Pathways of volcanic glass alteration in laboratory experiments through inorganic and...

Pathways of volcanic glass alteration inlaboratory experiments through inorganic

and microbially-mediated processes

J. CUADROS1 ,* , B . AFSIN1 , { , P . JADUBANSA2 , M. ARDAKANI3 ,

C . ASCASO4AND J . WIERZCHOS4

1 Department of Earth Sciences, Natural History Museum, Cromwell Road, London SW7, 5BD, UK, 2 Department of

Life Sciences, Natural HistoryMuseum,Cromwell Road, London SW7 5BD,UK, 3Department ofMaterials, 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)

ABSTRACT: 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, quartzformation, 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

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

* E-mail: [email protected]{ Present address: Department of Chemistry, Faculty ofScience and Arts, Ondokuz Mayis University, Samsun,55139 TurkeyDOI: 10.1180/claymin.2013.048.3.01

ClayMinerals, (2013) 48, 423–445

# 2013 The Mineralogical Society

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 fluid-

rock 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 vari-

able: 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 informa-

tion 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 experi-

ments, 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.

424 J. Cuadros et al.

EXPER IMENTAL

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 corre-

sponding 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 representa-

tive 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 experimentsLipari 76.9 0.252 11.9 3.63 0.106 0.111 1.67 4.28 2.83 101.70 n.d.Milos 78.6 0.166 12.8 1.10 0.062 0.213 1.31 3.90 3.56 101.80 n.d.Mixture 73.2 0.252 12.9 3.28 0.100 0.130 1.99 4.43 3.24 99.50 n.d.

6-year experiment76.7 0.07 12.0 2.22 0.06 0.04 0.69 3.99 4.77 100.54 40

n.d.: not determined.

Volcanic glass alteration through inorganic and microbially-mediated processes 425

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.

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 polypropy-

lene 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 experi-

ments. 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 experi-

ments 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

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 experi-

ments 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 (ICP-

AES, using a Varian VISTA PRO). Detection limits

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 experi-

ments. 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 crystal-

line 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 concen-

trated in the fine-grain fraction. Thus, glass grains,

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 experi-

ments) or grains with adhered biofilm (6-year

experiments) were sampled and placed in a water-

saturated atmosphere until analysis. Immediately

previous to analysis, they were frozen in sub-cooled

liquid N2, fractured to allow observation of mineral-

biofilm 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


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 experi-

ments 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 experi-

ments, 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 micro-

organisms 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 consump-

tion. 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 biologically-

mediated 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 black-

dark 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 experi-

ments (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 varia-

tions indicate modifications produced by the

biological activity and differences between the

specific biological colonies present in each


FIG.2.CationconcentrationsandpH

ofreactingwaters.

Theshort-term

experim

ents

(fourtoprows)

includeresultsfortheinitialconcentrations(fulltriangle

at

timezero)andafter6,10,14and18monthsofreaction.For18months,theexperim

ents

werereplicated3times.Forthe6-yearexperim

ents

(bottom

row)there

areresultsonly

attheendoftheexperim

ents,exceptforthemineralwater,forwhichdatafrom

thelabel

areincluded

(open

triangles).Someofthe6-yearplots

aredivided

intwo,wheretheaxis

foreach

groupofdatais

atthecorrespondingsideoftheplot.Modifiedfrom

Cuadroset

al.(2012,2013).


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 concen-

tration 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

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 experi-

ments 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 isprobable that the intimate contact between biolo-

gical 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 (Guven,

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


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

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 dioctahe-

dral (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.


(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 dioctahe-

dral 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 millimetre-

size 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

frequent, with a variety of size and morphology,

somet imes f ine ly mixed wi th kao l in i t e

(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 experi-

ments 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).


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 mono-

mineralic 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 hetero-

geneous 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 Ca-

rich 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) Hypersa-

line lake water, 18 months, biological experiment. The

edge of the particle has the typical smectitic morphol-

ogy 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 composi-

tion 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).


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).


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�3atoms. 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 mont-

morillonite 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 experi-

ments 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. Magnesium-

rich 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 corre-

spond 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 18-

month 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.

D I SCUSS ION

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, diocta-

hedral clay, suggesting chemical control from the

glass, which is much richer in Al than Mg

(Table 1). Very small grains (<10 mm) have a

chemical composition not very different from that

of the original glass but with signs of change that


FIG. 6. Plots of cation ratios from AEM values in clay particles, for 18-month (left) and 6-year experiments. The

structural formulas were calculated and the ratios in the plot use only octahedral Al, Mg and Fe (however, all Mg

and Fe is octahedral). The fields in the left-top to right-bottom diagonal correspond to smectite of trioctahedral,

tri- and dioctahedral, and dioctahedral composition.


seem to indicate the first stages of transformation

into clay (Fig. 3f,n). The morphology of these

particles, with angular corners, suggests glass rather

than clay. Whether originally present or generated

during the reaction, these particles with large

surface-to-volume ratios are likely to undergo

alteration and thus they are consistent with the

proposed major mechanism of in situ glass

alteration. The majority of clay particles analysed

with TEM-AEM have a composition of dioctahedral

character (Figs 5 and 6), even though they formed

in waters of very different chemistry. As the

common element within each of the two sets of

experiments (short-term and 6-year), biological and

inorganic, is the glass composition, it is then likely

that the main control on clay chemistry was by the

chemical composition of the glass, richer in Al than

Mg, and that the most important mechanism of clay

formation in most of the experiments is in situ

transformation of the glass.

The formation of Al-rich clay was only matched

by the formation of Mg-rich clay in two

experiments, fresh and hypersaline waters, both

18-month experiments. It is reasonable that the

hypersaline lake water produced the largest amount

of Mg-rich clay as the Mg concentration is solution

is the highest (Fig. 2). Biological experiments

produced a larger proportion of Mg-rich clays

than the inorganic tests (Fig. 6). This is likely due

to encapsulation of the glass grains in biofilms

(Cuadros et al., 2013) where the glass grains came

in contact with water enriched in Mg and other

soluble cations as compared to the bulk solution

(Fig. 3a). Such a conclusion is supported by the

results of the 6-year and short-term seawater

experiments because no encapsulation of the glass

grains took place and there is no apparent

difference of the clay composition from inorganic

and biological experiments. In the short-term spring

water experiments encapsulation did take place, but

the low Mg content in solution probably could not

produce sufficiently high Mg concentrations within

the biofilms to generate Mg-clay (Cuadros et al.,

2013). The short-term freshwater lake experiments

are different, with more Mg-rich clay formed in the

inorganic experiment. In this case, the pH seems to

play a decisive role in controlling the reaction

products. The original pH of this water was the

highest at 9. Cuadros et al. (2013) assessed the

mineral-water equilibrium conditions in this water

at several reaction times and found that the water

compositions were within or near the stability field

of saponite-talc, indicating that trioctahedral clay

formation was probably due to the water chemistry

in the bulk solutions. In the corresponding

biological experiments, the encapsulation of the

glass within biofilms, where the pH was probably

lower due to the biological activity, partially

prevented Mg-clay formation. Thus, the overall

picture is that the main chemical control on the

neoformed clay is glass composition unless water

chemistry becomes ‘‘extreme’’ (Mg concentration,

pH), in which case it exerts also a large control.

The water chemistry can be ‘‘extreme’’ for purely

inorganic causes or made such by biological

activity through the generation of concentrated

solutions within glass-entrapping biofilms.

These conclusions are in agreement with the

results of previous studies. Giorgetti et al. (2009)

found that alteration of glass in submarine

trachybasalt resulted in clay of beidellite to

montmorillonite composition for glass with Al2O3/

MgO% wt. ratios > 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 propor-

tion 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

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 transfor-

mation 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


each other. Sometimes, however, several crystal-

lisation points produced one single grain that was

not completed and had void space within it.

The formation of alunite in the 6-year experi-

ments 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, over-

coming the low S concentration in the glass

(Table 1) and solutions (Cuadros et al., 2012).

It remains an open question whether the short-

term 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, microcrystal-

line 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 character-

istic 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 cation-

depleted glass containing mainly or exclusively

silica has been found (Thorseth et al., 1995a).

Ghiara et al. (1993) mentioned that their hydro-

thermal 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 composi-

tion, with silica contents similar to those in our 6-

year 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 glass-

water 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 transfor-

mation 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 nano-

sized 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

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).

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