried at 40 uCovernight.

The X-ray diffraction data were collected on beam line BM16at the European Synchrotron Radiation Facility (Grenoble,France) under Q rotation at a fixed wavelength of 0.7378 Å,using an ADSC Q210r CCD detector with a pixel size of 51 mm.The size of the incident beam was about 200 6 200 mm2.Diffraction was thus taken from single aggregates in this work.XRD2DScan software was used to calculate the equivalentpowder diffractogram from the 2D diffraction pattern by radialintegrating pixel intensities.23 Mineral phases were identifiedby using X’Pert High Score Plus software and the ICSDdatabase. A Bruker SMART APEX diffractometer withMolybdenum Ka radiation (50 KV 30 mA) was also used forthe sample analysis.

Raman spectra were collected using a LabRAM-HR spectro-meter attached to a microscope (Jobin-Yvon, Horiba, Japan).The excitation line was provided by a diode laser emitting at awavelength of 532 nm and a Peltier cooled charge-coupledevice (CCD) (1064 6 256 pixels) was used as the detector. Theoptical microscope was equipped with 610 and 650objectives (Olympus, Japan). The acquisition time was 30 sfor the crystals growing inside the cell and 10 s for the crystalsextracted from the crystallization cell. The spectrometerresolution was better than 3 cm21. The structural homogeneityof a selected area of the surface of the monohydrocalcitecrystals was checked by ex-situ Raman mappings. A particulararea of the cross-section of the crystal was firstly selected formapping and its central core was defined as the origin of themap. For example, a 50 6 50 mm area was defined as the mapin Fig. 2a and divided into 20 6 20 square grids as shown inFig. 2b. The spectra of each grid were recorded three timeswith an acquisition time of 30 s. The most intense peaks of thedifferent phases in the spectra were converted to a color codeusing LabSpec Software (Horiba, Japan) so that each square ofthe grid is defined by the color of the main phase (Fig. 2c). Inparticular, the intensity of the peak appearing between 1080–1090 cm21 (the symmetric stretching of carbonate groups incalcite) was specified as red and the intensity of the peakbetween 1060–1070 cm21 (the symmetric stretching ofcarbonate groups in monohydrocalcite) was colored green.

3. Results and discussion

As described above, the concentration of the calcium andcarbonate solutions were chosen to be identical in order toallow a full variation of the Ca/CO3 ratio across the whole gel–solution crystallization cell. Consequently, calcium carbonateprecipitates not only in the gel (as in typical gel experiments)but also in the solution on top of the gel. The experimentlasted for four weeks, after which no obvious change in thecrystallization was observed. Inside the gel, only sheaf-of-wheat polycrystalline calcite formed in locations where theratio of CO3/Ca was larger than 1 and the pH was morealkaline. Aragonite crystals formed when the initial concentra-tions of both calcium and carbonate were set to 0.2 M (Fig. 3a).Close to the gel–solution interface where the ratio of Ca/CO3 isaround 1 and the pH is less alkaline, the crystals showedbizarre morphologies with a point symmetry group belongingto rhombohedral calcite (Fig. 3b). They are made of self-oriented nanocrystals with a rather unimodal size distributionof around 50 nm. However, in the part of the crystallizationcell filled with the calcium chloride solution, i.e. where the Ca/CO3 ratio is higher than 1 and the starting pH is neutral, manytiny micron-sized crystals distributed over the solution as wellas much larger spherical crystal were observed (Fig. 3c). Ex-situX-ray diffraction and Raman spectroscopy enabled theidentification of the former as calcite and the latter asmonohydrocalcite.

Spherical monohydrocalcite with diameters of 100–400 mmwere observed in the solution near the liquid–gel interface.The completion of the growth occurred after 3 weeks ofcrystallization. In fact, the structure was a hemisphere stuck tothe glass plate. When these hemispheres were removed andobserved upside down, a clear onion-like layered structurecould be observed. Further FESEM investigation suggestedthat the monohydrocalcite spheres could be divided into fourdifferent parts in terms of their texture: A) the core; B) thecompact layer; C) the loose layer, and D) the skin (Fig. 4a). Thedetailed study showed that the monohydrocalcite crystallitesof the core, the compact layer and the loose layer were built upof nanorods elongated along the c-axis that were typically 50nm by 20 nm in size; while in the skin, the nanoparticles werebigger spheres with a diameter of around 100 nm (Fig. 4f). Thecore has a peanut-like morphology and is formed ofdisordered nanocrystallites showing a rough surface(Fig. 4b). The compact layer contains radially arranged

Fig. 2 Raman mapping collected on the surface of a selected monohydrocalcitecrystal. (a) The optical microscopy image of the crystal, (b) selected points inwhich the Raman spectra were collected, (c) the results of the Raman map afterthe color conversion. The spherical-shaped crystal was analysed after 6 days ofcrystallization. (Scale bar: 10 mm.)

Fig. 3 Optical micrographs of the calcium carbonate crystals inside the crystal-lization cell: (a) elongated sheaf-of-wheat calcite in the gel, (b) calcite with abizarre stick morphology at the liquid–gel interface, (c) spherical monohydro-calcite together with tiny calcite crystals in the liquid. (Scale bars: 100 mm.)

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needle-like crystallites which are formed from ordered stacksof nanocrystallites. The length of the crystallites is at themicron scale but varies largely as they run from the base to thetop of each concentric sub-layer. This layered structure ofcompact needles made up the major component of thehemispherical structure of monohydrocalcite (Fig. 4c and S3in ESI3). As discussed above, this compact layer is notcontinuous along the radial direction but shows an onion-like concentric sub-layered structure, which seems to be theresult of oscillatory growth behavior.24–26 A further detailedstudy demonstrates that the needle-like regions are separatedby a thin (thinner than 5 microns) layer with a loose structure,i.e. of lower crystal density, which is also made of nanorods(Fig. 4a and c). The thickness of the needle-like and looseregions varies with time but the needle-like regions are alwaysthicker and this is why we call this the compact layer. The nextouter layer of the hemisphere is an arrangement of irregularnanorods that are similar to the loose regions of the compactlayer but much thicker (Fig. 4d). For this reason, we call thisthe outer loose layer. In summary, the core, the compact layersand the loose layers (the thinner and the thicker outer one)were all made up of the same nanorods, the difference beingthe compactness of the texture. Finally, a skin layer that is lessthan one micrometer thick covers the whole structure. Thisthin layer is made of monohydrocalcite nanospheres of about100 nm stacked together closely, forming a continuous surfaceentirely covering the hemispherical structure (Fig. 4e and f). Asshown in Fig. 4a, this skin layer is not stuck strongly to thesurface (Fig. 4a).

After growing for four weeks, the hemispherical crystalswere removed from the liquid part in the crystallization celland then analyzed by X-ray diffraction at beamline B16 of theEuropean Synchrotron Radiation Facility. Our experimentaldata in Fig. 5 show the main reflection peaks of monohy-drocalcite of the (100), (101), (102), (201), and (202) planeswhich are in good agreement with the standard data reported

Fig. 4 FESEM micrographs of monohydrocalcite: the entire structure (a), the center core (b), the compact layer and cracked band (c), the loose layer in the outer edge(d), and the skin upon the loose layer (e, f).

Fig. 5 The X-ray diffraction pattern of hemispherical monohydrocalcite particlesgrown in a silica-rich solution (black) and the standard pattern from the ICSDdatabase (red). The diffractogram shown in black was obtained by theintegration of intensity along the white line in the synchrotron X-ray diffractionimage shown in the insert.

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in the literature.27 X-ray characterization was also performedon samples stored in their mother solution at roomtemperature for one month using a Bruker SMART APEXdiffractometer (Molybdenum Ka, 50 KV 30 mA). The X-raypolycrystalline diffraction pattern proves that monohydrocal-cite was stable over that period in the silica-rich alkalinesolution. Furthermore, subsequent X-ray diffraction analysisof the particles harvested from the solution and stored at roomtemperature for 5 days also yields the same diffraction pattern,indicating that the monohydrocalcite phase was stable underatmospheric conditions. Therefore, it seems that silicaincreases the stability of monohydrocalcite as compared todata reported in previous studies.20 We believe that this effectas due to the well-known fact that silica is a dryer. We believethat the silica species adsorbed on the nanocrystals formingthe monohydrocalcite structure absorb water molecules fromthe atmosphere thus retarding the conversion to the anhy-drous forms.

Raman microspectroscopy provides chemical informationfrom the area accurately selected with the microscope. Fig. 6shows the Raman spectra collected on the surface of ahemispherical monohydrocalcite crystal (shown in the insetof Fig. 6). The spectra were collected along a straight line fromthe center core to the skin (from A to B in the inset of Fig. 6),thus covering the whole structure of the crystal. The Ramanspectra displayed clear bands at 1067 cm21, 874 cm21, 720 and696 cm21. These Raman shifts can be assigned to thevibrational modes of carbonate in monohydrocalcite

(Table 1).28 Table 1 summarizes the Raman shifts (in cm21)of the main peaks and their assignments to the vibrationalmodes of the carbonate groups in monohydrocalcite andcalcite according to Tlili et al. and Kontoyannis and Vagenas,respectively.28,29 Any of these peaks in the Raman spectra ofFig. 6 can be assigned to the calcite crystalline phase. Thisresult proved that monohydrocalcite was the only phaseforming the hemispherical structure. All the spectra here wererepeatedly measured at room temperature for 5 days, andproduced exactly the same Raman peaks of monohydrocalcite,thus confirming its stability. This has already been suggestedby the X-ray diffraction results described above.

The precipitated particles were characterized over timeusing in situ Raman microspectroscopy. At the beginning,some tiny calcite rhombohedral crystals nucleated in thesolution. After twelve hours the calcite rhombohedral crystalsceased to grow and we observed the formation of peanut-likemonohydrocalcite particles (Fig. S1 in ESI3). The number ofparticles increased with time for the next 24 hours suggestingthat monohydrocalcite is the stable phase under thesecrystallization conditions. Then the peanut-shaped particlesbecame firmly attached to the inner glass surfaces of thegrowth cell, adopting hemispherical shapes that reached adiameter of 50 mm after 3 or 4 days. Subsequently, the particlesgrew radially with an isotropic path keeping their hemisphe-rical shape until they had reached more than 200 micronsafter two weeks. Throughout the growth experiment wemapped the forming hemispherical structure by Ramanmicrospectroscopy. Fig. 7 shows the optical microscopypictures and the Raman map recorded at different times.The results clearly show that monohydrocalcite was the onlyphase crystallizing during the whole formation of the hemi-spherical particles, from the initial peanut-like core to thehemispherical multi-layered structure, including the skin. Theconcentric rings in the Raman maps in Fig. 7b2 and c2 areattributed to the variations in the intensity of the carbonatesymmetric stretching bands of monohydrocalcite. This couldprobably be explained by the different compactness of theminerals on the surface of the crystal. Since the incidentvisible light is polarized, these variations can be also related tochanges in the orientation of the nanorods on the surface.This fact could also explain the changes in the relative ratiobetween 696 and 720 cm21 (Fig. 7a3–c3).

These results demonstrate that monohydrocalcite can becrystallized and stabilized in a silica-rich alkaline solutionwithout the presence of magnesium by the counter diffusionmethod when the concentrations of calcium chloride and

Fig. 6 Raman spectra collected from the center (A) to the edge (B) of thehemispherical crystal shown in the inset. (Scale bar: 100 mm.)

Table 1 Raman shifts (in cm21) of the main peaks and their assignments to the vibrational modes of the carbonate groups in monohydrocalcite and calcite

Symmetric stretching (n1) Out of plane bending (n2) In plane bending (n4)

Monohydrocalcitea 1066 873 719, 694Calciteb 1085 711This work 1067 874 720, 696

a From Tlili.28 b From Kontoyannis.29

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sodium carbonate are both 0.05 M. In order to check the roleof silica in the formation of monohydrocalcite, we performed aparallel experiment with identical initial conditions of pH andCa and carbonate concentration but replaced the silica gelwith agarose gel. The Raman spectrum shows that only calciteforms in these experiments, demonstrating the key role ofsilica for polymorphic selection (Fig. S2 in ESI3). However,when we tried to repeat the experiment by mixing the reactantsin a silica-rich solution we failed to produce monohydrocal-cite, which demonstrate the key role of the proper rate ofmixing by diffusion to control the precipitation of thepolymorphs.

4. Conclusions

We have found a new way of producing monohydrocalcitecrystals without the presence of magnesium ions. Our results,supported by in situ and ex-situ XRD and Raman spectroscopy,demonstrate that monohydrocalcite hemispherical particlesform in a silica-rich alkaline solution using the two-layercounter diffusion method under precise starting concentra-tions of the calcium chloride and sodium carbonate solutions.FESEM observations show that the interior of the hemisphe-rical structure has an onion-like multi-layered structure ofmonohydrocalcite. In situ Raman microspectroscopy demon-strate that monohydrocalcite was the unique phase formedduring the whole crystallization process from the initialpeanut-like core to the external skin of the hemisphericalmulti-layered structure. Ex-situ Raman spectra and X-raydiffraction show that monohydrocalcite remains stable for

months both in the mother solution and when exposed to theair under atmospheric conditions.

Acknowledgements

The authors acknowledge the financial support from theMinisterio de Economıa y Competitividad (MINECO) throughthe Consolider-Ingenio 2010 project ‘‘Factorıa Espanola deCristalizacion’’ and the projects CGL2010-16882 and CGL2010-12099. GZ and JMDL also acknowledge the Consejo Superiorde Investigaciones Cientıficas for the JAE pre-doctoral grantand postdoctoral JAE-DOC contract, respectively, both withinthe program ‘‘Junta para la Ampliacion de Estudios’’ and co-financed with the European Social Fund (ESF). The authorsalso acknowledge Alicia Gonzalez Segura for her support withthe FESEM analysis and Dr. Alejandro Rodriguez Navarro forhis valuable advice on the XRD2DScan software.

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Fig. 7 In situ optical microscopy photographs of a hemispherical particle on the2nd (a1), 4th (b1) and 6th day (c1) of the experiment, showing its developmentin time. The corresponding Raman microspectroscopy mapping (a2–c2) of thesame particles. a3–c3 shows the Raman spectra obtained from the selectedwhite grids in the Raman mapping. (Scale bar: 10 mm.)

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