The Role of pH, Temperature, and NH4+ during Mica Weathering

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The Role of pH, Temperature, and NH4+ during Mica Weathering

Daniel Lamarca-Irisarri, Alexander van Driessche, Guntram Jordan, Chiara Cappelli, F. Javier Huertas

To cite this version:

Daniel Lamarca-Irisarri, Alexander van Driessche, Guntram Jordan, Chiara Cappelli, F. Javier Huer-

tas. The Role of pH, Temperature, and NH4+ during Mica Weathering. ACS Earth and Space

Chemistry, ACS, In press, �10.1021/acsearthspacechem.9b00219�. �hal-02350935�

HAL Id: hal-02350935

https://hal.archives-ouvertes.fr/hal-02350935

Submitted on 6 Nov 2019

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

The Role of pH, Temperature, and NH4+ during Mica Weathering

Daniel Lamarca-Irisarri, Alexander van Driessche, Guntram Jordan, Chiara Cappelli, F. Javier Huertas

To cite this version:

Daniel Lamarca-Irisarri, Alexander van Driessche, Guntram Jordan, Chiara Cappelli, F. Javier Huer-

tas. The Role of pH, Temperature, and NH4+ during Mica Weathering. ACS Earth and Space

Chemistry, ACS, 2019, �10.1021/acsearthspacechem.9b00219�. �hal-02350935�

1

The Role of pH, Temperature, and NH ₄ ⁺ during Mica Weathering

2

Daniel Lamarca-Irisarri,

^†

Alexander E. S. Van Driessche,*

^,‡

Guntram Jordan,

^§

Chiara Cappelli,

^∥

3

and F. Javier Huertas

^†

4^†

Instituto Andaluz de Ciencias de la Tierra (CSIC-University of Granada), Avda. de las Palmeras 4, 18100 Armilla, Granada, Spain

5^‡

Universite ́ Grenoble Alpes, Universite ́ Savoie Mont Blanc, CNRS, IRD, IFSTTAR, ISTerre, F-38000 Grenoble, France

6^§

Department fu ̈ r Geo- und Umweltwissenschaften, Ludwig-Maximilians-Universita ̈ t Mu ̈ nchen, Theresienstr. 41, 80333 Mu ̈ nchen,

7

Germany

8^∥

Department of Geological Sciences, The University of Alabama, 201 Seventh Avenue, 2003 Bevill Building, 35487 Tuscaloosa,

9

Alabama, United States

10

*

^S Supporting Information

11

ABSTRACT: Phyllosilicates are abundant materials both on Earth and Mars,

12

and the weathering of these minerals is an essential part of a wide variety of

13

geochemical cycles. Alteration mainly takes place at the solution − mineral

14

interface and needs to be fully understood in order to correctly model global

15

water − rock interactions. To directly link the physicochemical solution

16

properties to the dominant surface processes controlling phyllosilicate

17

alteration, we used a custom-built hydrothermal atomic force microscope to

18

study in situ the surface reactivity of biotite, phlogopite, and muscovite in

19

contact with aqueous solutions for a broad range of temperatures and pH

20

values. On the basis of our microscopic observations correlated with

21

previously obtained macroscopic dissolution rates, we have constructed a

22

tentative weathering diagram for mica minerals connecting the dominant

23

surface mechanisms and bulk dissolution behavior to the physicochemical

24

solution properties (pH, T, and speciation). The resulting diagram can be

25

divided into two main areas: low-grade weathering occurring at low temperatures and mildly acidic to neutral pH and high-

26

grade weathering taking place at high temperatures and low pH, separated by a transition zone. Each of these areas is

27

characterized by a series of chemical and physical surface processes, which can be related directly or indirectly to incongruent

28

and congruent bulk dissolution. The transition temperatures and pH values depend on the type of mica, with biotite being the

29

most reactive one and muscovite the least reactive one. It is noteworthy that for close to neutral pH conditions the presence of

30

NH

₄⁺

shifts the transitions from low- to high-grade weathering to a signi fi cantly lower temperature.

31

KEYWORDS: hydration, dissolution, (re)precipitation, biotite, muscovite, phlogopite, in situ observation, weathering diagram

1. INTRODUCTION

32

The weathering of phyllosilicates, e.g., clay minerals and micas,

33

is a ubiquitous process on the (sub)surface of Earth

^1,2

and

34

Mars.

³

In the latter case, the presence of phyllosilicates is used

35

as an important indicator of past climate.

^4,5

On Earth, clay

36

minerals (partially) determine many physicochemical proper-

37

ties such as porosity, ion exchange, and adsorption of

38

sediments and soils that in turn control the circulation of

39

fl uids and availability of nutrients and contaminants.

^6,7

Clay

40

minerals are also important in engineered environments, where

41

they are routinely used as binders for molding sands in

42

foundries,

^8,9

sealing liners in land fi lls and water dams,

¹⁰

or as

43

barriers in geological long-term nuclear waste disposal

44

facilities,

¹¹

to name just a few. Accordingly, the reactivity of

45

clay minerals has been studied in detail for a broad range of

46

conditions probing the in fl uence of pH, temperature, and

47

salinity, among others.

¹²⁻¹⁸

Micas are important rock-forming

48

minerals, whose dissolution directly in fl uences soil and

groundwater chemistry. In soils, for example, the release of

49

potassium from micas, such as biotite, is essential for plant

50

growth.

¹⁹

In deeper media the alteration of micas to

51

expandable smectites and vermiculite leads to changes in the

52

rock structure and sediment porosity.

²⁰ 53

The reactivity of mica has been extensively studied using

54

mainly macroscopic (i.e., bulk) and microscopic (i.e., nano-

55

scale) approaches. Di ff erent mica minerals, such as biotite,

56

phlogopite, muscovite, or other sheet silicates, such as

57

apophyllite, have been investigated.

²¹⁻²⁵

Bulk studies have

58

shown that a departure from neutral pH conditions always

59

leads to faster dissolution.

22,23,26−28

Also, at higher temper-

60

atures the dissolution rate is higher.

²⁷

On the other hand,

61

Received: August 7, 2019 Revised: October 18, 2019 Accepted: October 22, 2019 Published: October 22, 2019 A

62

single-crystal experiments unveiled that dissolution mainly

63

takes place at the crystal edges, while often the basal surface is

64

more resistant to dissolution showing for certain experimental

65

conditions hydration patterns, surface decomposition, and/or

66

etch pit formation.

²⁹⁻³²

In situ and ex situ atomic force

67

microscopy (AFM) studies at elevated temperatures (>80 ° C)

68

have shown that the reactivity of the basal surface increases

69

signi fi cantly,

^32,33

although it was observed that at temperatures

70

above 120 ° C the formation of secondary phases (e.g.,

71

gibbsite/kaolinite) coats the surface, blocking reactive sites

72

and thus lowering the basal surface reactivity.

³²

However,

73

despite the insights gained from the previous studies, a general

74

correlation between the physiochemical solution properties

75

(e.g., pH, temperature, and salinities) and the nanoscale

76

patterns at the mineral − solution interface that result from the

77

various alteration processes (i.e., dissolution, hydration, ion

78

exchange, and (re)precipitation) has not yet been established.

79

Moreover, the link between surface processes and the

80

measured bulk dissolution rates also warrants further

81

clari fi cation.

82

Recently, it was reported that mica minerals could act as an

83

important carbon reservoir. Detailed AFM observations of

84

phlogophite and muscovite revealed that nanoprotusions, i.e.,

85

bulges, located at the basal surfaces trapped signi fi cant

86

quantities of hydrocarbons

³⁴

and supercritical CO

₂

,

³⁵

87

respectively. At present, it remains unclear if these inclusions

88

are primary, i.e. syngenetic with the rock formation, or

89

secondary, i.e. related to weathering phenomena, and how the

90

hydrocarbons/scCO

₂

entered the interlayer of these non-

91

swelling phyllosilicates. Only the reaction with wet-scCO

₂

92

caused the formation of bulges, whereas scN

₂

, dry-scCO

₂

,

93

and scCO

₂

-saturated brine did not. This suggests that a scCO

₂

-

94

acidi fi ed water fi lm between bulk scCO

₂

and muscovite is

95

needed for CO

₂

molecules to enter the interlayers. These

96

observations further highlight the crucial role of surface

97

processes during phyllosilicate weathering.

98

To further our understanding of the complex interplay

99

between the physicochemical solution properties and the

100

process controlling mica alteration we directly observed the

101

surface reactivity of biotite, phlogopite, and muscovite in

102

contact with aqueous solutions for a broad range of pH values

103

(1−6.8) and temperatures (23−120 °C), making use of

104

hydrothermal atomic force microscopy (HAFM). These short-

105

term (up to ∼ 8 h) in situ experiments were complemented

106

with ex situ characterization of long-term (up to 50 days) and

107

high-temperature (up to 200 ° C) experiments conducted in

108

hydrothermal batch reactors. In ammonium-rich environments

109

(e.g., crustal rocks, diagenetic sediments), potassium of K-

110

bearing minerals is commonly substituted by ammonium

111

because of the similar charge and ionic radius.

³⁶

Despite this

112

abundant occurrence of mica with NH

₄⁺

fi xed in its interlayer,

113

and its capacity to act as a signi fi cant source of protons at

114

circumneutral pH, the potential in fl uence of this ion on

115

phyllosilicate weathering has been largely overlooked. There-

116

fore, in this study we have also conducted experiments at pH

117

6.8 using solutions containing 0.1 M NH

₄⁺

.

118

The obtained microscopic data were analyzed in detail and

119

correlated with previously obtained macroscopic dissolution

120

rates. On the basis of these insights we constructed a tentative

121

weathering diagram, linking the surface mechanism and bulk

122

dissolution behavior to the main physicochemical solution

123

properties.

2. MATERIALS AND METHODS

2.1. Natural Samples. The materials used in this study are

124

(1) Bancroft biotite, (2) Templeton phlogopite, and (3)

125

Madras muscovite. Their chemical bulk composition was

126

determined by X-ray fl uorescence spectrometry (XRF), and an

127

estimation of Fe

²⁺

and Fe

³⁺

was obtained based on literature

128

data.

^37,38 129

(1) Biotite: K

_0.97

(Mg

_1.40

Fe

²⁺_1.18

Fe

³⁺_0.11

Al

_0.04

Ti

_0.09

Mn

_0.09

)-

130

(Al

_0.97

Si

_3.03

)O

₁₀

(OH)

₂ 131

(2) Phlogopite: K

_0.93

(Mg

_2.63

Fe

²⁺_0.13

Fe

³⁺_0.02

Al

_0.16

)(Al

_1.03

-

132

Si

_2.97

)O

₁₀

(OH)

₂ 133

(3) Muscovite: (K

_0.9

Na

_0.08

)(Mg

_0.09

Fe

²⁺_0.17

Fe

³⁺_0.02

Al

_1.75

-

134

Ti

_0.03

)(Al

_0.94

Si

_3.06

)O

₁₀

(OH)

₂ 135

2.2. In Situ Characterization. In situ observation of the

136

mica-solution interface was carried out using a custom-built

137

contact-mode hydrothermal atomic force microscope

^39,40

and

138

uncoated Si-cantilevers with integrated tips (Nanosensors). A

139

schematic drawing of this experimental setup can be found in

140

ref 38. For the experiments, mica fl akes with an area of ∼ 25 −

141

35 mm

²

and thickness of ∼ 0.1 − 0.4 mm were exfoliated

142

immediately before fi xing them with a titanium wire within the

143

HAFM cell. Subsequently, the cell was fi lled with solution,

144

sealed, and pressurized to reach the desired temperature (80,

145

100, or 120 ° C). The cell has been connected to 3 reservoirs

146

containing solutions with di ff erent compositions, which could

147

be fl owed through the cell independently. The fl ow rates

148

ranged from 3 to 5 mm

³

· s

⁻¹

, allowing a rapid renewal of the

149

fl uid within the cell (volume ≈ 500 μ L). Because of rapid fl uid

150

renewal and a mineral surface area of only a few square

151

millimeters, the chemical composition of the fl uid was

152

negligibly a ff ected by mineral −fl uid reactions occurring in

153 154 t1

the cell. All solutions (Table 1) were prepared by dissolving

reagent grade chemicals into deionized water (18 M Ω· cm).

155

Using this setup, a total of over 60 experiments (Table S1)

156

were performed of varying duration (3 − 10 h).

157

Two di ff erent protocols were used for the in situ HAFM

158

measurements: (I) variable pH experiments to test the pH

159

e ff ect on the surface reactivity and (II) constant pH

160

experiments to study the long-term e ff ect of each experimental

161

solution on the alteration dynamics.

162

2.3. Ex Situ Characterization. To extend the observation

163

time ( ∼ 10 h max) of the HAFM experiments, batch reactors

164

were set up for posterior ex situ characterization of the reacted

165

surfaces. In each 45 mL reactor (Parr 4744), one mica fl ake of

166

∼ 25 − 35 mm

²

and ∼ 0.1 − 0.4 mm thickness was placed and 35

167

mL of reacting solution was added (Table 1). The reactors

168

were maintained in an oven at constant temperatures of 120

169

Table 1. Overview of the Aqueous Solutions Used To React with the Mica Surfaces

solution pH remarks

0.1 M HCl 1.0 For the pH 2 and 4 solutions, the ionic strength was keep constant, with respect to the pH 1 solution, by adding a corresponding amount of KCl.

0.01 M HCl 2.0 0.001 M HCl 4.0

deionized water

5.8 Always used as initial solution during pressurization and heating of the HAFM system.

0.1 M ammonium acetate

6.8

B

170

and 200 ° C ( ± 3 ° C) for 1, 2, 3, and 50 days. Mica samples

171

were recovered from the reactors and washed three times with

172

deionized water. Then, the basal surface topography of the

173

reacted mica fl akes was immediately characterized using AFM.

174

Measurements were done at room temperature ( ∼ 23 ° C) in

175

contact mode using nonconductive silicon nitride uncoated

176

tips (Bruker). For chemical characterization of the altered

177

surfaces, reacted samples were imaged using a scanning

178

electron microscope equipped with an energy-dispersive X-

179

ray spectroscope (SEM, CIC - University of Granada).

3. RESULTS

180

3.1. Hydration Processes. With our experimental setup,

181

two main features of mica hydration were observed: (I)

182

swelling f ronts, i.e., the fronts of more or less uniformly swelled

183

layers spreading laterally on the surface; the increase of the

184

layer height ranged between ∼ 0.5 nm and a few nanometers

185

and a ff ected large areas of the basal surface.

⁴¹⁻⁴³

(II) bulges,

186

i.e., abrupt and laterally con fi ned swelling of the surface in a

187

round or an elongated and partially branched shape. Bulges can

188

migrate on the surface and most likely result from an excessive

189

but spatially con fi ned hydration.

^28,39,40

190

3.1.1. Bulge Formation. At pH 1 and T > 80 ° C, the

191

reactivity of biotite was too high for evaluable AFM

192

observations for a su ffi ciently long period of time. At pH 1

and 20 − 80 ° C, the formation of point-shaped bulges (up to 25

193 194 f1

nm in height) was observed after ∼ 30 min (Figure 1A,B), which eventually broke open at the top. Similar observations

195

were made at pH 2 in the temperature range of 80 − 100 ° C

196

(data not shown). At pH 4 (80 and 100 ° C), the top of the

197

bulges became more rounded and remained unbroken. It is

198

worth noting that at pH 4 − 5.8, bulges still migrated across the

199

surface (Figure 1C − F). In NH

₄⁺

-solutions (pH 6.8), bulges

200

were less abundant but were also round-shaped and did not

201

break open.

202

203 f2

At pH 4 − 6.8 and all temperatures, bulges branched (Figure

204 f2

2A) following directions that roughly corresponded to the main PBC directions ⟨ 110 ⟩ , ⟨ 11 ̅ 0 ⟩ , and ⟨ 100 ⟩ .

⁴⁴

This indicates

205

a certain degree of structural control over the formation and

206

migration of bulges. Occasionally, bulges showed an open side

207

at steps (Figure 2B) and allowed comparison of the spacing

208

between the swelled and nonswelled layers revealing a roughly

209

doubled spacing of the altered layer (Figure 2C) at pH ≥ 4

210

and an expansion of 28 − 30 nm/layer at pH ≤ 2 (Figure S1),

211

due to complete disruption of the mica layered structure.

212

Within the pH range tested here, no bulge formation was

213

observed on muscovite and phlogopite. Previous studies of

214

cation exchange in phlogopite (using octylammonium and

215

NaCl solutions), however, did report the formation and

216

migration of bulges.

^42,43

Recently, bulges (referred to as

217

Figure 1.In situ HAFM images of initial surface patterns formed on biotite basal faces in contact with acidic solutions. (A) Typical bulges observed after 35 min at pH 1 and 80°C. (B) Cross sections of the bulge structures observed in image A, showing sharp profiles, indicating the initial stage of fracturing. (C) Cross sections and (D−F) time-lapse images of bulge formation and migration at pH 4 and 100°C. After 5 h and 20 min bulges of 30 nm developed. These structures moved at a rate of∼0.85μm/min toward the upper right corner of the image, leaving behind two elongated relict areas.

Figure 2.(A) Optical microscope image of a (001) surface of biotite reacted for 16 days at 50°C and pH 6.8, showing the regular alignment of the bulges, and (B) AFM deflection image of (001) face of biotite reacted for 3 days at 200°C and pH 6.8, (C) showing a∼200 nm high broken bulge (1) and a∼100 nm high nonswelled area (2). This points to a ratio of about 2 nm/layer (1 nm of swelling).

Article

C

218

nanoprotrusions) containing hydrocarbons were detected on

219

phlogopite from Ugandan kamafugite rock samples.

³⁴

220

Furthermore, bulges (referred to as blistering) were observed

221

on muscovite exposed to water-saturated scCO

₂

at 12 MPa and

222

90 ° C.

³⁵

223

3.1.2. Swelling Front Formation and Collapse Structures.

224

The second hydration process frequently observed during the

225

initial stages of biotite alteration is the development of swelling

226

fronts. At pH 1 − 2, the swelling fronts appeared static (by the

227

time AFM imaging started). However, at pH 4 − 5.8 and T =

f3 228

25 − 120 ° C, swelling fronts were moving still (Figure 3).

229

To further unravel the in fl uence of pH on the hydration

230

patterns, experiments were carried out with several sequential

231

in situ changes of solution pH. Three di ff erent solution

232

sequences were tested: (I) pH 5.8 → 6.8, (II) pH 5.8 → 6.8 →

233

2.0 → 5.8 → 6.8, and (III) pH 4.0 → 6.8. The most noticeable

234

surface process during these runs was the collapse of previously

235

formed swelled layers when an ammonium solution (pH 6.8)

236

was introduced (Figures 3D and S2). Experiments using

237

sequence I repeatedly allowed observation of the swelling and

238

fl attening of the surface (Figure S3). As a consequence of this

239

repeated sequence, swelling and collapse networks resulted on

240

the surface (Figure S4). A common structure associated with

241

the collapse of swelling fronts and bulges is the formation of

242

relict areas (circles in Figure 3D). Relict areas (also termed

243

engulfed areas

⁴³

) are small swelled remnants of previously

244

large swelled or bulged surface areas. Thus, the relict areas

245

likely represent isolated vertically expanded areas on the

246

surface in which water molecules from a previous large-scale

247

hydration event are trapped.

3.2. Dissolution Patterns. Given the dissolution dynamics

248

and the overall observed surface roughness, we have differ-

249

entiated two levels of surface alteration occurring during the

250

reaction of aqueous solutions with mica minerals: (I) low-

251

grade decomposition (LGD), which is characterized by

252

roughening (<1 nm height fl uctuations) and weakening of

253

the given surface layers (which decompose by the mechanical

254

impact of the AFM-tip), rather than by mono or multilayer

255

etch pit formation and subsequent step retreat, and (II) high-

256

grade decomposition (HGD), which is characterized by a

257

roughening of the surface due to the formation of a high

258

mono- or multilayer step density (via a high etch pit nucleation

259

rate).

260

Dissolution (ss) usually initiated after the above-described

261

hydration processes. With decreasing pH and increasing

262

temperature, however, delay of dissolution decreased and

263

both processes occurred increasingly contemporaneous. At pH

264

1 the surface reactivity is high and was best observed at room

265

temperature (23 ° C). For these conditions, dissolution is

266

dominated by step retreat ( ∼ 0.01 μ m/min), which often

267

advanced more rapidly in a speci fi c direction as a consequence

268

of coalescensing pits concentrated along that direction and

269 270 f4

eventually created dissolution channels (Figure 4 A,B). Also the nucleation of monolayer ( ∼ 1 nm) etch pits was frequently

271

observed (data not shown). At higher temperatures (80 ° C),

272

the formation of etch pits was very abundant and rapidly led to

273

a rough surface, i.e., HGD (Figure 4 C,D). At pH 2 and 100

274

° C, the surface reactivity was lower compared to pH 1 and

275

dominated by step retreat. This corresponds well with previous

276

in situ confocal microscopy observations of biotite flakes

277

reacting with nitric acid solutions at pH 1.

⁴⁵

We also detected

278

the development of fusiform-shaped swelled areas (see

279

previous section), which in some cases cracked open and

280

collapsed. Cracking of the swelled layers resulted in steps with

281

subsequent retreat (Figure S5). Starting from 120 ° C etch pit

282

formation became the dominant dissolution process (Figure

283

4G,H). Thus, at low pH, etch pit formation and step retreat are

284

the most common dissolution processes taking place on the

285

(001) surface of biotite, with temperature being the parameter

286

controlling which process will be dominant.

287

At pH 4, the reactivity of biotite is dominated by partial and

288

unordered destruction of the top TOT-layer (characterized by

289

<1 nm height fl uctuations of the surface layer), which

290

weakened the surface, and which we define as low-grade

291

surface weathering. Importantly, for these solution conditions

292 293 f5

heterogeneous surface reactivity was observed. Figure 5 shows how certain areas of a layer were dissolved because of etch pit

294

nucleation, while other areas of the same layer were less

295

reactive, remaining virtually unaltered for the duration of the

296

observation. These more reactive areas were previously

297

affected by bulging that did not fully collapse afterward. This

298

suggests that not fully collapsed layers are more prone to

299

dissolve, i.e. the bulging process weakened the structure of the

300

top layers.

301

Because of the low surface reactivity at pH 5.8, dissolution

302

patterns could not be observed during HAFM experiments. To

303

obtain information about the surface reactivity for these

304

solutions, mineral alteration was carried out in batch reactors

305

at 200 ° C and characterized ex situ. After 3 days of reaction,

306

mainly low-grade surface weathering was observed but no etch

307

pits could be detected.

308

In the solutions containing NH

₄⁺

(pH 6.8) at T ≤ 120 ° C,

309

hydration, dissolution, and precipitation processes occurred

310

Figure 3.In situ HAFM observation (6.3 h) of the interaction of a pH 4 solution at 100°C with a biotite surface: (A) initial biotite surface topography after reaching 100 °C and (B) formation of swelling fronts (white arrows) of 5 nm in height after 340 min. (C) Subsequently, bulges evolved during the following 25 min (white arrows). This bulging network was approximately 60 nm in height.

(D) General collapse after 10 min of ammonium solution treatment.

Several relict areas remained on the surface (white circles), and overall increase of the surface roughness is observed.

D

311

simultaneously with comparable intensity and obscured

312

observation of individual etch pits and/or retreating steps. At

T > 120 ° C, etch pits formed, although with a lower density

313

than in acidic solutions. The fact that at pH 5.8 and T > 120

314

°C no etch pits were observed (see above) points to a

315

remarkable promotion of the reactivity of biotite basal surface

316

by NH

₄⁺

.

317

Samples reacted with NH

₄⁺

solutions for longer times at

318

di ff erent temperatures (50 and 120 ° C) were analyzed ex situ

319

by SEM. This revealed the presence of Si-enriched layers

320

(Figure S6). Such layers were much more extensive for the

321

samples reacted at 50 ° C than those at 120 ° C, where Si-

322

enrichment was detected only at step edges. The long-term

323

experiments (50 days) at 200 ° C revealed intense pit

324

formation, reaching a depth of up to 10 nm (Figure S7).

325

The angles between intersecting edges of these pits correspond

326

with those of the structure of the basal surface, which indicates

327

that dissolution morphology is eventually controlled by

328

structural factors.

⁴⁴⁻⁴⁶ 329

3.3. Precipitates. At pH 1 − 5.8 and T < 100 ° C, no

330

precipitates were formed within approximately 5 h of HAFM

331

imaging. In contrast, in NH

₄⁺

solutions (pH 6.8), extensive

332

formation of a secondary phase(s) was observed during in situ

333

HAFM imaging of biotite and muscovite but never in the case

334

of phlogopite. Isolated particles of ∼ 1 nm height appeared on

335

biotite and muscovite after 140 min at 120 ° C and 5 h at 100

336

° C, respectively (Figure S8). In both cases, the individual

337

particles formed randomly, expanded laterally, merged, and

338

covered the entire surface while the height remained constant.

339

When a pH 2 solution was injected into the HAFM liquid cell,

340

the coating rapidly dissolved, exposing the pristine mica

341

surface. The persistence of the underlying mica surface clearly

342

shows that no surface amorphization took place under these

343

conditions (which would have led to fast and instantaneous

344

dissolution in the presence of an acidic solution).

345

Batch experiments on biotite reacted for 3 days at pH 6.8

346

and T = 120 ° C showed the presence of precipitates forming a

347

coating ( ∼ 1 nm) on the surface and areas depleted in

348

octahedral cations/enriched in Si (Figure S9). At pH 6.8 and T

349

= 200 ° C, the surface was partially coated with dendritic

350 351 f6

shaped precipitates after 1 day (Figure 6A). These precipitates had a height of ∼ 1.2 nm. After 2 days of reaction at 200 ° C,

352

the precipitates coated the surface while their height remained

353

unchanged. After 50 days at 200 ° C, large precipitates were

354

observed mainly composed of Fe and Al and some containing

355

Ti (Figure 6B).

356

Figure 4.In situ (H)AFM observation of biotite dissolution at low pH. (A and B) At pH 1 and 23°C, surface dissolution is dominated by step retreat and etch pit development (black arrows); (C and D) pH 1 and 80°C. Abundant surface swelling was observed 38 min after the onset of the experiment, and after 51 min, the roughness increased because of the formation of numerous small etch pits (inset E (deflection channel) and F (height channel) indicated by a black arrow), (G and H) pH 2 and 120 °C. After 47 min of reaction, abundant etch pits have developed (black arrow) and step retreat has occurred.

Figure 5.In situ HAFM images of the biotite surface reactivity at pH 4 and 120°C, which shows the link between hydration and dissolution dynamics. (A and B) Migration of bulges (b) and swelling fronts (s) on the (001) face of biotite. Once the bulges disappeared (C), the remaining layer was“weakened”in areas that are not completely collapsed and dissolved more readily than the rest of the surface; (D) a zoomed-in view of the dotted rectangular area of images A−C. Abundant etch pits (1−3 nm deep) developed in the area that was previously affected by two bulges.

The area unaffected by bulges, or affected by bulges that totally collapsed in a second step, shows only <1 nm heightfluctuations of the surface layer (no≥1 nm etch pits are observed).

Article

E

4. DISCUSSION

357

4.1. Hydration. During the initial stages of the mica surface

358

alteration, both bulges and swelling fronts were the most

359

common surface features. Crystalline swelling (intercalation

360

between individual 2:1 layers) is a process controlled by a

361

balance between attractive forces (i.e., Coulombic attraction

362

between the negative TOT-layers and the interlayer cations

363

and van der Waals interactions between adjacent layers) and

364

repulsive forces (mainly due to the hydration potential energy

365

of the interlayer cations

⁴⁷

). At acidic pH, a rapid K

⁺

− H

₃

O

⁺

ion

366

exchange likely takes place and an increase of interlayer space

367

occurs because of the readjustment between attraction and

368

repulsion forces and the entrance of water molecules (e.g., 41).

369

When ammonium is added to the solution, a general collapse

370

of the swelled layers occurred, most likely because strong

371

hydrogen bonds form among NH

₄⁺

, H

₂

O, and the negatively

372

charged mineral surface recreating a tight interlayer structure.

⁴⁷

373

In the case of bulges (i.e., nanoprotrusions or blistering), the

374

precise formation mechanism is still unclear, but it seems to be

375

related to the excess water uptake due to local variations of the

376

layer charge.

³¹

Indeed, the local decrease of negative layer

377

charge leads to a decrease of the Coulombic attraction between

378

the interlayer cations and the surface charge sites

⁴⁷

likely

379

causing a higher cation hydration. Bulges also have the capacity

380

to migrate across the surface until they reach a location where

381

the negative charge is high enough to stop them from moving

382

(Figures 3 and S2). However, this behavior has been observed

383

only between pH 4 and 5.8. At lower pH values, the

384

hydronium concentration is high enough to favor the exchange

385

between interlayer cations and hydronium (K

⁺

− H

₃

O

⁺

) and the

386

mica structure alteration by reactive sites protonation (proton-

387

promoted dissolution). Our observations during both fi xed and

variable pH experiments showed that the development of

388

bulges is mainly linked to previous mineral features such as

389

swelling fronts (e.g., Figures S2 and S3), cracks or layer edges

390

where charge imbalance and bond strain could exist.

391

Importantly, our observations also show that bulges play a

392

key role in the development of softened areas, which show

393

increased dissolution dynamics (see e.g. Figure 5).

394

The morphology of bulges changes as a function of pH, i.e.

395

at acidic pH they have a sharp pro fi le and are usually cracked

396

open along their axial plain, while the profile of bulges formed

397

in near neutral conditions tends to be rounded. This might be

398

the result of the higher capacity of hydronium to attack the Si −

399

O − Si and Si − O − Al groups of the tetrahedral sheet than water

400

molecules. This agrees with our measurements of the surface

401

topography of bulges; these expanded ∼ 2 nm per layer

402

between pH 4 and 6.8 and ∼ 16 nm per layer in acidic media

403

(pH < 4). The 2 nm per layer ratio can be linked to the

404

expansion capacity of phyllosilicates of low charge while the

405

much higher ratio in acidic media points to a generalized

406

destabilization of the crystalline structure, which eventually

407

leads to its disruption, e.g., cracking of the surface layers. Also

408

important to consider is the role of Fe

²⁺

oxidation in the

409

expansion of mica layers. During mica dissolution, Fe

²⁺

will

410

oxidize (even in very acidic HCl solutions), which should

411

reduce the TOT-layer charge and favor the release of interlayer

412

cations, making the structure unstable and favoring volume

413

expansion and cracking of the top layers. This process has been

414

observed in fi eld studies, where during the weathering of

415

quartz diorites Fe(II) oxidation in biotite led to volume

416

expansion and produced fractures which accelerated further

417

reactions.

⁴⁸ 418

Figure 6.(A) Ex situ AFM characterization of a biotite surface reacted with a pH 6.8 solution for 1 day at 200°C. Dendritic-shaped precipitates with a thickness of∼1.2 nm covered the basal surface of biotite. Height profile at the right side is that of the white dotted line shown in the 3D AFM image representing the black dotted square area in the 2D image. (B) AFM and SEM images of basal biotite surface reacted for 50 days at 200

°C and pH 6.8, showing (upper) plate-shaped precipitates composed of Fe−Al oxy-hydroxides and (lower) pseudohexagonal to rounded precipitates composed of a Fe−Al rich planar base and a Ti-rich core. Chemical analysis of the different points are (1) 39.3% Al; (2) 1.9% Al and 58.4% Fe; (3) 2.4% Al and 59.5 Fe; and (4) 4.7% Al, 17.1% Fe, and 38.4 Ti.

F

419

Bulges and swelling fronts were observed only for biotite and

420

phlogopite but not for muscovite. This is likely related to the

421

more stable octahedral aluminum in muscovite with respect to

422

iron and magnesium in biotite or phlogopite. Moreover, the

423

diversity of the crystal structure of micas (hydroxyl orientation

424

and tetrahedral twisting and tilting degree), together with their

425

difference in composition, explains the stronger interlayer K

⁺

426

retention and thus the higher stability of muscovite

427

structure.

^24,32,49

Notwithstanding, a recent study observed

428

abundant formation of bulges on muscovite when reacting at

429

high pressure (12 MPa) and 90 ° C with water saturated in

430

scCO2

³⁵

which may lead to the assumption that pressure

431

promotes bulge formation in the case of muscovite.

432

4.2. Dissolution. Our results show that the dissolution

433

dynamics are strongly influenced by both pH and temperature,

434

allowing distinction of two main regimes: (i) At moderate to

435

neutral pH and temperatures up to 100 ° C, low-grade

436

weathering of the mica was observed. This was characterized

437

by the development of shallow roughening ( ∼ 0.2 − 0.5 nm)

438

and weakening of the surface layers (the AFM cantilever tip

439

could easily disrupt the top layer of the basal surface). This is

440

probably the result of unordered destruction of bonds within

441

the TOT layer and incongruent release of material resulting in

442

a certain waviness and undulation of the top surface layer.

443

Hence, these alteration features can be linked to incongruent

444

dissolution previously reported for fl ow-through experi-

445

ments.

22,26,29,50

(ii) When the alteration reaction is accelerated

446

by using more aggressive dissolution conditions, i.e. decreasing

447

the pH and/or increasing the temperature (pH 1 and >23 ° C,

448

pH 2 and ≥ 80 ° C, and circumneutral ammonium media at

449

>100 ° C), high-grade decomposition of the surface sets in

450

during the initial stages being characterized by the formation of

451

abundant mono- and multilayer etch pits and step retreat. This

dissolution regime can be linked to congruent dissolution.

²⁴

It

452

is worth noting that formation of secondary phases on the

453

surface may cause retention of material from the e ffl uent

454

solution of the cell. A comparison of the chemical

455

compositions of input and e ffl uent solution in fl ow-through

456

experiments, therefore, might not be apt to reveal congruent

457

dissolution in all cases.

458

In addition, the results show that ammonium promotes the

459

dissolution of micas at circumneutral pH conditions. This can

460

be explained by the NH

₄⁺

capacity to dissociate into H

⁺

+ NH

₃ 461

(e.g., up to 22% at 200 °C), increasing locally the

462

concentration of protons in the medium,

⁵¹⁻⁵⁴

which are in

463

turn responsible for the attack of Si − O − Si and Al − O − Si

464

bonds.

465

4.3. Precipitates. During mineral weathering, it is not

466

uncommon to observe the formation of amorphous and/or

467

crystalline secondary phases on the dissolving surface. This

468

occurs because the ions that are released form the mineral can

469

interact with each other, or with ions already in the solution,

470

and precipitate as a new phase. This can even take place when

471

the bulk solution is undersaturated with respect to the

472

secondary phase, which is usually termed reprecipitation.

⁵⁵ 473

The precipitation of secondary phases during mica alteration

474

has been observed for a broad range of pH values and

475

temperatures (25 − 200 ° C) using di ff erent experimental

476

techniques.

^32,56,57

In this work, our goal was to constrain the

477

physicochemical solution parameters that promote the

478

formation of secondary phases; to observe the initial stages

479

of this process; and if possible, to identify the precipitates.

480

Only in neutral media containing NH

₄⁺

we observed the

481

formation of secondary Al − Fe phases on the mica surface (in

482

situ and ex situ). VP-SEM imaging combined with EDX

483

analysis also showed that step edges are preferential sites for

484

Figure 7. Schematic representation of the different processes detected for biotite weathering as a function of pH, temperature, and solution speciation. Two main areas, separated by a transition zone (gray), can be distinguished, i.e. high-grade (purple) and low-grade (pink) weathering.

Each area is characterized by a series of surface processes (CD, congruent dissolution; ID, incongruent dissolution). In the presence of NH⁴⁺, the enhancement of mica dissolution promotes the precipitation of secondary phases and formation of etch pits, and as such the high-grade weathering zone is shifted to lower temperatures.

Article

G

485

the deposition of Fe-, Al-, and Si-rich phases. Because these

486

components are present only in the mineral phase, the

487

precipitates must have formed through a dissolution − (re)-

488

precipitation process. Some studies have already pointed to the

489

e ff ect of ammonium in the precipitation of Al phases during

490

phyllosilicate alteration experiment due to competitive ion

491

exchange.

⁵⁸

These studies agree with our observations where

492

precipitates correspond to Al and Fe (oxy-)hydroxides, whose

493

formation is favored as solution pH shifts to neutral or alkaline

494

(due to reduced solubility of these phases).

495

In the case of Al-rich secondary phases, Johnsson and co-

496

workers

⁵⁶

proposed that the formation of these phases starts

497

with the exchange of the interlayer K

⁺

by H

₃

O

⁺

, which leads to

498

the protonation of basal sites where Al can be fi xated and thus

499

serve as a possible hotspot for the nucleation of Al (oxy-

500

)hydroxides. The exchange of K

⁺

for H

₃

O

⁺

also alters the

501

surface charge of the mica surface, which can promote the

502

polymerization of Al phases.

⁵⁹

Taking into account that

503

ammonium acts as a very weak acid, at higher NH

₄⁺

504

concentrations, more protons are available, and thus, more

505

secondary phases should be observed, which was the case in

506

our experiments. In the experiments with phlogopite no

507

precipitates were observed. This is a direct consequence of the

508

high solubility of Mg-phases for the entire range of tested pH

509

values.

5. CONCLUDING REMARKS

510

In this study, we have shown that during the interaction of

511

aqueous solutions with mica minerals several surface processes,

512

i.e., hydration, dissolution, and (re)-precipitation can occur

513

simultaneously. Temperature, pH, and the chemical composi-

514

tion of the mica surface and the solution determine which of

515

these processes are dominant. On the basis of our experimental

516

and previously reported data we have constructed for the fi rst

517

time a detailed weathering diagram for biotite, linking the

518

physicochemical solution properties with the prevailing surface

519

processes for a broad range of possible alteration conditions

f7 520

(Figure 7). Although the precise location of these boundaries

521

of this diagram will shift for phlogopite and muscovite, the

522

general trends are comparable.

523

Our nanoscale observations also allowed us to correlate

524

surface processes with the expected macroscopic outcome of

525

the alteration process, i.e., congruent or incongruent

526

dissolution. Low-grade weathering, representative of macro-

527

scopically observed incongruent dissolution, is favored under

528

less “ aggressive ” conditions (pH 4 − 6.8 and temperatures <100

529

° C), preferentially releasing interlayer and octahedral cations

530

over the tetrahedral compounds through the formation of

531

shallow etch pits. Under these conditions the tetrahedral

532

positions can be attacked, which will lead to congruent

533

dissolution, i.e., step retreat. Thus, macroscopically we will fi rst

534

detect incongruent dissolution, while during later stages

535

congruent dissolution will take over as already theorized by

536

Malmström and Banwart.

²⁶

At close to neutral pH in the

537

presence of NH

₄⁺

the high-grade weathering zone is shifted to

538

signi fi cantly lower temperatures.

539

Finally, the weathering diagram (Figure 7) also highlights

540

that during low-grade dissolution bulges are important features

541

on the basal surface. Hence, under these conditions it seems

542

more plausible that signi fi cant quantities of hydrocarbons

³⁴

543

and supercritical CO

₂³⁵

can be trapped.

■ ASSOCIATED CONTENT

544

*

^S Supporting Information 545

The Supporting Information is available free of charge on the

546

ACS Publications website at DOI: 10.1021/acsearthspace-

547

chem.9b00219.

548

Supporting Table S1 and Figures S1 − S9 (PDF)

549

■ AUTHOR INFORMATION

550

Corresponding Author 551

* E-mail: alexander.van-driessche@univ-grenoble-alpes.fr.

552

ORCID 553

Alexander E. S. Van Driessche:

0000-0003-2528-3425 554

Notes 555

The authors declare no competing fi nancial interest.

556

■ ACKNOWLEDGMENTS

557

This study was supported by funding from the Spanish

558

Government Contracts (MINECO CGL2011-22567 and

559

CGL2014-55108-P, with contribution of EU-FEDER funds)

560

and the PICS 2017 CNRS program (PICS07954). D.L.-I. was

561

supported by an FPI fellowship (BES-2012-058890) and

562

Grants (EEBB-I-16-11519 and EEBB-I-15-10043).

563

■

(1)

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