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1 Dipartimento di Scienze della Terra, Università di Cagliari, via Trentino 51-I-09127 Cagliari, Italy
2 Dipartimento di Scienze e Tecnologie Biomediche Sez. Microbiologia Applicata, via Porcell 4-I-09124 Cagliari, Italy
3 Dipartimento di Scienze Chimiche, Complesso Universitario, S.S. 554-I-09042 Monserrato, Cagliari, Italy
Correspondence: * E-mail: gbgiudic{at}unica.it
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ABSTRACT |
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 Top Abstract IntroductionExperimental methodsResults and discussionAcknowledgmentsReferences cited |
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SEM imaging shows that biomineral hydrozincite is made of small platelet-shaped crystallites having a 20–50 nm long side at the shortest and other sides measuring hundreds of nanometers long. These are interlaced to form sheaths several micrometers long. HRTEM analysis of the biomineral samples shows an imperfectly oriented aggregation of the nanocrystals that is discussed in terms of mesocrystals. Transmission electron microscopy (TEM) and XRD analysis indicate a progressive decrease in the size of the particles in the biomineral compared to the synthetic and geologic hydrozincite samples, with coherent diffraction domains in the biomineral hydrozincite that are smaller by 30–50% than in the other samples investigated in this study. 13C magic angle spinning (MAS) and cross polarization magic angle spinning (CPMAS) NMR spectra show more than one peak for all the investigated samples, despite the fact that carbon atoms have a unique crystallographic position in the hydrozincite structure. The additional peaks can reflect the presence of lattice defects typical of nanocrystals as indicated by the HRTEM images, where high concentration of lattice defects, such as grain boundaries and stacking modes, can be observed both in the biomineral and in the synthetic samples. Further additional peaks in the NMR spectra of biomineral samples are attributed to organic molecules, relicts of the biomineralization process, in agreement with the filaments observed in SEM images of biomineral samples.
Key Words: Biomineralization • hydrous carbonates • hydrozincite • nanocrystals • lattice defects
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INTRODUCTION |
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TopAbstractIntroductionExperimental methodsResults and discussionAcknowledgmentsReferences cited |
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A zinc carbonate, hydrozincite [Zn5(CO3)2(OH)6], in association with cyanobacteria (Scytonema sp.) and algae (Chlorella) (Podda et al. 2000), has been found in a mine environment at Naracauli Creek (Sardinia, Italy).
The crystal structure of hydrozincite was first studied in a geologic sample by using single-crystal X-ray diffraction (Ghose 1964). This author also found that synthetic crystals of hydrozincite have low crystallinity and speculated that plane defects are the likely cause of the observed decrease in crystal order. Infrared spectra from several hydrozincite specimens can differ significantly and show peak broadening (Jambor 1966; Zabinsky 1966). This difference was explained as the contributions of different plane defects. Jambor (1964) and, more recently, Hales and Frost (2007) proposed that hydrous zinc carbonate could have two or more polymorphs. In addition to those structural studies, hydrozincite has attracted the interest of many authors because of its role in the corrosion of Zn-rich materials (Stoffyn-Egli et al. 1998; Morales et al. 2006; Ghosh and Singh 2007), for its involvement in controlling the mobility of zinc in soils (Uygur and Rimmer 2000) and waters (Mercy et al. 1998; Podda et al. 2000; Zuddas and Podda 2005). The occurrence of hydrozincite forming in Zn-polluted calcareous soils was recently documented by Jacquat et al. (2008).
At Naracauli Creek, as already shown in the literature, the precipitation of this biomineral results in the reduction of zinc concentration from 348 to 2 ppm within a few hundred meters downstream. In addition, Pb concentration in the biominerals attains 6500 ppm, Cd concentrations 540 ppm, and many other heavy metals were detected in high concentration in Naracauli hydrozincite (see Table 1
in Podda et al. 2000). Naracauli biomineralization is thus effective also in the uptake of heavy metals other than Zn. Recent investigations on the in vitro reproduction of Naracauli hydrozincite biomineralization indicate that Scytonema sp. exerts a control on the morphology of hydrozincite crystals (De Giudici et al. 2007). These authors consider hydrozincite as a BCM.
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The objective of this work is to investigate the differences between the structure of hydrozincite crystals grown under the control of bacteria, geologic samples, and synthetic samples. Hydrozincite samples are investigated by both spectroscopic and microscopic techniques.
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EXPERIMENTAL METHODS |
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TopAbstractIntroductionExperimental methodsResults and discussionAcknowledgmentsReferences cited |
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Analytical techniques
Microscopic surface features of the samples were investigated with an environmental scanning electron microscope (ESEM QUANTA 200, FEI, Hillsboro, Oregon).
The samples were lightly ground in an agate mortar and ~200 mg of the powder of each sample was packed into the sample holder for the X-ray diffraction analysis. XRD was performed with a 
-2 conventional diffractometer (Siemens D-500) with MoK
radiation (0.709 Å). High-resolution NMR spectra were collected using a spectrometer with a 9.39 T wide-bore Oxford magnet (Varian UNITY INOVA). The same ground samples were analyzed by 13C magic angle spinning and cross polarization magic angle spinning by packing each sample (~100 mg) into 4 mm Si3N4 rotors at a spinning rate of 6 KHz. 13C MAS experiments were run with a recycle time of 5 and 1200 s, 90° pulse lengths (7.5 µs) and 50 kHz bandwidth, and 200 scans for each experiment. 13C CPMAS spectra were collected with contact time between 0.3 and 16 ms; recycle time was 4 s. 13C chemical shifts were referenced to that of tetramethylsilane.
The same ground samples were dispersed in octane and further submitted to an ultrasonic bath. The suspensions were then dropped on carbon-coated copper grids for high-resolution transmission electron microscopy. HRTEM images were collected using a JEM 2010UHR (Jeol) microscope with a LaB6 thermoionic source operating at 200 kV and equipped with a Gatan imaging filter (GIF). Energy-filtered images were acquired using a 3 mm GIF entrance aperture and a slit width of 15 eV. All high-resolution images were acquired digitally using a 1 or 2 s exposure and 1x binning (1024 x 1024 pixels) of the charge-coupled-device (CCD) camera. A fast Fourier transform (FFT) was calculated on the images by using the Digital Micrograph (JEOL) software. Because of the ~19x magnification between the TEM viewing screen and the CCD camera, a preliminary calibration using a standard gold sample was performed to obtain corrected values of d-spacing in the hydrozincite samples.
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RESULTS AND DISCUSSION |
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TopAbstractIntroductionExperimental methodsResults and discussionAcknowledgmentsReferences cited |
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) and crystals are characterized by a platelet shape (Fig. 1b), probably caused by the association of crystallographic forms {100} and {010}. Samples grown under a condition of inorganic synthesis show acicular crystals (Fig. 1c) having a short side typically 100 and 200 nm in length, while the largest side is 2–10 µm and platelet shaped (Fig. 1d). Synthetic crystals shown in Figure 1d are flattened on {100} and show shapes and sizes similar to the geologically occurring crystals shown in Figure 1b. The distinctive feature of the Naracauli samples is that hydrozincite forms a packed network (Fig. 1e) and encrusts cyanobacteria sheaths (Fig. 1f). The crystals of the Nar sample show a platelet shape (Fig. 1g) having the shortest side typically 50–100 nm long. These crystals are misaligned and form a mesoporous aggregate having a sponge-like surface. In addition, some filaments of organic material are clearly visible between the sheaths (Fig. 1g) and sometime protrude out from the inner sheaths.
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shows some of the collected XRD patterns. All observed Bragg reflections can be indexed as a hydrozincite single phase with a monoclinic structure, according to experimental (PDF Card 19-1458) and calculated (PDF Card 72-1100) reference patterns. No peaks of other crystalline phases were detected. The I100 peak appears at 2 
6°, and was indexed as the (200) hydrozincite main reflection. The (200) relative peak intensity is much higher than that reported for the monoclinc structure in the reference cards. This probably arises from a preferred orientation, in agreement with the {100} crystallographic form, as observed also by SEM imaging (Figs. 1a and b
), at least for the Geol1 and Geol2 samples.
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TEM analysis
A representative HRTEM micrograph of geologic samples is provided in Figure 3a, where a low magnification image of the Geol1 sample shows a euhedral particle. Due to the thickness of these well-crystallized particles, only the extreme edge is transparent to electrons in high-resolution mode at high magnification (Fig. 3b). The image of Figure 3b shows nanocrystalline domains larger than 5 nm with a grain boundary network. The observed nanocrystals exhibit the lattice plane distance of 2.85 Å, which corresponds to the (220) hydrozincite planes, as calculated by fast Fourier transform (FFT) in the inset (Fig. 3b).
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, with the FFT in Figure 3d (inset). The particle size distribution of the Nar sample, calculated on about 2000 particles observed in dark-field images, is reported in Figure 3d. The average particle size is about 3–4 nm and the particle size distribution is narrow. TEM results are in good agreement with FWHM from XRD; the value (3–4 nm), lower than the average crystallite size determined by XRD (7 nm), could be the result of the presence of some attached isoriented primary nanoparticles. In the literature, primary nanoparticles were reported to spontaneously self-organize to a superstructure with a common crystallographic orientation in a process called "oriented attachment" (see for the instance, Banfield et al. 2000; Meldrum and Cölfen 2007). Figure 3c (inset 1 and 3) clearly indicates that nanocrystals can aggregate by fusion at the grain boundaries. Sometimes misalignment between crystallites falls below 10°. More often nanocrystals sharing some of the atomic positions at their grain boundaries show misalignment higher than 10°. The fusion process should be able to decrease the excess of surface free energy and then stabilize the nanocrystals. According to Meldrum and Cölfen (2007, and references therein) the platelet observed by SEM can be considered as a mesocrystals produced by imperfect oriented aggregation of the nanocrystals observed by TEM. In the Nar sample, the aggregation and fusion process cannot be scaled up and mesocrystals are only the platelets observed in Figures 1g and 1h. In high-resolution mode, some nanocrystals are observed with the lattice plane distances of 2.72 and 3.14 Å corresponding to the (021) and (020) hydrozincite planes. In comparison with geologic samples, the Nar sample shows lattice defects, which can be related to the presence of stacking modes [Fig. 3c (inset 2)], in addition to nanocrystals in contact through grain boundaries.
An HRTEM image of the Synth2 sample is shown in Figure 3e with the FFT in the inset. The HRTEM image shows grain boundaries and lattice defects, which can be ascribed to the presence of stacking modes on the basis of the typical streaking of the reciprocal rows observed in the FFT.
Nuclear magnetic resonance analysis
In principle, the NMR spectral analysis can give information on the presence of polytypes, different numbers of lattice defects in the samples, namely plane defects such as stacking faults (Tateyama et al. 1997; Harris 2004). This is because NMR spectra are influenced by the local environment, extending to only a few spheres of coordination, so long-range order is not required to produce a signal. The local environment can lower the local symmetry, thus affecting the individual chemical shift and broadening of the observed peaks, which are the result of a superposition of the signals arising from all the C atoms in given sites (Mehring 1983; Engelhardt 1987).
The 13C MAS NMR spectra obtained for the five hydrozincite samples are shown in Figure 4. The spectra were collected at two relaxation delays of 5 s (Fig. 4a) and 1200 s (Fig. 4b). To the best of our knowledge, NMR spectra of hydrozincite have never been reported in the literature.
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, a peak with FWHM of ~1 ppm is present at 163.8 ppm. Figure 4a shows that no signals are observed in the spectrum of Synth1, while a large featureless signal is observed in the spectrum of Synth2 in the range 160–168 ppm. The spectrum of Nar sample (Fig. 4a) shows overlapping signals falling in the 162–169 ppm range, with a main signal at ~164 ppm. The spectra of Geol1 and Geol2 in Figure 4b show a main peak at ~164 ppm, such as that observed in the spectra of Geol1 and Geol2 in Figure 4a; however, small shoulders at both low and high frequencies can now be identified. In addition, a small high-frequency signal at ~168 ppm can be observed in the spectrum of Geol1 (Fig. 4b). The spectrum of Synth1 (Fig. 4b) presents overlapping signals in the 163–169 ppm range. In Figure 4b, the spectrum of Synth2 shows features similar to those of Synth1; however the peaks in the range 165.5–169 ppm are broader. The spectrum of the Nar sample shows the same features under both delay conditions (Figs. 4a and 4b).
As observed in Figure 4, even the use of a very long relaxation delay (1200 s) gives 13C MAS NMR spectra of low quality. However, the use of the 13C cross-polarization maginc angle spinning (CPMAS) technique gives spectra with a better signal to noise ratio (Fig. 5a). For this reason, we consider the analysis of CPMAS spectra suitable to discriminate the single components of the overlapped signals, at least for the chemical shift of the different observed peaks. Moreover, 13C CPMAS is recognized as a useful technique for detecting the presence of biopolymers (Ueyama et al. 1998; Takahashi et al. 2004). The CPMAS NMR spectra should be effective for the organic components, which have many protons, and the intense signal of carbonate carbon atoms could be reduced by ineffective cross polarization. In fact, in the 1H-13C cross-polarization experiment the proton magnetization transfer to a carbon nucleus occurs during the contact time period and depends on the distance between the protons and carbon nuclei; moreover, it is governed by the characteristic proton spin-lattice relaxation time in the rotating frame (T1
) and the carbon-proton cross polarization time (Mehring 1983; Slichter 1989). A quaternary or poorly protonated carbon is normally affected by slow rates of cross polarization from the few bonded or remote protons (Mehring 1983; Slichter 1989; Alemany et al. 1983).
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shows the spectra collected with a contact time of 4 ms (the maximum polarization time for Geol1, Geol2, Synth1, and Synth2 samples), 1 ms for the Nar sample, and Figure 5b shows the spectra collected with contact time of 10 ms for all the samples. Geol1, Geol2, Synth1, and Synth2 samples show overlapping signals in the 162–169 ppm range, whose main signal appears at ~164 ppm, at contact times of 4 and 10 ms. Also the Nar sample shows an intense peak at ~164 ppm chemical shift overlapping with other signals in the range 150–180 ppm at contact time of 1 ms, but in this sample no signal was cross-polarizable with a contact time of 10 ms.
The overlapped signals collected with 4 ms contact time (Geol1, Geol2, Synth1, and Synth2) were decomposed into individual Gaussians by using the software package Origin 7 from Microcal. This approach has been used in the literature either for the simulation of infrared and Raman spectra of hydrozincite and smithsonite systems (Hales and Frost 2007), or NMR spectra of silicon carbide (Mykhaylyk et al. 2002). The information obtained (i.e., position, FWHM, and chemical shift values of geologic and synthetic samples) are reported in Table 1. From the above considerations about the CPMAS experiments, the relative areas of the signals in Table 1 should be taken with caution and only as an indication of the relative amounts of the different structural conditions.
The fitting results clearly show that the signal at ~164 ppm is the main signal for all the samples, as observed in the MAS (Fig. 4) and CPMAS (Fig. 5) spectra. In the Geol1 sample two more signals were detected at ~163 and ~165 ppm, whereas in the Geol2 sample a further signal is detected at ~168 ppm. In the spectra of both synthetic samples, more components are present and a good simulation can be achieved using only five Gaussians at ~163, ~164, ~166, ~168, and ~169 ppm. Figure 6 shows the experimental and the simulated spectra of the Synth2 sample, whereas the results of the fitting of all the samples are reported in Table 1.
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. Additional broad signals at ~22, ~73, ~103, ~173, and ~181 ppm are clearly evident, whereas these signals were not observed in all the spectra of the other samples (spectra not shown). These additional signals may be attributed to the presence of organic material. Due to the complexity of the CPMAS spectrum of the Nar sample, the decomposition into individual Gaussians was not carried out.
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and 5 show several peaks in the range 150–180 ppm, more dominant in the biomineral and synthetic samples. Several reasons can be taken into consideration to explain the presence of these additional signals:
(1) The presence of paramagnetic metals can influence the chemical shift, the spin lattice relaxation time (T1) and the line width of the carbon signals (La Mar et al. 1973). The amount of paramagnetic metal impurities in the Nar sample is lower than 0.3%, is below 0.1% in the Geol1 and Geol2 samples, and far below 0.1% in the Synth1 and Synth2 samples (Table 2). As a consequence, since overlapping signals are present in Nar, Synth1, and Synth2 samples, the presence of paramagnetic metal impurities cannot be responsible for the appearance of additional signals. It is worth noting that the spectra of Synth1 and Synth2 clearly show a very broad signal at 5 s delay (Fig. 4a), whereas small peaks are observed with the 1200 s delay (Fig. 4b). Thus, large T1 characterizes the NMR signals of these samples. However, the spectrum of the Nar sample appears to be independent of the delay, showing that the NMR signals are affected by shorter T1. Geologic samples show a slight dependence on experimental delay. This seems to indicate that samples having low paramagnetic metal impurity content show dependence on relaxation delay (Synth1 and Synth2), samples having high concentration impurities content show very little dependence (Nar), and samples of intermediate content show a slight dependence.
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). Similar information is obtained from the CPMAS spectra (Figs. 5a and 5b).
It may be suggested that in Nar, the presence of a higher concentration of paramagnetic metal impurities, when compared to the other samples, induces a shortening of the T1 relaxation process, limiting the polarization transfer to C nuclei. This can explain the loss of cross-polarizable carbon signal in Nar sample collected at 10 ms contact time (Fig. 5).
(2) The appearance of extra peaks in the range 150–180 ppm could be explained by the presence of carbonyl/amide-carbons from organic biopolymers, as previously observed (Takahashi et al. 2004). However, the presence of the additional signals can be clearly excluded for the Synth1 and Synth2 samples, since they were synthesized in our laboratory in the absence of any organic molecule and no trace of organic filaments was observed in TEM images. As to the Geol1 and Geol2 samples, they show lower additional signals than the other samples analyzed, and, like the previous sample, the TEM images do not show traces of organic filaments. This evidence, and the fact that the amount of carbon estimated through CHNS-O analysis was in agreement with the stoichiometry of the hydrozincite formula (4.44%) for all the geologic and synthetic samples and for the Synth1 and Synth2 samples (Geol1 4.41%; Geol2 4.37%; Synth1 4.29%; Synth 4.34%) rule out this possibility.
Different considerations should be made regarding the Nar sample. In this sample, the C estimate through CHNS-O analysis revealed an excess of C (4.84%), which suggests the presence of biopolymer C. This evidence confirms the presence of some organic material in the Nar sample, as observed in SEM images, and is in good agreement with the signals observed in the CPMAS spectrum. In fact, the CPMAS spectrum (Fig. 7) of the Nar sample is characterized by broad signals that, in the absence of N, can be attributed, as revealed by CHNS-O analysis, to aliphatic chains (~22 ppm) and to O-aliphatic-carbons (~73 and ~103 ppm), whereas these signals were not observed in other samples. Carbonyl-C atoms would be expected in the range 160 < d 
190 ppm. All this evidence would suggest the presence of signals coming from the carbonyl C from organic material in the range 150–180 ppm or at least, and more importantly, that the organic material has some influence in the formation of this additional signals.
(3) The NMR technique is well known to be sensitive to crystal order and to the presence of lattice defects such as stacking faults. The relation between stacking and/or polytype and NMR peak formation is well known in the literature for several phases such as silicon carbide (Hartman et al. 1987; Tateyama et al. 1997; Harris 2004), calcium silicate hydrate (Cong and Kirkpatrick 1996), and saponite (Vogels et al. 2005). Depending on mineral synthesis, different stacking sequences or polytypes can possibly be achieved, (Vogels et al. 2005), and these result in different energy minima (Ryjá
ek et al. 2001).
Despite the fact that C has only one crystallographic position in the ideal structure of hydrozincite, 13C NMR spectra of our samples show up to five peaks depending on the mineral formation. These additional peaks can be ascribed to the presence of lattice defects, namely grain boundaries and stacking modes, that lower the crystal structure symmetry present in these hydrozincite crystals, in agreement with HRTEM analysis.
TEM analysis indicates that the crystallite size (i.e., the size of coherent diffraction domains) for synthetic and biomineral samples is between 3 and 5 nm. Thus, besides the high surface area of biomineral hydrozincite, also the high concentration in lattice defects could affect the surface reactivity, playing a role in the hydrozincite biomineralization.
Finally, the mechanism governing the hydrozincite biomineral formation at the molecular scale is not yet understood. Specifically, as for most of the biomineralization described in the literature (Skinner 2005; Meldrum and Cölfen 2007, and references therein), the organic molecules supposedly involved in the control of nanocrystal surfaces were detected by coupling microscopic and spectroscopic analysis. The role of organic matter in the biomineralization requires further investigation and will be the subject of a successive paper. The information acquired in this work will be useful for modeling the mechanisms of formation of biominerals and for predicting their reactivity and stability under specific environmental conditions, eventually developing in vitro tests and field scale experiments on biomineral production.
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ACKNOWLEDGMENTS |
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TopAbstractIntroductionExperimental methodsResults and discussionAcknowledgmentsReferences cited |
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Footnotes |
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MANUSCRIPT RECEIVED December 23, 2008; MANUSCRIPT ACCEPTED August 5, 2009
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