REVIEWS in MINERALOGY Volume 31

ChemicalWeathering Rates of

Silicate Minerals

Editors:

Arthur F. Whitev.s. Geological Survey, Menlo Park, CA

Susan L. BrantleyPennsylvania State Univer sity

F ro n t co ver: Denticulated margin on naturally weat hered horn­blende formed by side-by-side coalescence of lenticular etch pits.Prom a weathered corestone developed on the Carroll Knob Complexin the Appalachian Blue Ridge near Ott o, North Carolina. Field ofview = 60 u rn wide. Courtesy of M.A. Velbel, Michigan StateUniversity. See Velbel (1993) American M ineralogist 78:405-414.

Back Cover: SEM image of a naturally weathered (001) cleavagesurface of an al kal i felds par from gravels formed from the granite atShap, northern England. Scale bar: 2 urn, The paired triangular andtrapezoidal etch pits have developed at the outcrop of edge disloca­tions. These form extended 100ps around pert hi t ic albite lamellae inorthoclase. The lamellae extend parallel to b, the long axis of themicrograph. Courtesy of M.R. Lee and I. Parsons (s ee Chapt er 8).

Series Editor: Pani H. RibbeDepartment 01Geological Seiences

Virginia Polytechnic Institute & State UniversityBlacksburg, Virginia 24061 U.S.A.

Mineralogical Society of AmericaWashington, D.C.

Chemical Weathering Rates ofSilicate Minerals

TABLE OF CONTENTS, VOLUME 31Page

Foreword .. . . .. . .. . . . . . . . .. . .. . .. . iliAcknowledgments .. . . . . ... ... ..... ili

Chapter 1 A. F. White & S. L. Brantley

CHEMICAL WEATHERING RATES OF SILICATE MINERALS:AN OVERVIEW

Introduction .. IHistorical beginnings.. . ... ... . IRecent advances 2

Practical Importance of Chemical Weathering 3Sources of ino rganic nutrients in soils............................................. 3Role of chemical weathering in buffering watershed acidification 5The impact of chemical weathering on long terrn climate change........... . 6

Purpose and Content of Present Volume 7Fundamen~1 approaches in describing mineral dissolution

and prectpitauon rates... ..... ....................................................... 8Silicate rnineral dissolution as a ligand-exchange reaclion 8Chemical weathering rates of pyroxenes and amphiboles....................... 9The weathering of sheet silicates 10Controls on silica reactivity in weathering environments 11Dissolution rates of feldspars... 11Microscopic approaches ro weathering 12Weathering rates in soils 13Weathering rates in catchrnents , 14Methods for reconciling experimental and soil and catchment weathering

rates............... .. . . . . . . . .. . . . . . ..... ................................................ 14Chemical versus physical wealhering.......... ..................................... . 15Chemical weathering and its effect on atrnospheric COZ and climate.v. v.. . . 16

Weathering at Differenl Scales.......... .. .. . .. . .. .... .. .. ....... .. ............................ 17Philosophy of kinelics models. , 17Relaling model s at different scales .. 18Mineral surface area . .. 18Coupling of transport and reaclion .. . .. .. ...... . ...... ... ... .. . . .. ............ ... ... 19Transient versus steady slale . .. . .. .... .. . .. . .. . ... ... ... ... .. . .. . .. . ... ... . . . ... .... .. 20

Conclusions. 20References.. . .. 21

Chapter 2 A. C. Lasaga

FUNDAMENTAL ApPROACHES IN DESCRIBING MINERAL

DISSOLUTION AND PRECIPITATlON RATES

Introduction ... . ... ... .. . .. . .. . .. . .... . . ... . . .... .. . .. . .. .. .. .. ... . ... ... ... ... ... .. .. . . .. . .. . .. . .. . 23A simple rate model. .............. .. ............. .. .... ............ ......... .... ... ... . 27

Kink siles..... . ........... . .. . .. ... ... ...... . .... .. .... ....... ............ ....... 27Kinetic model... . ... .. . .. .. .. ........ ... . .. . . . .. . ... ............................ 29

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Caveat-Test Forms of the Function 31Other Forms of f(dG r) 32Surface Area............ ............ .. .... .. . ...... .. .. ...... ... .... ..... . .. . ... ....... ..... ... .... 33Activation Energies..... .... 35pH Dependence '" . .. . .. . . . . .. . 36Ionic Strength and Solution Composition ................ .. .. ................ ............... 37Ab Initio Approaches.. 39

Ab initio applications 44Ab initio studies and dissolution/precipitation rates.. ................ ......... ... 54

Dissolution under acid conditions...... .. .... 56Dissolution under basic conditions.... ...... .................... ............ 56

Effect of hydration layers. .. 61Reaction Mechanisms and Rate Laws... .. .. .. .... ...... .... ... .. ... . .... .................... 63Monte Carlo Methods and Rate Laws 70Summary 80Acknowledgments.............. ....... .... . ... . .. .... .. ... ....... ... ... ................. .......... 80References. .. .. ... ... ... ... ... .... ... .. ... . .. .. .. .. .. . ... . .. .. .. .. . .. ... ... . .. ... . ... .. ... . ... ... .. . 81

Chapter 3 W. H. Casey & C. Ludwig

SILICATE MINERAL DISSOLUTION AS

A LIGAND·EXCHANGE REACTION

Introduction .. .. .. . .. . .. . .. . 87Ligand-Exchange Kinetics.... .... .. ... .. ..... ... .. .. ...... .. ............ ...... .. ..... ... . ....... 88

What is a ligand-exchange reaction? 88Solvent exchange rates vary with M-O bond strengths 92Brensted reactions commonly proceed to equilibrium and affect rates.. .. ... 94Ligands can promote reaction.. .. ... . .... .. .. ... .. ... .. .... ... ... .. . ... .. .. ... .. .. .... 95Ligands have acid-base properties that affect reactivities.. .... .. ... .. ... .. ... ... 95M-O bond dissociation can control rates of chelation 97

Mineral Surface Reactions 98General comments....... . ... .. ... .. ..... .. ... .. .... .. . .... ... .. ...... . .. .. ... . .. . ..... .. 98Rate laws for surface reactions 98Solute adsorption/desorption is usually faster than dissolution 101Brensted reactions on mineral surfaces 102Proton-promoted dissolution 104Ligand-promoted dissolution 105

OrthosilicateMinerals 108Experimental dissolution of orthosilicates 109Orthosilicate mineral dissolution in nature 112

Conclusions and Predictions 113Acknowledgments 114References 114

Chapter 4 S. L. Brantley & Y. Chen

CHEMICAL WEATHERING RATES OF

PYROXENES AND AMPHIBOLES

Introduction 119Mineralogy of Common Soil Inosilicates 120Laboratory Chernical Reactors 121

Batch reactors 121

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Flow- through reactors 121Interpreting reactor experiments 122

Laboratory Dissolution Rates 123Measurement of true steady state 123Rate discrepancies 1'26

Run duraüon 130Mineral chemistry or structure 130Aging of powders 130Surface area 131Chemical affinity 131

Effects of varying solution composition 132Mineral surface area 133

Surface area of starting material 133Relationships between surface area and dissolution 133Surface roughness 137Etch pits 137

Relative elemental release rates 140Early observations 140Enstatite, bronzite, diopside, trernolite 140Wollastonite 141Rhodonite 141Hornblende 142

Surface Analysis of Fe-free Inosilicates 142Spectroscopy of laboratory-dissolved surfaces 142

Enstatite 142Diopside 142Wollastonite 143Rhodonite 143Tremoü\e 144

Surface Analysis of Naturally Weathered Minerals 145Coupled Oxidation-Reduction of Fe-containing Inosilicates 145Rate Laws for Dissolution 148

pH dependence 148Best estimates of empiricaI rate laws 151

Trends in dissolution rates 152Temperature Dependence of Dissolution 155Mechanism of Dissolution 157

Armoring surface layers 157Interface-controlled models 158

Steady state leached layers 158Helgeson model. 158Rimstidt and Dove model for wollastonite 159Stumm's surface protonation model. 159Surface charging of inosilicates 161Alkaline dissolution of inosilicates 162Point of minimum dissolution rate 162

Natural Inosilicate Weathering Rates 162Quantitative eval uation of etehing rates 162

Mean maximum etch depth 163Pit size distributions 164

Rate control in natural systems 165Conclusions 167Acknowledgments 168References 168

Chapter 5 K. L. Nagy

DISSOLUTION AND PRECIPITATION KINETICS

OF SHEET SILICATES

Introduction 173Bonding in Sheet Silicates 174

Sheet and layer struetures 175Composition 177

Cations. .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . .. 177Anions 177

Interlayer bonding 178Surfaee chemistry, bonding, and reaetion pathways 178

Dissolution Kineties 181Introductory eomments 181The dissolution reaetion 182Acid-leaching experirnents 183Dissolution data from solubility experiments 184Ion exchange and early dissolution experirnents 185Far-from-equilibrium rates and pH-dependenee 186

Aluminum hydroxide phases 188AI-oxide phases 189Brucite 190Kaolinite 190Chrysotile 192Antigorite 196Tale 196Muscovite 196lllite 197Montrnorillonite 198Biotite 198Phlogopite 199Chlorite 199

Effeet of ternperature 199Gibbsite 200Brueite ' " 200Kaolinite 201Muscovite 20 IPhlogopite ' " 20 IChrysotile 20 I

Stoichiometry versus nonstoiehiometry of dissolution 202Rate catalysts and inhibitors 205

Alurninum and silicon 205Other metals and anions 205Organie ions and molecules 207

Role of biota 209Dependenee on reaetion affinity or Gibbs free energy of reaetion 209Surfaee defects 211Oxidation of octahedral Fe 212Experimental approaehes and caveats 212

Type of experiment 212Solution composition 213Solids pretreatrnent 214

Nucleation Kineties 214Introductory comments 214Nucleation 214

Topotaxy 215

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Epitaxy 215Role of adsorption 217Gel recrystallization 217Organie templates 218

Precipitation (Growth) Kinetics 218Introductory comments 218Precipitation data 218

AI phases 218Kaolinite 219Sepiolite, palygorskite, and brucite 220

pH-dependence 220Rate catalysts and inhibitors 221

Inorganic ions 221Organic ions 221

Dependence on reaction affinity or Gibbs free energy of reaction 222Gibbsite 223Kaolinite 223

Surface defects 223Summary Remarks 224Acknowledgments 225References , 225

Chapter 6 P. M. Dove

KINETIC AND THERMODYNAMIC CONTROLS ON SILICA

REACTIVITY IN WEATHERING ENVIRONMENTS

Introduction 235Biogeochemistry of silica in terrestrial weathering environments 235Intent of this review 237

Silica Polymorphs in Weathering Environments 237Occurrence and forms 237Macroscopic to Microscopic: Grain textures and surfaces in weathering 239

Fragmentation 239Grain surface shapes 239Etch pits 240Correlation of grain textures and weathering histories 240

Silica polymorphs as coatings on other mineral constituents 240Reac tivity in Silica-Saturated Environments: Solubility 242

The dissolution reaction 242Solubility of silica phases 242

Effect of pH 244Effect of solution species 245Apparent solubility 246

Solubility as a function of particle size 247The Silica-Water Interface 247

Surface structures and properties 250Electrical double layer of the silica-water interface 250Ionization and surface charge 253

Reactivity in Non-Equilibrium Environments: Kinetics 254Basic principles 254

Aqueous diffusion 254Dissolution and precipitation 255Nucleation 260

Etch pit development 262Controls of solution composition and temperature on reactivity 263

Temperature dependence in deionized water 264

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Catalysis by alkali cations 265Solution pH 266Combined ternperature, electrolyte, and pH relations 268

Modifiers: Effects of sorbates and coatings on reactivity 268Organic acids 268Aluminum 269Ferrous-ferric iron 272Coatings on silica surfaces in natural settings 273

Surface and mechanistic controls on reactivity 274Development of surface complexation models of dissolution 274Solvent-surface controls on reactivity 277Theoretical mechanistic models 280

Regional Scale Weathering: Siliceous Fracture Kinetics 280Concluding Remarks 282Acknowledgments 282References 282

Chapter 7 A. E. Blum & L. L. Stillings

FELDSPAR DISSOLUTION KINETICS

Introduction '" 291Feldspar Mineralogy 292Feldspar Surface Chemistry during Dissolution 292

H+/OH- sorption and Ht/cation exchange reactions on the feldsparsurface.. ' " 293

Surface titrations 293Exchange and/or sorption reactions involving the interstitial

cations 294Adsorption and desorption reactions at dangling oxygens

at the surface 296Adsorption at bridging oxygens at the surface 297Alteration of the AI-Si tetrahedral framework 299

Experimental Determination of Feldspar Dissolution Kinetics 303Experimental teehniques 303Feldspar surface areas 303Rapid initial dissolution rates 304Steady-state feldspar dissolution rates 305

Albite dissolution kinetics 305Potassium-feldspar dissolution kinetics 305Plagioclase dissolution kinetics 306Reproducibility of feldspar dissolution rates 308

Effects of aluminum concentration and saturation state 308Effects of ionic strength on feldspar dissolution 310

Acidic pH solutions 310Basic pH solutions 311

Temperature dependence of feldspar dissolution 312Effects of organic acids upon feldspar dissolution 316

Models of Feldspar Dissolution Kinetics 324Leached-Iayer/diffusion models 324Atomistic models 325Surface speciation models 326

Applications of surface speciation models to feldspardissolution kinetics 327

Macroscopic models 331Surface nucleation models 331Dissolution at dislocations 331

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Limitations to modeling natural systems with experimentally-derived rates . 333References 335Appendix I. Dissolution studies of feldspar minerals in the presence of

organic acid solutions 343Appendix 11. Dissolution rates of felds pars in organic acid solutions 346

Chapter 8 M. F. Hochella, Jr. & J. F. Banfield

CHEMICAL WEATHERING OF SILICATES IN NATURE:

A MICROSCOPIC PERSPECTIVE

WITH THEORETICAL CONSIDERATlONS

Introduction 353Precursors to Weathering: Mineral Surfaces 355

General characteristics of mineral surfaces 355Mineral surface areas 357

Measuring surface areas 357Working definitions of external versus internal surfaces 359Evidence for the importance of internal surface area 361Implications for laboratory-based dissolution studies 363Reactive surface areas 364

Microscopic Observations: Rationale, Techniques, and Results 366The need for microscopic characterization of reacted surfaces 366The HRTEM approach 367Evidence of microtextural and defect controls of silicate dissolution and

weathering 368Perthitic alkali felds pars 369Plagioclase feldspars 373

The incipient stages of silicate mineral alteration 377Characteristics of Internal Weathering Fronts in Minerals 383

Some physical properties of water in confined spaces 384The structure of water near silicate surfaces 386

Water at the quartz/water interface 386Water at the kaolinite/water interface 387

The structure of aqueous solutions near silicate surfaces 388Aqueous solutions at the quartz/water interface 388Aqueous solutions at the smectite/water interface 388

The solvent properties of water in confined spaces 389Bronsted acidity 390Aqueous solution saturation states and nucleationlcrystallization 390

Consequences of Internal Weathering Front Characteristics on ReactionMechanisms and Rates 391

A qualitative model for weathering at internal surfaces 391Applications to stagnant water in the vadose zone 395

Beyond Internal Weathering Fronts: More Advanced Stages of MineralAlteration 396

Summary, Conclusions, and Final Thoughts 397A word about modeling 400

Acknowledgments 400References 401

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Chapter 9 A. F. White

CHEMICAL WEATHERING RATES OF SILICATE

MINERALS IN SOlLS

Introduction 407Weathering Environments Based on Soil Classification 409Mineralogical Mass Balances in Soils 411

Mass balance approaches 411Required assumptions for mass balance calculations 412Use of mass balance equations 413Elementallosses as functions of time 415Minerallosses as functions of time 416Volumetrie analysis and rates of soil formation 418

Soil Surface Areas 419Surface areas of silicate minerals in soils 420Surface area and particle size 422Surface areas of weathered silicates 422Internal porosity 426Calculation of combined surface roughness and internal porosity 426

Mineral Dissolution Rates Based on Primary Silicate Losses 428Coupling mineral surface area and dissolution rates 429Interactive approaches 431Calculation of dissolution rates 432Rates constants calculated from geometrie and BET surface area estimates 433

Weathering Rates Based on Etch Pit Formation 434Mineral Dissolution Rates Based on Solute Fluxes 435

Soil solution chemistry 438Comparison of soil solution and watershed chemistry 438Equilibrium controls on soil solution chemistry 440Role of solute chemistry and reaction affinity 444Soil water hydrology 445Estimating rates of fluid flow 445Example of estimation of soil water flux 447Soil mineral weathering rates based on solute fluxes 449

Interpretation of Weathering Rate Constants in Soils 450Surface area 451Soil age 452Solute chemistry 453Hydrologie heterogeneity 453Role of biological activity 454Climate 455

ConcIusion 457Acknowledgments 457References 458

Chapter 10 J. I. Drever & D. W. Clow

WEATHERING RATES IN CATCHMENTS

Introduction 463Measurement of Weathering Rates 463

By solute budgets 463Solutes in outflow 464Solutes from the atmosphere 465Changes in the exchange pool 466Changes in the biomass 466

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Chapter 11

Chemical weathering 468Rates Nonnalized to Mineral Surface Area 469

Catchments in the Czech Republic 469Coweeta Basin, North Carolina 470Bear Brooks, Maine 471Rocky Mountain National Park, Colorado 472Causes of discrepancies between field and laboratory rates 474

Rates Normalized to Land Surface Area 477Effect of climate 478Effect of lithology 478Relief 480

Summary 480Referenccs 481

H. Sverdrup and P. Warfvinge

ESTIMATING FJELD WEATHERING RATES

USING LA BORATORY KINETICS

Introduction 485The importance of models in geochemistry 485

Good and bad models 485SoH and catchment models 486

Process-oriented models 487Dynamic versus static models 488Spatial and temporal resolution of dynamic models 488Chemical weathering 489Other model type 491

Theoretical and Experimental Kinetics 491Introduction 491Abrief history of weathering kinetics 492

The long years of confusion 492Early rate equations 492

Chemic al dissolution reactions 493The Transition State Theory applied to weathering 493

The reaction between Hf-ions and a mineral. 497Organic acids 499The transition state theory rate expression 500Extension of the theoretical expression to field conditions 501Determining the surface area on soil sampies 503Soil wetness and surface area 503Considering the temperature 503

How rate coefficients were determined from data 504The reaction with Hr-ion 504The reaction with organic acid and C02 506The kinetic parameters 506

Modeling Kinetics of Field Weathering 507Introduction 507PROFILE and SAFE model description 508

Processes included 508Mass balance equations 508The pH-ANC relationship 510Cation exchange 511Nutrient uptake reactions 512Weathering limits forest growth 512Chemical feedback frorn soil chemistry 513Integrated feedback from soil chemistry and weathering 513

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Spatial structure 514Hydrological structure 514Computer appearance 514

The Gärdsjön Case Study 519Site characteristics 519Method 520Results 520

Modelling the field weathering rate with PROFILE 520Budget study method 522Strontium isotope method 523Total analysis correlation 525The historie weathering rate 526Historie weathering as reconstructed with the SAFE rnodel . 527Com paring methods for estimating the weathering rate 528

Model Validation 529Regional applications 531

Discussion 532The important minerals 532A simple model based on mineralogy 533The difference disappeared! 534

The importance of product inhibition under field conditions 535Taking away the difference step-by-step 538

Conclusions 538References 539

Chapter 12 R. F. Stallard

RELATlNG CHEMICAL AND PHYSICAL EROSION

Introduction and Background 543Sediments and the geologie record 543The present is unusual. 544

Providing a Framework: Steady-State Erosion 545Working hypotheses 545Collection of representative samples 546Caveats and corrections 547Steady-state denudation definitions 548Hillslope processes 549Erosion regimes 550Roles of organisms in physical erosion 551Deep erosion " 551Exceptional erosion 553Clastics and carbonates · 554Solid yields and silicate rocks 557Predicted solid yields 559

Summary Conclusions 561References 562

Chapter 13 Robert A. Berner

CHEMICAL WEATHERING AND ITS EFFECTON ATMOSPHERIC C02 AND CLIMATE

Introduction 565Weathering as a Feedback Control on C02 567Factors Affecting Weathering over Geologie Time 569

Continentalland area and lithology 569

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Mountain uplift. 570Climate 571Vegetation 574

Sensitivity of the GEOCARB Carbon Cycle Model 10 Some WeatheringParameters 578

Summary: Weathering, Climate, and C02 581Acknowledgments 581References 581

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