Applied Geochemistry 49 (2014) 46–56

Contents lists available at ScienceDirect

Applied Geochemistry

journal homepage: www.elsevier .com/ locate /apgeochem

Dissolution study of tremolite and anthophyllite: pH effect on thereaction kinetics

http://dx.doi.org/10.1016/j.apgeochem.2014.06.0090883-2927/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel.: +34 958 23 0000; fax: +34 958 552620.E-mail addresses: marisarozalen@ugr.es, mlrozalen@gmail.com (M. Rozalen).

M. Rozalen a,⇑, M.E. Ramos a, F. Gervilla a,b, T. Kerestedjian c, S. Fiore d, F.J. Huertas a

a Instituto Andaluz de Ciencias de la Tierra – IACT (CSIC-University of Granada), Avda. de las Palmeras 4, 18100 Armilla, Granada, Spainb Departamento de Mineralogía y Petrología, University of Granada, Avda. Fuentenueva 2, 18002 Granada, Spainc Geological Institute, Bulgarian Academy of Sciences, 24 Georgi Bonchev Str., 1113 Sofia, Bulgariad Istituto di Metodologie per l’Analisi Ambientale – IMAA (CNR), Tito Scalo, PZ Italy

a r t i c l e i n f o

Article history:Available online 18 June 2014

a b s t r a c t

The effect of pH on the kinetics of tremolite and anthophyllite dissolution was investigated at 25 �C inbatch reactors over the pH range of 1–13.5, in inorganic buffered solutions. Dissolution rates wereobtained based on the release of Si and Mg. Results obtained in this study show different behaviors forboth minerals. For tremolite, dissolution rates show a noticeable dependence on pH between 1 and 8,decreasing as pH increases and reaching a minimum around neutral conditions. At basic pH this depen-dence becomes even stronger, but dissolution takes place together with collateral effects of saturationand carbonation. A preferential release of Ca and Mg is observed in acid media, lowering the Mg/Si ratioto the extent that Mg solubility decreases with pH. For anthophyllite, dissolution rates also show a strongdependence on pH, between 1 and 9.5. At the same pH, anthophyllite dissolves up to 8 times faster thantremolite. For pH > 9.5 this dependence is smooth, and it is probably associated with effects of saturationand carbonation. Dissolution is also non-stoichiometric with a faster release of Mg with respect to Si inacid media. SEM observations show differences in the breakage mechanism of the fibers. The anthophyl-lite particle breakage during dissolution consists of the splitting of bundle fibers parallel to the fiberlongitudinal direction. However, for tremolite, other than fiber splitting, particles shorten induced bycoalescence of etch pits developed perpendicular to c axe.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Amphiboles are rock-forming minerals present at the Earth sur-face and formed at high temperatures in igneous and metamorphicrocks. Due to their instability under surface conditions, they areprimary sources of magnesium and calcium to surface waters, par-ticularly in areas where weathering rates are low (Clayton, 1986).In the case of calcium, their availability is essential as it is a neces-sary plant nutrient. However, high magnesium availability ham-pers the development of vegetation, inhibiting soil formation(Gunter et al., 2007). In this study, we have chosen two amphiboles(tremolite and anthophyllite) that are respectively representativeof the Ca and Mg series to study and compare their behavior duringdissolution kinetics.

Both minerals, anthophyllite and tremolite, when they occur inan asbestiform habit, are regulated as asbestos (Gunter et al.,2007). The environmental relevance of asbestos is very high asboth weathering and human activity can separate fibers and break

them down to fibrils of submicroscopic dimensions easily inhala-ble (diameters less than about 3 lm). Exposure to asbestos fibers,both for occupational and non-occupational activities, has beenlinked to numerous health problems and respiratory diseases asmesothelioma or asbestosis. Consequently, natural outcrops aswell as abandoned mines represent a potential health risk. Amphi-bole weathering contributes not only to chemical transformation,but also to breakdown of the fibers that may be spread out inthe environment and make them even more dangerous.

Despite its environmental relevance, little is known about thebehavior of tremolite and anthophyllite during weathering. Previousstudies of dissolution of other magnesium silicates in aqueoussolutions (forsterite, enstatite, and serpentine: Luce et al., 1972;bronzite: Grandstaff, 1977; hypersthene: Siever and Woodford,1979; enstatite, diopside and tremolite: Schott et al., 1981) mostlysuggest that the dissolution of magnesium silicates takes place incon-gruently with the formation of surface phases impoverished in mag-nesium that inhibit the dissolution reaction. Little is known about thedissolution mechanism and thus, the rate-controlling process.

Although amphibole dissolution rates have been reported in thepast (tremolite: Schott et al., 1981; anthophyllite: Chen and

M. Rozalen et al. / Applied Geochemistry 49 (2014) 46–56 47

Brantley, 1998; anthophyllite: Mast and Drever, 1987) the pH-dependence of the dissolution rate in a wide range of pH valuesis not well defined and the results are somehow incoherent.Schott et al. (1981) dissolved tremolite in aqueous solution at20–60 �C and pH 1–6, observing a slight depletion of Mg relativeto Si and a marked initial Ca depletion followed by congruentbehavior. According to the rate law R ¼ kHþ � an

Hþ, they obtained areaction order of n = 0.11. However, Mast and Drever (1987)reported a low pH dependence (from pH 2 to 5, n = 0). Regardinganthophyllite dissolution Chen and Brantley (1998) obtained avalue of n = 0.24 using flow-through reactors.

The aim of this study is to investigate the dissolution reaction oftremolite and anthophyllite (two representative amphibole miner-als) as a function of pH, in order to obtain dissolution rates and getsome insight about the dissolution mechanism. Results could beuseful as a background to evaluate amphibole weathering in differ-ent environments in addition to help develop remediation pro-cesses for potentially contaminated sites.

2. Materials and methods

2.1. Characterization of the mineral samples

2.1.1. TremoliteTremolite was collected in an old serpentine quarry next to A-

395 road, in Sierra Nevada (Granada, SE Spain). X-ray diffraction(XRD) was run on PANalytical X’Pert Pro diffractometer with anX’Celerator detector, operated at 45 kV and 40 mA, with Ni filteredCu Ka radiation. Patterns were recorded from 4 to 70 2h degrees.Only a pure tremolite phase without accessory/companying miner-als was identified. The natural tremolite was pretreated to enrichthe sample in fine particles (<4 lm equivalent spherical diameter)by repeated cycles of sedimentation-suspension in water. Althoughthe Stoke’s law cannot be properly applied to fibrous material, thisprocedure allowed us to reduce to minimum the amount of com-panying minerals. The supernatant was dried in an oven at 40 �Cand the particles were recovered with acetone. Finally, it wasstored in a polyethylene bottle as the starting material.

Chemical analysis of major elements was performed by X-rayfluorescence (XRF). Elements were analyzed using a S4 Pioneer(Bruker) wavelength dispersive X-ray fluorescence spectrometer,equipped with a X-ray tube with a Rh anode and a maximum volt-age/current of 60 kV/150 mA. The fundamental parameter methodusing Spectraplus software was used to quantify the elements. Thecalculated structural formula corresponds to a tremolite:

Ca1:83Na0:17ðMg5:05Fe2þ0:17ÞðSi7:85Al0:12ÞO22ðOHÞ2:

The corresponding atomic ratios Mg/Si, Ca/Si and Na/Si arerespectively 0.643, 0.233 and 0.022. The specific surface area ofthe tremolite was measured by the Brunauer–Emmett–Teller(BET) method, using 5-point N2 adsorption isotherms, after degas-sing the sample for 60 min at 120 �C. The specific surface area was0.32 m2 g�1, with an estimated uncertainty of 10%.

2.1.2. AnthophylliteAnthophyllite specimens were collected from an abandoned

mine, SW of Golyamo Kamenyane (Bulgaria). XRD analysis showsa pure anthophyllite phase without any accessory/companyingminerals. The natural anthophyllite was also pretreated to enrichthe sample in fine particles (<4 lm equivalent spherical diameter)by repeated sedimentation–suspension in water. The compositionof individual fibers was obtained by transmission and analyticalelectron microscopy (TEM–AEM; Philips CM20). The average com-position obtained from several tens of fiber analyses produced thefollowing anthophyllite structural formula,

Na0:11Ca0:08ðMg5:80Fe3þ0:61Mn0:02ÞðSi8:02Al0:07ÞO22ðOHÞ2

The corresponding atomic ratios Mg/Si, Ca/Si and Na/Si are respec-tively 0.723, 0.010 and 0.0137. The specific surface area of theanthophyllite was 2.38 m2 g�1, with an estimated uncertainty of 10%.

2.2. Experimental design

Short-term (30–35 days) dissolution experiments were carriedout in batch reactors. Batch dissolution experiments can provide dis-solution rates under far from equilibrium conditions, and equilib-rium data as the saturation state approaches equilibrium. Two setsof experiments were performed using tremolite (series Tr_1m) andanthophyllite (series An_1m) with a solid/solution ratio of0.5 g L�1. The mineral sample was placed in acid-cleaned PFA bottlescontaining 250 mL of buffered solutions from pH 1 to 13.5 (Table 1).Ionic strength was set between 0.01 and 0.02 M for pHs between 2and 11.5 in order to obtain a negligible effect into the dissolution rateequation. For extreme pH values, lower than 2 and higher than 11,the composition of the buffers makes experimentally unavoidableto get lower ionic strength. The temperature was controlled by par-tially immersing the reaction vessels in a water bath at room tem-perature (25 ± 2 �C). An aliquot of 20 mL of the suspension wasperiodically withdrawn and filtered through a 0.22 lm Duraporemembrane and poured into an acid cleaned polyethylene bottle.

The reaction bottles were only gently shaken during samplingto minimize eventual changes in the solid/solution ratio. SolutionpH was measured at room temperature immediately after filtrationusing a Crison combination electrode standardized with pH 4.01and 7.00 buffer solutions. The reported accuracy was ±0.02 pHunits. When necessary, the solutions were acidified with ultrapureHNO3 to pH 3 to prevent precipitations of Mg hydroxides/carbon-ates during storage for Si and Mg analysis. Silicon concentrationin every withdrawal was determined by colorimetry, using themolybdate blue method (Grasshoff et al., 1983) with a detectionlimit of 5 ppb and an associated error of 5%. Magnesium and cal-cium concentration in every withdrawal was determined by ionchromatography (IC) using a Metrohm 883 Basic IC Plus Ion Chro-matograph equipped with a Metrosep C3-250 column. The eluentwas prepared with 3.5 mmol L�1 HNO3. The detection limit andassociated error were 1 ppb and 3% respectively. Once the experi-ments were completed the remaining solids were filtered througha membrane filter and repeatedly rinsed with Milli-Q water andfinally with acetone to accelerate the drying process. They werecarefully collected and stored for further analyses.

Changes in particle morphology after experiments wereobserved using two apparatus: a Field Emission Scanning electronmicroscopy (FESEM), LEO Gemini 153 operating at 1–2 kV and aworking distance of 2 mm. The microscope was linked to an energydispersive X-ray spectrometer (EDS, Oxford INCA200). They werealso observed using a FESEM, Zeiss Supra 40 equipped with sec-ondary (SE) and back-scattered (BSE) electron detectors. Elementalanalyses were carry out with an EDS (Oxford Inca Energy 350)equipped with Si(Li) detector characterized by an ultrathin win-dow able to detect elements with atomic number Z > 4. Sampleswere diluted in Milli-Q water and exposed to low energy ultra-sound for 5 min using an ultrasonic bath. This method allows a dis-persion of fibers preserving their superficial features and avoidingrupture usually caused by particles collision after mechanical stir-ring. Few drops of these solutions were settle on aluminum stubsand covered with a carbon film before observation.

2.3. Kinetics calculations

In batch experiments the mineral dissolution rate (Rate,mol g�1 s�1) is calculated as variation in mineral component jconcentration in the reactive solutions as time progresses:

Table 1Composition of the initial solutions (mol L�1) used as buffers in the dissolution experiments.

Initial pH HNO3 CH3COOH KCH3COO KHCO3 K2CO3 KOH KNO3 Ionic strength

Series T (Tremolite)0.94 0.1 – – – – – 0.01 0.111.94 0.01 – – – – – 0.01 0.022.62 0.001 – – – – – 0.01 0.0114.02 – 0.0085 0.0015 – – – 0.01 0.024.94 – 0.0035 0.0064 – – – 0.01 0.025.53 – 0.0009 0.009 – – – 0.01 0.026.68 – – – 0.00025 – – 001 0.0108.17 – – – 0.0021 – – 0.01 0.01218.97 – – – 0.0095 0.0005 – 0.01 0.029.99 – – – 0.0068 0.0032 – 0.01 0.0211.58 – – – – – 0.005 0.01 0.01512.68 – – – – – 0.05 – 0.0513.53 – – – – – 0.5 – 0.5

Series A (Anthophyllite)0.91 0.1 – – – – – 0.01 0.111.91 0.01 – – – – – 0.01 0.022.93 0.001 – – – – – 0.01 0.0113.86 – 0.01 0.00115 – – – 0.01 0.0215.42 – 0.00026 0.0005 – – – 0.01 0.01086.03 – 0.00035 0.0076 – – – 0.01 0.0188.55 – – – 0.0021 – – 0.01 0.0129.48 – – – 0.0087 0.0015 – 0.01 0.020210.55 – – – 0.003 0.006 – 0.01 0.01911.75 – – – – – 0.005 0.01 0.01512.67 – – – – – 0.05 0.01 0.0613.62 – – – – – 0.5 0.01 0.51

48 M. Rozalen et al. / Applied Geochemistry 49 (2014) 46–56

Ratej ¼ �1v j

VM

dCj

dtð1Þ

where mj is the stoichiometric coefficient of component j in thedissolution reaction, M represents the mineral mass (g), V is thevolume of solution (L) and dCj/dt is the measured slope of concen-tration of j with time. Errors in the calculated rate is estimated byusing the Gaussian error propagation method and is less than 10%in all cases, which corresponds to approximately 0.10 logarithmicunits. Rates were normalized to mass (mol g�1 s�1) as it is discussedin Rozalen et al. (2008). The reported values of the BET surface area(given in material and methods section) would allow calculation ofarea-normalized (mol m�2 g�1) rates to be compared with literaturedata.

The saturation state of the solution with respect to solid phasesis calculated in terms of free energy of reaction, DGr:

DGr ¼ RT lnIAPKeq

� �ð2Þ

where R is the gas constant, T designates the absolute temperature,IAP and Keq stand for the ion activity product and the equilibriumconstant for the dissolution reaction respectively. Aqueous activi-ties and chemical affinities in the present study were generatedusing the EQ3NR geochemical code (Wolery, 1992) and the equilib-rium constants included in the software database for aqueous com-plexes and minerals. The hydrolysis reactions of the currenttremolite and anthophyllite specimens are the following:

Ca1:83Na0:17ðMg5:05Fe2þ0:17ÞðSi7:85Al0:12ÞO22ðOHÞ2 þ 14:6Hþ

! 0:17Naþ þ 1:83Ca2þ þ 5:05Mg2þ þ 0:17Fe2þ þ 7:85SiO2

þ 0:12Al3þ þ 8:30H2O ð3Þ

Na0:11Ca0:08ðMg5:80Fe3þ0:61Mn0:02ÞðSi8:02Al0:07ÞO22ðOHÞ2 þ 13:92Hþ

! 0:11Naþ þ 0:08Ca2þ þ 5:80Mg2þ þ 0:61Fe3þ þ 0:02Mn2þ

þ 8:02SiO2 þ 0:07Al3þ þ 7:96H2O ð4Þ

There is no specific value for the true equilibrium constant of thesehydrolysis reactions. As a proxy we have chosen the equilibrium con-stant for the dissolution reactions of the end-member of tremoliteand anthophyllite included in the EQ3 database. The correspondingvalues at 25 �C and 1 atm for tremolite and anthophyllite arelogKeq(Tr) = 61.24 and logKeq(An) = 66.80, respectively. The influ-ence of variations of minor cations (Al, Fe and Ca) concentrationson DGr was tested in EQ3NR using stoichiometric concentrations aswell as values 10 times lower and higher than stoichiometric values.Changing the concentration of minor cations by two orders ofmagnitude does not affect DGr in the second decimal figure.

3. Results

3.1. Tremolite (Serie Tr_1m)

The evolution of pH, Si, Mg and Ca concentrations in the batchexperiments are illustrated in Fig. 1 for some representative runs.The pH of the solutions remains constant during the experiments.As a general trend, Si concentration is lineally proportional to theelapsed time in the experiments run at pH below 5. Above pH 6there is a fast release of Si at the beginning of the experiments,and concentration tends to stabilize with time. This trend isemphasized as pH increases.

The release of Mg to solution at pH 2 is lineally proportional tothe elapsed time until the end of the experiments. Above pH 3 therelease of magnesium is very fast at the beginning of the experi-ments (first few days) but tends to stabilize as solution becomesmore saturated. From pH 1 to 7 Mg is preferentially released versusSi (Fig. 1). From pH 8 to 10 due to the decrease of the Mg2+ solubil-ity, silica is released to the solution preferentially. From pH 10 to13, Mg2+ concentration is below the detection limit.

A fast release of Ca2+ is observed during the first two days ofreaction and tends to stabilize with time. Between pH 1 and 7, apreferential release of Ca versus Si is observed (Fig. 1). This trendis reversed from pH 8 to13 due to the low solubility of Ca2+.

0

1

2

3

0

1

2

3

4

5

0 5 10 15 20 25 30

pH

Con

cent

ratio

n (

mol

L- 1

)

Time (days)

Si MgCa pH

(a) Tr_1m_2

0

1

2

3

4

0

0,5

1

1,5

2

0 5 10 15 20 25 30

Ca/

Si

Mg/

Si

Time (days)

Mg/Si

Ca/Si

(b)

0

1

2

3

4

5

6

0

1

2

3

0 5 10 15 20 25 30

pH

Con

cent

ratio

n (

mol

L-1

)

Time (days)

Si MgCa pH

(c) Tr_1m_4

0

1

2

3

4

5

6

0

1

2

3

4

5

6

0 5 10 15 20 25 30

Ca/

Si

Mg/

SiTime (days)

Mg/Si

Ca/Si

(d)

0

2

4

6

8

10

12

0

5

10

15

20

25

30

0 5 10 15 20 25 30

pH

Con

cent

ratio

n (

mol

L-1

)

Time (days)

Si MgCa pH

(e) Tr_1m_8

0

0,1

0,2

0,3

0,4

0,5

0

0,2

0,4

0,6

0,8

1

0 5 10 15 20 25 30

Ca/

Si

Mg/

Si

Time (days)

Mg/Si

Ca/Si

(f)

Fig. 1. Temporal evolution of the output pH, output concentrations of Si, Mg and Ca and Mg/Si and Ca/Si ratios of some representative tremolite dissolution experiments.Horizontal lines represent the ideal stoichiometric ratio.

M. Rozalen et al. / Applied Geochemistry 49 (2014) 46–56 49

3.2. Anthophyllite (Serie An_1m)

The evolution of pH and Si and Mg concentration in the batchexperiments for several representative runs are showed in Fig. 2. Bothsets of batch experiments (series An_1m and Tr_1m) showed similarbehavior. The pH of the solutions remains constant during the exper-iments, except in the experiments at pH 5 and 6, where pH increasesto remain constant at neutral values (approximately 7.5).

From pH 1 to 5, there is a lineal dependence of the released Siand the elapsed time (Fig. 2a). Above pH 5, the trend of Si releaseshows an asymptotic form. This trend is also found for Mg releasefrom pH 1 to 10. Above pH 11, Mg2+ concentration is below thedetection limit. Based on the Mg/Si ratio, between pH 1 and 7 thereis a preferential release of Mg with respect to Si (Fig. 2). At pH 8 theconcentration of Mg and Si released to solution is very close to theatomic stoichiometric value. From pH 9 to 10, silica is preferen-tially released to solution.

In the case of Ca, as a general trend, after a fast release of Ca tosolution at the beginning of the experiments, the concentrationremains constant. A preferential release of Ca versus Si is observedfor the complete range of pH (Fig. 2). Moreover, the experimentalCa/Si ratio increases as pH increases from 1 to 8. Above pH 9, theCa/Si ratio decreases as pH increases due to the low solubility ofCa2+ in basic media and easy carbonation processes.

3.3. Calculation of dissolution rate

According to Eq. (1) the dissolution rates are calculated from thevariation of measured concentrations with time (dCj/dt). Only datafrom a relatively short reaction interval in the beginning of theexperiments is used (Brantley and Conrad, 2008).

In this study dissolution rates were calculated at constant pHconditions using the Si concentrations in the time interval of linealrelease (Table 2). If dissolution takes place stoichiometrically, theconcentration of Mg2+ can be also used to calculate the dissolutionrate of tremolite and anthophyllite, so that the change in concen-tration of element j versus time can be fit to a lineal first-orderequation with a dC/dt derivative such as expressed in Eq. (1)(Brantley and Conrad, 2008).

4. Discussion

4.1. Stoichiometry

Despite of the difference in mineral compositions (the Ca con-tent per structural formula in anthophyllite is much lower thanin tremolite, and that of Mg is higher) the results of the presentstudy show a similar dependence of dissolution stoichiometry withtime and pH.

0

1

2

3

4

5

6

0

100

200

300

400

500

0 5 10 15 20 25 30 35

pH

Con

cent

ratio

n (

mol

L-1

)

Time (days)

Si MgCa pH

(a) An_1m_4

0

0,5

1

1,5

2

0

1

2

3

4

0 5 10 15 20 25 30 35

Ca/

Si

Mg/

Si

Time (days)

Mg/Si

Ca/Si

(b)

0

2

4

6

8

10

12

0

10

20

30

40

50

0 5 10 15 20 25 30 35

pH

Con

cent

ratio

n (

mol

L-1

)

Time (days)

SiMgCapH

(c) An_1m_8

0

0,5

1

1,5

2

2,5

3

0

0,5

1

1,5

2

0 5 10 15 20 25 30 35

Ca/

Si

Mg/

Si

Time (days)

Mg/Si

Ca/Si

(d)

0

2

4

6

8

10

12

14

0

5

10

15

20

25

30

35

40

0 5 10 15 20 25 30 35

pH

Con

cent

ratio

n (

mol

L-1

)

Time (days)

SiMgCapH

(e) An_1m_10

0

0,25

0,5

0,75

0

0,5

1

1,5

0 5 10 15 20 25 30 35

Ca/

Si

Mg/

Si

Time (days)

Mg/Si

Ca/Si

(f)

Fig. 2. Temporal evolution of the output pH, output concentrations of Si, Mg and Ca and Mg/Si and Ca/Si ratios of some representative anthophyllite dissolution experiments.Horizontal lines represent the ideal stoichiometric ratio.

Table 2Experimental conditions and results of the tremolite dissolution. Italicized values indicate supersaturation with respect to Mg-bearing minerals.

Batch pH in pH out average logRSi (mol g�1 s�1) logRMg (mol g�1 s�1) DGr (kcal mol�1)

Tr_1m_1 0.94 0.85 �12.29 – �206.17 �188.71Tr_1m_2 1.94 1.87 �12.50 �12.40 �183.16 �170.20Tr_1m_3 2.62 2.88 �12.62 �12.68 �157.18 �145.18Tr_1m_4 4.02 3.96 �12.85 �13.08 �141.85 �135.22Tr_1m_5 4.94 4.92 �13.03 – �127.39 �125.03Tr_1m_6 5.53 5.59 �13.18 �13.33 �112.12 �107.25Tr_1m_7 6.68 6.91 �13.08 �13.16 �70.00 �68.08Tr_1m_8 8.18 8.11 �13.16 – �34.17 �34.13Tr_1m_9 8.97 9.03 �12.77 – �22.26 �19.28Tr_1m_10 9.99 9.85 �12.22 – 12.41 13.87Tr_1m_11 11.58 11.71 �12.39 – 16.24 17.64Tr_1m_12 12.68 12.85 �11.95 – 7.82 8.94Tr_1m_13 13.53 13.55 �11.32 – �6.83 �11.64

50 M. Rozalen et al. / Applied Geochemistry 49 (2014) 46–56

As a general trend, high values of the Mg/Si and Ca/Si ratios fol-lowed by an asymptotic decrease with time can be observed in acidmedia (Figs. 1 and 2). This exponential dependence with time is inagreement with an early stage non-stoichiometric dissolution

observed for other chain-silicates at acidic pH (e.g., Luce et al.,1972; Sanemasa and Kataura, 1973; Grandstaff, 1977; Schottet al., 1981; Schott and Berner, 1983, 1985; Knauss et al., 1993;Chen and Brantley, 1998) and is consistent with surface

M. Rozalen et al. / Applied Geochemistry 49 (2014) 46–56 51

modifications during the initial dissolution stage (e.g. dissolutionof fine grained materials, highly strained areas on large grains ordefects).

The congruency or incongruency of dissolution reaction in thestage where ‘‘instant’’ dissolution rates are time-independent (lin-eal kinetics) is still under debate. Some authors (e.g., Schott et al.,1981; Chen and Brantley, 1998) argue that changes in surface areawith time and sample pretreatment are critical factors to reachcongruent dissolution. However, the ratio of dissolution ratesderived from the output Mg, Ca and Si concentrations in this studyshows a non-stoichiometric dissolution of tremolite and antho-phyllite with pH.

Regarding Mg a preferential release respect to Si is observed atacidic pH an Mg/Si ratio decreases at neutral pH conditions accord-ing to lower Mg solubility (Figs. 1a and c and 2a). The preferentialrelease of Mg over Si have been observed in many acid dissolutionstudies on phyllosilicates and has been attributed to rapid ion-exchange reactions between protons from the solution and Mglocated in the outermost part of the octahedral sheet (e.g., Kohleret al., 2005; Bibi et al., 2011). Incongruent dissolution generallyoccurs in magnesium silicate minerals under neutral or alkalineconditions, and it is related to magnesium solubility.

Regarding Ca, this cation is released in a greater extent than Mgfor tremolite and anthophyllite experiments. This is in agreementwith predictions based on site energies for Ca and Mg in tremolite(Schott et al., 1981). For amphiboles, cations in positions M4 (outercation sites) are released preferentially than cations in positionsM1, M2 and M3 (inner crystallographic sites) due to differencesin Madelung site energies. Consequently, Ca in the M4 site is nota-bly less strongly bonded than magnesium in M1, M2 or M3 sites(Schott and Berner, 1983, 1985; Brantley and Chen, 1995; Chenand Brantley, 1998).

Table 3Experimental conditions and results of the anthophyllite dissolution.

Batch pH in pH out average logRSi (mol g�1

An_1m_1 0.91 0.94 �10.19An_1m_2 1.91 1.96 �10.42An_1m_3 2.93 3.54 �10.59An_1m_4 3.86 4.07 �10.62An_1m_5 5.42 6.49 �11.32An_1m_6 6.03 6.92 �11.36An_1m_8 8.55 8.36 �11.61An_1m_9 9.48 9.44 �11.90An_1m_10 10.55 10.42 �12.01An_1m_11 11.75 11.69 �11.89An_1m_12 12.67 12.63 �11.82An_1m_13 13.62 13.54 �11.69

Table 4Calculated saturation state (DGr, kcal mol�1) of the first and last output solutions (1–30 dcorrespond to supersaturation conditions. Italicized values indicate supersaturation respec

Serie pH out Tr SiO2(am)

1 day 30 days 1 day 30 days

Tr_1m_1 0.85 �206.39 �188.82 �4.67 �3.50Tr_1m_2 1.87 �184.86 �171.85 �4.94 �3.75Tr_1m_3 2.88 �158.75 �149.74 �5.09 �4.02Tr_1m_4 3.96 �145.10 �137.67 �4.98 �4.24Tr_1m_5 4.92 �131.53 �129.71 �5.39 �4.87Tr_1m_6 5.59 �115.89 �111.95 �4.73 �4.33Tr_1m_7 6.91 �73.49 �72.73 �4.27 �4.00Tr_1m_8 8.11 �39.20 �39.17 �2.82 �2.79Tr_1m_9 9.03 �27.57 �24.81 �3.04 �2.96Tr_1m_10 9.85 6.22 7.84 �3.33 �3.11Tr_1m_11 11.71 9.89 11.31 �4.72 �4.48Tr_1m_12 12.85 1.06 2.21 �6.48 �6.31Tr_1m_13 13.55 �14.13 �19.15 �8.77 �9.53

Tr: Tremolite; SiO2(am): amorphous silica; Gb: Gibsite; Brc: brucite; Mgs: magnesite;; E

4.2. Effect of saturation on the dissolution rates

The calculated solution saturation (DGr) was used to evaluatewhether the experimental results are under far from equilibriumconditions. In this region, the degree of saturation has no directeffect on the dissolution rate (Lasaga, 1998) and the pH effectcan be evaluated independently.

The saturation state of all output solutions in both sets of exper-iments was calculated using the geochemical code EQ3NR.Although the DGr values were calculated for all the output solu-tions only the highest and lowest values are given in Tables 2and 3 for simplicity (the complete dataset is available at the sup-plementary online file).

With regard to tremolite, at pH 6 9, Table 2 shows undersatura-tion with respect to tremolite and undersaturation with respect toother minerals (gibbsite, brucite, magnesite, amorphous silica andcrystalline silica polymorphs) (Table 4). However there are no val-ues in literature to confirm that our experiments take place underfar from equilibrium conditions, since the value of DGr has not yetbeen estimated for far from equilibrium dissolution region oftremolite.

At pH > 9 Mg solubility decreases, and according to the EQ3NRcalculations, supersaturation with respect to tremolite and otherMg bearing secondary phases is reached (Tables 2 and 4). Solutionsupersaturation with respect to secondary phases may correlatewith slow dissolution of the primary phase due to lowered chem-ical affinity, precipitation of armouring phases, which complicatesdata interpretation. The proximity to equilibrium is experimentallyunavoidable under near-neutral conditions as many authors foundfor inosilicates like diopside (Schott et al., 1981), augite (Siegel andPfannkuch, 1984) or hornblende (Zhang et al., 1993). In basicmedia, Rozalen et al. (2014) reported that far-from-equilibrium

s�1) logRMg (mol g�1 s�1) DGr (kcal mol�1)

�10.18 �160.27 �135.77�9.86 �146.25 �121.15�10.31 �127.31 �75.22�10.31 �111.90 �95.51�11.27 �84.82 �61.28�11.13 �81.59 �72.36�11.51 �39.48 �53.85�11.97 �32.17 �46.62�13.26 �41.15 �25.44– �23.46 �17.86– �38.80 �33.23– �58.34 �52.82

ays) with respect to tremolite and other phases according to Eq. (3). Italicized valuest to the corresponding mineral phase.

Gb Brc Mgs

1 day 30 days 1 day 30 days

�18.95 �17.78 �28.77 �27.58 �25.09 �23.91�14.48 �13.52 �24.32 �24.04�10.45 �9.47 �22.44 �22.013�6.08 �5.34 �19.43 �19.42�2.91 �2.47 �17.08 �17.73�1.06 �0.73 �15.50 �15.54

0.37 0.72 �11.42 �11.700.50 0.37 �8.64 �8.79�1.04 �0.87 �6.90 �6.45�2.17 �1.68 0 0�3.89 �3.67 0 0�5.27 �5.15 0 0 –�6.43 �6.78 0 0 –

Q: mineral/solution equilibrium.

52 M. Rozalen et al. / Applied Geochemistry 49 (2014) 46–56

conditions cannot be obtained in using batch experiments. Ourexperimental conditions represent a compromise because thedecrease in the solid/solution ratio leads to cation concentrationsclose or below detection limit, which would increase the uncer-tainty associated with the calculated rate. Finally, dissolution ratesobtained in these conditions could be close to those in naturalenvironments, where precipitation of magnesium and calcium car-bonates could be expected.

With regard to anthophyllite, the calculated DGr values withrespect to anthophyllite show solution undersaturation under allthe pH range studied (Table 3). Although there is not a value toestablish far from equilibrium conditions for anthophyllite, ourresults (between �160 and �20 kcal/mol) are low enough toassume that DGr is not likely a critical variable in our rate lawequation. Solutions were also undersaturated with respect tochrysotile, amorphous silica and brucite for pHs below 10 (Table 5).Above pH 10 the solutions are saturated respect to brucite.

4.3. pH effect on the dissolution rate

4.3.1. TremoliteCalculated dissolution rates of tremolite are plotted as a func-

tion of solution pH (Fig. 3a). The rates exhibit the common trendobserved in log rate-pH plots of other silicates and complexmulti-oxides: rates decrease with increasing pH, reach a minimumvalue around neutral pH and increase again as pH increases.

In the acid pH range (from 1 to 6) the logarithm of dissolutionrates decreases when the pH increases. A simple linear regressionof log rate versus pH yields the following expression:

Table 5Calculated saturation state (DGr, kcal mol�1) of the first and last output solutions (1–35 d

Serie pH out An SiO2

1 day 35 days 1 day

An_1m_1 0.94 �160.97 �135.89 �2.33An_1m_2 1.96 �147.26 �116.55 �2.80An_1m_3 3.54 �128.10 �56.75 �2.89An_1m_4 4.07 �111.83 �82.37 �2.91An_1m_5 6.49 �82.80 �35.78 �2.84An_1m_6 6.92 �49.87 �49.22 �2.83An_1m_8 8.36 �31.13 �24.56 �3.11An_1m_9 9.44 �22.67 �14.80 �3.35An_1m_10 10.42 �32.92 �25.03 �4.12An_1m_11 11.69 �28.55 �15.33 �5.43An_1m_12 12.63 �23.89 �17.19 �7.35An_1m_13 13.54 �43.91 �37.76 �9.81

An: anthophyllite; SiO2 (am): amorphous silica; Chr: chrysotile; Brc: brucite.

Fig. 3. (a) Tremolite dissolution rates derived from the released Si and Mg using Eqconcentrations at every acid pH. The diagonal corresponds to stoichiometric dissolution

log RSi ¼ �12:13ð�0:02Þ � 0:19 � ð�0:01ÞpH r2 ¼ 0:995 ð5Þ

that, according to the empirical rate law of the form R ¼ KH � anHþ,

yield the following expression:

RSiðmol g�1 s�1Þ ¼ 10�12:13ð�0:02Þ � a0:19ð�0:01ÞHþ ð6Þ

A minimum rate value is reached around the pH range for 6–8in which the values are approximately constant with pH. From pH8 to 13.5 the rates increase as pH increases. As in the case of theacidic pH region, the empirical rate law is of the form:

RSiðmol g�1 s�1Þ ¼ 10�15:2ð�0:6Þ � a�0:26ð�0:06ÞHþ r2 ¼ 0:847 ð7Þ

The statistical analysis of variance (ANOVA) for lineal regres-sions Eqs. (5) and (7) yields values of regression coefficients(R2 > 0.9) and highly significant (P < 0.05) that indicate that the dis-solution rate variation with pH at the given acid and basic pHranges is significant.

Above pH 12 a step on the rates outline (Fig. 3a) and is observedprobably due to carbonation process of Mg during the aliquots with-draw, as documented by Halder and Walther (2011) for enstatitedissolution. They suggested the inhibition of dissolution rates abovepH 10 when experiments were open to the atmosphere. The carbon-ate dissolved in these solutions forms a > Mg2–CO3 surface complexat positively charged Mg surface sites on enstatite, resulting in sta-bilization of the surface Si–O bonds. Moreover, Mg can also precip-itate as microcrystalline brucite as it is described for alkalinedissolution of other silicates (e.g., Fernández et al., 2006).

ays) with respect to anthophyllite and other phases according to Eq. (3).

(am) Chr Brc

35 days 1 day 35 days 1 day 35 days

�0.53 �63.78 �55.61 �25.32 �23.80�0.88 �57.23 �46.81 �22.83 �20.62�0.97 �48.89 �21.06 �19.98 �11.98�1.13 �41.88 �31.80 �17.63 �15.46�1.73 �29.55 �10.98 �13.57 �8.12�1.72 �28.30 �16.76 �13.16 �10.05�2.26 �7.01 �5.41 �5.87 �5.91�2.62 �3.06 �0.72 �4.40 �4.11�3.33 �5.02 �4.76

0 0�6.51 0 0�9.09 0 0

. (1). (b) Comparison the logarithm of dissolution rates derived from Si and Mg.

M. Rozalen et al. / Applied Geochemistry 49 (2014) 46–56 53

4.3.2. AnthophylliteThe calculated dissolution rates of anthophyllite are plotted as a

function of solution pH in Fig. 4a. As in the case of tremolite a linealregression between the rates and pH in the acid and basic intervalsthe following expressions are obtained:

RSiðmol g�1 s�1Þ ¼ 10�9:92ð�0:06Þ � a0:20ð�0:01ÞHþ pH1� 9:5 r2 ¼ 0:983

ð8ÞRSiðmol g�1 s�1Þ ¼ 10�12:72ð�0:09Þ � a0:08ð�0:01Þ

Hþ pH > 9:5 r2 ¼ 0:983ð9Þ

(a)

Fig. 4. (a) Anthophyllite dissolution rates derived from the released Si using Eq. (1). (b) Cat every acid pH. The diagonal corresponds to stoichiometric dissolution.

a

c

e

20 μm

40 μm

20 μm

Fig. 5. SEM images of tremolite particles before and after the experiments: (a) and (blength:diameter ratios, (c) after 30 days of dissolution at pH 1 tremolite most aciculardirections, (e) after 30 days of reaction at pH 12 an intense dissolution show the developmaxe of the fibers, (f) after 30 days of reaction at pH 13 the precipitation of little particle

The statistical analysis of variance (ANOVA) for lineal regres-sions Eqs. (8) and (9) show values of regression coefficients(R2 > 0.9) and highly significant (P < 0.001) that thus indicate thatthe anthophyllite rate variation with pH at the given acid and basicpH ranges is significant.

The lineal dependence in the acid range can be extended up topH 9.5 where a minimum is reached, corresponding with the min-imum of solubility of Mg2+ and a change in the reaction stoichiom-etry, from congruent to incongruent (Fig. 4b). Above pH 9.5 the pHdependence is smoother and the reaction order is halved.

(b)

omparison the logarithm of dissolution rates derived from Si and Mg concentrations

b

d

f

20 μm

2 μm

10 μm

) starting material shows prismatic and acicular habit grains with wide variety ofparticles are dissolved, prevailing prismatic ones, (d) white arrows show breakageent of kinks (see arrows) due to formation of each pits perpendicular to longitudinal

s (white arrows) with prismatic habit could correspond with magnesite.

54 M. Rozalen et al. / Applied Geochemistry 49 (2014) 46–56

4.3.3. Insights about mechanismThe dissolution rates outline versus pH for anthophyllite and

tremolite exhibits different patterns (see Figs. 3a and 4a). In acidmedia anthophyllite dissolves faster than tremolite, reaching val-ues of almost two orders of magnitude higher. Rate pH dependenceis similar for both minerals (nH+ = 0.2) and this result is in accor-dance with the results for anthophyllite by Chen and Brantley(1998). It is generally accepted that for 2:1 phyllosilicates, theoctahedral cations are released to the solution more readily thantetrahedral ones (e.g., Brantley and Chen, 1995; Lasaga, 1995;Nagy, 1995; Hume and Rimstidt, 1992; Zysset and Schindler,1996; Jurinski and Rimstidt, 2001; Kalinowski and Schweda,2007; Rozalen et al., 2008; Bibi et al., 2011). Thereby, a generalmechanism for dissolution of tremolite and anthophyllite in acidmedia can be proposed: the relative ease of breaking of Mg–Obonds compared to that of Si–O bonds and the evidence of Mg–Hexchange reactions (Oelkers and Schott, 2001; Saldi et al., 2007)support that breaking of Si–O bonds is the limiting step of the pro-cess. This is sustained by our experimental results were measuredrates from released silica are lower than rates measured fromreleased Mg: RSi < RMg (see Tables 3 and 4 and Figs. 3b and 4b).

At around neutral pH the dissolution behavior of both mineralsis clearly different: the minimum of dissolution rate shifts from pH8 to pH 10 for tremolite and anthophyllite, respectively. In additionthe EQ3NR outputs might indicate precipitation of Mg carbonatesor hydroxides in tremolite dissolution experiments.

The main structural difference between tremolite andanthophyllite is found on the cations in M4 sites, Mg2+ and Ca2+,

Fig. 6. SEM images of the anthophyllite particles before and after the experiments: (a)form bundles with split ends, (c) and (d) fiber bundles start to splayed parallel to the fibeat pH 13 most of acicular particles have disappeared and prismatic particles are shortercould correspond with magnesite growing on the surface of anthophyllite fibers, (f) det

respectively. Differences in dissolution rate values may arises fromdifferences in chemical composition and structure.

The main difference in composition is the amount of Ca, whichis higher in tremolite and the amount of Mg and Fe, higher inanthophyllite (Eqs. (3) and (4)). Madelung energies vary for Si–O(1), Fe–O (0.5) and Mg–O (0.33) and Ca–O (0.33) bonds, and Mgand Ca are easier to detach from the structure. Schott et al.(1981) observed a preferential release of Ca and Mg with respectto Si, arguing a greater mobility of the cations in the M4 site overthe M1, M2 and M3 site in amphiboles.

Surface area of anthophyllite is almost a factor of 8 higher thanthat of tremolite. Higher the area, higher the number of reactivesurface sites exposed to the attack of H+, OH- and H2O moleculesand consequently dissolution rates are higher. Finally, differencesin the particles morphology of these two minerals also affect theirbehavior during dissolution. Figs. 5 and 6 show SEM pictures ofaltered samples observed after experiments finish at different pHs.

SEM image of non-reacted tremolite sample (Fig. 5a and Fig. 5b)shows highly variable grain size. Tremolite grains display mainlyprismatic and acicular habits, that form fiber bundles with a widevariety of length:diameter ratios. The material is highly friable anda large number of extremely fine fibers are observed.

SEM images of reacted material show morphological changesdue a process of particle breakage. These changes are more evidentin very acid/basic media. Fibers length decreases by splittingperpendicular to the main cleavage plane (see arrows in Fig. 5din acid pH and Fig. 5e for basic pH), decreasing the length:diameterratio. Coalescence of etch pits perpendicular to the c axis induces

starting material shows mainly acicular and needle-like particles, (b) most of themr longitudinal direction after 30 days of reaction at pH 1, (e) after 30 days of reaction

than in starting material. Little particles with rhombohedral habit (white arrows)ail of magnesite particles growing into a fiber after 30 days of reaction at pH 10.

M. Rozalen et al. / Applied Geochemistry 49 (2014) 46–56 55

breakage and particle shortening (Fig. 5e) as it is observed by thedevelopment of kinks. Finally, observation at pH 13 (Fig. 5f) showsstrong changes in morphology. Most of acicular particles have dis-appeared and prismatic particles are shorter than in starting mate-rial. Moreover little particles with rhombohedral habit couldcorrespond with magnesite growing on the surface of tremolitefibers. Precipitation of magnesite is consistent with solution super-saturation and other observations reported in literature (Halderand Walther, 2011).

Furthermore, the particle morphology of the non-reactedanthophyllite sample (Fig. 6a and b) shows mainly acicular andneedle-like particles. Most of them form bundles with split ends.Compared with non-reacted tremolite fibers, anthophyllite fiberslook longer and thinner. SEM images of dissolved mineral at veryacidic pH show fiber bundles start to splayed parallel to the fiberlongitudinal direction (Fig. 6c and d). Particle breakage results invery thin and fragile fibers with pronounced elongation in the cdirection. SEM images at pH 10 and 12 show the presence of smallparticles with rhombohedral habit of deposited material that prob-ably correspond to magnesite (Fig. 6e and f).

Microstructural differences arising during primary growth orduring subsequent alteration of a particular amphibole result indifferences in the behavior and properties (Veblen, 1980). More-over, Banfield and Barker (1994) reported that chain with defectsin amphibole increase the resistance of the amphibole to weather-ing, whereas stacking faults are sites of enhanced dissolution. Moredetailed microscopic studies can help to understand closely theprocess of dissolution.

4.3.4. Implication for remediation proceduresResults obtained in this study show different behavior in the

dissolution of these two minerals. Anthophyllite dissolves in acidmedia. As a consequence the remaining material, after 30 days ofreaction at room temperature, lost most of the fine material,considered to be the most dangerous (easy breathable fraction).Alkaline treatments would result less efficient as dissolution ratesdecrease between one and two orders of magnitude.

Tremolite dissolution seems to be faster in alkaline media, andfinal destruction of the material is even stronger than in the case ofanthophyllite. However, the effect of potential carbonation andprecipitation of secondary phases on the amphibole surface mayaffect the overall process.

This study shows that a simple acid/basic leaching at room tem-perature, could contribute to reduce the dangerousness of contam-inated sites by the presence of these amphiboles, in a similar wayas it was proposed for chrysotile by Rozalen and Huertas (2013).Moreover the complete outline of dissolution rates with pH pro-vides an excellent background to develop remediation process ofabandoned mines, contaminated areas and repositories of regu-lated wastes.

5. Conclusions

In the present study the effect of pH on tremolite and antho-phyllite dissolution rates was investigated at 25 �C, using batchreactors. Results from these experiments support the followingconclusions:

� Dissolution rates of tremolite in buffered inorganic solutionshave a noticeable dependence with pH between 1 and 8,decreasing when pH increases and reaching a minimum aroundneutral conditions. At basic pH conditions this dependencebecome even stepper. Effects of potential saturation and car-bonation were observed.

� Dissolution rates of anthophyllite have also a strongdependence on pH between 1 and 10, dissolving up to a factorof 8 faster than tremolite in similar conditions. At pHs > 10 thelog rate-pH dependence is slow. Carbonation was also observed.� SEM images show that the anthophyllite particle breakage con-

sist of the splitting parallel to the fiber longitudinal direction.However, for tremolite in addition to splitting, coalescence ofetch pits perpendicular to c axe also induces breakage and par-ticle shortening.� Anthophyllite dissolution is faster in acid media and tremolite

dissolves faster in basic media. Both treatments strongly dis-solve fine material, removing the most dangerous fraction ofthese materials and represent an excellent background todevelop remediation processes of contaminated sites.

Acknowledgements

Financial support was obtained from project P07-RNM-02772(Junta de Andalucía); MR benefited from a JAE-Doc contract fromCSIC. EU contribution through FEDER funds is acknowledged. Theauthors thank Eduardo Flores for assistance with laboratory work.FJH, amdg.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.apgeochem.2014.06.009.

References

Banfield, J.F., Barker, W.W., 1994. Direct observation of reactant-products interfaceformed in natural weathering of exolved, defective amphibole to smectite:Evidence of episodic isovolumetric reactions involving structural inheritance.Geochim. Cosmochim. Acta 58, 1419–1429.

Bibi, I., Singh, B., Silvester, E., 2011. Dissolution of illite in saline-acidic solutions at25 �C. Geochim. Cosmochim. Acta 75, 3237–3249.

Brantley, S.L., Chen, Y., 1995. Chemical weathering rates of pyroxenes andamphiboles. In: White, A.F., Brantley, S.L. (Eds.), Chemical Weathering Ratesof Silicate Minerals, vol. 31. Mineralogical Society of America, pp. 119–172.

Brantley, S.L., Conrad, C.F., 2008. Analysis of rates of geochemical reactions. In:Brantley, S.L., Kubicki, J.D., White, A.F. (Eds.), Kinetics of Water–RockInteraction. Springer, New York, pp. 1–37.

Cama, J., Metz, V., Ganor, J., 2002. The effect of pH and temperature on kaolinitedissolution rate under acidic conditions. Geochim. Cosmochim. Acta 66, 3913–3926.

Chen, Y., Brantley, S.L., 1998. Diopside and anthophyllite dissolution at 25 �C and90 �C and acid pH. Chem. Geol. 147, 233–248.

Clayton, J.L., 1986. An estimate of plagioclase weathering rate in the Idaho Batholithbased upon geochemical transport rates. In: Colman, S.M., Dethier, D.P. (Eds.),Rates of Chemical Weathering of Rocks and Minerals. Academic Press, Orlando,pp. 453–467.

Fernández, R., Cuevas, J., Sánchez, L., Vigil de la Villa, R., Leguey, S., 2006. Reactivityof the cement–bentonite interface with alkaline solutions using transport cells.Appl. Geochem. 21, 977–992.

Grandstaff, D.E., 1977. Some kinetics of bronzite orthopyroxene dissolution.Geochim. Cosmochim. Acta 41, 1097–1103.

Grasshoff, K., Ehrhardt, M., Kremling, K. (Eds.), 1983. Methods of Seawater Analysis,second ed. Verlag Chemie.

Gunter, M.E., Belluso, E., Mottana, A., 2007. Amphiboles: environmental and healthconcerns. In amphiboles: crystal chemistry, occurrences, and health concerns.Rev. Mineral. Geochem. 67, 453–516.

Halder, S., Walther, J.V., 2011. Far from equilibrium enstatite dissolution rates inalkaline solutions at earth surface conditions. Geochim. Cosmochim. Acta 75,7486–7493.

Hume, L.A., Rimstidt, J.D., 1992. The biodurability of chrysotile asbestos. Am. Miner.77, 1125–1128.

Jurinski, J.B., Rimstidt, J.D., 2001. Biodurability of talc. Am. Miner. 86, 392–399.Kalinowski, B.E., Schweda, P., 2007. Rates and nonstoichiometry of vermiculite

dissolution at 22 �C. Geoderma 142, 197–209.Knauss, K.G., Nguyen, S.N., Weed, H.C., 1993. Diopside dissolution kinetics as a

function of pH, CO2, temperature and time. Geochim. Cosmochim. Acta 57, 285–294.

56 M. Rozalen et al. / Applied Geochemistry 49 (2014) 46–56

Kohler, S.J., Bosbach, D., Oelkers, E.H., 2005. Do clay mineral dissolution rates reachsteady state? Geochim. Cosmochim. Acta 69, 1997–2006.

Lasaga, A.C., 1995. Fundamental approaches to describing mineral dissolution andprecipitation rates. In: White, A.F., Brantley, S.L. (Eds.), Reviews in Mineralogy:Chemical Weathering Rates of Silicate Minerals, vol. 31. Mineralogical Society ofAmerica, Washington, DC, pp. 23–86.

Lasaga, A.C., 1998. Kinetic theory and applications in earth sciences. Princeton Press,Princeton.

Luce, R.W., Bortlett, R.W., Parks, G.A., 1972. Dissolution kinetics of magnesiumsilicates. Geochm. Cosmochim. Acta 36, 35–50.

Mast, M.A., Drever, J.I., 1987. The effect of oxalate on the dissolution rates ofoligoclase and tremolite. Geochim. Cosmochim. Acta 51, 2559–2568.

Nagy, K.L., 1995. Dissolution and precipitation kinetics of sheet silicates. Rev. Miner.31, 173–233.

Oelkers, E.H., Schott, J., 2001. An experimental study of enstatite dissolution rates asa function of pH, temperature, and aqueous Mg and Si concentration, and themechanism of pyroxene/pyroxenoid dissolution. Geochim. Cosmochim. Acta 65,1219–1231.

Rozalen, M., Huertas, F.J., 2013. Comparative effect of chrysotile leaching in nitric,sulfuric and oxalic acid at room temperature. Chem. Geol. 352, 134–142.

Rozalen, M.L., Huertas, F.J., Brady, P.V., Cama, J., Garcia-Palma, S., Linares, J., 2008.Experimental study of the effect of pH on the kinetics of montmorillonitedissolution at 25 �C. Geochim. Cosmochim. Acta 72, 4224–4253.

Rozalen, M., Ramos, M.E., Fiore, S., Gervilla, F., Huertas, F.J., 2014. Effect of oxalateand pH on chrysotile dissolution at 25 �C: an experimental study. Am. Miner.99, 589–600.

Saldi, G.D., Kohler, S.J., Marty, N., Oelkers, E.H., 2007. Dissolution rates of talc as afunction of solution composition, pH and temperature. Geochim. Cosmochim.Acta 71, 3446–3457.

Sanemasa, I., Kataura, T., 1973. The dissolution of CaMg(SiO3)2 in acid solutions.Bull. Chem. Soc. Jpn. 46, 3416–3422.

Schott, J., Berner, R.A., 1983. X-ray photoelectron studies of the mechanism ofiron silicate dissolution during weathering. Geochim. Cosmochim. Acta 47,2233–2240.

Schott, J., Berner, R.A., 1985. Dissolution mechanism of pyroxenes and olivinesduring weathering. In: Drever, J.I. (Ed.), The Chemistry of Weathering, vol. 149.Reidel, Dordrecht.

Schott, J., Berner, R.A., Sjoberg, E.L., 1981. Mechanism of pyroxene and amphiboleweathering: I. Experimental studies of iron-free minerals. Geochim.Cosmochim. Acta 45, 2123–2135.

Siegel, D.I., Pfannkuch, H.O., 1984. Silicate mineral dissolution at pH 4 andnear standard temperature and pressure. Geochim. Cosmochim. Acta 48,197–201.

Siever, R., Woodford, N., 1979. Dissolution kinetics and the weathering of maficminerals. Geochim. Cosmochim. Acta 43, 717–724.

Veblen, D.R., 1980. Anthophyllite asbestos: microestructures, intergrown sheetsilicates, and mechanism formation. Am. Miner. 65, 1075–1086.

Wolery, T.J., 1992. EQ3NR, a computer program for geochemical aqueousspeciation-solubility calculations: theoretical manual, user’s guide and relateddocumentation (version 7). UCRL-MA-110662 PT III. Laurence LivermoreNational Laboratory.

Zhang, H., Bloom, P.R., Nater, E.A., 1993. Change in surface area and dissolutionrates during hornblende dissolution at pH 4.0. Geochim. Cosmochim. Acta 57,1681–1689.

Zysset, M., Schindler, P.W., 1996. The proton promoted dissolution kineticsK-montmorillonite. Geochim. Cosmochim. Acta 60, 921–931.