Role of Bulk pH during Witherite Biomorph Growth in Silica Gels

Sep 10, 2009 - The precipitation of alkali-earth carbonates, such as barium carbonate (witherite), in mildly alkaline silica gels occurs in the form o...
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DOI: 10.1021/cg9005967

Role of Bulk pH during Witherite Biomorph Growth in Silica Gels E. Melero-Garcı´ a, R. Santisteban-Bail on, and J. M. Garcı´ a-Ruiz*

2009, Vol. 9 4730–4734

Laboratorio de Estudios Cristalogr aficos, IACT (CSIC-UGR), Avda. del conocimiento s/n, P.T. Ciencias de la Salud, 18100 Armilla, Granada, Spain Received June 2, 2009; Revised Manuscript Received August 26, 2009

ABSTRACT: The precipitation of alkali-earth carbonates, such as barium carbonate (witherite), in mildly alkaline silica gels occurs in the form of crystalline aggregates that display external morphologies resembling those encountered in crystalline biomaterials and which have been named silica-carbonate biomorphs. In this work, we have followed, spatially and temporally, the evolution of pH inside silica gels during the precipitation of witherite biomorphs, in correlation with a photographical record of the spatial and temporal evolution of the morphologies of the aggregates. By combining the two resulting observations, we have determined that the formation of witherite biomorphs is most efficient under a pH that is less alkaline than it was previously believed. We discuss the implications of our findings and how they fit in the frame of the recently proposed model for the formation of these materials.

1. Introduction Living organisms manage to create functional structures out of polycrystalline materials through the organization and assembly of small single crystal building blocks for which nucleation, growth, and crystallographic orientation are carefully controlled.1,2 To understand the biomineralization processes that give rise to these hierarchical materials systems like carbonate-silica biomorphs constitutes invaluable laboratory models. The reason for this is that silica biomorphs mimic the internal texture found in polycrystalline biomaterials. Biomorphs are made of crystalline nanorods of orthorhombic carbonate, arranged with a large degree of orientation along the c-axis and embedded in an amorphous silica matrix.3-7 This layout of the building units allows for the formation of external morphologies and shapes that share many of the characteristics generally associated with biomineral materials: silica biomorphs are bounded by sinuous smoothly curved surfaces instead of flat faces and do not have defined edges or angles. In fact, they have been used as a proof that morphology and/or chemical composition, by themselves, are not a sufficient basis for claims of detection of ancient primitive life.8,9 However, silica biomorphs not only mimic the internal layout of polycrystalline biomaterials. The organization and assembly of their building units proceeds through self-assembly and self-organization processes driven by simple inorganic chemical interactions, in which no complex biomolecule, or even organic compound, is present. Silica biomorphs are purely inorganic systems, hence their interest for the advance in the understanding of the processes of formation of polycrystalline materials in living organisms, since they are a much simpler system that nevertheless produces identical mineral textures or architectures. The details of the mechanism of formation of barium carbonate (witherite) silica biomorphs have been recently published.10 In summary, witherite silica biomorph morphologies arise from a single crystal core that suffers the relaxation of its symmetry along two different but consecutive steps: a *Corresponding author. Tel: (34) 958181644. Fax: (34) 958181632. E-mail: [email protected]. Web: http://laue.lec.csic.es. pubs.acs.org/crystal

Published on Web 09/10/2009

first stage, in which polymeric silica induces splitting at noncrystallographic angles of the initial single crystal core, and thus a fractal-like growth, and a second stage that produces the characteristic curved morphologies. This second stage is proposed to be the consequence of a coupled precipitation of silica and carbonate through their inverse solubilities with pH (silica supersaturation increases with decreasing pH while carbonate supersaturation increases with increasing pH) and their inverse buffering effect on the bulk pH (precipitation of silica rises the pH while precipitation of carbonate decreases it). The resulting feed-back oscillation of the local pH values and their influence on carbonate and silica precipitation produce what the authors have termed a fibrillation of the growth front. This process consists of the cessation of the physical continuity of the crystalline building so that the material starts growing by three-dimensional (3D) nucleation, at the growing boundary, of myriads of largely oriented nanocrystallites that are rapidly covered and cemented by silica. The continuation of this process yields a composite polycrystalline aggregate that can grow free of symmetry constraints imposed by crystal symmetry. From the early works of Garcı´ a-Ruiz et al.,11 it was acknowledged the large influence of pH in the development of silica biomorphs. Growth of biomorphs in solutions and gels was found to occur only at initial pH values higher than 8.5. Moreover, at pHs between 8.5 and 9.5, crystal precipitates are reported to show spicular dendritical and/or densely branched globular fractal structures, while the characteristic biomorph morphologies, like sheets and braids, are obtained at higher pH values (ref 8 and ref therein). This major role played by pH can be now better understood in the frame of the proposed model, as this parameter both affects and reflects the speciation of silica and carbon in aqueous systems and, therefore, the concentration of the relevant chemical species, namely, carbonate ions and silicic acid (Si(OH)4). In all previous studies, however, pH values quoted in experimental protocols have been the initial values either of the solution or of the gel precursor solution. As pH is an intrinsically timevarying parameter in both solution (diffusion of atmospheric CO2 in an open system) and gel (diffusion of a alkaline-earth soluble salt solution into a silica gel) growth methods, its r 2009 American Chemical Society

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Figure 1. pH of gels obtained by acidification of a sodium metasilicate solution with 1 M HCl, after 5 days. Points corresponding to HCl additions larger than 4.75 mL, and smaller than 2.5 mL, produced mixtures that did not gellify after 5 days. The error bars are the standard deviation calculated from the total number of measurements performed for each point, which is indicated in the table at the right (N). This number varies from point to point since we were more interested in a specific range of pH (see main text). To ensure the validity of the data two different batches of silica and two different pH probes and pH meters were used and treated independently. In this way the variability associated to all these factors are included as well in the calculated error bars. The inset shows an enlarged view of the conditions that yielded gels of high pH.

specific influence on the formation mechanism is still unknown. The aim of this work is to clarify the role of this parameter on the formation mechanism of this interesting material. 2. Experimental Methods Witherite biomorphs have been reported to grow indistinctively in silica solutions and gels. The general procedure to grow biomorphs in silica gels is quite simple. It starts from a diluted solution of commercial sodium silicate, typically, 0.6 M in SiO2, to which hydrochloric acid (HCl) is added. The acidification increases the [SiO2]/[NaOH] ratio making it less soluble in the solution. As a result, the condensation reaction induces the polymerization and finally the gelling of the silica solution.12 After the gelling process is finished, a barium chloride (BaCl2) solution of a certain concentration is poured on top. As the barium ions diffuse through the boundary into the gel they cause the precipitation of barium carbonate crystals together with carbonate ions that are present as a result of dissolved atmospheric carbon dioxide in the gel and in the solution. The diffusion of the barium solution and the consequent precipitation of carbonate contribute to create a time-dependent gradient of pH along the diffusion direction in the gel that will affect the development and ulterior growth of silica biomorphs. By measuring the gradient along the gel at different times, we could correlate the morphologies of biomorphs in the gel, and their evolution, with pH in a reliable way. In the following sections, we outline the experimental difficulties related to the control and measurement of pH in silica gels, the details of the experimental procedure followed to obtain spatiotemporal pH measurements with a correlated spatiotemporal morphogram, and the materials used. 2.1. pH Measurements in Silica Gels. Measuring the pH of silica gels is a problematic issue for several reasons. First, common glass electrodes cannot be used since most of them rely on the formation, when they are brought into contact with the solution, of a very thin wetted silica gel around the glass bulb for ion transport. Because of this, alkaline silica gels interact strongly with the glass bulbs sensibly decreasing their reliability after a few measurements. Second, the pH of the interstitial solution of the gel rises during the gelling process in an amount that cannot be easily predicted, as it depends on the solubility equilibrium conditions sought out by the gel. The

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Figure 2. Scheme of the crystallization cells. Two rectangular glass covers of 55  110 mm are pressed against each other with a rubber frame, of about 2-3 mm thick, as a spacer. The frame is covered with vacuum grease to ensure the tightness of the cell, and the glasses are held using foldback clips. The liquid precursor of the gel is poured up to a height of 4 cm through the middle needle (red), which is then removed (panel A). After 5 days the barium chloride solution is poured on top through the upper needles, which are also removed (panel B), therefore preventing further diffusion of atmospheric CO2 into the system. In panel C, we see how the gel was divided to get a spatial record of pH values. In panel D, it is shown a general view of a gel showing the typical spatial pattern of morphologies after several days. The flatness of the glass covers and the thin space between them allow for easy photographic record of the biomorphs in the gel. reason for this is that the process of gelling in alkaline silica solutions after an increase of the [SiO2]/[NaOH] ratio by acidification is determined by two processes:12 a protonation reaction that generates silanol groups ... Si-OH, which for the case of the monomeric molecule can be represented as SiO2(OH)2-2 þ H3Oþ T SiO(OH)3- þ H2O; a condensation reaction involving two silanol groups that creates a siloxane bond ...Si-O-Si ... with the release of a water molecule ...Si-OH þ OH-Si... T ... Si-O-Si ... þ H2O Therefore, the precipitation of silica implies a net removal of acidic -OH groups from the solution, which causes the rise of pH. Third, to measure pH in a gel means to deteriorate the gel since it needs to be mechanically manipulated in some way to ensure good contact with the probe. This means that we cannot measure the pH of a gel and use that same gel to further grow biomorphs in. For our purposes, it is important to know the initial pH of the gel just before the addition of the BaCl2 solution. Hence, we carried out a study of the pH of silica gels formed after different additions of HCl. Silica gels were prepared from commercial sodium trisilicate solutions from Sigma-Aldrich (approximately 27% in SiO2 and 13-14% in NaOH, which means a [SiO2]/[NaOH] molar ratio of approximately 1.33) diluted 1:10 in volume with bidistilled water. 1 M HCl in different amounts was added to aliquots of 10 mL of the silica dilution, freshly prepared. The mixture was used to fill Linbro well plates that were then covered, sealed with paraffin, and let stand for 5 days to ensure the completion of the gelling reaction. After this period the pH of the different gels was measured by punctuation with commercial pH microprobes and pH-meters from Sentron, calibrated twice before each run of measurements. These microprobes use solid-state technology for the sensor and are unaffected by the chemistry of silica, being therefore very suitable to measure pH in silica gels. The result of the study is shown in Figure 1. As it can be seen, this study allows us to control, within an error, the pH of the gel after the gelling process, and therefore to reliably obtain silica gels of any pH between 9.5, or even 8.5, and 11, which is the range where biomorphs have been found to grow. 2.2. Spatiotemporal pH Measurements Correlated with Spatial and Temporal Morphogram. For growing witherite biomorphs in silica gels we used the homemade cells depicted in Figure 2. The pH gradient of the gels formed in these cells can be obtained as a function of the distance from the interface by dividing the gel into suitably small pieces, measuring their pH, and associating the value to the coordinates of the center of the piece. The detailed procedure is as follows: The cell is opened and the upper solution (if any) is poured away taking care that it does not cover nor it is spilled over the gel. The gel is divided into slices and each slice is pushed inside a small eppendorf tube where we insert the pH microprobe to measure the pH. The small size of the microprobe means that only a small

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Table 1. Observed Initial pH Gradients of Silica Gels in the Growth Cellsa distance to interface (cm)

fresh silica dilution þ 3.5 mL of HCl

aged silica dilution þ 3.25 mL of HCl

0.5 1.5 2.5 3.5

10.19 ( 0.05 10.34 ( 0.05 10.42 ( 0.06 10.43 ( 0.08

10.38 ( 0.04 10.42 ( 0.07 10.47 ( 0.05 10.43 ( 0.04

a

To perform confirmation test larger slices than in the actual experiment were used.

Figure 4. Time development of the pH values versus distance to the interface. The blank curve corresponds to the unfilled cells. See main text for details on the experimental procedures. Figure 3. Spatial and temporal morphogram made with photographs of representative aggregates. The photographs were taken at the indicated time after the beginning of the experiment, and among those aggregates located at the corresponding distance from the interface. Beneath each photograph it is indicated the correlated value of the pH (see main text and Figure 4 for more details). Black bar is 50 μm long. Black crosses indicate the absence of aggregates. volume of gel is needed, and therefore it was possible to make three measurements at each distance point from the interface (see Figure 2C). The full actual experimental setup is as follows: We prepared 12 of the homemade crystallization cells and half-filled them with the same silica gel precursor solution prepared as described further in the text. After the 5 days, a solution of BaCl2 0.25 M was poured on top of 10 of these cells so that the cells were completely filled. Thus, uptake of CO2 is prevented from this very moment. The only sources of carbonate are therefore the CO2 already dissolved in the silica and in the BaCl2 stock solutions, and that uptaken from the atmosphere during the 5 days of waiting time. The filling with BaCl2 signaled the beginning of the experiment. The two leftover cells were used as controls for the expected initial pH and were thus opened to measure their pH right before the filling of the rest of the cells with BaCl2. Eight of the filled cells were used to obtain in duplicate the pH as a function of distance to the interface at 7 h, 1 day, 2 days, and 4 days. The two remaining filled cells were used to record the spatial and temporal evolution of the aggregates for the whole duration of the experiment. Photographs from both cells were taken to be equivalent. For the pH of the gels, we selected the value of 10.5 as this value has been reported as being the one around which the more biomorphic morphologies are obtained, and has been extensively used by researchers.6,13,14 However, it happened that the condition of our study corresponding to this pH (3.5 mL of 1 M HCl added to 10 mL of a commercial sodium silicate freshly diluted 1:10 in water to yield a pH of 10.47 ( 0.19) was not totally accurate for the gels formed inside our cells. We observed that a small gradient of pH was formed in the gel when using this condition. To eliminate that gradient, we found out that we had to use an old 1:10 aliquot (about one month old) instead of a fresh one, and also to use a smaller amount of HCl, 3.25 mL instead of 3.5 mL for each 10 mL of silicate solution, to attain the correct pH value (see Table 1). We could not find an

explanation for the difference observed between the two protocols, but we speculate that the reason might be the presence of silica particles and silica oligomeric species, coming from the very concentrated mother solution, which would have more time to effectively dissolve in the aged dilution than in the freshly prepared. As a consequence the pH of the aged aliquot should be lower than that of the freshly prepared, and this difference could account for the smaller amount of acid needed to achieve gelation at the desired pH for the aged aliquots. These species in the fresh dilution would tend to sediment inside the cells (they are left to gel standing up) creating a spatial dependence on the gelling process and, as suggested by one referee, an enhanced syneresis in the gelation of freshly prepared dilutions that would account for the observed gradient. The sedimentation effect would have been hidden in the study of pH due to small height, and the single measurement, of each well in the Linbro plate. The silica species would be completely dissolved in the aged dilution, therefore affecting less the amount of acid needed to reach a certain pH.

3. Results The recorded spatial and temporal morphogram, with the spatially and temporally correlated pH measurements, can be seen in Figure 3. The resulting morphological pattern agrees with those previously reported in the literature.8,14 Aggregates near the interface appear in a large number at very early stages. They show a globular shape and on some of them there can be seen incipient laminae that characterize growth under the second stage of the formation mechanism. They stop evolving before the first 7 h since they are observed to be very similar from the beginning to the end of our experiment. On the contrary, aggregates far from the interface, and especially at the end of the cells, are larger and show the characteristic curvilinear morphologies like large sheets and helicoids. In general they display more developed and differentiated morphologies with increasing growth time. In Figure 4, we have plotted the measured pH curves at the different times. Each point is the average of all the measurements done at the corresponding distance from the interface

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(six in total, three for each cell) and the error bars are the calculated standard deviation. It can be seen that the control cells (blank curve) corroborate our expected initial pH of 10.4 and that after the addition of the BaCl2 solution a temporal varying pH gradient, rising to the initial pH at the end of the cell indeed forms inside the silica gel. The influence of the diffusion of the barium solution in the pH gradient is clearly seen in the differences between the curves at 7 h, 1 days, and 2 days, from which pH at a fixed distance from the interface decreases from the initial value of 10.4. Undoubtedly, the acidification due to carbonate precipitation also contributes to this decrease in the areas where it occurs. The curve at 7 h misbehaves at large distances from the interface due to some experimental problems that took place with the two cells corresponding to this time point. However, the relevant measurements of these cells are only those corresponding to less than 1 cm where aggregates are present at this early stage. Here the general behavior agrees with the rest of the measurements and with a diffusion-influenced gradient for distances close to the interface. The curve of 4 days neither rises to the initial pH value at the bottom of the gel but from its measured gradient it is clear that equilibration of the whole cell is very advanced at this time. 4. Discussion The spatial and temporal development of the pattern of precipitates (Figures 2D and 3) is clearly related to the diffusion of the barium ions through the gel. This can be anticipated from the geometry of our experimental setup and it has been even reported in a previous study.15 However, the gradient of pH that is formed inside the gel is not only influenced by the diffusion of the slightly acid barium solution, but it is also affected by the precipitation of barium carbonate. Therefore, pH inside the silica gel is a parameter that cannot be easily predicted and it needs to be measured directly. Through our observations and measurements we have been able to follow the correlated evolution of morphology and pH in space and time. The most important result of our experiment is that, despite starting from an initial pH value of the gel of 10.4, the formation mechanism seems to be at work only at lower pH values. From Figure 3, aggregates appearing at distances of 1.25 cm and beyond nucleated at pHs lower than 10, in fact lower than 9.8 for most of the cases. Hence, aggregates at these distances necessarily formed and evolved their morphologies under a pH below that value. Moreover, from Figure 4, the growth occurred under a pH that is decreasing with time. Nevertheless, aggregates at these locations are the largest and the ones showing the most biomorphic morphologies. This upper pH bound for the formation mechanism is more acidic than it was previously expected.5 However, it is in agreement with the proposed model and can be understood when analyzed within its frame. Since the pKa of silicic acid is reported to be of 9.9,12 this means that any decrease in local pH, such as the one caused by carbonate precipitation, produces an increase of the concentration of Si(OH)4 (and therefore a precipitation of amorphous silica) with a proportionality that is larger the lower the bulk pH is. This phenomenon is one of the fundamentals of the fibrillation process formulated in the proposed model. Precipitation of SiO2 as a response of carbonate precipitation must be sufficiently fast and large to rapidly cover the nuclei and to keep their size sufficiently small so that crystal symmetry cannot

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Figure 5. Plot of the logarithm of the molar concentration of the different ionic species against pH for the example case of a total amount of dissolved CO2 and SiO2 of 1 and 0.1 mM in water, respectively. Together we have plotted the observed highest and lowest pH values in which biomorphs in our experiments in gels have grown. The curves are calculated from equilibrium equations taking into account only the two first deprotonations of the acids with constants, pK1 and pK2, of pKC1 = 6.35 and pKC2 = 10.33, and pKSi1 = 9.9 and pKSi2 = 11.7.12,16

direct the growth of the aggregate, and we end up with a polycrystalline material. However, the bulk pH cannot be arbitrarily low. While still below the pKa of Si(OH)4, it must be sufficiently high so that precipitation of carbonate, as a response to the local increase of pH caused by the precipitation of amorphous silica, can take place and the two precipitations may become effectively coupled. The above discussion is better understood if we represent together our observed upper limit pH for biomorph formation and the equilibrium concentration curves in water of the species of the carbon and silica systems, as functions of pH (see Figure 5). In Figure 5 the rectangle indicates the region where we have observed the formation mechanism to be at play, namely, pH 9.3-9.8. While our data indicate that the coupled precipitation does not occur at pH higher than this value in our experiments, the value of 9.3 is the minimum value recorded in our experiments. At that value the formation mechanism was still working, so that the pH boundary where the precipitates enter again into the fractal route10 needs to be lower than that value. Another important result of the experiment concerns the possibility, sought out in previous studies, of using the pH as a mean to control the resulting morphologies of the aggregates. Within the proposed model the different morphologies, even the most characteristic ones like leaves and helicoids, are considered to be circumstantially different manifestations of the same growth mechanism and therefore no direct relation between pH, initial or actual, and a specific morphology should exist. Our own data support this perspective as, from Figure 3, aggregates showing sheets and helicoids appear at different positions, different times, and different pH values, most of the time in the same aggregate. From the previous arguments, it could be thought that a carefully controlled gel at, for example, pH 9.8, or a solution experiment at that pH, would be good systems to grow biomorphs regardless of other chemical parameters like dissolved CO2 concentration, silica concentration, or barium concentration. However, for anyone that has ever tried to grow biomorphs this is clearly not true. And the reason is another of the fundamentals of the proposed model. Carbonate silica

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biomorphs are the result of self-assembly and self-organization processes occurring in a system out of equilibrium. Here, by system, we refer to the macroscopic assembly of the cell, with the solution on top that causes the concentrated barium solution to diffuse into the gel, thus time-dependently affecting the pH and supersaturation of barium carbonate inside the gel. The evolution to equilibrium is thus the driving force for the formation of biomorphs and, therefore, the role of the initial chemical conditions is to ensure that the re-equilibration history of the system passes through the appropriate window of conditions. What is more, since biomorph formation occurs through the degradation of single crystal cores through two successive stages, the initial core needs to pass first through the fractal stage for a latter fibrillation to occur. Therefore, good initial chemical conditions are those that allow, during the re-equilibration, initially the production of sufficient precipitation of carbonate so that when the system reaches later a pH below 9.9, there are already fractal aggregates that can fibrillate. This is the reason for the high initial pH that has been reported, for both gel and solution methods, needed to obtain the biomorphs. In fact, the best initial conditions to get large and greatly developed biomorphs would be those in which the coupling conditions are reached when the disequilibrium of the system is small, so that the passage time through the window of coupling precipitation is maximized. From Figure 3, we can see this is precisely the case for the conditions reached in our cells at distances greater than 1.75 cm, where the largest and more developed aggregates are found. The more specific dependences of the growth mechanism on the rest of the chemical parameters are however still unknown. For systems in which uptake of atmospheric CO2 is not possible, like the cells used in this work, pH only interrelates carbonate and silica precipitation while the gross amount of precipitation is still determined by the specific supersaturation values which are complex functions of the initial chemical conditions and of the geometry of the system. Moreover, growing aggregates tend to suffer perturbations of their growing fronts that cause diverse accidents critical for the morphogenesis of the most ordered morphologies,10 like variations of the speed of growth, sudden sealings of previously active fronts, curlings, and more. The chemical environment intuitively influences these perturbations, but the details of the very complex dynamics of the growth mechanism are still unknown and are a subject for future works. 5. Conclusions We have studied the temporal and spatial evolution of pH inside silica gels during witherite biomorph growth, in correlation

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with a spatial and temporal photographical record of the resulting morphologies. This correlation has allowed us to study the influence of pH on the temporal morphological development of witherite biomorphs with great detail. We have determined that, despite setting an initial pH of 10.4 for the silica gel, nucleation and evolution of the larger aggregates with the most developed morphologies actually took place under a decreasing with time pH below 9.8. This upper value is discussed in the frame of the proposed model for the mechanism of formation and it is found in agreement with it. Another corollary of the model, namely, the expected absence of a direct relation between specific morphologies (leaves, helicoids, etc.), was also confirmed by our observations. Finally, our measurements have provided insight into the complex aspects of biomorph formation as a self-organizing outof-equilibrium inorganic system. Acknowledgment. This work was supported by the Spanish Ministry of Science and Innovation (project MAT2006-11701) and is part of the Consolider-Ingenio 2010 project Factorı´ a Espa~ nola de Crystalizaci on. E.M.-G. acknowledges financial support from the program Juan de la Cierva (Ministerio de Innovaci on y Ciencia). R.S.-B. acknowledges financial support from the I3P (CSIC) program.

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