The role of intragranular microtextures and microstructures in chemical and mechanical weathering:...

PII S0016-7037(98)00200-2

The role of intragranular microtextures and microstructures in chemical andmechanical weathering: Direct comparisons of experimentally and naturally

weathered alkali feldspars

MARTIN R. LEE,1 MARK E. HODSON2,* and IAN PARSONS

1

1Department of Geology and Geophysics, University of Edinburgh, West Mains Road, Edinburgh EH9 3JW, UK2MLURI, Craigiebuckler, Aberdeen, AB9 2QJ, UK

(Received November25, 1997;accepted in revised form June5, 1998)

Abstract—Electron microscopic observations of alkali feldspars from soils show that intragranular micro-textures, such as exsolution lamellae, and microstructures, primarily dislocations, are both highly significantdeterminants of the weathering behaviour of these minerals. In particular, strained structure around intersect-ing edge dislocations in the plane of exsolution lamellae,;(6#01), dissolves at a rate which is orders ofmagnitude greater than unstrained feldspar, producing a mesh of intersecting etch tubes extending.5 3 1023

cm into the crystal. As a result, dissolution at dislocations is the major source of solutes during initial stagesof chemical weathering in the field. With progressive chemical weathering, the most highly reactive feldsparis consumed by growth and coalescence of etch tubes, but outer parts of the grain are physically weakened,leading to mechanical flaking that increases available surface area and exposes further reactive sites. Incontrast, previous dissolution experiments, and microscopy of reacted surfaces, have shown little or nocorrelation between dissolution rate and dislocation density and few visible signs of dissolution at particularlyreactive sites.

To resolve the apparent discrepancy between field and laboratory behaviour we have carried out flow-through dissolution experiments using pH 2 HCl at 25°C on three alkali feldspars with carefully characterizedintragranular microtextures and microstructures. These alkali feldspars were: (1) Eifel sanidine, an alkalifeldspar that has no microtextures at the TEM scale and a low dislocation density (,106 cm22), (2)unweathered alkali feldspars from the Shep Granite, which have a mainly coarse exsolution microtextures andhigher dislocation density (.2–3 3 108 cm22), and (3) naturally weathered alkali feldspars, also from theShap Granite, which have the same microtextures as unweathered Shap Granite alkali feldspars but, becausethey have been weathered, have a lower density of dissolution reactive dislocations exposed on grain surfaces(,2–3 3 108 cm22). Results from the experiments are ambiguous. If the rate data are normalised to thepowder’s initial BET surface area, dissolution rates increase with dislocation density. Normalisation to thepowder’s BET surface area as it is inferred to have changed during the experiments, yields no correlation withdislocation density. SEM and AFM images of reacted grain surfaces show that dislocation outcrops and albiteexsolution lamellae have both etched more rapidly than tweed orthoclase, but dissolution at these sites makesa quantitatively insignificant contribution to the overall rate of laboratory dissolution of the feldspar powders.Major differences in the importance of dislocations to rates of early chemical weathering in field andlaboratory contexts probably result from corresponding contrasts in the saturation state of ambient solutions.Observations of naturally weathered alkali feldspars show that microtextures and microstructures have thegreatest impact on mineral weathering rates during advanced stages of dissolution when grain surfaces startto disintegrate. Hundreds of years of dissolution under the laboratory conditions used here would be requiredto reach this stage.Copyright © 1998 Elsevier Science Ltd

1. INTRODUCTION

The role played by intragranular microtextures (e.g., exsolutionlamellae) and microstructures (e.g., dislocations) in influencingthe mechanisms and rates of mineral weathering is poorlyunderstood, but potentially of high significance. Their signifi-cance arises from the fact that such microtextures and micro-structures are created during the growth of mineral grains andtheir subsequent geological history, mainly during the coolingof igneous and metamorphic rocks. If microtextures and mi-crostructures have no effect on mineral weathering, all grains ofa given mineral species will, under the same conditions,weather at the same rate, regardless of their geological back-

ground. If, however, these variables are significant, the weath-ering rates of grains of a single mineral species, again under thesame conditions, may be very different, depending at least inpart on their geological background. Such a variability wouldsignificantly complicate the task of accurately understandingand modelling mineral weathering.

The almost ubiquitous presence of etch pits on the surfacesof naturally weathered mineral grains shows that certain dis-crete areas of the surface dissolve at a faster rate than surround-ing regions. Most workers suggest that these reactive sitesoccur where highly elastically-strained crystal structure inter-sects the grain surface (Wilson, 1975). Crystal structure maybecome strained as a result of stresses accompanying externalprocesses such as tectonic activity and also by spontaneousinternal forces, such as those generated by exsolution (Brownand Parsons, 1993). Strain generated by both of these processes

*Present address:Department of Mineralogy, The Natural HistoryMuseum, Cromwell Road, London SW7 5BD, UK.

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Geochimica et Cosmochimica Acta, Vol. 62, No. 16, pp. 2771–2788, 1998Copyright © 1998 Elsevier Science LtdPrinted in the USA. All rights reserved

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2771

is commonly accommodated by the formation of dislocations.The nature of strain energy associated with dislocations and thecontribution of dislocations to the total energy of a mineralgrain in the context of weathering behaviour has been previ-ously discussed in detail (e.g., Helgeson et al., 1984; Lasagaand Blum, 1986; Blum et al., 1990; Blum, 1994; Blum andStillings, 1995). If the most reactive areas of grain surfacesoccur at dislocation outcrops, as observations of naturallyweathered grains have suggested (Wilson, 1975), mineralweathering rates should vary with dislocation density.

A number of workers (Table 1) have sought to quantify thecontribution of dislocation density to weathering rates by per-forming laboratory experiments on samples of minerals withlow and high dislocation densities. In most studies, high dis-location densities have been created using experimental shocktechniques. Results from dissolution experiments using a vari-ety of mineral species (Table 1) reveal either no correlationbetween dislocation density and dissolution rate or an enhance-ment of dissolution rate by a factor of 2–3 with elevateddislocation density. As a number of workers have suggested,the discrepancies between observations of naturally weatheredgrains and results from laboratory experiments are likely to liein the very significant differences in chemical and physicalconditions between field and laboratory environments. This isexemplified by Casey et al. (1993) who concluded, from acomparison of field and laboratory weathering, that experi-ments have yet to reproduce, even qualitatively, many texturesobserved in naturally weathered minerals. Despite this state-ment, there are no studies known to us which have directlycompared textures on the surfaces of naturally and experimen-tally weathered mineral grains from the same parent rock.Without such work, results from laboratory dissolution studieswill be of only limited use for predicting weathering in fieldenvironments.

The present paper describes results of a study whose aim wasto examine the role of intragranular microtextures and micro-structures in influencing mechanisms and rates of mineralweathering in both field and laboratory contexts. We haveaddressed the problem by using two complementary ap-proaches. First, we studied in detail the behaviour of alkalifeldspar grains from a single parent rock, the Shap Granite,

during all stages in their natural weathering cycle, from beforerelease from their parent rock to final disintegration withinsoils. Some preliminary results from this part of the study havealready been reported (Lee and Parsons, 1995) and the readershould consult this paper and also Lee et al. (1995) for illus-trations of the microtextures which are present in Shapfeldspars. Second, we performed laboratory dissolution exper-iments on powders of alkali feldspars with very different mi-crotextures and dislocation densities. These samples includedboth naturally weathered and unweathered alkali feldspars fromthe Shap Granite. We compared these dissolution rates withthose of a gem quality sanidine from the Eifel, Germany, whichhas an extremely low dislocation density. The aim of theseexperiments was to determine if dissolution rates could becorrelated with microtextures and dislocation density and alsoto examine the extent to which textures observed on the sur-faces of naturally weathered Shap Granite alkali feldsparscould be recreated by laboratory dissolution of unweatheredalkali feldspars from the same parent rock.

SYMBOLS USED

Ar1 BET surface area at the beginning of experimentalrun 1 (cm2)

Ar2 BET surface area at the end of experimental run 2(cm2)

At BET surface area of a feldspar powder at timet(cm2)

Cd Mean cross-sectional area of highly strained felds-par surrounding an edge dislocation (cm2)

Dab-trgh Depth of a trough formed by selective dissolutionof an albite lamella (cm)

Dd-pit Depth of a dislocation etch pit (cm)F Flow rate of acid in flow-through experiments

(ml s21)n Number of ions of Si per feldspar formula unitO Output solution minus acid blank for Si (gmL)

rab The dissolution rate of albite per unit area of albitelamellae (moles cmab

22 s21)Rab The dissolution rate of albite lamellae per unit area

of grain surface (moles cmtotal22 s21)

Table 1. Details of previous studies of the effect of dislocation density on rates of experimental dissolution.

Authors Minerals used

Dislocation densities Experimental conditionsResulting changein dissolution rateUndeformed Deformed Acid Conc. Temp.

Murr and Hiskey(1981)

Chalcopyrite 107 cm22 1011 cm22 K2Cr2O7 andH2SO4

0.05M0.5M

50°C Factor of;2

Casey et al. (1988) Rutile 106 cm22 4 3 1010 cm22 HF 1.065 N 25°C Factor of 2–3Holdren et al.

(1988)Calcicplagioclase ,106 cm22 109 cm22 not stated pH 2.7 25°C ,a factor of 3

Cygan et al. (1989) Hornblende not specified not specified 2-(HO2C)C6H4CO2K pH 4 25°C Factor of 2.1–3.7Labradorite #106 cm22 108–109 cm22 2-(HO2C)C6H4CO2K pH 4 25°C Factor of 2–3.5Oligoclase not specified not specified 2-(HO2C)C6H4CO2K pH 4 25°C Factor of 10.4–13.3

Murphy (1989) Sanidine ,106 cm22 10829 cm22 HCl pH ;3 95°C NoneSchott et al. (1989) Calcite 106 cm22 5 3 108 cm22 KCl pH 4–7.4 3°C Factor of 0.95–1.73

106 cm22 109 cm22 KCl pH 5–7.4 25°C Factor of 1.48–2.19106 cm22 5 3 108 cm22 KCl pH 4.2–7.4 80°C Factor of 1.18–1.38

Blum et al. (1990) Quartz ,105 cm22 5 3 1010 cm22 H2O pH ;7 80°C IndistinguishableHF 0.2 M 22°C Indistinguishable

2772 M. R. Lee, M. E. Hodson, and I. Parsons

rab'to The dissolution rate of albite and tweed orthoclaseper unit area of albite lamellae and tweed ortho-clase (moles cmab,to

22 s21)Rab,to The dissolution rate of albite lamellae and tweed

orthoclase per unit area of grain surface (molescmtotal

22 s21)rd The dissolution rate of dislocation outcrops per

unit area of dislocation outcrops (moles cmd22 s21)

Rd The dissolution rate for dislocation outcrops perunit area of grain surface (moles cm22

total s21)rto The dissolution rate of tweed orthoclase per unit

area of tweed orthoclase (moles cmto22 s21)

Rto The dissolution rate of tweed orthoclase per unitarea of grain surface (moles cmtotal

22 s21)Rdeep Rate of deepening of etch pits (cm s21)Rfield Rate of chemical weathering in the field

(moles cmtotal22 s21)

Rlab Rate of chemical weathering in the laboratory cal-culated from Si data (moles cmtotal

22 s21)Rz Rate of chemical weathering in the laboratory

calculated from Al, K or Na data (molescmtotal

22 s21)Rlab,field Rate of chemical weathering in the laboratory or

field (moles cmtotal22 s21)

RRRZ Relative release ratio of element Z (Al, K or Na)Sab Area of a grain’s surface occupied by albite lamel-

lae (cmab2 cmtotal

22 )Sab1to Area of a grain’s surface occupied by albite lamel-

lae and tweed orthoclase (cmab1to cmtotal22 )

Sd Area of a grain’s surface occupied by dislocationoutcrops (cmd

2 cmtotal22 )

Sto Area of a grain’s surface occupied by tweed ortho-clase (cmto

2 cmtotal22 )

t Time of collection of solutions from flow-throughexperiments (s)

Tlab Duration of laboratory dissolution (s)Tfield Duration of natural weathering (s)Vab The total volume of feldspar removed from the

outcrops of albite lamellae over a given area of thegrain’s surface (cmab

3 cmtotal22 )

Vab-trgh The volume of troughs that have formed by disso-lution at the outcrop of albite lamellae over a givenarea of the grain’s surface (cmab

3 cmtotal22 )

Vd The total volume of feldspar removed from dislo-cation outcrops over a given area of the grain’ssurface (cmd

3 cmtotal22 )

Vd-pit The volume of pits that have formed at dislocationoutcrops over a given area of the grain’s surface(cmd

3 cmtotal22 )

Vlayer The volume of a uniform layer of feldspar that hasbeen removed from a grain’s surface during theduration of an experiment (cm3 cmtotal

22 )Vmol Molar volume of albite (cm3 mol21)

Vto The volume of feldspar removed from an area oftweed orthoclase over a given area of the grain’ssurface (cmto

3 cmtotal22 )

W Atomic weight of Sird Dislocation density (cm22)

2. MATERIALS AND METHODS

2.1. Collection, Preparation and Imaging of NaturallyWeathered Alkali Feldspars

Naturally weathered alkali feldspars were collected from soils thathave formed on glacially-scoured surfaces of the Lower Devonian ShapGranite in northwest England. These weathered feldspar grains arefragments of distinctive large, pink phenocrysts. The granite has es-caped significant tectonic deformation, and its petrology has beendescribed by Grantham (1928) and Caunt (1986). For descriptions ofthe microtextures of unweathered alkali feldspar phenocrysts see thefollowing discussion and Lee et al. (1995). The soil which has formedon the granite is a raw oligo-fibrous peat, part of the Winterhillassociation (Jarvis, 1984). The soil ranges in thickness from a fewcentimetres around granite tors to 1 m or more in other areas; in allcases it is unstratified. The soil is almost permanently waterlogged (onesample of the soil had a water content of 84.1% by weight), and surfaceflow can occur during wet periods. Importantly, chemical weatheringwithin these saturated soils will be more comparable to conditions usedin laboratory dissolution experiments than mineral soils that are typi-cally much less saturated with water. Millimetre-sized feldspar grainswere hand-picked from samples of soil collected from within a fewcentimetres of the soil-granite interface. The feldspar grains were thenimaged using a Cambridge Instruments S250 SEM operated at 20 kV.The grains received no ultrasonic cleaning or any other processing priorto gold coating and imaging. Stereopairs of some grains were obtainedby recording two SEM images of the same field of view, one tilted by6° relative to the other. For further details of the preparation ofstereopairs, see Lee (1997).

The morphology and interconnectivity of etch pits in grain interiorswas characterized by making resin casts of weathered grains using atechnique described by Lee and Parsons (1995). Grains which had beenimaged by SEM were impregnated with Araldite under vacuum. Afterit had cured, the resin block was polished to expose the underside of thefeldspar grain which was then completely dissolved using concentratedHF acid. The resin casts were then gold coated and imaged by SEM. Afew of the resin-impregnated grains were not dissolved but wereprepared for imaging by TEM. The resin block was polished to a wafer;0.1mm in thickness whose surfaces were parallel to the former grainsurface. The wafer was then Ar ion beam thinned and the foil wasimaged using a Philips CM12 Transmission Electron Microscope(TEM) operated at 120 kV.

2.1.1. Soil water compositions

Natural waters were extracted from soils containing the weatheredalkali feldspars for chemical analysis. Approximately 0.125 m3 of soilwas collected in March 1997, and waters were extracted using acentrifuge operated for 1 hour at 10003 g, then passed though a 11mmfilter and collected. The water was brown in colour after centrifugingbut colourless following filtering; we are, therefore, confident thatsuspended organics were removed. Cation concentrations were deter-mined by inductively coupled plasma-optical emission spectroscopy(ICP-OES) and Cl, SO4, NO3, and NH4 were measured by highdefinition ion chromatography (HDIC) analysis at the Macaulay LandUse Research Institute, Aberdeen. The pH of a number of different soilwater samples was also measured using a digital pH meter which wascalibrated using commercial buffers (pH 4.006 0.02 and 7.006 0.02).Saturation indices of solutions extracted from the soils and from thelaboratory dissolution experiments were calculated using MINTEQA2(Allison et al., 1991).

2.2. Alkali Feldspars Used in the Experiments

2.2.1. Selection of alkali feldspars

A single size fraction (106–53mm) of powders of three differentalkali feldspars was used for the laboratory experiments. Thesefeldspars were (1) Eifel sanidine, (2) unweathered alkali feldspar phe-nocrysts from the Shap Granite at outcrop in an active quarry (hereaftertermed UW-Shap for brevity) and (3) naturally weathered Shap Granitealkali feldspar phenocrysts (W-Shap).

Eifel sanidine is a gem quality volcanic alkali feldspar which was

2773Role of intragranular microtextures and microstructures in chemical and mechanical weathering

ejected from the Quaternary Laachar See volcano in the Eifel region ofnorthwest Germany. Samples of this feldspar were obtained from thecollection of the Department of Geology and Geophysics, EdinburghUniversity. The feldspar is glass-clear in hand specimen and is notedfor its low dislocation density (,106 cm22, Murphy, 1989) and ab-sence of any exsolution microtextures, even at the nanometre-scale.Eifel sanidine has been used in previous laboratory experiments byMurphy (1989) and Schweda (1989). UW-Shap is a pink-colouredalkali feldspar with a predominantly coarse perthitic microtextures andrelatively high dislocation density (.2–33 108 cm22, Lee and Par-sons, 1995; Lee et al., 1995). The microtexture of this sample isdescribed below. W-Shap is a sample of naturally weathered alkalifeldspar phenocrysts collected from gravels (grus) overlying weatheredsurfaces of the Shap Granite (it is thus a naturally weathered equivalentof UW-Shap). As the weathered surfaces have been glacially-scoured,all weathering must postdate the end of the last glaciation in this area(;10 kyr ago). The W-Shap powder was produced by grinding natu-rally weathered grains 4–8 mm in size. Some grains in the powder willhave been completely unweathered (coming from the interiors ofweathered grains) whereas other grains will have been naturally weath-ered (from the surfaces of weathered grains). As described by Lee andParsons (1995) and reiterated below, dissolution during natural weath-ering of Shap Granite alkali feldspars is most rapid at dislocationoutcrops on grain surfaces, forming discrete etch pits. Thus, surfaces ofgrains in the W-Shap powder will on average have a lower density ofpotentially reactive dislocation outcrops than surfaces of grains fromthe UW-Shap powder. Anbeek (1992a, b) produced detailed equationsfor calculating the proportions of weathered and unweathered grainsurfaces in experimental powders from BET data. We, however, de-termined the proportion of grain surfaces that had been weathered byexamining randomly-selected grains using SEM; results showed that15% of grain surfaces have etch pits characteristic of natural weather-ing.

2.2.2. Preparation and characterisation of feldspar powders

Each feldspar sample was manually crushed and dry sieved to obtainthe 106–53mm fraction then ultrasonically cleaned in distilled water,wet sieved, and oven dried. Eifel sanidine was assumed to be miner-alogically pure and so received no subsequent processing, but bothUW-Shap and W-Shap were further purified by high gradient magneticseparation (Hillier and Hodson, 1997).

The mineralogical composition of the three powders was determinedby electron probe analysis. For this work, grains from each powderwere mixed with an epoxy resin and mounted on a glass slide. Afterpolishing, 100 randomly-selected grains in each mount were chemi-cally analysed using a Cameca Camebax electron probe operated at20kV with a 10 nA beam current. These data (Table 2 ) confirm that theEifel sanidine powder is pure feldspar, whereas both the UW-Shap andW-Shap powders have minor quartz contamination. The bulk molecularcomposition of the feldspar component of each powder was alsodetermined from electron probe analyses of randomly-selected grainsin the polished samples (Table 2). Note that these electron probeanalyses only give bulk compositions of the grains. We assume that thesurfaces of grains exposed to acid in the laboratory had an identicalcomposition to the bulk grain interior. This is a reasonable assumptionfor the Eifel sanidine and UW-Shap powders, but surfaces of somegrains from the W-Shap powder which had been previously naturallyweathered may have had a modified composition (i.e., a Si-rich leachedlayer). As only 15% of grain surfaces exposed to laboratory acids hadalso been naturally weathered (see above), such a layer, even if present,is unlikely to have had a major impact on the chemical composition ofoutput solutions from the experiments.

Following the experiments, reacted grains were imaged using back-scattered electrons in a Philips XL30 CP SEM operated at 20kV incontrolled pressure mode (;0.2 mBar). This imaging technique obvi-ated the need for a conductive coating and proved highly effective inthe identification of very small etch pits. Grain surfaces were alsoimaged using a NanoScope Scanning Probe Microscope (SPM) oper-ated in contact atomic force (AFM) mode at the University of Arizona.In order to investigate the possible presence of Si-rich layers on reactedgrain surfaces, depth profiles through the surfaces of grains from theUW-Shap powder were also produced using a Cameca IM4f ion probe

in the Department of Geology and Geophysics, University of Edin-burgh. Owing to the small size of the grains relative to the rastered ionbeam, results were ambiguous and will not be discussed further.

2.2.3. Surface area determinations

BET surface areas of the three powders were measured both beforeand after the experiments. Measurements were performed on 0.9–1.2 gpowders using N2 single-point gas adsorption at a partial pressure of0.3 atm and applying the BET isotherm (Gregg and Sing, 1982). Ninedeterminations were made for each sample. For the instrument used inthis study (a Quantachrome Quantasorb sorption analyser) a singlepoint is as accurate as carrying out triple point determinations (J. Marr,pers. commun., 1996). Surface area data for the three powders aresummarised in Table 3.

2.3. Flow-Through Experiments

Two runs of the flow-through experiments were performed, both at25°C and with the same samples. The first run used a continuous flowreactor system in which each reactor vessel contained 50 mL of acidwith the feldspar powder, the remaining volume (5–10 mL) being filledwith air. Reactor vessels were sealed with neoprene bungs containingtwo tubes, one delivering fresh acid (pH 2 HCl), the other removingacid with which the powders had reacted. The powders were held in 20mm mesh polypropylene bags which were suspended from the top ofthe reactor vessels. Fresh acid was pumped into the vessels at a rate of1.46 3 1023 6 1 3 1023 mL s21 where it was agitated by amagnetically-driven stirrer. Reacted acid was driven out of the vesselsby the force of the incoming fresh acid and collected in clean beakers.

Table 2. Mineralogical composition of the three powders used in theexperiments and bulk chemical compositions of their alkali feldspar

component

Eifelsanidine UW-Shap W-Shap

Mineralogy of powder (%)1

Alkali feldspar 100 96 94Quartz 0 6 6

Bulk chemical compositionof alkali feldspar (wt%) 2

SiO2 64.04 64.87 65.13A1rO3 18.42 18.49 18.56FeO 0.19 0.11 0.10CaO 0.01 0.13 0.13BaO 0.86 0.45 0.45Na2O 1.75 3.14 2.93K2O 13.74 11.98 12.18Total 99.00 99.18 99.47

cations per 8 OSi 2.98 2.99 2.99A1 1.01 1.00 1.00Fe21 0.01 0.01 0.01Ca 0.00 0.01 0.01Ba 0.02 0.01 0.01Na 0.16 0.28 0.26K 0.82 0.70 0.71Total 4.99 5.00 4.99n 11 25 25

molecular proportionsOr 83.8 (0.5) 71.1 (6.5) 72.8 (2.8)Ab 16.2 (0.5) 28.2 (6.3) 26.6 (2.7)An 0.0 (0.0) 0.7 (0.2) 0.6 (0.2)

1 Determined by electron probe analyses of 100 randomly-selectedgrains.

2 Mean of wavelength-dispersive electron probe analysis of random-ly-selected grains. Figures in parentheses are standard deviations.


Solutions were collected every 14 days and analysed for Si, Al, Na, Ca,and K by ICP-OES. Two problems with this initial reactor design wereencountered. Firstly, the powder sank to the bottom of the polypro-pylene bags, preventing the incoming fluid from circulating freelybetween grains, and secondly some feldspar grains escaped from thebags. After 2.773 106 s the UW-Shap experiment was stopped, but thepowder was left in the reactor vessel; all three experiments were haltedafter 4.243 106 s and the experimental design was modified.

In the second run, which used the same powders, pH 2 HCl wasagain pumped into the reactor vessels at a rate of 1.463 1023 6 1 31023 mL s21 via a tube which reached to the bottom of the vessel. Thistime, the powder was not contained within a bag. The reacted acid wasexpelled through a tube at the top of the vessel by the incoming fluidand passed through an external 4mm filter before collection. The filterswere examined by SEM for particles, but none were found. Eachreactor vessel was agitated for 5 min twice daily to ensure completemixing between incoming acid and the feldspar powder. Eifel sanidineand UW-Shap were reacted for 7.203 106 s and W-Shap for 7.903106 s in this run.

Ideally, less concentrated acids would have been used for the exper-iments to more accurately simulate weathering conditions in nature.However, owing to the slow rates at which alkali feldspars dissolve, itwas necessary to use a concentrated acid in order for appreciablereaction to have taken place at the grain surface. In addition, our resultscan be directly compared with other laboratory dissolution experimentson alkali feldspars which have used the same pH and temperatureconditions (e.g., Schweda, 1989).

3. NATURAL WEATHERING OF SHAP GRANITEALKALI FELDSPARS

3.1. Microtextures of Unweathered Feldspars

Previous work on unweathered Shap Granite alkali feldspars(Lee et al., 1995) has shown that each phenocryst contains threedifferent microtextures which are intergrown on a sub-mmscale. As these microtextures are composed of the same phasesand have similar bulk chemical compositions, but differ incoarseness and coherency (dislocation density), Shap Granitealkali feldspars are ideal minerals to study in order to determinethe effects of microtextures and dislocation density on weath-ering rates.

Characteristics of the three microtextures will only be sum-marised below because they have been described in detailelsewhere (Lee et al., 1995; Lee and Parsons, 1995, 1997a, b,1998). Cryptoperthites (bulk chemical composition

;Or86.8Ab30.5An1) constitute;2 vol% of each phenocryst andare composed of small albite exsolution lamellae (,;0.075mm in width by a few micrometres long) called platelets, intweed orthoclase. These platelets have fully coherent interfaceswith orthoclase (i.e., no dislocations). The dominant microtex-ture is microperthite (;58 vol%) which has a bulk chemicalcomposition of;Or71.8Ab27.6An0.7 and is composed of albiteexsolution lamellae.;0.075–;0.5 mm in width by manymicrometres long, called films, in tweed orthoclase. The albitefilms are semicoherent, with subregularly spaced pairs of edgedislocations along their interface with orthoclase. Patch per-thites cross-cut cryptoperthites and microperthites as patches orirregular veins and constitute;40 vol% of each phenocryst.They are composed of micropore-rich intergrowths of albiteand microcline subgrains. The subgrain boundaries are semi-coherent or incoherent and have many dislocations. Thus,coarseness and dislocation density differ in the order patchperthite. microperthite. cryptoperthite.

3.2. Natural Weathering Behaviour

The natural weathering behaviour of Shap Granite alkalifeldspars in three different microenvironments has been stud-ied: (1) within lichen-encrusted granite surfaces at outcrop, (2)in gravels (grus) overlying exposed rock surfaces at outcrop(Lee and Parsons, 1995), and (3) within peat soils formed onthe granite at outcrop (Lee and Parsons, 1995 and previouslyunpublished work below). These three microenvironmentscover the entire weathering history of a grain, from initialexposure to weathering solutions within the parent rock, to finaldisintegration in soils. This weathering history is summarisedin Fig. 1 and described below. Emphasis is placed on theevolution of grain surfaces that intersect microperthites becausethese surfaces evolve in a highly regular and predictable man-ner.

With initial chemical weathering of microperthites, etch pitsform by selective dissolution at the sites where edge disloca-tions along albite films intersect the grain surface (stages a to bin Fig. 1). Each pair of dislocation etch pits then merges acrossthe albite films to form a single rectangular pit (stage c). The

Table 3. Summary of data related to the laboratory experiments.

Eifel sanidine UW-Shap W-Shap

Initial sample mass (g) 3.95 3.07 3.65

Tlab

run 1 4.243 106 4.243 106* 4.243 106

run 2 7.203 106 7.203 106 7.903 106

Total 1.143 107 1.143 107 1.213 107

BET surface area (m2 g21)Ar1 0.1096 0.001 0.1046 0.002 0.2456 0.001Ar2 0.1286 0.001 0.3136 0.001 0.3936 0.001Ar2/Ar1 1.17 3.01 1.60

Rlab (10215 moles feldspar cms2 s21)†

Normalised toAr1 1.24 (0.22) 2.83 (0.26) 1.61 (0.26)Normalised toAt 1.22 (0.27) 1.13 (0.16) 1.11 (0.22)

RRRAl 0.53 (0.21) 0.89 (0.06) 0.75 (0.19)

*Measurements of solution composition ceased at 2.773 106 s, but powder remained in acid until 4.243 106 s.†Expressed as the mean of the last five analyses. Figures in parentheses are standard deviations of the mean.


rectangular pits enlarge by dissolution of intervening albiteuntil only narrow (,0.5 mm thick) pillars of albite remainbetween them (stage d). These stages are also seen on thesurfaces of alkali feldspars from lichen-encrusted granite sur-faces, grus, and soils.

With further chemical weathering of microperthites, the pil-lars of albite between etch pits are removed by dissolution andthe pits coalesce to form continuous channels (stage e). Re-moval of the albite pillars weakens the grain surface so muchthat it eventually becomes mechanically unstable and disinte-grates like a house of cards (Fig. 2a–d ). Micrometre-sizedfragments of orthoclase litter grain surfaces (Fig. 2a, b). Thethickness of these orthoclase fragments shows that disintegra-tion strips away the top;2–10mm of the grain surface; a goodimpression of the depth to which fragmentation occurs can alsobe gained from stereopairs (Fig. 2c). Areas of grain interiors

that have been newly-exposed by fragmentation are lessheavily weathered than former grain surfaces and have stage detch pits (Fig. 2b). These pits will continue to enlarge until theyreach stage e and the grain surface again disintegrates. Mi-croperthite surfaces are, therefore, progressively lowered byalternating periods of dissolution (stage d to e) and fragmen-tation (stage e to d). In many cases, fragmentation may betriggered by organic materials adhering to the weakened grainsurfaces (Fig. 2d). Although the etch pit sequence in Fig. 1 isconstructed from images of many different grain surfaces, eachdiagram can also be considered as a section through a weath-ered grain. Thus, when a grain surface is at stage e, at progres-sively greater depths beneath the surface will be etch pitswhose cross-sections are characteristic of stages d, c, b, andfinally unweathered feldspar of stage a.

Resin casts of grains whose surfaces are at etch pit stage eshow that the pits extend for$50 mm into grain interiors asnetworks of tubes (Fig. 3a ). Each network is composed of aparallel array of major tubes 0.4–0.6mm wide that are orientednormal to the grain surface and are connected to each other bytubes 0.6–1.8mm long by 0.08–0.3mm wide (Fig. 3b). TEMimages of the interior of weathered grains confirm the one-to-one relationship between edge dislocations along albite filmsand etch tubes (Fig. 3c). The smallest tubes observed wererectangular in cross-section and 15 by 30 nm in size (i.e.,equivalent to stage b on the grain surface). These tubes readilyenlarge and coalesce to form a single tube (equivalent to stagec on the grain surface; Fig. 3c). Importantly, no precipitatessuch as clay minerals were found within any of the tubes,despite an extensive search by TEM (note that these grainswere never ultrasonically cleaned). SEM images of grain sur-faces do, however, suggest that bacteria may be abundantwithin the networks of etch tubes (Fig. 3d). Although suchimages also suggest that bacteria could play a role in chemicalor even mechanical weathering, direct evidence for their in-volvement in breakdown of the feldspar is absent.

Cryptoperthites are free of etch pits, even when microper-thites exposed on different areas of the same grain surface areat stage e (Fig. 4a ). Although cryptoperthite surfaces appearsmooth, they have shallow grooves which are comparable insize and shape to albite platelets (Fig. 4b, c). As deep etch pitsare absent, bands of cryptoperthite do not undergo mechanicalweathering and are often elevated considerably above sur-rounding microperthite (Fig. 4b, c). Veins of patch perthitewhich cut microperthite surfaces at stage e are represented bydeep, narrow troughs which contain a delicate honeycomb ofhighly etched feldspar (Fig. 4d).

4. EXPERIMENTAL RESULTS

4.1. Solution Chemistry

Dissolution rate data for the three feldspar powders aresummarised in Table 3 and plotted in Fig. 5. The data areexpressed in two forms. Firstly, we have normalised the ratedata to the BET surface area of the powder which was mea-sured at the start of run 1 (Ar1, cm2), using Eqn. 1

Rlab 5O 3 F

Ar1 3 W 3 n(1)

Fig. 1. Schematic illustration of the weathering sequence observedon microperthite grain surfaces. Stages a to d were illustrated by Leeand Parsons (1995). When the grain surface reaches stage e it disinte-grates, returning to stage d. Cycling between stages d and e progres-sively lowers the surface.


Fig. 2. Secondary-electron (SE) SEM images of stage e naturally weathered microperthite surfaces of Shap Granite alkalifeldspars. (a) Fragmentation of orthoclase selvages remaining between etch channels (oriented N-S in the image) hasproduced innumerable lath-shaped feldspar fragments which litter the grain surface. Note that most of the information onthis very fragile surface would be completely destroyed if the grain were ultrasonically cleaned prior to imaging. Scale bar5 10 mm. (b) Area of the grain surface where orthoclase fragments have been removed by natural processes, exposing theless heavily weathered grain interior. The interior (I, lower right) has albite pillars indicative of weathering stage [d]. Scalebar 5 10 mm. (c) Stereopair of an area of grain surface undergoing fragmentation. These images give a good impressionof the depth to which fragmentation has taken place. Scale bar5 10 mm. (d) Stereopair illustrating how fragmentation ofthe grain surface can occur, in this case possibly related to a plant root forcing its way along the surface. Scale bar5 10mm.


Fig. 3. Images of naturally weathered Shap Granite alkali feldspars. (a) SE image of a resin cast of a stage e naturallyweathered microperthite. The former grain surface is parallel to the right hand edge of the image. Resin has penetrated alongmajor etch tubes (oriented E-W in the image) that are interconnected by minor etch tubes (oriented N-S). Together, theresin-filled etch tubes form a mesh-like structure which extends a minimum of 50mm beneath the weathered grain surface.Scale bar5 10 mm. (b) SE image of one part of a resin etch tube cast; major tubes are again oriented E-W and minor tubesN-S. The smallest tubes along which the resin has penetrated are 0.08mm in width. Scale bar5 2.5 mm. (c) Bright-fieldTEM image of the interior of a grain whose surface was at stage e. In this image are three albite exsolution lamellae (darkgrey, oriented N-S) within tweed orthoclase (light grey). The narrowest of the three lamellae is fully coherent; the other twowider lamellae are semicoherent. A few unweathered edge dislocations (D) straddle the coarser lamellae. Highly elasticallystrained feldspar surrounding all of the other dislocations has been selectively dissolved; the holes in these images (whitein colour) are, therefore, cross-sections of etch tubes, equivalent to the smallest resin-filled tubes in (a) and (b). Althoughthese etch tubes may have been slightly enlarged by ion thinning, the close correlation between the tubes and dislocationsshows that they are original features of the weathered grain’s interior. Scale bar5 250 nm. (d) Stereopair of SE images ofa partially-fragmented stage e grain surface. Numerous rod- and disc-shaped bacteria occupy the etch channels, but note thatthey will have shrunk in size under high vacuum conditions. This image suggests that microorganisms may play a role inmineral weathering, but provides no proof. Scale bar5 5 mm.


Fig. 4. SE images of stage e naturally weathered Shap Granite alkali feldspar grain surfaces. (a) Area composed ofmicroperthite, with characteristic etch channels oriented N-S, which contains two parallel bands of cryptoperthite (orientedE-W). The microperthite is at stage e and disintegrating, whereas the cryptoperthite’s surface is very smooth and free of etchpits. Scale bar5 50 mm. (b) SE image of the interface between cryptoperthite (lower left half of the image) andmicroperthite (upper right). The cryptoperthite is free of etch pits and elevated considerably above microperthite; themicroperthite surface is inferred to have lowered faster than the adjacent cryptoperthite by a combination of chemical andmechanical weathering. Scale bar5 10 mm. (c) Stereopair of SE images of the interface between cryptoperthite and stagee microperthite (etch channels in the microperthite oriented E-W). This image illustrates the abrupt nature of the interfacebetween the two microtextural regions. Scale bar5 10mm. (d) Stereopair of SE images of a vein of patch perthite (orientedNE-SW) cross-cutting stage e microperthite (etch channels oriented E-W). The patch perthite has clearly weathered muchfaster than surrounding microperthite. The former patch perthite vein is occupied by a very delicate honeycomb of feldsparand, possibly, reaction products. Scale bar5 10 mm.


whereRlab 5 dissolution rate at timet calculated from Si data(moles cmtotal

22 s21), O 5 output solution minus acid blank forSi (g ml21), F 5 flow rate of acid (ml s21), W5 atomic weightof Si, andn 5 number of ions of Si per feldspar formula unit(on the basis of 8 O). Final dissolution rates are expressed asthe mean of the last five analyses in run 2 (Table 3, Fig. 5). Asthe BET data show that the surface area of all three powdershad apparently increased between the start of run 1 and end ofrun 2 (Table 3), we have also, following Stillings and Brantley(1995), normalised the rate data to BET surface area as it isinferred to have changed over time. The BET surface area of apowder at timet (At, cm2) is calculated using Eqn. 2

At 5 SAr2 2 Ar1

TlabD 3 t 1 Ar1 (2)

whereAr2 5 BET surface area at the end of run 2 (cm2), Tlab 5total duration of the experiment (s), andt 5 time of samplecollection (s). As noted by Stillings and Brantley (1995), im-plicit in this equation is the assumption that BET surface areaincreases linearly between the two measurements. Dissolutionrates were then calculated using Eqn. 3

Rlab 5O 3 F

At 3 W 3 n(3)

Importantly, Eqns. 1 and 3 assume that all areas of the grain’ssurface are equally dissolution-reactive and that this reactivesurface area equals surface area measured by BET. In addition,Eqn. 3 assumes that any new surface area created during theexperiments, for example by the growth of etch pits, will beequally as dissolution-reactive as the rest of the grain’s surface.These assumptions, which relate to the relative reactivities ofdifferent sites on a grain’s surface, are discussed in later sec-tions.

Both plots for Eifel sanidine show that rates initially de-creased rapidly (within the first;1.00 3 106 s), then settleddown so that most subsequent analyses were within standarddeviation of the mean of the last five measurements (Fig. 5a, b).These two separate periods of behaviour correspond to theparabolic and steady-state dissolution kinetics noted in manyprevious studies of feldspar dissolution (see Blum, 1994 andBlum and Stillings, 1995 for comprehensive reviews). The plotfor UW-Shap which has been normalised toAr1 shows an initialparabolic phase, but then an increase in rate during the steady-state phase (Fig. 5c). Stillings and Brantley (1995) also re-ported an increase in steady-state dissolution rates with time forrate data normalised toAr1. They concluded that such anoma-lous results demonstrate the importance of normalising data toAt. The plot for UW-Shap which is normalised toAt (Fig. 5d)

Fig. 5. Plots of the dissolution rates of the three feldspar powders derived from Si concentrations,Rlab, over the durationof runs 1 and 2. Two plots for each powder are presented, one showing rate data normalised toAr1, the other showing ratedata normalised toAt. The mean of the last five analyses in run 2 for each powder is shown as a solid horizontal black lineand the standard deviation of this mean is represented by the grey shading on either side of the line.


shows a gradual decrease in rate during run 1 and little overallchange during run 2, again corresponding to parabolic andsteady-state kinetics. Data for W-Shap do not show such apronounced drop in rate at the start of run 1 but also show asignificant change between run 1 and run 2 (Fig. 5e, f). Despitethese differences, the majority of measurements lie within thestandard deviation of the mean of the last five data points andso are representative of steady-state rates.

The stoichiometry of the dissolution reaction is expressedusing the equation below (after Holdren and Speyer, 1985):

RRRz 5Rz

Rlab(4)

whereRRRZ is the relative release ratio of elementZ (Na, Al,or K); values of 1 indicate stoichiometric dissolution. Data forRRRAl are summarised in Table 3 and plotted in Fig. 6 . Thesegraphs show that at the start of run 1 the dissolution of all threepowders was nonstoichiometric with respect to Al, but settleddown within;7.203 105 s to a value of close to 1. At the endof run 2, RRRAl values are,1, indicating that Si was beingreleased into solution preferentially to Al for all three powders(Fig. 6). Stillings and Brantley (1995; Table 2) also noted thatRRRAl for steady-state dissolution of microcline in 0.001M HClwas,1 (0.856 0.44).

4.2. SEM and AFM Characterisation of Reacted GrainSurfaces

Secondary electron SEM images of the surfaces of UW-Shapgrains after run 2 failed to reveal any of the regularly-arrangedetch pits that are so characteristic of early stages of weatheringin nature (Fig. 1). However, BSE images of uncoated grainsurfaces using the Philips XL30 CP operated at a pressure of0.2 mBar showed that very small etch pits do occur at dislo-cation outcrops along albite exsolution lamellae (Fig. 7a). Fur-ther imaging of UW-Shap grain surfaces by AFM revealed thecharacteristics of the dislocation etch pits in great detail (Fig.7b–d). These pits are at least 8 nm in depth, and their size andshape is closely comparable to the distribution of highlystrained feldspar surrounding the edge dislocations seen inTEM images (Lee et al., 1995; Lee and Parsons, 1995). TheAFM images also show that albite exsolution lamellae havedissolved more rapidly than surrounding tweed orthoclase,forming troughs 5 nm deep (Fig. 7c,d). Note that AFM imagesof freshly-broken grains acquired during this study demonstratethat troughs at the outcrop of albite lamellae did not form bysome selective fracturing or plucking when the feldspar grainswere broken during grinding to a powder.

5. DISCUSSION

5.1. Role of Different Surface Sites in InfluencingMineral Weathering Rates

Observations above show that naturally weathered and ex-perimentally reacted feldspars have three major types of sur-face site that may have a different response to weathering: (1)dislocation outcrops, (2) albite exsolution lamellae, and (3)tweed orthoclase. The rate at which a grain dissolves in thelaboratory or field (Rlab, Rfield, moles cmtotal

22 s21) can be ex-pressed as the sum of the individual dissolution rates of allthree sites:

Rlab,field 5 Rd 1 Rab 1 Rto (5)

whereRd, Rab, andRto is the chemical weathering rate of alldislocation outcrops, albite lamellae, and all areas of tweedorthoclase, respectively, per unit area of the grain surface(moles cmtotal

2 s21). Rd, Rab, andRto are calculated using thefollowing equations:

Rd 5 ~r d 3 Sd! (6)

Rab 5 ~r ab 3 Sab! (7)

Rto 5 ~r to 3 Sto! (8)

where rd, rab and rto are the dissolution rates of dislocationoutcrops per unit area of dislocation outcrops (moles cmd

22

s21), albite lamellae per unit area of albite lamellae (molescmab

22 s21), and tweed orthoclase per unit area of tweed ortho-clase (moles cmto

22 s21), respectively, andSd, Sab, andSto is thearea of the grain’s surface occupied by dislocation outcrops(cmd

2 cmtotal22 ), albite lamellae (cmab

2 cmtotal22 ), and tweed ortho-

clase (cmto2 cmtotal

22 ), respectively. By combining the previousfour equations,Rlab,field can be calculated:

Rlab,field 5 ~r d 3 Sd! 1 ~r ab 3 Sab! 1 ~r to 3 Sto! (9)

Fig. 6. Plots ofRRRAl for the three feldspar powders over theduration of runs 1 and 2. The horizontal black line in each plot lies ata RRRAl of 1 (i.e., stoichiometric dissolution).


Fig. 7. Images of surfaces of grains from the UW-Shap powder after run 2. (a) BSE image of an uncoated grain surfaceat a pressure of 0.2 mBar. Albite exsolution lamellae are oriented NNE-SSW in the image. All of the lamellae are straddledor cut by regularly-spaced dislocation etch pits, here imaged as discrete black areas. Scale bar 1mm. (b) AFM image of agrain surface at approximately the same magnification as in (a). The outcrops of albite lamellae, again oriented NNE-SSW,are in all cases;5 nm lower than the surrounding grain surface, forming shallow troughs. All but the finest lamellae arestraddled by regularly-spaced pairs of short and narrow dislocation etch pits whose bases of which are at least 8 nm lowerthan surrounding tweed orthoclase; a value of 20 nm is probably more realistic owing to difficulties the AFM tip would havehad in reaching the pit bases. Note that the grain surface has many steps, the smallest of which are approximately one unitcell in height. Scale bar 1mm; height differences in the image are calibrated using the grey scale on the right hand side.(c) Higher magnification AFM image of a grain surface. In the centre of the field of view is a trough that has formed at theoutcrop of an albite lamella which is straddled by regularly-spaced dislocation etch pits. Very fine lamellae on the left handside of the image have no dislocation etch pits because they are coherent. Scale bar 1mm. (d) Tip deflection image of thesame area as in (c). In this image the troughs at the outcrops of albite exsolution lamellae and dislocation etch pits are easierto appreciate. Scale bar 1mm.


5.2. Natural Weathering of Shap Granite Alkali Feldspars

5.2.1. Mechanisms of chemical weathering

In order for data from SEM images to be of use in predictingthe chemical weathering rates of Shap Granite alkali feldsparsin the field, it is first necessary to understand the mechanismsby which grain surfaces interact with natural acidic solutions.Figure 1 shows that the weathering history of Shap Granitealkali feldspars in the field is dominated by the growth andcoalescence of etch pits at dislocation outcrops. During earlyphases of weathering (i.e., stages a to b), dissolution at dislo-cation outcrops will be a major source of solutes. As the etchpits enlarge and coalesce, the total surface area of a grain willincrease significantly so that dissolution at dislocations willmake a progressively smaller contribution to the total flux ofsolutes from each grain. Qualitatively, therefore, dislocationswill make the most important contribution to chemical weath-ering rates during the early stages.

During the initial periods of chemical weathering when etchpits are shallow, diffusion distances between etch tube bases,the sites of rapid chemical weathering, and solutions at grainsurfaces will be relatively short, so that no gradient in soluteconcentration between the base and top of an etch pit would beexpected. As a grain progresses towards stage e and etch tubesreach$5 3 1023 cm in depth, however, a gradient in soluteconcentration may develop. Although the mouths of etch tubesare relatively broad (Fig. 2a–d), TEM results have shown thatthe bases of etch tubes, which have formed by dissolution of themost highly elastically strained feldspar surrounding edge dis-locations, are 15 by 30 nm in cross-sectional area (Fig. 3c); thesmallest tubes along which resin has penetrated are 80 nmwide. Such small and isolated sites have been collectivelytermed the internal zone by Hochella and Banfield (1995).Weathering reactions in the internal zone may be fundamen-tally different to reactions at the grain surface or external zone.Within the external zone fluid movement is, or is potentially,hydraulically-driven, whereas fluid flow cannot be supportedwithin the internal zone owing to the narrow, tortuous, anddead-end nature of the etch tubes. Aqueous species move in theexternal zone by advection and through static fluids in theinternal zone by diffusion (Hochella and Banfield, 1995).

The concentration of solutes within internal zone solutionsmay differ from external zone solutions if ions are beingliberated from the bases and walls of etch pits at a greater ratethan they can be removed from the internal zone by aqueousdiffusion. Although even the most reactive parts of alkalifeldspars are likely to dissolve relatively slowly, diffusion willbe hindered if paths are narrow and tortuous. In addition,internal zone and external zone solutions are very unlikely to bein contact with each other throughout the weathering history ofa grain because, as described by Velbel (1993), movement ofwaters through soils is often restricted to discrete channels sothat at any one time the surfaces of the majority of grains areby-passed by advecting soil waters. If solutes cannot be re-moved from the internal zone, they will accumulate and even-tually slow rates of dissolution as saturation is approached.Although details are unclear, it is apparent from this qualitativeexamination of weathering mechanisms that the solutionswhich mediate chemical weathering are likely to be closer tosaturation with respect to the reactant phases, especially during

relatively advanced stages in a grain’s weathering history, thanintergranular soil waters. Note that the high concentration oforganic acids in the soil waters could have inhibited precipita-tion of clays that may have otherwise have formed within theintragranular pores.

In the sections below, we use information from SEM imagesto quantify chemical weathering rates. Although these calcula-tions are for an early stage in the weathering history of a grain(approximately stage b in Fig. 1), which is most comparable tothe degree of dissolution experienced by UW-Shap grains usedin the laboratory experiments, the calculations use informationon the behaviour of grains throughout their weathering history.

5.2.2. Rates of early chemical weathering

Lee and Parsons (1995) demonstrated that the three differentmicrotextures in Shap Granite alkali feldspars have contrastingdensities of etch pits. Specifically, patch perthites have agreater density of etch pits than microperthites which havemore etch pits than cryptoperthites (etch pits comparable tothose in patch perthites and microperthites are in fact absentfrom cryptoperthites). These observations suggest that chemi-cal weathering rates vary in the order patch perthite. mi-croperthite. cryptoperthite. Previous observations by Lee andParsons (1995) and images of deep dislocation etch pits pre-sented here show thatrd must be greater than bothrab and rto

for microperthites and patch perthites. Dislocations must, there-fore, have an effect on the chemical weathering rate of thesefeldspars but, as described above, it will be greatest duringearly stages of weathering. The quantitative effect of disloca-tions is calculated below, and these calculations are also sum-marised in Table 4 .

As Eqns. 5–9 demonstrate, the quantitative importance ofdislocations to overall rates of chemical weathering will dependon the values ofrd, rab, rto, and Sd, Sab, Sto. Although thepresence of dislocation etch pits shows thatrd . rab andrd .rto, the difference in relief on naturally weathered surfacesbetween albite lamellae and surrounding tweed orthoclasewhen dislocations are absent (Fig. 4a–c) is very small; thus, wecan assume for simplicity thatrab ' rto (hereafter expressed asrab'to) and thatSab 1 Sto (hereafterSab1to) 5 12Sd. The valueof rd during early natural weathering of microperthites, thedominant microtextures in Shap Granite alkali feldspars, can beestimated from observations of etch pits in naturally weatheredgrains using Eqn. 10:

r d 5Rdeep

Vmol(10)

whereRdeep5 the rate at which dislocation etch pits deepen(cm s21) andVmol 5 molar volume of feldspar (cm3 mol21).Rdeepcan be calculated using Eqn. 11:

Rdeep

Dd-pit

Tfield(11)

whereDd-pit 5 depth of dislocation etch pits (cm), andTfield 5duration of natural weathering (s). Resin casts (Fig. 3) showthat by weathering stage eDd-pit 5 5.003 1023 cm (50mm).Tfield is one of the variables in this model which is most difficult


to accurately determine. It may be anything between 3.1531011 s (10 kyr; the maximum age of their host soil) and;3.153 1010 s (1 kyr); for brevity, we assume an intermediate valuefor Tfield of 1.583 1011 s (5 kyr). If the etch pits have grownat an equal rate throughout the 5 kyr weathering history of thegrain,Rdeep5 3.173 10214 cm s21 (10 nm yr21). Using thisvalue of Rdeep in Eqn. 10, and ifVmol 5 100.07 cm3 mol21,yields rd of 3.173 10216 moles cmd

22 s21. Note that the truevalue of Rdeep during early stages of chemical weathering islikely to have been greater than 10 nm yr21, which is anaverage for 5 kyr of weathering. This is becauserd should haveslowed over time as etch tubes deepened and solute concentra-tions within the tubes increased (see previous discussion).

If Sd can be measured and the difference betweenrd andrab'to is known, Rfield can be determined using a modifiedversion of Eqn. 9:

Rfield 5 ~r d 3 Sd! 1 ~r ab'to 3 Sab1to! (12)

Note that (rab'to 3 Sab1to) 5 Rab,to (moles cmtotal22 s21). Sd is

the product of the cross-sectional area of highly-strained felds-par surrounding a dislocation,Cd (cm2), and dislocation den-sity, rd (cm22). Here we assume an early stage of naturalweathering wherebyCd 5 1.90 3 10210 cm2 (the cross-sectional area of stage b etch pits in naturally weathered mi-croperthites measured by Lee and Parsons (1995) and illus-trated schematically in Fig. 1) andrd 5 2.00 3 108 cm22

(determined using TEM, Lee and Parsons, 1995); thus,Sd 50.038 cmd

2 cmtotal22 andSab 1 Sto 5 0.962 cmd

2 cmtotal22 .

As there is no way to independently determine the differencebetweenrd andrab'to, we will assume two very different ratiosand assess their effect onRfield . If rd 5 2 3 rab'to, Rfield 5 1.643 10216 moles cmtotal

22 s21; if, however, rd 5 103 3 rab'to,Rfield 5 1.23 3 10217 moles cmtotal

22 s21. Although the two

ratios of rd to rab'to differ by a factor of 500, they only yielda factor of 13 difference inRfield. Significantly, if rd 5 2 3rab'to, Rfield is a factor of 12.2 lower thanRlab for UW-Shap of;2 3 10215 moles cmtotal

22 s21 and is close toRlab for K-feldspar in pH 4 acids of;10216 moles cmtotal

22 s21 (Blum,1994). As the weathering rate of a given mineral in the field isknown to be 1–3 orders of magnitude slower than rates for thesame mineral in a laboratory environment (see reviews byBlum, 1994 and Blum and Stillings, 1995), we suggest that ifrd 5 2 3 rab'to, Rfield is unrealistically high and thatrd 5 $103

3 rab'to gives more reasonable values ofRfield.Results from these calculations also demonstrate that ifrd 5

103 3 rab'to, dissolution at dislocations during early stages ofchemical weathering yields 1 order of magnitude more feldsparin solution than all other areas of the grain surface combined(Rd 5 39.473 Rab,to, Table 4). Note that as chemical weath-ering progresses, new surface area is created in etch pit wallsand the highly reactive dislocation outcrops are consumed; thusdissolution at dislocations will progressively become a lessimportant source of solutes with time.

5.2.3. Mechanical weathering of grain surfaces

Mechanical weathering of heavily-etched grain surfaces islikely to amplify contrasts in the rates of chemical weatheringof different microtextural regions. Thus cryptoperthites, whichare free of etch pits, show no signs of mechanical weathering(Fig. 4a, b, c). By contrast, heavily pitted microperthites readilydisintegrate when they reach weathering stage e. Disintegrationwill elevate overall rates of chemical weathering by increasingthe surface area/volume ratio of the grains and by exposinggrain interiors that have been less weathered chemically. Patchperthites, which have a higher dislocation density than mi-croperthites, also disintegrate readily, again facilitating rates ofsubsequent chemical weathering. It is likely, therefore, thatdifferences inRfield between mineral grains with contrastingdislocation densities will become even greater as their grainsurfaces disintegrate.

5.3. Laboratory Dissolution: Influence of Microtexturesand Microstructures

5.3.1. Interpretation of rate data

Laboratory dissolution rates of the three feldspar powdersdescribed here are comparable to rates determined by Schweda(1989) from dissolution of alkali feldspar (microcline frompegmatites at Stormyra, Sweden) in pH 2 HCl and LiOHsolutions at 25°C. Rates from two experiments were 1.12310215 and 2.203 10215 moles cmtotal

22 s21 (Schweda, 1989).If laboratory dissolution of alkali feldspars in our study had

been strongly influenced by microtextures and dislocation den-sity, Rlab should have varied in the order UW-Shap. W-Shap. Eifel. As Table 3 shows, this is the case for dissolution ratedata nomalised toAr1, but not the data normalised toAt. Foraccurate interpretation of these data it is necessary to under-stand why BET surface area increased during the experiments,especially for the UW-Shap powder.

The very small pits and troughs on UW-Shap grain surfacesimaged by AFM after the dissolution experiments (Fig. 7)cannot have been responsible for the significant increase in

Table 4. Data and calculations for the dissolution rates of ShapGranite alkali feldspars in field and laboratory contexts.

Natural weathering Experimental dissolution†

Depth of etch pits (cm)Dd-pit 5.003 1023 Dd-pit 2.003 1026

Dab-trgh 5.003 1027

Area of the surface of a grain occupied by each site (cm2ab,d,tocmtotal

22 )Sd 0.038 Sd 1.303 1023

Sab1to 0.962 Sab 0.1484Sto 0.8503

Dissolution rate of each site per unit area of the site (moles cm22ab,d,tos21)

rd 3.173 10216 rd 3.683 10215

rab'to* 3.173 10219 rab 2.373 10215

rto 1.933 10215

Dissolution rate of each site per unit area of grain surface (moles cmtotal22 s21)

Rd 1.203 10217 Rd 4.783 10218

Rab,to* 3.043 10219 Rab 3.523 10216

Rto 1.643 10215

Rd/Rab,to* 39.47 Rd/(Rab 1 Rto) 2.403 1023

Overall dissolution rate of a grain (moles cmtotal22 s21)

Rfield* 1.233 10217 Rlab 2.003 10215

* assumes thatrd 5 103 3 rab,to.† Data and calculations for the UW-Shap powder.


BET surface area of this powder. One possible explanation forthe apparent increase in surface area is that each grain haddeveloped a leached layer, which was porous to N2. However,it is difficult to quantify the role of such a layer in gas adsorp-tion. A second possibility is that some grains in all threepowders had fragmented during the experiments. However,there is no obvious reason why grains in the UW-Shap powderwould have fragmented much more readily than in the twoother powders. A third possibility is that secondary phases hadformed during the experiments, but no such precipitates wereobserved by SEM, and the experimental solutions were under-saturated with respect to potential precipitates (Table 5 ). Thus,the increases in BET surface area of the three powders aredifficult to account for. We believe that this in part reflectswider problems in understanding the precise nature of thesurface area of mineral powders that is measured by the BETmethod (Hodson et al., 1997; Hodson, 1998).

5.3.2. Quantitative role of microtextures and microstructures

The AFM images show that albite exsolution lamellae are;5 nm lower than surrounding tweed orthoclase and the basesof dislocation etch pits are;8 nm lower than orthoclase; thusrd . rab . rto on UW-Shap grain surfaces. Owing to thenarrowness of dislocation etch pits, it is likely that the AFM tipcould not reach and accurately record the bases of the pits. Asa result, the pits are probably considerably deeper than mea-

sured; here we assume a value of 20 nm. A schematic illustra-tion of an experimentally-reacted grain surface is shown in Fig.8a . With these AFM data, the contribution of dissolution at thethree different sites toRlab can be quantified in detail (see Table4 for a summary of the calculations).

If we first, and incorrectly, assume that all areas of a grain’ssurface dissolve at an equal rate (i.e.,rd 5 rab 5 rto), thevolume of a hypothetical uniform layer of feldspar that hasbeen removed from the surface during the duration of anexperiment (Vlayer, Fig. 8b) can be calculated using Eqn. 13:

Vlayer 5 Rlab 3 Tlab 3 Vmol (13)

If we assume thatRlab 5 2.003 10215 moles cm22 s21 (closeto the mean of the two possible values of the UW-Shap powder,Table 3) andTlab 5 1.14 3 107 s (duration of the UW-Shapexperiment),Vlayer 5 2.28 3 1026 cm3 cmtotal

22 (this corre-sponds to a layer with a uniform thickness of 22.8 nm). As theAFM results have demonstrated, the grain surface did notretreat uniformly and dissolved more rapidly at the outcrops ofedge dislocations and albite exsolution lamellae than at theoutcrop of tweed orthoclase (Fig. 8a, b). The volume of pits andtroughs at the outcrops of dislocations and albite lamellae(Vd2pit, cmd

3 cmtotal22 andVab2trgh, cmab

3 cmtota22, Fig. 8c) can be

calculated using Eqns. 14 and 15:

Vd2pit 5 Dd2pit 3 Sd (14)

Vab2trgh 5 Dab2trgh 3 Sab (15)

Here we take values forSd and Sab from Fig. 7b; these datatogether with values forDd-pit andDab-trgh are summarised inTable 4. From Eqns. 14 and 15Vd-pit 5 2.60 3 1029 cmd

3

cmtotal22 and Vab-trgh 5 7.42 3 1028 cmab

3 cmtotal22 . The total

volume of feldspar removed from the outcrops of dislocations(Vd, cmd

3 cmtotal22 , Fig. 8d), albite lamellae (Vab, cmab

3 cmtotal22 ,

Fig. 8d), and tweed orthoclase (Vto, cmto3 cmtotal

22 , Fig. 8d) duringthe UW-Shap experiment can be calculated using the followingthree equations:

Vd 5 @Vlayer 2 ~Vd2pit 1 Vab2trgh!# 3 Sd 1 Vd2pit (16)

Vab 5 @Vlayer 2 ~Vd2pit 1 Vab2trgh!# 3 Sab 1 Vab2trgh

(17)

Vto 5 @Vlayer 2 ~Vd2pit 1 Vab2trgh!# 3 Sto (18)

Data forSto (12[Sd 1 Sab]) are given in Table 4. From Eqns.16, 17 and 18,Vd 5 5.463 1029 cmd

3 cmtotal22 , Vab 5 4.013

1027 cmab3 cmtotal

22 andVto 5 1.87 3 1026 cmto3 cmtotal

22 . Fromthese data,rd, rab andrto can be calculated using Eqns. 19, 20,and 21, respectively:

r d 5Vd

Tlab 3 Vmol 3 Sd(19)

r ab 5Vab

Tlab 3 Vmol 3 Sab(20)

r to 5Vto

Tlab 3 Vmol 3 Sto(21)

These equation give the following results:rd 5 3.683 10215

moles cmd22 s21, rab 5 2.37 3 10215 moles cmab

22 s21 and

Table 5. Chemical composition, pH, and saturation indices of outputsolutions from the laboratory experiments and peat soil water.

Soilwater*

Eifelsanidine† UW-Shap† W-Shap†

Chemical composition(mg L21)

Cl 28.16 n.a. n.a. n.a.N in NO3 0.14 n.a. n.a. n.a.S in SO4 2.9 n.a. n.a. n.a.Ca 2.05 0.05 0.08 0.05Na 12.11 0.16 0.22 0.24K 6.97 0.08 0.11 0.17Mg 1.75 0.00 0.00 0.00Al 1.41 0.06 0.17 0.25Si 2.71 0.33 0.6 0.88Fe n.a. 0.00 0.00 0.00P 0.26 0.00 0.01 0.00pH 3.4* 2.1 2.1 2.1

Saturation indices‡

Albite 210 220 219 218Anorthite 223 237 235 235Microcline 28 219 218 217

Al(OH)3 26 210 210 210Gibbsite 24 29 28 28Halloysite 27 218 217 216Imogolite 29 219 217 217Kaolinite 24 215 214 213Montmorillonite 26 223 221 224

* Average of four separate determinations on different samples ofsoil water using a 0.01 M CaCl2 electrolyte.

† Values quotes are means of the last five analyses of run 2. n.a.denotes not analysed for.

‡ Saturation indices for laboratory experiments assumed 25°Cwhereas calculations for the soil water assumed 7°C.


rto 5 1.933 10215 moles cmto22 s21; thus,rd 5 1.6 3 rab and

rd 5 1.9 3 rto (Table 4). The contribution of each of the threesites to the overall dissolution rate of the UW-Shap powder can

be calculated using Eqn. 9. Data forRd, Rab, and Rto aresummarised in Table 4 and show that dissolution at dislocationoutcrops makes an insignificant contribution to overall rates

Table 6. Calculations ofRlab andRfield given different values ofrd.

rd* 105 107 109 rd 105 107 109 1011

Cd

(cm2) 1.903 10210 1.903 10210 1.903 10210Cd

(cm2) 8.003 10212 8.003 10212 8.003 10212 8.003 10212

Sd

(cmd2 cmtotal

22 ) 1.903 1025 1.903 1023 0.19Sd

(cmd2 cmtotal

22 ) 8.003 1027 8.003 1025 8.003 1023 0.80rd

(moles cmd22 s21) 3.173 10216 3.173 10216 3.173 10216

rd

(moles cmtotal22 s21) 3.683 10215 3.683 10215 3.683 10215 3.683 10215

Rd

(moles cmtotal22 s21) 6.023 10221 6.02t3 10219 6.023 10217

Rd

(moles cmtotal22 s21) 2.943 10221 2.943 10219 2.943 10217 2.943 10215

Sab1to

(cmab1to2 cmtotal

22 ) 0.9999 0.9981 0.81Sab

(cmab2 cmtotal

22 ) 0.1486 0.1486 0.1474 0.0297rab'to

†

(moles cmab,to22 s21) 3.173 10219 3.173 10219 3.173 10219

rab

(moles cmab22 s21) 2.373 10215 2.373 10215 2.373 10215 2.373 10215

Rab,to†

(moles cmtotal22 s21) 3.173 10219 3.163 10219 2.573 10219

Rab

(moles cmtotal22 s21) 3.523 10216 3.523 10216 3.493 10216 7.043 10217

Sto

(cmto2 cmtotal

22 ) 0.8514 0.8513 0.8446 0.1703rto

(moles cmto22 s21) 1.933 10215 1.933 10215 1.933 10215 1.933 10215

Rto

(moles cmtotal22 s21) 1.643 10215 1.643 10215 1.633 10215 3.293 10216

Rd/Rab,to 0.02 1.91 234.24 Rd/(Rab 1 Rto) 1.483 1026 1.483 1024 0.02 7.36Rfield

†

(moles cmtotal22 s21) 3.233 10219 9.183 10219 6.053 10217

Rlab

(moles cmtotal22 s21) 1.993 10215 1.993 10215 2.133 10215 3.343 10215

* If rd 5 1010 dislocations occupy.1 cmd2 cmtotal

22 of the grain surface.† Assumes thatrd 5 103 3 rab'to.

Fig. 8. Schematic cross-sections of the surface of a UW-Shap grain after run 2 that illustrate the different terms used inthe calculation ofRd, Rab, andRto from AFM data. (a) Cross-section of the grain surface imaged by AFM showing how thepits and troughs relate to underlying microtexture. (b) Illustration of the same area as in (a), showing thatVlayer is an averageof the volume of feldspar removed from all three surface sites. (c) Illustration of the relationship ofVab-trgh andVd-pit to theetched surface. (d) Illustration of the relationship ofVab, Vd, andVto to the etched surface.


during a laboratory experiment (Rd/(Rab1R) 5 2.40 3 1023,Table 4).

6. IMPORTANCE OF DISLOCATION DENSITY INWEATHERING

6.1. Impact of Dislocation Density onRlab and Rfield

The effect of different dislocation densities onRlab can becalculated using Eqn. 9: results of these calculations are listedin Table 6 . Here we assume the same values ofCd andrd as forthe UW-Shap powder (i.e.,Cd 5 8.003 10212 cm2 andrd 53.683 10215 moles cmd

22 s21). We also assume that the grainsare microperthites so thatSab 1 Sto 5 1-Sd (with Sab andSto inthe same proportions as in UW-Shap measured by AFM, Table4) and thatrab andrto are the same as for UW-Shap. Results ofthese calculations are summarised in Table 6. They demon-strate that under the experimental conditions used by us, itwould have been difficult to distinguish the dissolution rates ofpowdered feldspars with dislocation densities ranging over 6orders of magnitude (rd 5 10521011 cm22). Only whenrd 5$1011 cm22 are more solutes contributed from dislocationsthan from all other areas of the grain surface combined (i.e.,Rd/(Rab 1 Rto) 5 .1). These findings confirm conclusions fromprevious work (Table 1) that dislocation density is a variablethat has little or no measurable effect on dissolution rates ofrock-forming minerals under laboratory conditions.

Repeating the same calculations for early natural weatheringof Shap Granite alkali feldspars shows that a difference indislocation density between 105 and 109 produces a corre-sponding change inRfield of two orders of magnitude (Table 6).When rd 5 $107 cm22 dislocation etch pits are a moreimportant source of solutes than all other areas of the grainsurface combined. Thus, dislocation density is likely to be ahighly important variable during early natural weathering ofperthitic alkali feldspars.

6.1.1 Explanation for contrasts in microtextural andmicrostructural control on chemical weathering ratesbetween the laboratory and field

The calculations in previous sections have shown thatrd 5$103 3 rab'to in the field whereasrd 5 1.63 rab andrd 5 1.93 rto in the laboratory. When these rates are normalised to theproportion of a grain’s surface that the different sites occupy,they show that dislocations will have very little influence onrates of chemical weathering in the laboratory unlessrd 5$1011 cm22, but may be the major source of solutes during theearly stages of natural chemical weathering ifrd 5 $107

cm22.A number of authors have suggested that dislocations should

play a more important role in chemical weathering in the fieldrather than the laboratory owing to distinct contrasts in thesaturation state of weathering solutions between the two envi-ronments (Lasaga and Blum, 1986; Velbel, 1986; Blum et al.,1990; Hochella and Banfield, 1995). In highly undersaturatedsolutions, which typify the laboratory experiments, many vari-eties of surface sites, such as impurities and vacancies inaddition to dislocation outcrops, actively participate in thedissolution reaction (i.e.,rd ' rab ' rto; Schott et al., 1989). Atconditions closer to saturation with respect to feldspar, whichoccur in soil waters and especially within intragranular micro-

environments, the driving force for dissolution will be lowerand so only the sites with the lowest activation energy fordissolution (such as highly strained feldspar surrounding dis-location outcrops) will contribute significantly to dissolutionrates (i.e.,rd ° rab ° rto). Saturation indices for natural watersextracted from soils formed on the Shap Granite and outputsolutions from the three experiments are presented in Table 5.These data clearly show that soil waters were closer to satura-tion with respect to both reactant phases and potential precip-itates than the experimental solutions.

6.2. Natural Mechanical Weathering

Although calculations show that dislocation density is animportant variable during early chemical weathering of mi-croperthitic alkali feldspars in the field (Table 6), microtexturesand dislocation density are likely to have a much more signif-icant impact on weathering rates during advanced stages ofchemical weathering when dislocation-rich microtextures (mi-croperthites and patch perthites) begin to disintegrate but dis-location-poor microtextures remain intact. This phase of disin-tegration will not be recorded in laboratory experimentsbecause of their relatively short duration and the fact thatrd

under laboratory conditions (i.e., far from saturation) is lessthan 1 order of magnitude greater thanrab andrto. The impor-tance of time and saturation state is exemplified by the follow-ing calculation. Microperthite surfaces of Shap Granite alkalifeldspars start to disintegrate when they reach stage e; at thisstage Dd-pit 5 5 3 1023 cm. The duration of continuouslaboratory dissolution that would be required for etch pits togrow to this depth can be estimated from the AFM data. IfDd-pit 5 2 3 1026 cm whenTlab 5 1.143 107 s, 2.853 1010

s (0.9 kyr) of continuous laboratory dissolution will be requiredbefore Dd-pit 5 5 3 1023 cm (i.e., 1.143 107 3 [5 31023/(2 3 1026)]).

Although some laboratory experiments have been conductedunder conditions whereby a significant proportion of the start-ing material has been dissolved (e.g., Blum et al., 1990, fordissolution of quartz with low and high dislocation densities inHF acid), the solutions would again have been far from satu-ration. Under such conditions,rd would have been relativeclose to the dissolution rates of other sites on the surfaces sothat dislocation etch pits would have deepened too slowlyrelative to the surrounding grain surface for significant frag-mentation to have taken place.

7. CONCLUSIONS

This study has reconciled the apparent contradiction betweenthe strong effect of intracrystal microtextures and microstruc-tures on dissolution seen in partly dissolved, naturally weath-ered alkali feldspars, and the lack of correlation with disloca-tion density found in previous laboratory dissolution studies. Itshows that extreme caution must be exercised when usingdissolution rate data derived from laboratory experiments topredict rates of chemical weathering of silicate minerals in thefield. Owing to the highly undersaturated conditions underwhich most experiments are performed, sites with a range ofactivation energies for dissolution dissolve at rates which differby less than 1 order of magnitude. Some hundreds of years oflaboratory attack would be required before microtextures had a


significant role in dissolution. In contrast, in the restricted poresin organic-rich soils studied, and in the internal zone withincrystals, fluids are closer to saturation with respect to reactantsand potential precipitates. As a consequence, sites with lowactivation energy for dissolution, such as dislocation outcrops,dissolve at a rate which is several orders of magnitude greaterthan the surrounding grain surface. Dislocation density is thusa highly important variable during early stages of chemicalweathering in the field. Mechanical weathering further exag-gerates the effect of dislocations on weathering rates via apositive feedback mechanism. As etch pits enlarge, the grainsurface becomes weak and eventually disintegrates, increasingthe surface area/volume ratio and exposing new sites, thusreinvigorating chemical weathering processes. Deep networksof etch pits may also provide niches for bacteria which mayaccentuate rates of chemical and mechanical weathering. Ul-trasonic cleaning of mineral grains prior to imaging will com-pletely destroy the fragile textures of naturally weathered sur-faces which are such a rich source of information onmechanisms and rates of mineral weathering.

Acknowledgements—We thank Chris Jeffree for provision of SEMservices and John Findlay for assistance with image acquisition. We arealso grateful to Tony Fraser, Pat Cooper, Malcom Coull, and LynnClark for looking after the dissolution experiments during MEH’sabsences and Jim Marr (Department of Chemistry, University of Ab-erdeen) for use of the gas adsorption equipment. We are especiallygrateful to Fernando Camara and Peter Buseck for their invaluable helpwith the AFM work. MRL and IP are funded by NERC (GR3/10290)and MEH by National Power–PowerGen joint environmental researchprogramme. This manuscript benefited considerably from careful re-views by three anonymous referees.

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