Forming Cohesive Calcium Oxalate Layers on Marble Surfaces for

Article pubs.acs.org/crystal

Forming Cohesive Calcium Oxalate Layers on Marble Surfaces for Stone Conservation Helen E. King,*,† Dorothea C. Mattner,† Oliver Plümper,‡ Thorsten Geisler,§ and Andrew Putnis† †

Institut für Mineralogie, University of Münster, Corrensstrasse 24, 48149 Münster, Germany Department of Earth Sciences, Utrecht University, Budapestlaan 4, 3584 CD Utrecht, The Netherlands § Steinmann Institut für Geologie, Mineralogie und Paläontologie, University of Bonn, Poppeldorfer Schloss, 53115 Bonn, Germany ‡

ABSTRACT: Batch experiments were conducted with Carrara marble cubes to examine the replacement of calcite by calcium oxalate, a proposed method of protection for marble used as building stone. Coherent oxalate coatings formed on the marble surface during reactions with >10 mM oxalic acid. The replacement rim contained an inner layer that remained attached to the marble surface and was composed of submicron-sized, rounded grains of calcium oxalate with minimal interconnected porosity, although open fluid pathways (inherited grain boundaries from the underlying marble) were present. In contrast, the outer rim comprises large, individual crystals and is easily removed. Raman spectroscopy identified the mineral in both layers as whewellite (CaC2O4·H2O). Raman mapping revealed that the rims have zones of different crystallographic orientations contributing to the friability of the outer layer. Mapping of 18O incorporation into the replacement rim indicates that the outer layer formed from the inner layer via a fluid-mediated dissolution−reprecipitation mechanism. This suggests that the textures of precipitated oxalates could be tailored to different marble protection applications through changes in solution chemistry.

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phases that convert to whewellite with time.9 Both the precipitation of weddelite and whewellite could potentially form a completely passivating replacement layer on calcite surfaces as there is a replacement-induced solid volume increase of 56 and 44%, respectively. However, formation of these phases along grain boundaries or within pores of the original calcite could also create additional irreversible damage. To understand how to create a coherent layer on the surface of these calcite-bearing materials as well as where the oxalate precipitates will form, the replacement mechanism itself needs to be better understood. This study examines the formation of calcium oxalate replacement layers by reacting Carrara marble cubes with various concentrations of oxalic acid for different durations at temperatures between 20 and 80 °C. By using a combination of different microanalytical techniques, we were able to investigate the evolution of the replacement in space and time and how it is controlled. This knowledge provides new insights into how to tailor this replacement reaction to cultural heritage applications.

INTRODUCTION Calcium carbonates, in particular calcite (CaCO3), are an important component of building materials such as marble and limestone. However, calcium carbonate is highly soluble in acidic solutions and thus leaves it vulnerable to acid attack, especially from rain. To protect the building material, a variety of treatments have been proposed. One of the treatments that has shown promising results on real monuments1 is the use of calcium oxalates. These oxalates can develop naturally through the production of oxalic acid during the respiration of lichens,2 fungus,3 algae,4 and bacteria5 and form a layer of crystallites on the marble surface.1 However, experiments indicate that although oxalates can precipitate epitaxially on the calcite (104) surface the oxalate rim formed is fragile and easily removed, and the replacement reaction causes cracking within the underlying calcite substrate.6 Formation of a coherent rim on calcite-rich building material is critical for sustainable rim development that can protect the building stone effectively over long time periods. Calcium oxalates can be present in three main forms: monohydrated whewellite (CaC2H4·H2O), dihydrated weddelite (CaC2H4·2H2O), and trihydrated coaxite (CaC2H4·3H2O). At atmospheric conditions, whewellite is the most stable phase, whereas the other hydrates typically form at lower temperatures7 and under specific solution conditions, for example, under high supersaturation in solution.8 Weddelite and coaxite have also been observed to form as precursor, metastable © 2014 American Chemical Society

Received: April 10, 2014 Revised: June 14, 2014 Published: July 7, 2014 3910

dx.doi.org/10.1021/cg500495a | Cryst. Growth Des. 2014, 14, 3910−3917

Crystal Growth & Design

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Table 1. Experimental Conditions with Solution Ca Content after the Experiment As Measured by ICP-OES oxalic acid concentration (mM)

solution pH (at 25 °C)

temp (°C)

duration (days)

3.51 1.70

60 20 40 60

29 7 7 7 29 7 29 7 7 1 2 4 7 29 7 29 7 29 7 29 7 29 7 29 7 7

1 10

80 100

1.11

20 40 60

80 250

1.05

60 80

500

0.78

60 80

1000

■

0.64

60 80

Caaq (mol/L) 2.07 2.76 3.76 4.50 1.94 2.02 2.19 6.84 8.08 1.48 1.84 2.61 4.01 3.67 3.93 1.74 1.74 2.09 2.99 3.37 1.93 1.36 8.61 9.92 1.63 1.61

× × × × × × × × × × × × × × × × × × × × × × × × × ×

10−4 10−5 10−5 10−5 10−5 10−5 10−5 10−5 10−5 10−4 10−4 10−4 10−5 10−5 10−5 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−5 10−5 10−4 10−4

error (mol/L) 1.44 1.44 1.44 1.44 7.20 7.20 7.20 1.44 5.76 5.88 9.3 1.18 4.32 2.88 1.44 1.44 7.20 4.32 1.39 5.76 2.88 7.20 1.44 1.44 1.44 1.44

× × × × × × × × × × × × × × × × × × × × × × × × × ×

10−8 10−8 10−8 10−8 10−9 10−9 10−9 10−8 10−8 10−8 10−8 10−7 10−8 10−8 10−8 10−8 10−9 10−8 10−7 10−8 10−8 10−9 10−8 10−8 10−8 10−8

used to mill away two trenches on either side of the area of interest and a larger third trench from the front of the area of interest to expose the first segment of the slice-and-view section (see Figure 4d). Milling was followed by a low current (0.5 nA) polishing step of the front face. For the slice and view investigation 150 cross sections of approximately 50 nm thickness were milled using an ion current of 0.3 nA with the same 38° beam angle. Each cross-section was imaged in BSE mode using a 2 keV beam (0.16 nA) with a pixel size of 3.5 nm (2048 × 1768 pixels; immersion lens). To ensure that the material did not change its microstructure during the slice-and-view series, we monitored the slicing process through frequent live imaging with the electron beam. The stack of SEM images created in the FIB-SEM were combined into a single three-dimensional volume and manually aligned using the tools available in the IMOD image modeling program package.11 A three-dimensional representation of the pore space was subsequently created using the Avizo Fire program. For phase identification, a marble cube that had been reacted in 1 M oxalic acid for 7 days at 60 °C was crushed using an agate pestle and mortar and analyzed using the Cu−Kα line in a Philips X’Pert X-ray powder diffractometer. Data were collected between 5 and 60° in 2θ with a step size of 0.014° for a total time of 45 min. Spatially resolved phase identification and mapping of the 18O content within the reaction rims was conducted using a Horiba Scientific LabRam HR800 confocal Raman spectrometer. Raman scattering was excited with a solid state Nd:YAG laser (532.09 nm) with about 50 mW at the sample surface. A 100 times objective with a numerical aperture of 0.9 was used for all measurements, resulting in a theoretical lateral resolution of 720 nm. The confocal hole was left fully open (1000 μm), resulting in a depth resolution of a few micrometers. The scattered Raman light was collected in a 180° backscattering geometry by an electron-multiplier charge-coupled device detector after having passed through a 20 μm entrance slit and being dispersed by a grating of 1800 grooves/mm. With these settings the spectral resolution was 0.4 cm−1 in the frequency range of interest, as given by the measured full width at half-maximum of Ne emission lines. Such high spectral resolution was necessary to separate individual 18O isotopologue bands of the oxalate group in the reaction product formed in 18O-labeled

MATERIALS AND METHODS

Polished cubes (1.5 mm side length) of Carrara marble were reacted with various concentrations of oxalic acid that had been dissolved in Milli-Q water (double deionized water). Isotope tracer experiments were carried out using 0.1 M oxalic acid solutions with 50% 18Oenriched water. The experiments were conducted in tetrafluoroethylene-lined steel autoclaves held in an oven at 40 to 80 ± 1 °C for 7−29 days. Experiments conducted at 20 °C were treated the same way as the oven experiments but were left in a temperature controlled room. After the experiments, the solutions were filtered and then analyzed for Ca concentration using a Varian inductively coupled plasma optical emission spectrometer (ICP-OES). Errors in the concentrations were calculated using the standard deviation of the concentrations based on the three peaks related to Ca measured using the ICP-OES. Experimental conditions and the corresponding Ca concentrations in solution after the experiments are summarized in Table 1. For each experiment 1.5 mL of solution was added to the autoclave containing the marble cube, which was then sealed and weighed. The oven was preheated to the required temperature before the autoclaves were placed inside. The samples were weighed after 24 h and at the end of the experiment after being cooled in air to ensure that no fluid had been lost. After the experiments the crystals were removed from the solution, washed with ethanol, and dried. To examine the reaction products, entire cubes and cross sections embedded in epoxy were examined using a JEOL 6610-LV scanning electron microscope (SEM) equipped with a backscattered electron (BSE) and energy-dispersive X-ray (EDX) detector. Slices of material were cut normal to these polished cross sections using a FEI Nova Nanolab DualBeam focused-ion-beam SEM (FIB-SEM) and studied using high-resolution secondary electron (SE) imaging. Before the slice-and-view procedure a local Pt layer (approximately 8 × 8 μm with a 1 μm thickness) was deposited in situ on the polished surface to protect the area of interest from the ion beam and to prevent curtaining during the FIB cutting process. The sample slices through the polished cross sections were milled at 30 keV acceleration voltage with the ion beam angled at 38 to the sample.10 A 20 nA current was 3911



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solutions. The spectrometer was calibrated using the first order Raman band of silicon at 520.7 cm−1 and neon emission lines. The spectral signal was accumulated 40 (for mapping) to 500 times (for single measurements) for 1 s.

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RESULTS AND DISCUSSION Replacement of Marble by Ca Oxalate. Experiments at the lowest oxalic concentrations (1 mM) produced a replacement rim that encased the marble cube after 7 days of reaction at 60 °C (Figure 1a). The overall rim thickness increased with increasing oxalic acid concentration (Figure 1a− d). At low concentration of oxalic acid (1 mM), the reaction rim formed at 60 °C for 7 days was a single layer made up of an intergrowth of larger crystals between 1 and 2 μm held together by a matrix probably made up of smaller crystals (Figure 1a). However, reaction with solutions of 10 mM or greater produced a replacement rim with two definable layers (Figure 1b−d). An increased thickness of the outer rim indicates a greater extent of reaction at higher oxalic acid concentrations (Table 2). In comparison, the size of the inner rim only increased slightly in more concentrated oxalic acid solutions. This layering of the replacement rim is visible at all temperatures examined (Figure 1e−h), although the crystals of the outer rim are often smaller at lower temperatures. In addition to the clear replacement rims that seem to retain the straight sides from the marble cube, the presence of larger, euhedral crystal forms can be observed on the surface of the cube reacted with 10 mM or greater concentrations of oxalic acid. Whewellite was the only phase detected in X-ray diffraction (XRD) analysis of the replacement material from a 1 M experiment conducted at 60 °C for 7 days. Spatially resolved Raman spectroscopy confirmed that all the rims precipitated during the replacement of marble in oxalic acid were composed of whewellite, including both rims and the external crystals. This is clearly seen in the Raman spectra shown in Figure 2a by the presence of a doublet band associated with the C−O symmetrical stretching mode at 1463 and 1490 cm−1, which occurs as a singlet in other Ca oxalate phases such as weddellite.2 Retention of the shape of the marble cube implies that the replacement of marble by whewellite is pseudomorphic. However, widths measured across cross sections of the replaced cube show an increase of between 0.4 and 0.1 mm compared to the original size depending on the reaction time and oxalic acid concentration demonstrating that extensive reactions are associated with a volume increase (Figure 3). The formation of an initial pseudomorph in conjunction with the sharp interface between the replacement rim and the marble as well as the evidence for etch pits (e.g., the inset of Figure 1a and undulating marble surface shown in the insets of Figure 1e−h) indicates that the replacement of marble by whewellite occurs via an interface-coupled dissolution−reprecipitation mechanism12 consistent with previous investigations.6 In this mechanism, the dissolution of calcite quickly supersaturates the interfacial solution with respect to the Ca oxalate phase. Precipitation of whewellite removes Ca from the solution, promoting further dissolution by increasing the undersaturation of the interfacial solution with respect to calcite. Thus, dissolution and precipitation are coupled in space and time within the interfacial solution. The formation of porosity is an integral feature of interfacecoupled dissolution−reprecipitation replacement reactions that

Figure 1. BSE cross-section images showing the effect of oxalic acid concentration (a−d) and temperature (e−h) on marble replacement by calcium oxalate. Inset images are magnifications of the rim highlighted by the white box in the corresponding image. Experiments examining the effect of concentration were conducted for 7 days at 60 °C with 1 mM (a), 100 mM (b), 250 (c) and 500 mM (d) oxalic acid. Reaction with higher oxalic acid concentrations produces two distinct, increasingly thick layers as well as large external crystals (white arrow in b). A 10 mM oxalic acid solution was used to examine the effect of temperature in 7 day experiments. Temperature was varied between 20 °C (e), 40 °C (f), 60 °C (g), and 80 °C (h). At all temperatures with the 10 mM solution a similar rim structure was observed to the double layer structure produced in b.

provides fluid pathways to replenish the interfacial solution allowing the reaction front to move through an entire crystal.13 Porosity can be generated due to the reduction of molar volume upon replacement, e.g., during the replacement of aragonite by apatite14 or differences in solubility between the 3912



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Table 2. Replacement Rim Layer Thickness under Different Experimental Conditions oxalic acid concentration (mM) 1 10

temp (°C)

duration (days)

inner rim thicknessa (μm)

standard deviation (1σ)

outer rim thicknessa (μm)

standard deviation (1σ)

60 20 40 60

7 7 7 7 29 7 29 7 7 1 2 4 7 29 7 29 7 29 7 29 7 29 7 29 7 7

n.d.b 8 6 11 12 12 17 8 8 5 6 12 23 29 22 46 43 57 48 65 44 59 47 50 n.d.c 20

n.d. 2 1 3 3 2 5 1 2 2 2 3 6 7 10 12 13 15 10 14 12 30 13 18 n.d. 5

n.d. 7 4 5 5 5 10 8 19 3 4 5 16 13 8 21 59 124 63 297 62 420 112 183 n.d. 228

n.d. 3 2 2 2 2 4 1 4 1 2 2 6 5 3 12 20 35 30 47 36 112 41 53 n.d. 25

80 100

20 40 60

80 250

60 80

500

60 80

1000

60 80

a

Averaged from a total of 20 measurements. Five measurements were taken at approximately equal distance along each of the sides of the cube visible in the SEM cross section image. bNot determined because the rim was inconsistent in texture. cNot determined as the sample was used for XRD analysis.

Figure 2. Raman spectra of whewellite formed as a replacement product around marble. (a) Raman spectra from three points along a line through the reaction rim produced in an experiment reacted for 29 days with a 100 mM oxalic acid solutions at 60 °C. (b) Raman spectra of the three zones (inner and outer rim, external crystals) that formed in 18O-labeled solution. The splitting of the νs1(C−O) and νs2(C−O) stretching bands near 1463 and 1490 cm−1, respectively, into five bands, reflecting the different oxygen isotope-related isotopologues of the two nonequivalent oxalate groups, is clear evidence of the incorporation of significant amounts of 18O into the whewellite structure (see text). Note that the fifth νs2(C−O) band reflecting the fully 18O occupied second oxalate group is located below the νs1(C−O) band near 1463 cm−1.

parent and product phases.15 As the molar volume of whewellite (65.77 cm3/mol) is ∼40% larger than that of calcite

(36.96 cm3/mol), the formation of porosity and retention of the cube shape implies that not all calcium released during 3913



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Figure 3. Extensive replacement of a marble cube in 500 mM oxalic acid at 80 °C for 29 days.

calcite dissolution is reprecipitated at the reacting interface as Ca oxalate. This is consistent with the presence of Ca in the reacted solution (Table 1). The SEM images in Figures 1 and 3 indicate that the outer rim has quite a high number of fluid pathways or porosity between the large crystals; however, the inner rim texture cannot be resolved from these SEM images. Therefore, high resolution images of the inner rim were obtained and 3D FIB tomography (Figure 4) was conducted to examine the inner rim structure. These investigations show that the inner rim comprises very small, rounded whewellite crystals (Figure 4c) that create nanoporosity due to their imperfect packing. This porosity has minimal connectivity, but the locations of previous calcite grain boundaries remain open within the inner rim. The open channels at the previous calcite grain boundary locations imply that the initial reaction is pseudomorphic, and thus Ca must be lost to the solution. However, with the exception of the channels, the transport of fluid through the inner rim is expected to be limited. Using Raman Spectroscopy To Provide an in-Depth Analysis of the Replacement Rim. Raman spectroscopy is a powerful tool to examine mineral replacement reactions as shifts in band position can provide information about ion substitution16 and oxygen isotope incorporation,17 relative band intensities can be related to crystallographic orientation18 and changes in frequency and bandwidth can be used to determine the atomic ordering/crystallinity of a material.19 Recent studies into isotope exchange in solutions and solids have shown that Raman spectroscopy can provide vital information that is both spatially resolved and pinpoints the specific site of isotope incorporation within a crystal structure.14 Specifically, using 18 O-enriched water as a tracer for dissolution and reprecipitation has been shown to be effective for a range of mineral systems20−22 including calcite replacement.23 Furthermore, with the determination of rates for 18O exchange into different chemical moieties, such as phosphate14 and carbonate,17 recent work has also shown that this method can be used as a chronometer for mineral replacement reactions.24 The exchange of 16O by 18O in a solid is evident by a massdependent frequency shift of those Raman bands that are associated with vibrations within the solid that involve the displacement of O atoms. In some compounds, however, the incorporation of the heavier oxygen isotope results in localized vibrations of oxygen-based isotopologues and thus produces distinct new Raman bands.14 Band splitting due to isotope incorporation was also observed in 18O-rich whewellite that formed as a replacement product in 18O-labeled solutions (Figure 2b), which allows a precise and accurate quantification of its 18O content. As mentioned above, the bands near 1463

Figure 4. Examining the inner rim: (a) backscattered electron image of potential nuclei of the larger outer rim crystals (white arrows) within the inner rim during an experiment with a 100 mM solution run for 7 days at 80 °C. High resolution SEM secondary electron images of the rim from the 100 mM oxalic acid, 29-day experiment at 60 °C from which the Raman spectra in Figure 2 were taken showing the interface between the inner and outer rims (b) and the nanometer to micrometer crystals that make up the inner rim (c). Mottled texture of the large outer rim crystals in (b) is due to beam damage. (d) Area of rim chosen for slice and view imaging of the inner rim using 3D FIB tomography from a cross section of a reacted marble cube, open grain boundary indicated by arrow. (e) Single slice of area sampled in (d) showing the retention of an open grain boundary, also visible in (d), inherited from the marble and the presence of pores in the inner rim structure. (f) 3D reconstruction of the inner rim pores visible in the area highlighted by the box in (e) from successive, parallel 3D FIB tomography slices.

and 1490 cm−1 of whewellite (Figure 2a) were assigned as the symmetrical C−O stretching frequencies νs1(C−O) and νs2(C−O)25 of the two nonequivalent oxalate groups in the whewellite structure,26 respectively. Both bands split into five distinct bands in 18O-rich whewellite (Figure 2b), supporting their assignment. Moreover, it follows that the four new bands can be assigned to the four 18O-bearing isotopologues of the oxalate group. Such assignment is supported by the good agreement of the observed frequencies with those that were calculated from simply reduced mass behavior and the assumption that the νs(C−O) frequency of an isotopologue is the average of the four harmonic C-mO (m = 16 and 18) stretching frequencies in the oxalate group (Table 3). In the following, we will use νs1(C−O)n to refer to the symmetrical C−O stretching modes of the five isotopologues C216O4−n18On, whereby n represents the number of 18O atoms in the oxalate group and thus varies between 0 and 4. Note that the fifth νs2(C−O)4 band, related to the C218O4 isotopologue of the 3914



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spectrum of whewellite is beyond the scope of this contribution but will be discussed in more detail elsewhere. The integrated intensity of the isotopologue bands is related to (1) the isotopologue concentration, (2) the scattering efficiency, (3) the self-absorption of the sample, (4) the depolarization ratio of the molecular species, (5) the excitation frequency and intensity, and (6) the instrumental response function. As we are usually dealing with relative intensities of Raman bands that (i) are obtained from a single sample, (ii) occur within a narrow frequency range, and (iii) that are depolarized to the same extent, factors (2) to (6) cancel out or can be neglected. The integrated band intensities are thus proportional to the fraction of the molecular isotopologue species, and the 18O content can be calculated by considering the number of 18O atoms in each isotopologue. For this, the deconvolution of the nine detectable Raman bands between 1400 and 1500 cm−1 had to be carried out first by least-squares fitting of nine Voigt functions, which is representatively shown in Figure 2b (lower gray and black spectra). During fitting of the map spectra, the position of the Voigt functions were allowed to vary ±0.5 cm−1 from the position determined from

Table 3. Reduced Mass (μ), Shift Factor and the Calculated and Observed C−O Stretching Frequencies of the OxygenBased Isotopologues of the Oxalate Group in 18O-Rich Whewellite

C216O4 C216O318O C216O218O2 C216O18O3 C218O4

μ

shift factor

shift (cm−1)

ν*calc (cm−1)

ν*obs (cm−1)

6.857 6.943 7.029 7.114 7.200

0.9938 0.9877 0.9818 0.9759

9.1 18.0 26.7 35.3

1453.9 1445.0 1436.3 1427.7

1463 1454 1446 1439 1431

second oxalate group, is not resolved as it has to be located directly below the νs1(C−O)0 band of the first oxalate group at 1463 cm−1 so that only nine bands are resolvable. We note here that the band located near ∼900 cm−1 (Figure 2a) also split into five bands in 18O-enriched whewellite, indicating that the previous assignment of this band to the oxalate C−C stretching motion is incorrect. A detailed discussion of the Raman

Figure 5. Raman mapping of the replacement rim. (a) Reflected light microscope image of the reaction rim area mapped in (b) and (c) from the 18 O-enriched solution experiment. (b) Map of 18O incorporation measured using the splitting of the νs1(C−O) band in the whewellite spectra (see text and Figure 2b). (c) Map of the intensity ratios between the νs1(C−O) and νs2(C−O) stretching modes of the two nonequivalent oxalate groups in the crystal structure of whewellite (crystal orientation map), reflecting changes in its crystal orientation with respect to the laser polarization. (d) BSE image of a second mapped area from the experiment with 18O enriched solution. (e) Map of 18O fraction incorporated into the oxalate group in whewellite from the area imaged in (d). (f) Crystal orientation map of area mapped in (d). (g) BSE image of part of the rim generated in the equivalent experiment without 18O enrichment. (h) Map of the νs1(C−O) bandwidth across the whewellite rim for area in (g). (i) Crystal orientation map of the rim from (g). 3915



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there is minimal spatial coupling during the first part of the reaction, or grow progressively during the reaction due to the diffusion of Ca-rich solutions from the interfacial solution through the reacting layer. The 18O fraction within these crystals is the lowest observed in the samples, blue areas in the Raman map (Figure 5e), indicating that they nucleated immediately upon reaction of the calcite with the oxalate solution. These external crystals then continued to grow, as evident from the progressive change in the fraction and color on the map from a dark blue to green, coinciding with the enrichment observed in the inner rim. Although the outer rim is made up of large crystals, their ineffective packing (e.g., inset in Figure 1d) produces easy pathways for fluid infiltration, especially when cracks are produced. In contrast, fluid transport in the inner rim is limited mainly to contact areas (grain boundaries) between the whewellite crystals. Fluid movement in this area is therefore restricted, with the exception of relic grain boundaries inherited from the underlying marble, which remain open in the inner rim (Figure 4e). Evidence for fluid movement along these pathways is observed in all of the experiments with a well-developed inner rim as fingers of the outer rim extending toward grain boundaries (Figure 1b,c). The ability of the fluid to infiltrate the relic grain boundaries is also highlighted by the 18O fraction along one of the outer rim fingers (Figure 5e). This indicates that the oxalate located in the fingers formed from a solution with a similar 18O fraction to the outer rim. In this area of the rim, the formation of more crystalline particles right at the calcite-replacement rim interface (white arrow in Figure 5d) and their higher 18O fraction compared to the surrounding inner rim show that the more equilibrated solution could also access the reacting interface via the outer rim finger. In contrast the fraction of 18O at the calcite-inner rim interface in Figure 5b is lower than the inner rim, indicating that the equilibrating bulk solution cannot freely diffuse through the entire replacement rim to access the calcite interface. If the transport within the inner rim is limited, the dissolution of calcite would also release carbonate ions that have a natural, i.e., 16O dominant signature that would compete with the oxalate during exchange with the 18O water. This would result in a decreased 18O fraction in the precipitated whewellite. Thus, formation of the inner rim with no fingers limits fluid movement to the calcite interface and diffusion of excess Ca away from the dissolving calcite interface. Structural Relationships within the Replacement Rim. Ruiz-Agudo et al.6 have shown that there are two epitaxial relationships between the (104) surface of calcite and growing whewellite crystals. However, these surfaces are not always exposed at the cut surfaces of the calcite crystals present in marble due to the large spread of calcite crystal orientations (often between 60 and 90°). Different orientations of the whewellite crystals that grow on the marble cube will influence the friability of the replacement rim thus exploration of this is important when determining the feasibility of using oxalate as a protective layer. Raman spectroscopy is sensitive to different crystal orientations which produces a variation in band intensities.18 Thus, by examining the difference in peak intensities at specific locations in the replacement rim we can predict whether the whewellite crystals have orientations that are controlled by the different orientations of the underlying calcite crystals. As can already be seen from Figure 2, the C−O symmetrical stretching bands at 1463 and 1489 cm−1 vary in relative intensity in different sections of the replacement rim above the

the first spectrum of the maps which exceeds the maximal shift of the spectrometer (±0.2 cm−1) during long-time map acquisition. However, to calculate the 18O content from the integrated intensities, the overlap of the νs2(C−O)4 intensity of the second oxalate group on the νs1(C−O)0 band at 1463 cm−1 had to be corrected. Assuming a statistical distribution of the isotopologues, the overlap intensity was calculated from the ratio between the integrated intensities of the νs1(C−O)4 and νs1(C−O)3 isotopologue bands. The ratio was multiplied with the νs2(C−O)3 intensity to obtain the νs2(C−O)0 intensity that was then subtracted from the νs1(C−O)4 intensity. The 18O content was finally calculated for each point measurement and 2D 18O distribution maps were created (Figure 5b,e). Relative Timings of Rim Formation and Transformation. Previous experiments determining the kinetics of oxygen exchange between oxyanions, such as phosphate14 and carbonate,17 and 18O enriched water demonstrate that O atom exchange can be measured on experimental time scales. If the exchange occurs at a similar rate as a replacement reaction, for example, calcite replacement by apatite, oxyanion 18O signatures within the precipitated replacing phase can be used as a chronometer.24 Similar 18O gradients to those observed during apatite replacement of calcite were observed in the Raman spectroscopy map after experiments of oxalate reactions with marble implying that oxalate also undergoes a gradual exchange with water O atoms. We can utilize this phenomenon to examine the processes occurring during the replacement of marble by whewellite in two ways. First, potential nuclei of larger crystals can be observed within the inner rim, particularly close to the interface between the inner and outer rims (Figure 4a). This observation in conjunction with the retention of a much thinner inner rim despite the different durations and oxalic acid concentrations used in the experiments implies that the larger crystals in the outer rim are formed from whewellite within the inner rim. The textural re-equilibration of the whewellite within the reaction rim could occur via a solid-state mechanism or a dissolution−precipitation process. A solid-state transformation mechanism should produce a gradual gradient of increasing 18O fraction toward the calcite crystal as the oxalate in solution equilibrates with the enriched water. In contrast, if the process occurs via a fluid-mediated mechanism, dissolution and reprecipitation of the whewellite would allow oxalate with a more equilibrated 18O signature to be incorporated into the solid phase at the outer-inner rim interface during the transformation. This can clearly be seen in Figure 5b,e as the outer rim has a higher 18O fraction, yellow or orange respectively, directly at the inner rim-outer rim interface compared to the lower 18O fraction, green in the maps, of the inner rim. Thus, the textural re-equilibration must occur via fluid-mediated dissolution-reprecipitation. The textural re-equilibration is probably governed by Ostwald ripening processes; however, an additional driving force could be the increased crystallinity associated with the larger crystals formed in the outer rim. The Raman map in Figure 5h shows that the full-width half-maximum of the ν1 oxalate bands at 1463 cm−1 from an experiment without 18O enriched water is larger in the inner rim, especially at the calcite-inner rim interface, indicating a lower crystallinity in these regions compared to the homogeneous and much more crystalline whewellite of the outer rim. Second, it is not evident from the 16O-rich Raman spectra or SEM investigations of the replacement process whether the external crystals form at the beginning of the experiment; i.e., 3916



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ACKNOWLEDGMENTS This project was funded by a Deutsche Forschungsgemeinschaft (DFG, German Research Council) grant (PU153/16-1), Marie Curie International Outgoing Fellowship (TMuPiFe PIOF-GA-2012-328731), and a NWO Veni grant awarded to O.P. All experimental work was performed at the Institut für Mineralogie, University of Münster, Germany. Low magnification SEM work was conducted at the University of Münster, whereas the FIB-SEM work was performed at the Center for Electron Microscopy Utrecht (EMU), Utrecht University, The Netherlands. Raman mapping of the samples was done at the Steinmann Institut, University of Bonn, Germany. The authors would like to thank Dr. P. Schmid-Beurmann for assistance with the XRD analysis and Dr. E. Ruiz-Agudo and Dr. C. V. Putnis for helpful discussion.

same calcite crystal in the marble. This suggests that they have a different orientation and is contrary to the hypothesis that this doublet band arises due to two slightly different sites with the same symmetry.25 In contrast, the sensitivity of the two doublet bands indicates that the oxalate group can occupy two different symmetries within the whewellite structure, as indicated from XRD data.26 Figure 5c,f,i shows that although the orientation of the inner rim whewellite is quite homogeneous, the crystals in the outer rim can have completely different orientations within the rim above a single calcite crystal. The inner rim orientation is also not related to the outer rim orientation indicating that any epitaxial relationship between the whewellite and the marble calcite is probably lost during the textural reequilibration. However, this would need to be confirmed using other techniques such as electron backscatter diffraction. These differences in orientation prevent the outer rim from growing into a continuous layer that could effectively protect the underlying marble and probably increase the friability of the outer layer.

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REFERENCES

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CONCLUSIONS The investigation into the replacement of marble by whewellite indicates that a cohesive layer of whewellite can be formed on the surfaces of calcite crystals using a 10 mM oxalic acid solution at 20 °C. This rim has limited transport properties and is composed of small crystals with a mixture of orientations dictated by the underlying calcite. Therefore, the inner, cohesive oxalate rim can form a more durable replacement layer for marble protection than Ca oxalate layers formed at lower oxalic acid concentrations. The higher temperature and higher concentration experiments indicate that this layer can evolve to form a more porous Ca oxalate layer where the misorientation of the whewellite crystals and increase in the overall volume of the layer makes it highly friable. Furthermore, the evidence for different orientations of whewellite within both layers prevents the Ca oxalate forming a homogeneous layer with minimal to no porosity during textural re-equilibration, which would be optimal for stone conservation. The evidence from the 18O experiments indicates that this transformation is fluid-mediated. Thus, the oxalate layers can potentially be engineered to prevent the formation of the less useful outer layer by changing the fluid chemistry or limiting the amount of fluid that the stone is initially exposed to. Unlike previous experiments with single calcite crystals,6 no cracking was observed in the marble experiments. This further increases the potential for the use of oxalic acid in the treatment of building stone. However, further research needs to be conducted into how to tailor the reaction to different applications as well as inhibiting the Ca oxalate textural reequilibration and the effect of the open “grain boundaries” that are retained within the inner rim on the durability of the whewellite layer and therefore the degree of marble protection.

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Article

AUTHOR INFORMATION

Corresponding Author

*Present address: Department of Geology and Geophysics, Yale University, 210 Whitney Avenue, New Haven, CT 06511, U.S.A. E-mail: [email protected]. Notes

The authors declare no competing financial interest. 3917


Forming Cohesive Calcium Oxalate Layers on Marble Surfaces for

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