Surface area and porosity of primary silicate mineralsepsc511.wustl.edu/Brantley_p1767-1783_00.pdf · BRANTLEY AND MELLOTT: SURFACE AREA AND POROSITY OF SILICATES 1769 mesh (44–37

American Mineralogist, Volume 85, pages 1767–1783, 2000

0003-004X/00/1112–1767$05.00 1767

INTRODUCTIONLaboratory-ground and naturally weathered primary silicates

show generally similar trends in specific surface area (SSA) asa function of mineralogy. White (1995) and White et al. (1996)concluded that SSA of naturally weathered PSMs generallyincreases for a given grain size from quartz to potassium feld-spar (denoted K-spar) to plagioclase, and a similar trend wasobserved for SSA of laboratory-ground samples based upon acomparison of literature data by Brantley et al. (1999). Thisincreasing SSA parallels an increase in rate of dissolution acrossthe same compositional trend (Brantley et al. 1999), but littleis known regarding the effects of mineralogy on SSA in labo-ratory-ground or naturally weathered silicate grains.

Although trends in SSA of laboratory-ground and naturallyweathered silicates may be similar, many studies of weatheredsoil silicate grains (e.g., White and Peterson 1990; Anbeek1992a, 1992b, 1993; Anbeek et al. 1994) have shown that spe-cific surface area of silicates is larger for naturally weatheredas compared to laboratory-ground samples. Some of this in-

creased surface area was attributed to increased roughness ofthe external surface. However, some of the increased surfacearea may be due to internal surface related to porosity devel-oped during crystallization or during solution etching (e.g.,Montgomery and Brace 1975; Worden et al. 1990; Anbeek1992a, 1992b, 1993; Walker et al. 1995; Lee and Parsons 1995;White 1995).

The contribution of surface area from porosity may be mea-sured using standard techniques of nitrogen adsorption. Whereinvestigators have used nitrogen adsorption isotherms to inferpores of diameter 2–50 nm (mesopores), the evidence suggeststhat such mesoporosity is a significant contributor to specificsurface area (SSA) in many naturally weathered samples. Forexample, Titley et al. (1987), Mayer (1994), and Werth andReinhard (1997) concluded that pores contribute significantlyto surface area of several estuarine and marine sediment samplesand several soils. Porosity in weathered quartz and feldsparwas similarly documented by Ball et al. (1990), Wood et al.(1990), and Yau (1999) using gas adsorption, mercuryporosimetry, and diffusion analysis on glacial outwash sedi-ments. Although mesoporosity was not specifically measured,White and coworkers also inferred the presence of porosity in

Surface area and porosity of primary silicate minerals

SUSAN L. BRANTLEY AND NATHAN P. MELLOTT*

Department of Geosciences, Pennsylvania State University, University Park, Pennsylvania 16802, U.S.A.

ABSTRACTSurface area is important in quantifying mineral-water reaction rates. Specific surface area (SSA)

was measured to investigate controls on this parameter for several primary silicate minerals (PSM)used to estimate rates of weathering. The SSA measured by gas adsorption for a given particle sizeof relatively impurity-free, laboratory-ground samples generally increases in the order: quartz ≈ oli-vine ≈ albite < oligoclase ≈ bytownite < hornblende ≈ diopside. Reproducibility of BET SSA valuesrange from ±70% (SSA < 1000 cm2/g) to ± 5% (SSA > 4000 cm2/g) and values measured with N2were observed to be up to 50% larger than values measured with Kr. For laboratory-ground Ameliaalbite and San Carlos olivine, SSA can be calculated using log (SSA, cm2/g) = b + m log (d), whered = grain diameter (µm), b = 5.2 ± 0.2 and m = –1.0 ± 0.1. A similar equation was previouslypublished for laboratory-ground quartz. Some other samples showed SSA higher than predicted bythese equations. In some cases, high SSA is attributed to significant second phase particulate con-tent, but for other laboratory-ground samples, high SSA increased with observed hysteresis in theadsorption-desorption isotherms. Such hysteresis is consistent with the presence of pores with diam-eters in the range 2 to 50 nm (mesopores). In particular, porosity that contributes to BET-measuredSSA is inferred for examples of laboratory-ground diopside, hornblende, and all compositions ofplagioclase except albite, plus naturally weathered quartz, plagioclase, and potassium feldspar. Pre-vious workers documented similar porosity in laboratory-ground potassium feldspar.

Surface area measured by gas adsorption may not be appropriate for extrapolation of interface-limited rates of dissolution of many silicates if internal surface is present and if it does not dissolveequivalently to external surface. In addition, the large errors associated in measuring SSA of coarseand/or impurity-containing silicates suggest that surface area-normalized kinetics in both field andlaboratory systems will be difficult to estimate precisely. Quantification of the porosity in labora-tory-ground and naturally weathered samples may help to alleviate some of the discrepancy betweenlaboratory- and field-based estimates of weathering rate.

* E-mail: [email protected]

BRANTLEY AND MELLOTT: SURFACE AREA AND POROSITY OF SILICATES1768

weathered samples of plagioclase, K-spar, and hornblende basedupon microscopic observations and variations in surface areameasured by gas adsorption as a function of grain size for soilsfrom Puerto Rico (Schulz et al. 1999) and Merced, California,(White et al. 1996). In addition, porosity in alkali feldspars hasbeen documented by many workers (e.g., Worden et al. 1990;Walker et al. 1995; Lee and Parsons 1995). Pores have alsobeen documented in laboratory-ground silicates: for example,Hodson and coworkers (Hodson et al. 1997; Hodson 1998)documented porosity in potassium feldspar and Brantley et al.(1999) documented mesoporosity in hornblende.

A better understanding of the controls on mineral surfacearea and porosity in surficial environments is important forseveral reasons. For example, if internal surface area (e.g.,Gregg and Sing 1982; Hochella and Banfield 1995) is presentin primary silicate minerals, and if internal surface area is moreor less reactive than external surface area, then quantificationof the ratio of internal vs. external surface area could be impor-tant in the extrapolation of mineral-water kinetics from onesystem to another (see, for example, Lee and Parsons 1995;Hodson 1998). Whereas several authors have suggested thatweathered and laboratory-ground samples differ with respectto the relative importance of internal and external area (e.g.,Anbeek 1992a, 1992b, 1993; Anbeek et al. 1994), few havequantified the contribution from these two types of surface forminerals which are commonly used in comparisons of fieldand laboratory weathering rates (e.g., plagioclase, hornblende,olivine, diopside). Specifically, if the ratio of these two typesof surface area differ between laboratory-ground and naturallyweathered plagioclase, hornblende, olivine, and diopside, thenperhaps these differences contribute to the large (up to fiveorders of magnitude) discrepancies between laboratory- andfield-derived dissolution rates for these minerals (White andBrantley 1995).

In addition, if porosity is present in silicate grains then theobserved aging or sequestration of contaminants in soils andaquifers may be due to sequestration inside these pores (e.g.,Farrell and Reinhard 1994; Werth and Reinhard 1997; Luthy etal. 1997). In a series of papers, Mayer (1994, 1999) also pro-posed that sequestration of organic matter into small pores mayallow preservation of the organic matter on continental shelfsediments since hydrolytic enzymes should be excluded fromsuch small pores. Again, however, little is known about min-eral controls on the presence of porosity in the silicates mak-ing up the sediments.

This paper addresses the following questions concerningSSA of primary silicates: What is the range of SSA measuredusing BET for laboratory-ground primary silicate powders? Canwe infer the presence of porosity from adsorption data for labo-ratory-ground and weathered primary silicates? What do theseobservations imply for reaction kinetics in the laboratory andin the field? Our approach was to investigate relatively pristinemineral samples that have traditionally been used by geochem-ists to investigate mineral dissolution (samples largely derivedfrom pegmatite deposits) using standard BET techniques foranalysis of gas adsorption-desorption. We also used the sameBET technique on seven naturally weathered samples to inferthe presence or absence of porosity.

BACKGROUNDThe specific geometric surface area, SSAgeo, can be ex-

pressed as a function of grain diameter d:

SSAgeo = adD–3⁄ρ (1)

where ρ is the density of the solid and a is a geometric param-eter. For Euclidean solids (i.e., solids with no fractal proper-ties), D = 2.

Adsorption of gas on to a powder surface is extensivelyused to measure powder surface area (Gregg and Singh 1982;Lowell and Shields 1991). The ratio of the measured surfacearea using gas adsorption, SSAads, to the geometric surface area,SSAgeo, has typically been called the surface roughness, λ, bygeochemists (Helgeson et al. 1984):

λ = SSAads/SSAgeo (2)

The surface roughness differs from 1 because of surface to-pography and porosity. White (1995) and White et al. (1996)produced a general expression for the physical surface of amineral grain in terms of external surface roughness, λext, andinternal surface area, SSAint:

SSAads = λext a/(ρd) + SSAint (3)

This equation can be applied to a suite of samples of vary-ing grain size if surface roughness and internal surface area areboth assumed to be independent of grain size.

METHODS

Samples

The minerals were used previously to measure dissolutionkinetics (e.g., White and Brantley 1995). Primary silicate min-erals include examples of anorthite (Miyake Jima, Japan), an-orthite (Grass Valley, California, provided by R. Holdren,Pacific Northwest National Laboratory, Washington), albite(Amelia Courthouse, Virginia), diopside (Herschel, Ontario,Canada), oligoclase (Madawaska, Ontario, Canada), labradorite(Labrador, Canada), bytownite (Duluth, Minnesota), olivine(San Carlos, Arizona), olivine (Twin Sisters Range, Washing-ton), and hornblende (Gore Mountain, New York). Where spe-cific collections are not indicated above, samples were obtainedfrom Wards Scientific Inc. Glass of albite composition was alsoprepared by melting Amelia albite (Hamilton 1999). The GrassValley anorthite was investigated by Holdren and Speyer(1987); sample preparation is described in that paper.

Although we review some literature data for microcline,we make no attempt to summarize or investigate the potassiumfeldspars and we refer the reader to other publications (e.g.,Worden et al. 1990; Walker et al. 1995; Lee and Parsons 1995;Hodson et al. 1997; Hodson 1998).

Sample fragments were ground with an agate mortar andpestle and dry sieved until 1 to 5 g of each of the following sizefractions were collected: 20–35 mesh (840–500 µm), 35–60mesh (500–250 µm), 140–200 mesh (105–74 µm), 230–270mesh (62–53 µm), 270–325 mesh (53–44 µm), and 325–400

BRANTLEY AND MELLOTT: SURFACE AREA AND POROSITY OF SILICATES 1769

mesh (44–37 µm). For some samples, additional size fractions(e.g., mesh size 100–200 or 75–150 µm) were also collected.Data for a labradorite, summarized by Brantley et al. (1999),are here included for completeness. Each size fraction of thissample and the Miyake Jima anorthite were also run through amagnetic separator up to three times to attempt to remove im-purities. All powders were ultrasonically cleaned in deionizedwater several times for 6–9 minutes until supernatant fluid con-tained little if any fine material, and rinsed with Optima Ac-etone (Fisher Scientific) before surface analysis.

Bulk chemical analysis or electron microprobe analysis wascompleted for samples lacking published analysis (Table 1).Grain mounts were prepared, polished, and carbon-coated foranalysis with a Cameca SX-50 electron microprobe. In somecases, grain mounts were imaged on a Philips XL-20 scanningelectron microscope (SEM). For several minerals in Table 1,small amounts of disseminated second phase inclusions arepresent. In other cases, second phase particulates were foundin samples, despite careful cleaning (e.g., Grass Valley andMiyake Jima anorthite, Twin Sisters olivine).

Adsorption-desorption isotherms were also measured onseveral weathered samples from the Merced soil chro-nosequence (White 1995; White et al. 1996). Samples describedby White (1995) of the 500 to 1000 µm fraction of quartz, po-tassium feldspar, and plagioclase from two soils (Modestosampled at a depth of 413 cm, Turlock Lake sampled at a depthof 375 cm) were analyzed. Samples of quartz were also ana-lyzed from the China Hat soil from the same chronosequence.These samples were the identical samples, cleaned using a ci-trate-dithionite extraction, analyzed by White et al. (1996). Theages of the surfaces of these soils were 40 ka (Modesto), 600ka (Turlock Lake), and 3000 ka (China Hat).

Surface area measurement

SSA was determined with a Micromeritics ASAP 2010 sur-face area analyzer. Unless noted otherwise, ~1 to 5 g of sample

were degassed at 300 °C for 4–12 h (until the pressure of thesystem reached 30–40 mm Hg) before Kr measurement. In mostcases, Kr and He were used as the analysis and backfill gases,respectively. Kr is recommended for determination of SSAs aslow as 10 cm2/g, and is thus preferred over argon or nitrogenfor PSM, which characteristically have low specific surfaceareas. Where noted, N2 was used as the adsorbate. Multi-pointsurface area (8 points recorded) was calculated for relative pres-sures of 0.010 to 0.25, and adsorption isotherms were drawnusing the Brunauer, Emmet, and Teller (BET) method (Brunaueret al. 1938; Lowell and Shields 1991). A commercially avail-able alumina-silica nonporous reference powder (P/N no. 004/16816/00, 46F-BA-106-6, available from Micromeritics withquoted surface area of 4600 cm2/g) was run periodically as astandard.

The BET equation (Gregg and Sing 1982) models the mea-sured number of moles, n, adsorbed on 1 g of adsorbent:

p p

n p p n c

c

n cp p

m m

//

/00

011 1

−( ) = +− ( ) (4)

where nm is the calculated number of moles adsorbed as a mono-layer on 1 g of adsorbent, p is the gas pressure, p0 is the satura-tion vapor pressure of the gas, and c equals:

c = exp(∆H/RT). (5)

Here ∆H is related to the net heat of adsorption of the gas onthe surface, R is the gas constant, and T is the absolute tem-perature. In practice, (p/p0)/n(1–p/p0) is plotted against p/p0,yielding values for (c–1)/nmc (the slope, s) and 1/nm c (the in-tercept, i) for values between p/p0 = 0.05 and 0.3 (Webb andOrr 1997). The values of nm of SSA can then be calculated:

nm = (s + i)–1 (6)

TABLE 1. Composition of samples

Mineral and location Weight % oxides Estimated* AnalysisSiO2 Na2O CaO FeO Fe2O3 Al2O3 MgO K2O H2O (LOI) % method

Albite (Amelia)† 68.12 11.50 0.39 N/A N/A 19.59 N/A 0.20 N/A rare EMOligoclase (Madawaska) 62.90 9.01 4.02 N/A 0.07 22.40 0.05 0.70 N/A N/ABytownite (Duluth) 48.81 2.57 15.49 0.38 N/A 32.82 N/A N/A N/A rare EMAnorthite (Miyake Jima)‡ 43.45 0.41 18.91 0.44 N/A 37.45 N/A 0.02 N/A 12 EMAnorthite (Grass Valley)§ 44.78 0.89 18.52 0.59 N/A 34.53 0.02 0.02 N/A >5 EMForsteritic olivine (San Carlos)|| 40.72 N/A N/A 9.21 N/A N/A 49.24 N/A N/A rare XRFOlivine (Twin Sisters) 40.27 N/A N/A 8.26 N/A N/A 47.46 N/A N/A 25 EMEnstatite# 57.21 N/A 0.37 5.58 N/A N/A 36.65 N/A N/A – EMDiopside (Herschel) 54.41 0.25 24.85 2.18 N/A 0.43 17.14 N/A N/A rare EMHydrated Phase** 55.72 0.59 13.00 3.25 N/A 2.15 22.17 N/A N/A – EMHornblende (Gore Mt.) 46.00 3.16 8.34 6.9 7.66 17.60 10.20 0.40 1.65 5 XRFHydrated Phase** 35.08 0.04 0.31 18.71 N/A 10.72 21.53 0.01 N/A – EMNotes: N/A = not analyzed; rare = only a few or no second phase inclusions were observed; LOI = loss on ignition.* % of impurity, estimated visually from back-scattered electron photomicrograph or based upon literature descriptions.† Analysis from Stillings and Brantley (1995).‡ Sample contained dark red to gray secondary particulates that could not be probed (probably a Mg-Ca-Fe oxide). Black glassy inclusions were alsopresent along grain boundaries.§ Analysis from Casey et al. (1991). Sample was provided by R. Holdren, PNNL. Sample contained significant iron-rich mineral (Holdren, personalcommunication).|| Analysis from Wogelius and Walther (1992).# Second phase particulates observed with the previous phase.** Impurity inclusions observed with the previous phase.


SSA = Lnmam (7)

where am is the area occupied by the adsorbate molecule (aconstant for a given gas, calculated theoretically from the liq-uid density) in the monolayer and L is Avogadro’s number.

Porosity measurement

Adsorption-desorption hysteresis was measured for severalminerals and for one blank (no sample) run with the Micro-meritics ASAP 2010 surface analyzer. Adsorption-desorptionisotherms and BET SSAs were determined using 1 to 5 g of100–200 mesh sample, with N2 and He as the analysis and back-fill gases. Kr is not used for hysteresis measurements.

We use the IUPAC (1972) convention for nomenclature ofporosity even though other conventions have been used in themineralogical literature (e.g., Montgomery and Brace 1975;Worden et al. 1990; Walker et al. 1995). By IUPAC convention,pores are characterized by their diameters, where micro- (50 nm) are identified. Theshape of the isotherm (see Lowell and Shields 1991; Gregg andSing 1982; Hodson 1998) for PSMs in our experience can beclassified as Type I (interpreted to contain significantmicroporosity), Type II (typically interpreted as nonporous), orType IV (interpreted to contain significant mesoporosity).

In brief, microporous samples (Type I) show enhanced ad-sorption below p/p0 = 0.1–0.2 and an adsorption plateau abovethis value (Lowell and Shields 1991). In addition, a value of cwhich is high (>100) or negative indicates micropores (Webband Orr 1997). Conceptually, a Type I isotherm documentsadsorption of gas into micropores at very low gas pressures.Mesoporous samples generally show hysteresis in the adsorp-tion and desorption branches above p/p0 = 0.4, attributed tomultilayer formation and, especially, capillary condensation inmesopores (Type IV). Nonporous samples show neither typeof enhanced adsorption, and instead resemble curves for non-porous standards (Type II, no hysteresis, no enhanced adsorp-tion). Combinations of pore types can make interpretation ofisotherm shape difficult: for example, Gregg and Sing (1982)point out that a Type II isotherm can still have microporosity.For a Type II isotherm measured on a microporous sample,however, adsorption will be exceptionally high in the low pres-sure region and the value of c will be anomalous (several hun-dreds or negative). For such a case, the SSA calculated fromBET analysis will be erroneously high (Eq. 4 cannot be used,Webb and Orr 1997).

The presence of porosity is also documented using t-plotscomparing the volume of nitrogen adsorbed on a sample tothat adsorbed on a nonporous standard. The t-value is the sta-tistical thickness of the adsorbed gas layer as calculated at agiven value of p/p0 for a nonporous standard adsorbent (Webband Orr 1997). The volume of microporosity can be determinedfrom the intercept of the first straight line segment (Brunauer1969; Webb and Orr 1997; Hodson 1998). If the sample iso-therm is identical with that of the standard, the t-plot will belinear and pass through the origin. Gregg and Sing (1982) pointout that a t-curve should be based upon a standard sample thatis chemically similar to the study sample. For example, a stan-dard sample could be chosen such that the BET c value of the

standard and sample are similar (Brunauer 1969).Because the t-plot relies upon a model parameter, nm, Sing

(1969) advocates using a different method, the α-plot, as pre-sented here to compare standard isotherms. In essence, t- andα-plots are identical, representing comparisons between thesample and a nonporous standard. Sing (1969) defines α = n/n0.4, where n is the number of moles of gas adsorbed at anyrelative pressure, and n0.4 is the number of moles adsorbed at p/p0 = 0.4. In effect, α equals the moles gas adsorbed normalizedby the moles adsorbed at a relative pressure where mesoporosityshould start causing enhanced adsorption. These α curves showvolume of gas adsorbed on a sample for a given p/p0 plotted vs.the α value for a standard for the same p/p0. Both α and t-plotshave the same shape, but axes on each plot are scaled differ-ently (Webb and Orr 1997).

If the standard and the sample are both nonporous, then theα (or t) plot should be linear and should pass through the ori-gin. Microporous samples will show an α (or t) plot with alarge slope for α 100 µm, Table 2).

The average of the values of the surface area of the Si- andAl-containing standard measured using N2 was 40% higher thanthe average measured using Kr. Similar comparisons for PSMsshowed a range wherein surface areas measured with N2 were20–50% larger than those measured by Kr. The value measuredfor the standard using Kr was within 4% of the accepted value.

Porosity in primary silicate grains

The volumes of gas both adsorbed and desorbed per gramsof powder were measured for albite, bytownite, anorthite(Miyake-Jima and Grass Valley), olivine (Twin Sisters and SanCarlos), diopside, and hornblende (e.g., Figs. 3 and 4), as well


TABLE 2. BET surface area results

Name Gas Grain size Mean diameter* Mass for BET SSA RSD† Time||(µm) (µm) g (cm2/g) (%)

Albite (Amelia) Kr 840–500 648 1.97 225 ~4.5Kr 500–250 354 1.45 497 ~4.5Kr 105–75 88 1.67 1000 ~4.5Kr 62–53 57 1.67 1670 ~4.5Kr 53–44 48 1.18 2520 ~4.5Kr 44–37 40 1.18 5450 ~4.5Kr 44–37 40 1.57 5570 ~4.5Kr 44–37 40 2%N2 150–75 106 3.16 1190 ~4.5

Albite Glass Kr 840–500 648 1.55 86 ~4.2Kr 500–250 354 1.75 197 ~4.2Kr 105–75 88 1.61 838 ~4.2Kr 62–53 57 1.41 1800 ~4.2Kr 53–44 48 1.22 2680 ~4.2Kr 44–37 40 1.06 6000 ~4.2

Oligoclase (Madawaska) Kr 840–500 648 1.48 927 ~0.2Kr 500–250 354 1.35 1080 ~0.2Kr 105–75 88 1.28 1500 ~0.2Kr 62–53 57 1.16 2760 ~0.5Kr 53–44 48 0.94 2680 ~0.5Kr 44–37 40 0.98 7260 ~0.5

Labradorite (Labrador) Kr 505–297 387 5.00 1460 ~2.0Kr 297–250 272 5.00 1230 ~2.0Kr 250–210 229 5.00 1690 ~2.0Kr 210–149 177 5.00 1620 ~2.0Kr 149–105 125 5.00 1720 ~2.0Kr 105–74 88 5.00 2190 ~2.0Kr 74–53 63 5.00 3150 ~2.0Kr 53–44 48 5.00 4310 ~2.0

Bytownite (Duluth) Kr 840–500 648 1.18 1020 ~0.2Kr 500–250 354 1.88 1180 ~0.2Kr 150–75 106 1.73 1530 ~0.2Kr 105–75 88 1.47 2380 ~0.2Kr 62–53 57 1.11 2790 ~0.2Kr 53–44 48 1.02 5440 0.03Kr 44–37 40 1.14 1280 ~0.5N2 150–75 106 4.85 1850 ~0.5

Anorthite (Miyake-Jima) Kr 840–500 648 1.52 414 1.5Kr 840–500 648 1.47 750 1.7Kr 840–500 648 41%Kr 500–250 354 1.61 1200 1.6Kr 500–250 354 1.92 420 2.0Kr 500–250 354 68%Kr 105–74 88 1.39 2830 1.6Kr 62–53 57 1.48 7290 1.6Kr 53–44 48 1.23 12000 1.6Kr 44–37 40 0.98 17900 1.6N2 150–75 106 2.50 1750 2.0

Anorthite (Grass Valley)‡ N2 75–37 53 1.77 6550 >12 yAnorthite (Grass Valley)§ Ar 75–37 53 ? 10,200 ?

75–37 53 31%

Olivine (San Carlos) Kr 840–500 648 1.25 223 ~0.2Kr 500–250 354 1.70 329 ~0.2Kr 150–75 106 2.35 1040 ~0.2Kr 105–75 88 1.20 3550 ~0.2Kr 62–53 57 1.12 3050 ~0.2Kr 53–44 48 1.24 4620 ~0.2Kr 44–37 40 1.03 520 ~0.5N2 150–75 106 3.85 1500 ~0.5

Olivine (Twin Sisters) Kr 840–500 648 1.67 3190 ~4.8Kr 500–250 354 1.85 2340 ~4.5Kr 150–125 136 1.69 2290 ~4.8Kr 105–74 88 1.99 1850 ~4.5

* Diameter calculated as the antilog of the mean of the log (maximum) and log (minimum) diameters. This calculation assumes that the particledistribution is log-normal.† Where multiple measurements were made, the relative standard deviation is noted (see text).‡ Original sample as reported by Holdren and Speyer (1987) measured in our laboratory in 1998.§ Same sample as above, measured and reported by Holdren and Speyer (1987).|| Approximate time since grinding (months unless otherwise noted).


as for the soil grains. Hysteresis (where volume of gas adsorbedat a given relative pressure is not equal during adsorption anddesorption) was observed for every laboratory-ground sampleexcept olivine (San Carlos and Twin Sisters) and Amelia albite(Figs. 3 and 4). Observed curves with hysteresis can be clas-sified as Type B based upon their geometries (Gregg andSing 1982; Lowell and Shields 1991). Such curves have beenclassically interpreted to indicate the condensation of gas inslit-like mesoporosity (Gregg and Sing 1982). For all labora-tory-ground minerals, adsorption for gas at p/p0 < 0.1 or 0.2was small and the BET c value (Eqs. 4 and 5) was


FIGURE 1. SSA plotted vs. grain size (micrometers): (a) microcline, (b) albite, (c) oligoclase, (d) labradorite, (e) bytownite, and (f) anorthite.Stars and crosses are used for BET (Ar), solid symbols for BET (N2), and open symbols for BET (Kr).


FIGURE 2. SSA plotted vs. mean grain size for (a) olivine, (b)diopside, (c) hornblende. Solid symbols are used for BET (N2), andopen symbols or open symbols with a cross/slash embedded for BET(Kr).

TABLE 3. Porosity results

Pore volume* % Porosity % Pore % Pore c Time sinceMineral Phase × 10–4 (cm3/g) as area as area as grinding

mesoporosity† mesoporosity† microporosity‡ (months)Albite (Amelia) 2.8 37% 93% bd 34 4.5Bytownite (Duluth) 6.9 37% 80% 11% 27 –0.5Anorthite (Miyake-Jima) 9.3 56% 80% 17% 49 2Anorthite (Grass Valley) 24.9 40% 76% 17% 43 >12 yOlivine (San Carlos) 2.4 57% 81% 17% 11 0.5Diopside (Hershel) 9.5 41% 80% 12% 28 0.5Hornblende (Gore Mtn.) 8.3 51% 88% 8% 37 17Hornblende (Gore Mtn.) 22.1 26% 86% 8% 59 3.5Olivine (Twin Sisters) 5.7 56% 89% 8% 43 0.25Potassium Feldspar (Modesto) 14.8 22% 84% bd 65 fieldPotassium Feldspar (Turlock Lake) 41.6 36% 92% bd 90 fieldPlagioclase (Modesto) 19.2 47% 70% 25% 27 fieldPlagioclase (Turlock Lake) 60.7 54% 90% 6% 143 fieldQuartz (Modesto) 6.0 40% 87% 13% 14 fieldQuartz (Turlock Lake) 6.6 42% 77% 13% 27 fieldQuartz (China Hat) 4.2 51% 76% 20% 17 field* Cumulative pore volume as calculated using BJH model for adsorption isotherm (Micromeritics, 1995).Note that


DISCUSSION

Reproducibility of SSA measurements

Reproducibility of measurement of SSA for coarse grainsizes of PSMs was disappointing, considering that the SSA ofthe alumina-silica standard can be reproduced within


measured SSA decreased with increasing time since grinding(Table 2). For these samples, the calculated pore volume alsodecreased (Table 3). In contrast, Miyake Jima anorthite showedboth an increase and a decrease in SSA with time since grind-ing for individual samples. Especially for samples with impu-rities, lack of reproducibility may be related to entrainment offine second phase particles. For example, as indicated in Table2, SSA of Miyake Jima anorthite was measured for a sample(75–150 µm) using N2 as 1750 cm2/g; another sample, groundseparately to the same grain size and cleaned similarly to thefirst, revealed a SSA of 3760 cm2/g. This second sample, al-though cleaned similarly to the first sample (and not reportedin the table), was visibly more contaminated with second phase

particles, consistent with the suggestion that high SSA of theMiyake Jima anorthite is at least partially explained by the pres-ence of a second phase, and furthermore suggesting that samplepreparation can drastically change measured SSA.

The BET method measures surface area with a “ruler” equiva-lent to the area of the adsorbate molecule: for example, 16.2 vs.15.2 A2 (N2 vs. Kr at 78 K, Gregg and Sing 1982). For this reason,and because of nonidealities in adsorption, different molecules yielddifferent values for the SSABET, with Kr < N2 for most materials(Gregg and Sing 1982), and with Ar < N2 (Brunauer 1969). In agree-ment with this, we observed that BET (N2) surface areas were largerthan Kr areas by up to 50% (Table 2). For the low SSAs observedfor these samples, Kr is more reproducible than N2 (Table 2).

FIGURE 4. Volume of gas sorbed (at STP) during N2 adsorption and desorption plotted vs. relative pressure for (a) olivine (b) diopside (c)hornblende. Characteristic error bars are shown for a blank run on the same instrument at various values of relative pressure, normalized for themass of sample used. This error was always positive as shown. α plot showing volume adsorbed for a sample vs. the α value for the samerelative pressure as measured for hydroxylated silica (see text) for (d) olivine, diopside, and hornblende. α = 1 implies a relative pressure of 0.4.All grains were 75–150 micrometers in diameter.


High SSA values

The SSA generally increases in the order albite ≈ olivine <oligoclase ≈ bytownite < hornblende ≈ diopside (Fig. 6A). Ifall mineral samples had been ground identically for identicalperiods of time (probably not strictly valid), then the SSA of agiven grain size should vary inversely with fracture energy ofthe sample. For a given expenditure of energy during grinding,the surface area created should be inversely proportional to thefracture energy. The data available contradict this hypothesis:if anything, SSA increases rather than decreases with increas-ing fracture energy (Atkinson and Meredith 1987).

However, as shown in Figure 6, SSA of both laboratory-

ground and naturally weathered samples generally increaseswith increasing hysteresis in adsorption-desorption. For sev-eral of the samples, hysteresis in adsorption-desorption quali-fies as Type B hysteresis which is commonly interpreted asevidence for slit-shaped meso- or macropores (Lowell andShields 1991). Assuming the presence of pores for samples withsignificant hysteresis, we can use the BJH method to calculatemodel pore diameters (Table 3). The BJH method calculatesthe presence of meso- and microporosity, and for the samplesinvestigated here, mesoporosity contributes dominantly to thesurface area.

The presence of meso- and/or macropores in the labora-tory-ground plagioclase (excluding albite), diopside, and horn-

FIGURE 5. Volume gas sorbed (at STP) during N2 adsorption anddesorption plotted vs. relative pressure for plagioclase from the (a)Modesto and (b) Turlock Lake soil (see text). Soil grains were collectedand prepared by White et al. (1996).

FIGURE 6. (a) Log (SSA) vs. log (diameter) plotted for selectedminerals as measured in our laboratory. Estimated regression line forquartz derived from Parks (1990). (b) Volume of gas sorbed duringdesorption, Vdes, minus volume of gas sorbed during adsorption,Vads at p/p0 =0.6. Error bars estimated for a blank run assuming 3 g of sample.


blende samples is inferred to at least partially explain the rela-tively high measured SSA in those minerals.

One laboratory-ground sample showed high SSA but insig-nificant hysteresis: Twin Sisters olivine (Fig. 6). It is possiblethat the Twin Sisters olivine contains mesoporosity, since somemesoporous samples show no hysteresis (Lowell and Shields1991). However, this olivine contained a significant volume ofsecond phase particulates (Table 1). It may therefore be sim-pler to assume that the second phase in the Twin Sisters sampleis either finer or more platy (e.g., with higher SSA) than theolivine, and that the impurity explains the high SSA observed(Table 2). In fact, every sample with a high content of secondphase (Twin Sisters olivine, Grass Valley and Miyake Jimaanorthite) also showed a high SSA.

Two minerals from this study with significant impurity con-tent also showed hysteresis in sorption behavior consistent withthe presence of mesopores (both anorthite samples). Pores withdiameters on the order of nanometers can form at triple junc-tions between some mineral grains in rocks if reactive fluidsare present long enough for equilibration (e.g., Lee et al. 1991).For the impurity-containing laboratory-ground minerals thatshowed hysteresis in this study, SSA may therefore documentporosity at triple junctions between the impurity and primaryminerals. Such triple junctions are not, however, limited tomultimineralic samples: interconnected triple junctions couldalso contribute measurable porosity in the impurity-free speci-mens that show hysteresis.

Standard isotherms

One way to analyze adsorption isotherms to determine thepresence of porosity is to plot Vads for one sample against α fora nonporous reference sample for the same value of p/p0. Suchan α plot (Gregg and Sing 1982; see also description in Meth-ods section) should be linear if both samples are nonporous,but should show a deviation for porous samples compared tononporous standards. The data for the standard phase used hereto make the α plots, based on work by Sing and coworkers (seeoriginal references in Gregg and Sing 1982, Table 2.14), iscollated from nitrogen isotherms for phases of SiO2 includingquartz with surface areas ranging from 1–200 m2/g.

For α plots for all laboratory-ground silicates investigated(Figs. 3 and 4), a regression line was calculated through thefirst three data points. In every case, the intercept of this lineequaled zero within error or was negative, consistent with ab-sence of microporosity. From the α plots, the Vads at α = 1.0(relative pressure = 0.4) increased from albite ≈ olivine → di-opside → hornblende → anorthite, consistent with increasingmesoporosity in these samples.

Regression of the first linear region of any α plot will yielda positive intercept if micropores are present or zero if nomicropores are present (Webb and Orr 1997). The followingobservations therefore imply that the contribution of microporesto the total surface in our laboratory-ground samples is small(


porosity is inferred in every primary silicate sample investi-gated except olivine and albite: in a pyroxene (diopside), in anamphibole (hornblende), and in three plagioclase feldspars (oli-goclase, bytownite, anorthite). Zhang et al. (1993) also usedhysteresis in gas sorption behavior to argue that slit-like poreswere present in hornblende. His observations of porosity weresimilar to those reported here for hornblende.

Mesoporosity also contributes SSA to natural samples ofsoils and sediments (e.g., Titley et al. 1987; Mayer 1994, 1999).Consistent with our observations, Mayer (1994) argued thatthe contribution of micropores (pores with diameters


cating that the model does not fit the data. In particular, SSA offinest fractions are high compared to the prediction.

The nonlinearity of inverse plots might be explained if Aintwere a function of grain size. For example, if pore spacing av-eraged l, and if grains were ground until diameter d < l, thenAint would be a function of grain size for d< l because such finegrains might contain no pores. Such a model would predict,however, that SSA of fine grains should be unusually small,contradicting Figure 8. However, if internal surface area weregrinding-induced, and if finer grains contain more grinding-induced cracks than coarser grains as suggested for some al-kali feldspars by Hodson (1998), then inverse plots such asthose shown might be produced.

Another explanation for high SSA for fine fractions is thatλext increases with decreasing grain size. Hodson (1998) re-ported changing λext as a function of grain size for four samples

of alkali feldspar; however, the question of roughness as a func-tion of grain size has not been investigated for the mineralsreported here. To document such roughness, one needs a probecapable of measuring roughness at the same scale as the BETadsorbate. SEM work will only, under optimal conditions, re-veal features down to tens of nanometers and thus cannot yieldinsight into roughness measured at the scale of the BET mea-surement (e.g., Hodson 1998). Perhaps the only probe that canmeasure such atomic roughness is the atomic force microscope(AFM); however, the AFM is only useful in measuring rough-ness on surfaces that are relatively flat.

Extrapolation of kinetics

Variations in SSA for grain sizes above 75 µm measured indifferent laboratories and for different compositions range from


dorite),


weathered samples (see for example, Lee et al. 1998), then theBET surface area may be an inappropriate parameter to use forextrapolating interface-limited kinetics from laboratory to field.

If transport in and out of a pore is slow compared to inter-face-limited reaction within the pore, then solution chemistryin the pore will differ from bulk chemistry. How small must apore be to create slow transport in and out of the pore? Derjaguinand Churaev (1986) argued on the basis of a capillary modelthat solute transport from pores smaller than 1000 nm in diam-eter must be controlled by diffusion. If this model is applicablehere, then all pores identified in this study by gas adsorptionwould be accessible only by diffusional transport. Solutionchemistry and reaction kinetics inside the pore probably differfrom chemistry and kinetics outside the pore. In such a system,the BET surface area includes the pore area and is not appro-priate for extrapolation of reaction kinetics based upon mea-surements of dissolution of external surface area.

Although other authors have discussed this problem, datareported in this work document that for several silicates forwhich field and laboratory dissolution rates have been com-pared, porosity may contribute significantly to BET SSA. Ifmineral-water reaction within micro- and mesoporosity is trans-port-limited and if such reaction does not significantly con-tribute to the flux of dissolved constituents in a dissolvingsystem, then laboratory and field dissolution rates should becompared only after normalization by SSAexternal rather thanSSABET. For some minerals, such as the laboratory-groundquartz, albite, and San Carlos olivine samples reported here,external surface area may equal BET SSA. However, for othersilicates such as the diopside, hornblende, and other plagio-clase compositions analyzed here, the BET SSA differs fromthe external surface area. For such minerals, external SSA ratherthan BET SSA might be more useful to compare rates, and itmay be possible to estimate the external SSA by using the SSAof Amelia albite, quartz, or San Carlos olivine.

Normalization by external SSA will increase the apparentrates of dissolution compared to BET-normalized rates if SSABET> SSAexternal. To quantify these factors, we can assume that ex-ternal surface area of the primary silicate equals the SSA ofalbite, quartz, or San Carlos olivine: with this assumption, theratio of SSABET/SSAexternal for laboratory-ground diopside, horn-blende, and plagioclase at a given grain size lies between ~2–8(Table 2). The so-called field-lab discrepancy in which fieldrates are slower than laboratory rates will be alleviated only ifthe ratio SSABET/SSAexternal is larger for the field than for thelaboratory, and if external grain surfaces dissolve more rapidlythan internal pore surfaces.

As an upper limit estimate of this ratio for naturally weath-ered samples from the Merced soils, we assume that the exter-nal surface area of the soil samples equals SSAalbite as measuredfor laboratory-ground samples of the same grain size. Usingthis assumption, the SSABET/SSAexternal for the Turlock Lakesamples varied from 10 (quartz) to 49 (potassium feldspar) to109 (plagioclase). These values are much larger than the ratios(SSABET/SSAexternal) estimated from data in Table 2 for labora-tory-ground plagioclase samples (≤5). If dissolution rates wereestimated for Merced plagioclase using BET surface area, andif the internal surface area was insignificantly reactive, then

the field rate would be underestimated by about a factor of 20with respect to the laboratory rate because field samples haveabout a factor of 20 more internal surface area than laboratory-ground samples. Furthermore, if some of the porosity in PSMsis grinding-induced, or if the internal surface area of labora-tory-ground samples is more reactive than naturally weatheredsurfaces, such a discrepancy could further contribute to theobserved laboratory-field problem (e.g., Anbeek 1992a, 1992b,1993; Anbeek et al. 1994; Lee et al. 1998; Hodson 1998).

CONCLUSIONSIf internal and external surface is not equally reactive in

primary silicates, then porosity in PSM samples as documentedhere implies that BET surface area measurements may not beappropriate for extrapolation of geochemical kinetics. Reac-tive surface area may be dominated by external surface areabecause reaction in pores may be rate-limited by transport ratherthan interfacial reaction.

Research should focus on attempting to estimate the reac-tive surface area and porosity in field systems, and learninghow to accurately estimate these parameters in the laboratoryfor use in extrapolation to the field.

ACKNOWLEDGMENTSDon Voigt, Drew Stolar, Michelle Smith, Peter Heaney, Carlo Pantano, Rich

Holdren, Art White, Jorie Schulz, and Mark Hodson are acknowledged for ex-perimental work, access to instrumentation, samples, and/or advice. Fundingfrom the Dept of Energy Office of Basic Energy Sciences grant DE-FG02-95ER14547.A000 is also acknowledged. Ian Parsons and Mark Hodson pro-vided extremely valuable reviews.

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