Pathways of volcanic glass alteration in laboratory ... - Clay Minerals

Clay Minerals, (2013) 48, 423–445

Pathways of volcanic glass alteration in laboratory experiments through inorganic and microbially-mediated processes J. CUADROS1,*, B. AFSIN1,{, P. JADUBANSA2, M. ARDAKANI3, C . A S C A S O 4 AND J . W I E R Z C H O S 4 1

Department of Earth Sciences, Natural History Museum, Cromwell Road, London SW7, 5BD, UK, 2 Department of Life Sciences, Natural History Museum, Cromwell Road, London SW7 5BD, UK, 3 Department of Materials, Faculty of Engineering, Imperial College London, London SW7 2AZ, UK, and 4 Department of Environmental Biology, National Museum of Natural Sciences, CSIC, Serrano 115, 28006 Madrid, Spain

(Received 12 September 2012; revised 17 January 2013; Editor: John Adams)

AB ST R ACT : Rhyolitic obsidian was reacted with natural waters to study the effect of water chemistry and biological activity on the composition and formation mechanisms of clay. Two sets of experiments (18 months, 6 years) used fresh, hypersaline water (Mg-Na-SO4-Cl- and NaCl-rich) and seawater. The 6-year experiments produced the transformation of obsidian into quartz, apparently by in situ re-crystallization (Cuadros et al., 2012). The most abundant neoformed clay was dioctahedral (typically montmorillonite), indicating chemical control by the glass (where Al > Mg). Altered glass morphology and chemistry in the 18-months experiments indicated in situ transformation to clay. Magnesium-rich (saponite) clay formed under water-chemistry control in the bulk and within biofilms with elevated Mg concentration (Cuadros et al., 2013). The contact between microbial structures and glass was very intimate. Glass transformation into quartz may be due to some characteristic of the obsidian and/or alteration conditions. Such combination needs not to be uncommon in nature and opens new possibilities of quartz origin.

KEYWORDS: biologically-mediated formation of clay, cryo-SEM, mineral-microbe interaction, quartz formation, TEM-AEM, volcanic glass alteration to clay.

The inorganic processes producing clay minerals have been studied ever since these minerals started to be sufficiently characterized in the 1930s. Much has been learned about clay formation in these decades and there exists a framework for our understanding of these processes. However, the complexity and variety of both clays and the processes that produce them mean that we are not yet in possession of the whole picture. Besides, for * E-mail: [email protected] { Present address: Department of Chemistry, Faculty of Science and Arts, Ondokuz Mayis University, Samsun, 55139 Turkey DOI: 10.1180/claymin.2013.048.3.01

some time now, scientists have increasingly recognized that biological activity contributes significantly to the production of clays (Konhauser & Urrutia, 1999). At first sight, this biological influence may appear to be of minor importance as compared to the role of inorganic processes. However, biological processes are driven by metabolism and their corresponding effects are accelerated. Thus, it has been assessed that 20 30% of stone weathering is produced by biological activity (Wakefield & Jones, 1998). In addition, in the last few decades extreme condition habitats have been found (Rothschild & Mancinelli, 2001) and the dimension of the subsurface microbial population estimated to be up to

# 2013 The Mineralogical Society

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15 times larger than that at the surface (Whitman et al., 1998), indicating that the mass of living organisms and its influence on earth processes is larger than thought in the past. The inorganic processes generating clay minerals have been studied from both natural settings and laboratory experiments. Two major elements are recognized as affecting clay mineral formation: the composition of the rocks in contact with fluids and the physicochemical conditions in which the fluidrock interaction occurs. The latter includes water chemistry, pH, temperature, pressure, water/rock ratio, porosity, etc. At the same time, rock type and physicochemical conditions control another variable: reaction kinetics, which are important in the formation of most clay minerals, as they are produced in near-surface environments where reactions are slow. The currently available information on clay formation from natural settings is blurred by the difficulty to discriminate between the controls mentioned above. This problem can be partly alleviated by carrying out laboratory experiments, although these have the disadvantage of the necessary short reaction times, which places more weight on the reaction kinetics control. In fact, although experiments broadly confirm the findings from field studies, there are certain important inconsistencies between their respective results. For example, the types of clays formed in natural environments are usually controlled by water chemistry (Cerling et al., 1985), whereas those from experiments are controlled by rock chemistry (Thomassin et al., 1989; de la Fuente et al., 2002). However, there are exceptions, such as the case of early weathering of amphibole in a natural setting that produced different types of clay in different crystallographic faces, indicating a rock-controlled process in a natural setting (Proust et al., 2006). At the moment, it is recognized that the necessarily different conditions operating in natural and experimental settings, especially reaction time, are responsible for many of these differences, although it is not clear how the different conditions cause the contrasting results. The literature on the effect of biological activity on clay formation is growing quickly. It ranges from microbial (e.g. Konhauser & Urrutia, 1999; Hama et al., 2001; Thorseth et al., 2003; Tazaki, 2005) to animal (Swinbanks, 1981; Nooren et al., 1995; Needham et al., 2006) and plant (Bormann et al., 1998) mediated processes. Most of the work is centred on microbial activity as it should be the

most influential given its large mass (C in prokaryotes is estimated to be 60 100% of total C in plants; Whitman et al., 1998) of which virtually all is in contact with mineral surfaces (97% of prokaryote cells live in soil and the subsurface; Whitman et al., 1998). It is estimated that 70 80% of the alteration of submarine volcanic glass in the upper 250 m of the oceanic crust is due to microbial activity (Staudigel et al., 2008). Additionally, microbial experiments are easier to perform than those with superior animals and plants. Microbial effect on clay formation has been studied in the frame of specific single water chemistry environments (e.g. Hama et al., 2001; Thorseth et al., 2003; Tazaki, 2005). The studies agree in observing an acceleration of mineral alteration although clay formation is not always apparent (Thorseth et al., 1995a, b; Staudigel et al., 1995). Where clay formation is observed, it may occur by alteration of the mineral in direct contact with the microorganisms (Alt & Mata, 2000; Barker & Banfield, 1996) or by precipitation of the clay from solution (Tazaki, 2005; Ueshima & Tazaki, 2001) and from the cation-enriched medium within the biofilms (Sanchez-Navas et al., 1998). Clay formation may be induced by the metabolic activity of the microorganisms (Tazaki, 2005) or that of the chemical interaction of biological secretions and rock (Barker & Banfield, 1996; Ueshima & Tazaki, 2001). The clays produced are typically of very low crystallinity (Sanchez-Navas et al., 1998; Konhauser & Urrutia, 1999) and their composition follows broadly the chemistry of the solution and/or original minerals, although sometimes the neoformed clays have a range of compositions (Alt & Mata, 2000). The present contribution is a study of the effect of four different types of waters with their microbial content on the formation of clay from volcanic glass as a reactive silicate rock. These experiments are complemented by a similar set originally designed only to test the ability of microbiota to survive long experiments of glass alteration. Some of the results have been published before (Cuadros et al., 2012, 2013) but are included here to provide a complete, stand-alone description of the present investigation. The emphasis of this contribution is on morphological features that show the type of interaction of microbes and their secretions with the original glass and the mineral reaction products, as well as the mechanisms of glass alteration.

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EXPERIMENTAL Two different sets of reactions were carried out. The first set (short-term experiments) was a group of tests where water chemistry was analysed at several stages, and where the maximum duration was 18 months. These experiments were designed to study glass alteration and there is a thorough characterization of original materials and products. The second set (6-year experiments) was designed with the sole goal to determine whether or not the microbiota could survive long batch experiments consisting of water, volcanic glass and organic nutrients. As they were not intended as alteration experiments as such, the original waters are poorly characterized and there is no information from any intermediate stage of the reactions. The corresponding final products were studied because of the interest of an experiment of such (originally unintended) length.

Short-term experiments Rhyolitic obsidian with significant Fe and Mg contents was chosen in order to have a more complete range of inorganic nutrients. Two obsidian samples, from Lipari and Milos (collections of the Department of Geology and Palaeontology, and of the Museum of Mineralogy, Faculty of Geology and Geography, both at the University of Sofia) were mixed in order to obtain the amount required for the experiments. Their mass ratio in grams is Lipari/Milos = 48.5/14.5. They were ground together and homogenized, taking care to produce a progressively decreasing grain size and avoid very

small particles. The reason for this was to separate the clay products more easily from the original glass particles after reaction. The final particle size, as determined by dry-sieving, was 150 250 mm. The glass mixture was chemically analysed (Table 1) after acid attack with HF-HClO4-aqua regia in closed bottles in a microwave oven (Thompson & Walsh, 2003), using inductively coupled plasma-atomic emission spectrometry (ICP-AES, in a Varian Vista PRO). Analytical errors were 0.4 5 % of the measured values. The detection limits ranged from 5 ppm for Sr to 0.05 wt.% for CaO. Four types of natural water were used: spring water (Compton-Abdale, Cheltenham, UK), seawater (Brighton Marina, UK), freshwater (West Reservoir, London, UK) and hypersaline water (Las Saladas de Chiprana, Spain). They are representative of different types of surface environments where clay forms, with respect to both water chemistry and type of microbiota. Water in the lakes and in the sea was collected from rocky shores. Care was taken that the water was clean from sediment to avoid contamination of the experiment with pre-existing clay. Also, the water was left to sediment any unseen mineral content for a few hours before it was filtered (8 mm pore size) and transferred to the experiments. The time from water collection to transfer to the experiments was below 48 h to avoid the original microbial population dying out. The experiments were performed with and without biological activity, termed biological and inorganic, respectively. One gram of the volcanic glass was placed in Nalgene sterile bottles and

TABLE 1. Composition of the obsidian rhyolitic glass used for the experiments. All values are in wt.% except S. The mixture in the short-term experiments corresponds to a mass ratio Lipari/Milos of 3.34. Obsidian

SiO2

TiO2

Al2O3

Fe2O3

MnO

MgO

CaO

Na2O

K2O

Sum

S (ppm)

Short-term experiments Lipari 76.9 Milos 78.6 Mixture 73.2

0.252 0.166 0.252

11.9 12.8 12.9

3.63 1.10 3.28

0.106 0.062 0.100

0.111 0.213 0.130

1.67 1.31 1.99

4.28 3.90 4.43

2.83 3.56 3.24

101.70 101.80 99.50

n.d. n.d. n.d.

0.07

12.0

2.22

0.06

0.04

0.69

3.99

4.77

100.54

6-year experiment 76.7 n.d.: not determined.

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250 ml of natural water were added after filtration. The inorganic experiments were then closed and kept in the dark. Although the waters were not sterilized, the lack of light and organic nutrients were sufficient to avoid any apparent (visually observable) development of life in the inorganic experiments. Organic nutrients, 1 3 mg of glucose and peptone, were added to the biological experiments every 2 weeks. The caps of the bottles were placed but not tightened, to allow gas diffusion. For the first few weeks, the caps were removed daily for a few hours, to allow microbial contamination and encourage biological activity. The bottles were illuminated 12 h each day with artificial greenhouse white light. Both biological and inorganic experiments were carried out in the same room. The temperature in the room varied from 18.0 to 28.8ºC with an average of ~22ºC, and most time remaining in the 20 24ºC range. Experiments were set for 6, 10, 14 and 18 months, of which the l8-month tests, biological and inorganic, were carried out in triplicate.

the short-term experiments and thus experienced identical conditions (then inorganic experiments were in the dark). During the first 54 months, the temperature of the 6-year experiments was not recorded but was similar to that of the short-term experiments, probably with a cooler average of ~20ºC. The caps of the 6-year biological experiments were removed as with the short-term tests, and there was no stirring of the solutions. No replicates were carried out.

Water analyses The water of the short-term experiments, both biological and inorganic, was analysed before and at the end of the experiments lasting 6, 10, 14 and 18 months, and the waters from the 6-year experiments at the end of the experiments only. The analyses included pH (0.02 pH units uncertainty) and a complete suite of cations (ICPAES, using a Varian VISTA PRO). Detection limits

6-year experiments A rhyolitic obsidian from the Lipari Islands (London Natural History Museum collection) containing only volcanic glass as shown by X-ray diffraction analysis (XRD, Fig. 1) was used. The major element components of the glass were analysed after fusion with LiBO2 flux, dissolution in dilute HNO3 and then using ICP-AES as for the glass in the short-term experiments. The sulfur content was determined using the dissolution and analysis method used for the glass in the short-term experiments (Table 1). A piece of the obsidian was broken into chips of 2 8 mm size. The chips were placed in four different types of water (volumes 130 925 mL, in non-sterile HDPE and polypropylene bottles): seawater (off the coast of Cornwall), spring water (unknown location in Scotland), mineral drinking water (Evian brand) and brine water (Wieliczka salt mine, Poland). The waters were not chemically analysed before the experiments. Tests were carried out with and without organic nutrients (peptone and glucose; 1 3 mg added every 15 days, as above). The experiments were conducted for 6 years. During the first 54 months, both inorganic and biological experiments were exposed to the natural cycles of sunlight and dark in the laboratory. For the last 18 months, they were placed in the same room with

FIG. 1. XRD powder analysis of the original obsidian (Cu-Ka radiation) and products (Co-Ka radiation) after reaction with the mineral water in the 6-year experiments. The products were the same and the X-ray patterns very similar for the four types of water. Q: quartz, A: alunite, C: calcite. Modified from Cuadros et al. (2012).


were 0.002 0.1 mg/L depending on the cation, and uncertainty 0.2 20% of the measured value, depending on element concentration. Anions were measured in the original water of the short-term experiments only, using ion chromatography (Dionex DX300). Detection limits in the spring and freshwater lake were 0.02 0.17 mg/L, depending on the anion, and for sea and hypersaline water 1 8.5 mg/L. Uncertainty was 6% of the measured value for fluoride and 2% for the other anions. In all cases the water was double filtered (8 mm pore size).

Analysis of biofilms for cation adsorption The biofilms of the short-term experiments were sampled at the end of the 6, 10 and 14 month experiments to investigate any possible cation adsorption or accumulation. After sampling of the water for analysis, the biofilms were broken up with a spatula and pieces of the biofilms dispersed in the water were collected with a micropipette and deposited on Al holders covered with C-coated adhesive tape. Care was taken to avoid volcanic glass grains in this operation. Then the deposited biological material was washed three times placing a few drops of distilled water and removing it by suction by capillarity after a few minutes. This washing was carried out to remove crystallized salts after water evaporation. The entire process was repeated multiple times to accumulate a sufficient and representative sample of the biological mat. Finally, the preparation was left to dry, C-coated and investigated using an SEM Leo 1455VP microscope, in back-scattered electron mode, equipped with an Oxford energy dispersive X-ray analyser (EDX).

X-ray diffraction The original obsidian samples of both short- and long-term experiments were analysed with XRD to ensure that they only contained glass. The glass was finely ground with pestle and mortar and analysed as powder in the range 2 60 or 2 80º 2y, using a Philips PW1710 at 54 kV, 40 mA, and graphite monochromator, with Cu-Ka radiation. No crystalline phases were observed (Fig. 1, for the glass in the 6-year experiments). After the reactions, XRD analysis was intended to investigate the neoformed clay. It was assumed that clay would be concentrated in the fine-grain fraction. Thus, glass grains,

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part of the biological mat (in the case of biological experiments) and a fine sediment that was found at the end of the 6-year experiments were transferred to a plastic vial and sonicated with an ultrasound probe at 60 watts for 1 1.5 minutes, with the intention of detaching clay particles from larger mineral grains or biofilm tissue. After stirring, 2 ml of the fine suspension were pipetted onto glass slides and allowed to dry. The oriented mounts were analysed from 2 to 15º 2y in search of 001 clay peaks, with the equipment indicated above. During the course of the study, TEM analysis indicated that the 6-year experiments had produced quartz, which fact prompted the analysis of the corresponding solids as a whole, and not only the fine fraction. For this study, a large part of the mineral chips from the 6-year experiments and of the fine sediment, generated during the tests, were ground together and analysed as powders in the range 2 80º 2y, with the same apparatus as above and Co-Ka radiation (rather than Cu).

Cryo-SEM One of each of the biological 18-month experiments and the biological 6-year experiments were analysed using a cryo-SEM system to investigate the relation between the biofilms, glass, and neoformed minerals. Pieces of the biofilm containing glass grains (18-month experiments) or grains with adhered biofilm (6-year experiments) were sampled and placed in a watersaturated atmosphere until analysis. Immediately previous to analysis, they were frozen in sub-cooled liquid N2, fractured to allow observation of mineralbiofilm contact, etched at 70ºC for ~5 min (sublimation of surface ice to allow observation of the sample surface), and Au sputter-coated. All operations after freezing were carried out in an Oxford Cryotrans CT-1500 unit, attached to the microscope (Zeiss 960). Samples were viewed using both back-scattered and secondary electrons, and chemically analysed using an Oxford Link Isis EDX detector.

TEM-AEM One of each of the 18-month samples and 6-year samples, from biological and inorganic experiments, were analysed using TEM-AEM to chemically characterize the type of clay formed. Some glass grains and part of the biological mat, in the case of

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the biological experiments, were transferred to a wide plastic vial. In the case of the 6-year experiments, part of the fine-grained material found as a sediment at the end of the tests and one or two mm-sized grains were transferred, together with part of the biological mat. These samples were completely dry. The biological mat of the biological experiments was gently broken using a spatula. Ethanol of reagent grade was added and the suspension was sonicated with an ultrasound probe at 60 watts for 30 s. Immediately before sampling, the vials were shaken, the suspension allowed settle for a few minutes, a few drops from the upper part of the suspension were sampled and deposited on a Cu microgrid with a Formvar film stabilized with C. The study was carried out in a Jeol 2010 TEM apparatus at 200 kV. Chemical analysis (AEM) was carried out using an X-Max 80 mm2 Oxford Instruments detector with Inca software, with acquisition live time of 60 s. No short-time analysis was carried out to prevent partial loss of alkaline elements. Quantitative optimization was performed before the analyses with the Cu microgrid.

RESULTS Visual observation of biomat development The short-term (6 18 months) biological experiments developed evident microbial biofilms in a matter of days. Except in the seawater experiments, the glass grains were enveloped in a thick mat that encapsulated them completely. After these experiments, the mat had to be broken with a spatula to release the glass grains. This microbial mat was thickest in the hypersaline water. In seawater, the grains were coated with a loose microbial mat that did not retain the grains inside. Effects of the biological activity could be observed in the formation of large gas bubbles growing from the microbial mat at the bottom of the bottles. The glass grains were not observed to change in colour or appearance. The identification of the microorganisms in the short-term experiments is provided by Cuadros et al. (2013). No identification was carried out for the 6-year experiments. In the 6-year biological experiments, substantial microbial colonies developed, within days for the spring water, weeks for sea and mineral water and after 2 years in the case of brine water. Presumably, the mineral water was free from microorganisms

originally as it was intended for human consumption. There was no encapsulation of the glass grains by the biological colonies in any of the 6-year experiments. The colonies appeared to develop without physical contact with the mineral grains. However, care was taken that the colonies made contact with the grains to facilitate biologicallymediated alteration. The spring water developed a very large microbial mat that occupied most of the water volume. Thus, although the mat did not entrap the glass grains, it completely surrounded them. The slow development of visible biological colonies in the brine experiment is attributed to a low microbial content in the original water and the aggressive conditions generated by the high NaCl concentration. It is remarkable, however, that the development took place. At the time in which the blooming was observed, the 18-month hypersaline experiments had not started, and no contamination from them could take place. The species that developed in the 6-year brine experiment were either (1) originally present, and it is unclear why it took so long for them to develop, (2) caused by contamination from the other experiments, likely from the seawater, or (3) by contamination from the atmosphere. In the two latter cases, the adaptability of species from other environments to develop in the harsh conditions of the NaCl brine would be remarkable. The glass grains in the 6-year experiments changed colour gradually, from blackdark brown to grey and white-gray. The grains underwent no other apparent change and preserved their size. At the end of the experiments, a white to colourless fine-grained sediment of inorganic appearance was found (Cuadros et al., 2012).

Water chemistry The cation concentrations in the solutions of the short-term experiments (6 18 months) have been described in Cuadros et al. (2012). They were typically similar (Fig. 2) for biological and control experiments at all reaction times. Some variations existed between biological and inorganic experiments (e.g. Ca in spring water tests, Fig. 2) and between the replicas of the 18-month biological experiments (e.g. Na in spring water tests, Fig. 2). Calcium displayed a significant decrease in all the biological spring water experiments. These variations indicate modifications produced by the biological activity and differences between the specific biological colonies present in each

FIG. 2. Cation concentrations and pH of reacting waters. The short-term experiments (four top rows) include results for the initial concentrations (full triangle at time zero) and after 6, 10, 14 and 18 months of reaction. For 18 months, the experiments were replicated 3 times. For the 6-year experiments (bottom row) there are results only at the end of the experiments, except for the mineral water, for which data from the label are included (open triangles). Some of the 6-year plots are divided in two, where the axis for each group of data is at the corresponding side of the plot. Modified from Cuadros et al. (2012, 2013).


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experiment. The most defined chemical trend was the exponential increase of dissolved Si with time, except for seawater. Other cations displayed cycles of decreasing and then increasing concentrations. The original pH of the water from the freshwater lake was the highest, at 9. The pH values during the experiments were slightly higher (up to 0.6 units) in the biological tests. Iron and Al concentrations were typically below their detection limit, 0.01 0.5 mg/L for Fe, and 0.04 1.0 mg/L for Al, depending on the dilution used in the analysis due to water salinity. Measured Fe contents were 0.016 mg/L (original lake fresh water) and 6.9-9.3 mg/L (biological and inorganic 18-month experiments with hypersaline water). The few Al concentrations measured were in the range 0.041 0.063 mg/L. In the 6-year experiments, there is only one measurement for each test, at the end of the experiment. The values labelled as ‘‘original’’ in the mineral water (open triangles, Fig. 2) were taken from the bottle label. The composition of the waters with and without microbial activity was similar except for some variation in Na and Ca in the mineral and spring water (Fig. 2). The pH of all solutions at the end of the tests was > 7 except in the biological spring water test, where it was 3.5 (Fig. 2). This is the experiment that produced the largest biological development and where the bio-mat occupied the greater portion of the water volume. The low pH at the end of this experiment is thus assigned to the biological control. The comparison of cation concentrations and pH values of the short-term and 6-year experiments shows a good agreement between seawater tests and those of the spring water (short-term) and mineral and spring water (6 years). The specific K concentrations in the latter (the two freshwater samples of 6-year experiments) are in the range 0.61 1.94 mg/L. The calcium concentration in the 6-year mineral water experiment dropped significantly from the original value (taken from the bottle label), similar to the less accentuated drop observed in the short-term spring-water experiments for the biological tests. The silicon concentration increased 4 5 times over the 6-year mineral water experiment, and thus behaved in a similar way to the short-term experiments, where an exponential increase with time was measured.

XRD The XRD investigation of fine material as oriented mounts from the 18-month and 6-year

experiments displayed no clay peaks. The neoformed clay (see below) was below the XRD detection limit. However, the analysis of powders from the 6-year experiments in all waters and from biological and inorganic tests produced the surprising result that the glass had been transformed into quartz (Cuadros et al., 2012) with minor amounts of alunite and calcite with a varying Mg content (Fig. 1). No remaining glass was detectable (lack of elevated background between 20 and 40º 2y in Fig. 1). As these results were common to inorganic and biological experiments, the processes that produced them must be inorganic. The magnesium content in the calcite was assessed by the position of the peak at ~35º 2y. The assessed values approximately followed the Mg/Ca concentration in the waters measured after reaction (Appendix A in Cuadros et al., 2012).

Cation adsorption or precipitation on biological tissue One of the SEM-EDX studies (performed only for short-term experiments) focused on the search for possible precipitation of mineral phases or cation adsorption on the biological tissue. Care had been taken to avoid glass grains in the sampling of biological tissue and to wash the preparation to avoid salt precipitation; however, microscopic mineral grains (glass and reaction products) were present in most of the investigated areas, and salt grains were found in the seawater preparations and were ubiquitous in those from hypersaline water experiments. The presence of glass grains in this SEM study led to observations related to glass alteration which are included below. The investigated areas of biological tissue, seemingly clean from mineral grains and salts derived from evaporation, were either free of inorganic content or contained very small amounts of Ca and/or Mg, with and without S. Where S was not present, two possibilities exist, (1) the presence of carbonates or oxalates with varying Mg content, and (2) direct Ca and/or Mg adsorption on the biological tissue. If S was present, Ca-Mg sulfate was probably present. On some occasions the measured concentration of S was much higher than those of Ca or Mg, which was interpreted as due to biological tissue with S-containing proteins. The S-Ca-Mg combination consistent with Ca and Mg sulphates was abundant in seawater and very abundant in the hypersaline water preparations, in


agreement with the chemistry of these waters. This S-Ca-Mg phase formed a very fine extended film that could not be seen by SEM but was observed in the TEM analyses, most probably the result of precipitation during water evaporation. It is thus not possible to extract conclusions from the SEM study of sea and hypersaline water samples about cation adsorption or precipitation on biological tissue. However, the study of the freshwater samples indicated that Ca and Mg, especially the latter, were frequently present on the tissue, whether adsorbed on it or forming minute carbonate or oxalate crystals that could not be discerned at the conditions used for the analysis. If Ca and Mg were adsorbed as cations, the specificity of this selection over other cations such as Na and K suggests direct biological activity. However, if Ca and Mg were in a carbonate or oxalate phase, the presence of such a phase could be due either to bio-precipitation or to passive adsorption of a phase that formed abundantly in the water, with particle size and surface properties that facilitated adsorption on biological tissue.

Biological interaction with glass and glass alteration The cryo-SEM and EDX analysis showed a number of features that are relevant for the understanding of the biological and inorganic processes that caused glass alteration and how they took place, especially the differences between alteration in the 18-month and 6-year experiments (Fig. 3). The chemical EDX analyses were evaluated with respect to newly formed silicate phases considering the modifications from the composition of fresh glass surfaces. Analyses of these surfaces are included for comparison (Fig. 3). The chemical compositions were normalized to 100 Si atoms (i.e. Si abundance is 100 in all the spectra in Fig. 3). The glass in the 18-month experiments had typically compositions of Al 15 20, Fe 0 3 and Mg 0, with Al+Mg+Fe values of 15 20. Strong indication of the presence of clay or glass alteration towards clay was provided by values of Al+Mg+Fe > 20. Besides, clearly contrasting individual cation contents, including K and Na, with respect to analyses of fresh glass were also considered to indicate the presence of clay or glass alteration. With a few exceptions, the cryo-SEM study did not allow the establishment of whether the alteration processes were unique for the several types of waters, which is

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reasonable given the large variety of the microbial species in the experiments (Cuadros et al., 2013) and the resulting large number of chemical variables operating, many of which vary locally within a single experiment. The differences in the chemistry of the neoformed clay, between biological experiments and controls, and between the several water experiments, were made evident in the TEM study, described in a different section. An important element of the alteration process was encapsulation of the glass grains within the biofilm (Cuadros et al., 2013). As indicated above, this took place in all 18-month experiments except for seawater, where the biofilm was loose and did not confine the glass grains. No encapsulation took place in the 6-year experiments. The encapsulation is visualized in Fig. 3a, which shows a glass grain surrounded by the crystallized SO4-Cl-Mg-Na brine frozen in the cryo-stage. The fluids within the biofilms were probably more concentrated and of different composition from those in the bulk solution, due to the microbial activity (Aouad et al., 2006). Biological material including cells, EPS and fungal hyphae, were in close contact with the mineral grains in many cases (Fig. 3h l,z,aa). It is probable that the intimate contact between biological material and glass promoted dissolution of the glass in the contact points, as described previously for certain corrosion features in microbially colonized glass (Staudigel et al., 1995; Ullman et al., 1996; Brehm et al., 2005). Although such case could not be totally ascertained, some instances may indicate local glass dissolution resulting from the effect of the immediate contact with cells and diatoms (Fig. 3i,l,m). Thorseth et al. (1995a) showed that unaltered glass presents grooves and it is not clear whether similar features in our experiments (Fig. 3m) have a biological origin. Regarding the mode of glass alteration, it was observed in some cases that lines on the glass surface, produced presumably by fracture before the experiments (Thorseth et al., 1995a), developed into laths (Fig. 3b). The composition of these laths could not be determined because of their small size and the glass background. It is, however, plausible that they are clay particles because the lines in the glass probably represent stress areas that are more liable to alteration. Certainly the lath morphology is frequent in clays of smectitic composition (Gu¨ven, 1988). In the short-term experiments, in situ alteration of the glass was observed in some instances. In some of these cases the alteration was

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made evident by a darker contrast of the glass surface, as observed in back-scattered electron mode, due to the hydration of the glass (Fig. 3d). This specific type of alteration was detected only in

hypersaline water. The corresponding EDX analyses probably included unaltered glass under the altered spot, but they unmistakably indicated a transition towards Mg-rich clay. In other cases, the in situ

FIG. 3 (this and following three pages). SEM micrographs of the solid products of reaction of biological experiments, in both back-scattered and secondary electrons mode. All are from the cryo-SEM study except (d) and (e), that correspond to conventional SEM (see methods). Relative cation concentration values from EDX analyses are shown in some cases. The concentrations are normalized to Si = 100 in all cases. Mature clay should have Al+Mg+Fe550. (a) Hypersaline water, 18 months. Glass grain trapped in brine (frozen by the cryogenic treatment) within the biofilm. The round structure to the left of the glass grain is probably a cell. (b) Hypersaline, 18 months. Glass grain covered by biological film (darker contrast in the back-scattered electron image) on the left and top. The free glass surface shows lines probably caused by rupture in the sample preparation. The lines altered into detaching laths, that may be clay products. (c) Hypersaline, 18 months. Glass surface showing alteration that generates tightly-packed scales of enriched Mg composition (bottom figures), possibly towards formation of trioctahedral clay. The composition of the glass is shown for reference (top figures). Minute, isolated, scale-like structures are present on the pristine glass surface at the bottom of the picture, some of them at the end of a step, perhaps corresponding to in situ alteration to clay. (d) Hypersaline, 14 months. Surface of the glass showing dark contrast (back-scattered electron image) areas produced by alteration towards Mg-rich clay; the composition of the glass is also shown. (e) Hypersaline, 14 months. Clay formed within a sheath of Ca-rich composition, probably calcite. The EDX results are an average from 4 spots and indicate a dioctahedral smectite with significant Mg content. (f) Seawater, 18 months. Inorganic and biological debris. The analysed grains consist of a Ca-rich and a silicate phase. The interpretation of the EDX data assumes that the Mg is mainly located in the silicate phase, and this would indicate alteration towards Mg-rich clay.


alteration produced a roughening of the surface of the particles and compositions that approached that of clay. The most frequent composition was indicative of transformation towards dioctahedral smectite (Fig. 3c,p) but there were also cases of a trend towards trioctahedral composition (Fig. 3p). More frequent, both in the 18-month and 6-year experiments, were groups of silicate grains that may have been produced by precipitation from solution or by accumulation of grains detached from the glass

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during the alteration. Frequently, these silicate grains were mixed with other phases, the most common of which were a Ca-rich phase interpreted as calcite, in the 18-months experiments, and alunite in the 6-year experiments. Silicate grains of such characteristics in the 18-month experiments had glass composition (Fig. 3o) or components of smectite-like dioctahedral (Fig. 3n) and intermediate Al-Mg composition (Fig. 3f), and they had a kaolinitic (sometimes beidellitic) composition in the 6-year experiments

FIG. 3 (contd.). (g) Seawater, 18 months. Diatoms and biological debris on the glass surface. Scale-like structures develop apparently on the glass surface. (h) Seawater, 18 months. Glass grain partially covered by biofilm with cells and EPS. Small scale-like structures occur on the exposed glass surface. (i) Seawater, 18 months. Glass grain with signs of corrosion, partially covered by diatoms and, perhaps, EPS. There are scale-like structures on the surface. Some of the diatoms (arrow) appear to penetrate the surface of the glass (biologically enhanced glass dissolution?). (j) Freshwater lake, 18 months. Remains of biofilm, showing EPS and tubular structures that probably correspond to fungal hyphae, attached to a glass grain. (k) Freshwater lake, 18 months. Fungal hyphae on glass. Arrows show points of evident attachment to the glass. (l) Freshwater lake, 18 months. Glass grain in contact with biofilm. Numerous mineral grains have precipitated in the contact. The glass appears to be corroded (arrows). (m) Freshwater lake, 18 months. Grooves on the surface of the glass that may have been produced by microorganisms. EPS fibres are also evident. (n) Spring water, 18 months. Lump of grains, held together by EPS, of a Ca-rich (possibly calcite; not shown in the analysis) and a silicate phase, possibly altering glass, as indicated by the Fe and K contents, which are higher than in the glass (bottom figures). (o) Silicate grains of glass composition sandwiched within biofilm fragments.

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(Fig. 3t v,y). One special case was the observation of mature montmorillonite apparently precipitated within a sheath of a Ca-rich phase, probably calcium carbonate (Fig. 3e). One other image showed possible evidence for the precipitation of dioctahedral clay and quartz (quartz is assumed because of the ubiquitous presence of this mineral in the products of the experiment) on the surface of a cell (Fig. 3aa) in one of the 6-year experiments. Alteration was also observed in large areas of the surface of the glass in the 18-month experiments,

generating minute scale-like structures (Fig. 3c,g i). The composition of these structures could not be established because of their small size and location on the glass surface. Similar structures were observed by de la Fuente et al. (2000) and Fiore et al. (2001) in hydrothermal alteration of rhyolitic volcanic glass and were interpreted as in situ transformation of the glass into clay. Here, they are also interpreted to represent such transformation. In the 18-month experiments, most of the glass was not altered, whereas in the 6-year experiments

FIG. 3 (contd.). (p) Springwater, 18 months. In situ alteration of glass generating a different surface morphology and chemical composition of varying Al-Mg-Fe contents. The glass analysis (bottom figures), shown for reference, is from a nearby area. (q) Brine, 6 years. Biological tissue with shrunken cells (algae or cyanobacteria) covering a mass of mineral grains. (r) Brine, 6 years. Ca-rich phase (probably calcite) deposited on a tubular biological structure. (s) Brine, 6 years. Quartz on the surface of the transformed obsidian. The grains are welded and appear to be corroded. (t) Alunite grain (chemistry not shown) surrounded by quartz. Kaolinite or beidellite (analysis corrected for alunite in the background) have formed on the alunite grain. Quartz contains Al traces. (u) Seawater, 6 years. Internal side of a fracture (produced during the cryo-SEM analysis) of a quartz grain (originally obsidian), showing large conduits within the grains. The film covering grains and generating a 3-dimensional structure is most probably NaCl, that typically takes this morphology in cryo-SEM conditions. Alunite, halite and clay were detected. After subtraction of the spectra of halite and alunite, the clay appears to be kaolinite, probably mixed with other Mg-containing clay.


it was thoroughly transformed into quartz. SEM micrographs show the morphology of the neoformed quartz. The external surface of the millimetre-size chips appeared as an accumulation of quartz grains of very different size (Fig. 3x), some of them with apparent dissolution features (Fig. 3s). However, the fracture of the millimetresize grains in the cryo-SEM analysis revealed a uniform mass terminating into facets and riddled by numerous channels (Fig. 3u,w,x). The channels were in fact coincident with the faces of individual quartz grains (Fig. 3w,x). Alunite grains were

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frequent, with a variety of size and morphology, sometimes finely mixed with kaolinite (Fig. 3t v,y). A Ca-rich phase, interpreted as calcite, was observed in some occasions in the 6-year experiments, connected with biological tissue (Fig. 3r). X-ray diffraction analysis indicates that calcite precipitated inorganically because it is present in both biological and inorganic experiments in similar amounts. Thus, the biological tissue may have retained precipitating calcite or facilitated the precipitation process, rather than being the cause of precipitation.

FIG. 3 (contd.). (v) Seawater, 6 years. Detail of a cavity in quartz with NaCl of foam-like morphology and neoformed alunite and kaolinite or beidellite (analysis after alunite subtraction). (w) Spring water, 6 years. Internal side of a fractured quartz grain. (x) Spring water, 6 years. Fractured quartz grain showing the internal surface of the fracture (top) and external surface of the grain (bottom). This image with (u) and (w) above show that the original mm-size obsidian grains remained a single mass at which surface the quartz crystal facets developed. (y) Spring water, 6 years. Granulated kaolinite or beidellite and alunite deposited on quartz crystals. The bottom values are the direct result of the analysis; the top values were corrected for alunite. (z) Spring water, 6 years. Biofilm with cells engulfing and penetrating between mineral grains. (aa) Spring water, 6 years. Biofilm with cells engulfing mineral grains, mainly quartz. One cell (chemical analysis) shows a few crystals attached to it, interpreted to consist of a mixture of quartz and dioctahedral clay. Micrographs (a) and (c) modified from Cuadros et al. (2013). Micrograph (x) from Cuadros et al. (2012).

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TEM-AEM The TEM study was mainly intended to obtain AEM chemical data of the composition of the clay particles, and thus was conducted on small and transparent grains, where the approximately monomineralic nature of the grain could be ascertained, or on the edges of larger, opaque particles, in search of glass-to-clay alteration on grain rims. The latter type of analyses revealed some interesting features (Cuadros et al., 2013). The surface of altered glass grains typically had a beidellitic composition and high K content, whereas small, detached particles had a greater and more varied Mg content and lower K content (Fig. 4b). In some cases, the original glass grains, still preserving their morphology and sharp edges, had been completely altered to clay, also with a beidellitic composition (Fig. 4c). Some particles in the hypersaline water experiments had typical smectite morphology and contrast, and variable chemistry depending on the analysed spot, especially in their Mg content (Fig. 4a). This variable chemistry may be due to a number of reasons, such as advanced but heterogeneous alteration of glass, aggregation of clay particles of different composition, co-precipitation of several clay phases, etc. Apart from phases related to the glass and its alteration products, TEM-AEM analysis of particles in the 18-month experiments detected numerous Carich crystals, assumed to correspond mainly to carbonates, as observed in infra-red data (Cuadros et al., 2012). In addition, the hypersaline water experiments contained numerous particles rich in Mg, Na, S and Cl, the result of precipitated sulfate and chloride phases. The seawater experiments also had significant, but much less abundant, NaCl crystals and, still less abundant, S-rich crystals. The 6-year experiments were entirely dominated by quartz grains (they produced sharp electron diffraction patterns), and had numerous alunite and calcite particles. Grains containing Si and other cations, such as would correspond to clays or altered glass, were more difficult to find than in the 18-month experiments and fewer of such analyses could be recorded.

Chemistry of the neoformed clay The composition of silicate grains (containing Si and other cations) were plotted using atomic ratios that indicate the nature of the phases (Fig. 5), as in

Cuadros et al. (2013). It was assumed that the most likely clay to form in the experiments is smectite, of dioctahedral or trioctahedral nature. Considering the structural formula based on O10(OH)2, Si in

FIG. 4. Selected TEM micrographs of clay particles with the corresponding AEM values as atomic percent and normalized to Si = 100. The sum of Fe, Al and Mg should be greater than 50 for mature clay. (a) Hypersaline lake water, 18 months, biological experiment. The edge of the particle has the typical smectitic morphology and a high Mg content, somewhat in excess of a purely trioctahedral phase, for which reason some other Mg phase is suspected; the interior of the particle showed variable Al and Mg composition in different spots (only one spot shown). (b) Seawater, 18 months, biological experiment. Glass grain altered in one point of the surface to a smectite of beidellitic composition, and small clay particles of montmorillonitic composition with different Mg and Al contents. Beidellitic smectite generated from direct glass alteration had high K contents. (c) Spring water, 18 months inorganic experiment. Glass grain that has been thoroughly altered to beidellite and preserves the original morphology. Micrograph (c) modified from Cuadros et al. (2013).


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FIG. 5. Plots of cation ratios from AEM values in silicate particles, for 18-month (left) and 6-year experiments. The Si/(Al+Fe+Mg) ratio provides the approximate limits for smectite composition. The Al/Si ratio provides an approximate characterization of the smectite in terms of Al vs. Mg+Fe content. The composition fields are marked on the plot and correspond, from left to right, to nontronite and saponite, montmorillonite, montmorillonite + beidellite and beidellite, with kaolinite at the right end of the beidellite field. The composition of the original glass is marked with a circle. Data points above the smectite field and below the original glass are altered glass, presumably in a process of transformation into clay. Data points above the original glass correspond to cation-depleted glass. Values below the smectite field indicate contamination with other Al, Mg or Fe phases. Modified from Cuadros et al. (2013).

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smectite may be considered to range between 3 and 4 atoms. The sum of Al + Mg + Fe, which are mainly octahedral cations, should be roughly 2 3 atoms. However, this range is widened by the possibility of tetrahedral substitution (mainly Al for Si) to 2 4. Thus, smectite particles will be approximately within the chemical range 0.754Si/(Al+Fe+Mg)42. The Al/Si ratio allows a rough characterization of the smectitic phase from nontronite or saponite, with low Al, to montmorillonite and beidellite, with increasing Al. For the case Al/Si=1 and Si/(Al+Mg+Fe)=1, the clay is kaolinite. In all experiments, there is a main line of change of composition of silicate particles observed, indicating two opposed trends (Fig. 5). The first trend is the formation of particles increasingly enriched in Fe, Mg and, mainly, Al, that results in the formation of dioctahedral clay. The second trend is the loss of Al, Mg and Fe (also Na, K and Ca, although not shown in Fig. 5), resulting in cation-depleted glass in the short-term experiments and quartz in the 6-year experiments. In the latter, these quartz particles were very abundant and typically contained Al traces, whether retained from the original glass or present in some other neoformed phase. Separate from the above two trends of chemical change is the formation of low-Al clay (Al/Si < 0.3; Fig. 5), which is Mg-rich as shown below. The data points in the nontronite/saponite field and those approaching it from the original glass composition (Fig. 5) do not show a specific reaction path, as was the case with the Al-rich clays. Rather these data points are distributed as a cloud. They are most numerous among the 18-month hypersaline and freshwater lake experiments. All the other experiments exhibit a great majority of Al-rich clay particles. Thus, the picture that emerges from these results is that Al-rich smectite is the favoured clay alteration phase in the experiments. Magnesiumrich clay (see below) is abundant in some experiments. Kaolinite could be present according to Fig. 5 but further analysis of AEM results (see below) indicate that none of them corresponded to this mineral. Some of the SEM-EDX results from some 6-year experiments also appeared to correspond to kaolinite (Fig. 3v,y) but beidellite may not be ruled out. The clay nature of the particles within the clay fields in Fig. 5 was corroborated by calculating structural formulas (not shown) that matched well those of smectite (for 18-month experiments, see

Cuadros et al., 2013). Besides, particle morphology was typical of clay, with thin, flaky, irregular shapes; although, in some cases, the particles were not thin, as in the glass grains thoroughly altered described above (Fig. 4c). Electron diffraction patterns (SAED, not shown) of a few of these particles displayed the typical hexagonal patterns of clay minerals viewed down the c axis, with variable streaking, or weak diffraction rings (Cuadros et al., 2013). The analyses with good clay formulas were further investigated by plotting Mg/Si vs. (Al+Fe)/ Si ratios (Fig. 6), which allow the delimitation of the dioctahedral and trioctahedral compositional fields in smectites. Magnesium, Fe and Al are all octahedral in Fig. 6. No interlayer Mg was present in the formulas. Tetrahedral Al was the cause that some analyses in Fig. 5 appeared to correspond to kaolinite. The variable Si content in the smectite particles introduces a slight uncertainty in the exact di- or trioctahedral character of the data points in or near the Di/Tri field in Fig. 6. Most experiments produced Al-rich clay, with the exceptions of the freshwater lake inorganic and hypersaline water experiments. There is a wider Mg range in the composition of the dioctahedral clay in the 18month experiments than in the 6-year experiments. In the latter, most of them have very low Mg. As indicated above, all Al-poor analyses correspond to Mg-rich, trioctahedral phases. Thus, apparently, no nontronitic phases formed. Of all the data, only a handful of them had Fe > Al, of which only one was within the dioctahedral field. The data points within the Di/Tri field may correspond to truly intermediate phases (Deocampo et al., 2009) or particle aggregates.

DISCUSSION Clay formation and biological influence The SEM-EDX and TEM-AEM analyses show compelling evidence of a mechanism of in situ alteration of the volcanic glass to clay, with concomitant changes in surface morphology and composition (Fig. 3c,d,p; Fig. 4). This type of alteration produced more typically Al-rich, dioctahedral clay, suggesting chemical control from the glass, which is much richer in Al than Mg (Table 1). Very small grains ( 1.7, whereas the product was saponite and occasional montmorillonite for ratios < 1.7. The Al2O3/MgO ratios in our experiments were well above 1.7. Alt & Mata (2000) found that alteration of basaltic glass with Al2O3/MgO = 1.74 2% wt. ratio produced consecutive layers of pristine glass, altered glass and Al-rich clay, mainly smectite, where the latter is interpreted to be the result of in situ glass alteration given the textural continuity with it. In void spaces on the surface of the grains and in the centre of fractures they found smectite with larger Mg content, interpreted as precipitated from solution. It is interesting to note that the glass controlled the composition of the clay formed in situ in the study of Alt & Mata (2000) even though the calculated seawater/rock mass ratio that produced the alteration was 43 or above. Thus rock control can take place even in large fluid/rock alteration regimes (de la Fuente et al., 2002). Considering that the 18-month freshwater lake experiment produced Mg-rich clay, it can be questioned why the mineral water in the 6-year experiments produced only little trioctahedral clay (Fig. 6) in spite of the high Mg concentrations in solution and high final pH values (Fig. 2). If the pH data in the label (on the bottle) of the mineral water are to be trusted, the reason is most probably that the original neutral pH (Fig. 2) created conditions far from the stability field of saponite, and well within the montmorillonite field. As there are no data for intermediate stages, it is not possible to assess how the chemical conditions in the water evolved. It is


also the case that Mg content in the volcanic glass is about 3.3 times higher in the short-term experiments than in the 6-year ones (Table 1), which facilitates the formation of Mg-rich clay in the short-term experiments over the 6-year experiments. It was more difficult to find particles with clay composition in the 6-year experiments than in those of 18 months, as can be observed by the lower number of data points in the AEM chemical data (Figs 5 and 6). There can be several reasons for this fact. One of them could be that alunite in the 6-year experiment products sequestered a certain proportion of Al and hindered clay formation. The proportion of alunite in the final products was assessed from the relative intensity of the X-ray peaks to range between 10 and 5 wt.% (Cuadros et al., 2012), which would take 4 2 wt.% of Al2O3, from the total 11.9 % in the glass. Another possibility is simply that, in the 6-year experiments, there is a larger proportion of non-clay (quartz, alunite and calcite) particles of fine size than in the short-term experiments. This fact made clay less concentrated in the fine-size portion that was collected for TEM-AEM analysis and statistically more difficult to find.

Different trends of glass alteration between the short-term and 6-year experiments Surprisingly, the 6-year experiments produced the thorough alteration of the glass to quartz and minor alunite and calcite, which has been addressed elsewhere (Cuadros et al., 2012). These changes took place in both inorganic and biological experiments, and thus they were driven by inorganic processes. Such a transformation is surprising because alteration of volcanic glass typically produces clay with a number of other minor phases and because quartz precipitation is extremely slow at low temperatures (Cuadros et al., 2012). The short-term experiments produced the more habitual results of intact or surface-altered glass, with alteration compositions typical of clay or indicating transformation towards clay (Figs 3 6). Typically, the glass would become hydrated and the external surface and surface within cracks would develop a layered structure with clay and possibly other mineral phases (Thomassin et al., 1989; Magonthier et al., 1992; Verney-Carron et al., 2008; Staudigel et al., 2008). Alternatively, depending on conditions, glass hydration and dissolution are the only processes that take place

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and no deposition of alteration products occurs (Mazer et al., 1992; Thorseth et al., 1995a; Staudigel et al, 1995). In the 6-year experiment, however, the mass of glass was transformed into quartz. This fact indicates that glass alteration is affected by not yet recognized variables that can alter dramatically the reaction products (Cuadros et al., 2012). The chemical composition of the glass in both experiments is quite similar (Table 1). If the different behaviour between the two sets of experiments were due to differences in the glass chemistry (e.g. Ti, Mg, K; Table 1), the implication would be that small chemical differences can result in completely different ways of alteration and alteration products. For the 6-year experiments, Cuadros et al. (2012) proposed a mechanism of glass-quartz replacement via in situ glass transformation that starts on the surface and propagates within the grains, through the solid and channels generated by the reaction (although some of these channels may have been present originally). There is evidence of the existence of these channels, as SEM photographs show that the quartz is composed of numerous ‘‘welded’’ crystals with interstitial space between them (Fig. 3s,u,w,x). There are also much larger channels within the quartz (Fig. 3u). The creation of channels during the reaction is also deduced from the mass loss in the transformation from glass to quartz and the preserved volume of the altering grains during the entire reaction. Some quartz grains showed apparent dissolution features (Fig. 3s). However, in the light of the mechanism described above, these features may not be the result of dissolution but of quartz growth from direct transformation of the glass, forming morphological types with similarities to skeletal crystals. In most cases this mechanism produced grains with void space between them, but in other occasions it may have produced void space within the grains. Such could be the case where quartz growth occurred in two dimensions rather than three. In this case, the grains may be the result of merging of quartz layers propagating from different points and in different directions. Such growth may generate void space within the grains. Thus, the void space generated by mass loss in the transformation was also created within grains. The in situ glass-to-quartz transformation suggests a template control on the crystallization of quartz, where multiple points of quartz growth are active at the same time. Generally the points of quartz growth resulted in complete grains separated from

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each other. Sometimes, however, several crystallisation points produced one single grain that was not completed and had void space within it. The formation of alunite in the 6-year experiments is also surprising in that the formation of this mineral typically requires relatively acid conditions, whereas our experiments were under neutral to alkaline pH (Fig. 2), except for the biological experiment with spring water. This fact is another indication that mineral formation was controlled by local conditions at the mineral-solution interface, rather than the bulk conditions. Cuadros et al. (2012) proposed that the local acidic conditions necessary for alunite formation were the result of proton-for-Na exchange within the hydrated glass at an early alteration stage. As Na is exchanged by proton ions earlier than K during glass alteration (Cerling et al., 1985), the conditions of K, Al and proton concentrations in the reacting glass were sufficient to trigger alunite precipitation, overcoming the low S concentration in the glass (Table 1) and solutions (Cuadros et al., 2012). It remains an open question whether the shortterm experiments would have also produced quartz if they had run for a longer time. The question can be asked because the composition of the glass is similar as well as the chemistry of most of the waters and the physical conditions in which the reactions took place. Is it possible that, at some stage in the alteration reaction, certain chemical and physical conditions were created at the glass-water interface that would result in the production of quartz and alunite, rather than continue with the initiated production of clay? Or is there something completely specific in the glass used in the 6-year experiments that is the cause of the alteration into quartz and alunite? There is the possibility that the latter contained microcrystalline quartz that may have acted as a template and promoted further crystallization of quartz. If such quartz existed, it was not observable using XRD, which implies a very low quartz content in the glass because any crystalline phase would have been easily detected against the background of the low-intensity scattering of glass. This would be true especially of quartz, which has two peaks of high intensity in the 20 40º 2y region (Fig. 1). Thus, microcrystalline quartz was not abundant in the glass. If it was not abundant, then it could not be ubiquitous within the glass. However, the conversion to quartz was complete and affected the entire mass of the glass, which suggests that nucleation points were indeed

ubiquitous. The texture of the quartz (Fig. 3s,u,w,x) also suggests numerous crystallization points, as discussed above. Accordingly, this argument suggests that quartz formation was not due to the existence of quartz seeds in the original glass, which could not be sufficiently abundant to produce the resulting transformation. It is also necessary to remark that the possible existence of quartz nucleation points does not explain the rapid quartz formation according to our present knowledge, because the known rates of quartz crystallization on quartz (growth of quartz grains, deposition of new quartz layers in pre-existing ones in fissures, etc.) at low temperature are much slower than those in our experiments (Rimstidt & Barnes, 1980; Cuadros et al., 2012). Another specific characteristic of the glass in the 6-year experiments may be a special texture that promotes a specific reaction path resulting in the formation of quartz. Unfortunately, no information is available about the original texture of the glass and this possibility cannot be explored further. It has been found that the silica content in glass affects the rate of glass dissolution (Wolff-Boenisch et al., 2004), with higher silica causing slower glass dissolution. This fact may imply that silica content modifies the mechanism of glass-water reactions. The glass used in the 6-year experiments has a high silica content, which will promote a slower dissolution rate. Many of the glass alteration experiments or studies of naturally altered glass, with or without biological component, have been carried out on basaltic glass (e.g. Thomassin et al., 1989; Ghiara et al., 1993; Abdelouas et al., 1994; Thorseth et al., 1995a, b; Staudigel et al., 1995; Alt & Mata, 2000; Giorgetti et al., 2009) and no quartz formation has been reported, although cationdepleted glass containing mainly or exclusively silica has been found (Thorseth et al., 1995a). Ghiara et al. (1993) mentioned that their hydrothermal fluids become saturated with respect to quartz during the inorganic alteration of basaltic glass at 200ºC for up to 175 days, but they found no quartz due to, they suggest, the slow quartz precipitation kinetics. Studies of naturally or experimentally altered glass of rhyolitic composition, with silica contents similar to those in our 6year experiments (Magonthier et al., 1992; Fiore et al., 2001; Kawano & Tomita, 2002; Verney-Carron et al., 2008; in these studies the SiO2 wt.% range is 70 75), including our own 18-month experiments (Table 1), did not report quartz generation.


Considering the above results of rhyolitic glass alteration, which cover a wide range of conditions and also long times in some cases, the glass composition alone does not seem to be the determining element in the production of quartz. The cause of the transformation of glass into quartz and alunite may then not be unique but a combination of causes (glass chemistry and texture, reaction conditions). However, there is nothing so extremely special about the physico-chemical characteristics of the experiments, as far as can be seen, that may not be reproduced in nature. Rhyolitic glasses are abundant in the continents, and they frequently alter at low-temperature. The chemistry of the waters is not an issue, as all four waters in our 6-year experiments produced the same results. Perhaps the alteration system must be closed to favour silica accumulation at the glasswater interface, conditions that can be reproduced in nature where rhyolitic glass alteration takes place in stagnant or slowly-percolating waters, a situation that does not suggest itself as necessarily rare. Besides, the short time necessary for the transformation into quartz means that such conditions need not to prevail for long, which makes the natural occurrence of this reaction more probable. Even if the right hydrological conditions were reproduced only temporarily during the seasonal cycles it could be envisaged that the accumulation of many of these cycles would produce a significant proportion of glass alteration into quartz. It is possible that such a transformation does occur in nature but has not been recognized because the present knowledge of quartz origin does not contemplate it. Recently, Lindgreen et al. (2010, 2011) have described nanosized quartz associated with chalk from the North Sea. The quartz is interpreted to have formed through direct precipitation on the seafloor rather than as a result of later diagenesis. Silica was provided by radiolarians, according to the authors, although nearby volcanism suggests that volcanic ash may have also been a source. In any case, this study introduces the concept of quartz crystallized in a very short time, during the deposition event, from a silica-rich source. The parallel with the results from our 6-year experiments is obvious. These findings indicate that the origin of quartz in certain young sedimentary environments should not be automatically considered as detrital from magmatic, hydrothermal or diagenetic rocks, or that quartz in volcanic rocks is necessarily produced before the eruption, all of which may influence

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deeply the interpretation of some geochemical and geological processes. ACKNOWLEDGMENTS

The authors thank T. Wing-Dudek for her contribution in planning the experiments and collecting some of the waters, V. Dekov and E. Neykova for providing some of the volcanic glasses, F. Pinto for his expert technical support in the cryo-SEM study, and two anonymous reviewers for their careful work and suggestions. This work was funded by the Marie Curie Fellowship programme, project Bio-Clays (2009-2011). REFERENCES

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