American Mineralogist, Volume 76, pages211-217, 1991 Dissolution rates of plagioclase at pH : 2 and 3 Wrr-r-rau H. C.nsnv,flnNnv R. Wnsrnrcn Geochemistry Research,Sandia National Laboratories,Albuquerque, New Mexico 87185, U.S.A. GBoncn R. Holonnx NSI Technology ServicesCorporation, E.P.A. Environmental ResearchLaboratory, 200 Southwest35th Street, Corvallis, Oregon 97333, U.S.A. Ansrru,cr The dissolution rates of plagioclasein acids are very sensitive to the mineral composi- tion. At pH : 2.0 and25 oC, rates range from =l x l0 15feldspar-mol/cm2/s for albite to greater than I x l0 r, feldspar-mol/cm2lsfor anorthite. At pH : 3.0, the respective ratesrange from =3 x lQ-to feldspar-mol/cm2/sto=7 x l0-t4 feldspar-mol/cm2ls.The relationship betweenmineral composition and dissolution rate, however, is not simple. In order to better define such a relation at pH : 2, rateswere measuredon eight plagioclase minerals and compared to existing data. In general, the dissolution rates of a given plagioclasemineral estimated by different researchersgenerally agree to within a factor of two to five. The uncertainty is much larger for Ca-rich minerals such as bytownite and anorthite, and reasonsfor the disag"reement are unclear. Even with this uncertainty, however, it is apparent that dissolution rates vary nonlinearly with composition and that the rates for the Ca-rich minerals vary more with composition than Na-rich minerals. INrnonucrroN last, 1984, 1985;Holdren and Speyer,1985; Casey et al., 1988,r989). It has long beenobserved that mineral weatheringrates The disparity in reactivity reflects differencesin the vary considerably with the mineral structure and com- bonding characterof Al and Si to O atoms at the mineral position. Goldich (1938), for example, noted that silicate surface. Furthermore, becausehydrolysis reactions are materials that dissolve slowly tend to contain a highly catalyzedby H and hydroxyl ions (Wendt, 1973), The polymerized (e.g., tectosilicate) structure (seealso Hurd disparity in reactivity reflectsdifferences in the acid-base et al., 1979; Lin and Clemency, 198l). These materials properties of the exposedaluminate and silicate sites.Al also tend to be selectively leachedby aqueoussolutions, is rapidly leachedfrom the surfaceoffeldspar in acid but as there are sufficient unreactive bonds near the mineral is not detectableat near-neutraland basic pH conditions surface to maintain integrity once the reactive constitu- (Caseyet al., 1988, 1989). The leaching reaction creates ents are removed (Murata, 1943). Under certain condi- an Al-, Na-, and Ca-depletedlayer on the surfaceof some tions, thesematerials form a thick leachedlayer adjacent plagioclaseminerals in the early stagesof a dissolution to the aqueoussolution. Conversely,silicates with a poor- experiment. ly connectedfabric, such as olivine, tend to dissolve rap- This selectivehydrolysis not only affectsthe chemistry idly and uniformly (Schott and Berner, 1985; Petit et al., of the mineral surfacebut should also be manifested in 1989) as long as the adjacent solution is undersaturated the bulk dissolution ratesofminerals, suchas plagioclase, with respectto secondaryphases. which have a wide range in Si and Al concentrations. Both the leaching behavior and bulk dissolution rate Plagioclase ranges in composition from albite (NaAl- of minerals are controlled by the relative concentration SirOr) to anorthite (CaAl$irO8). This variation corre- of reactive and unreactive sites at the mineral surface. sponds to a coupled, isostructural substitution of alumi- Aluminate (AlO;5) sites on an aluminosilicate mineral nate (AlO;') groups andCa2'for silicate (SiO;a) groups surface tend to be more rapidly hydrolyzed in a strong and Na". acid than the silicate (SiO;') sites (Murata, 1943). Acid The actual variation in plagioclasedissolution rate at treatment, for example,is usedto dealuminate zeolitesin a constant pH is poorly known, as much previous work order to make hydrocarbon-crackingcatalysts (e.g., Bar- was limited to a small range of mineral compositions. rer, 1978; Man et al., 1990). Similar dealumination of Even in more comprehensive studies, however, impor- clay has been proposed as a nonbauxitic source of AI tant variables such as the mineral surface area and purity (Aglietta et al., 1988).Acid dealumination reactionsalso were not determined, or the solution composition was affectthesurfaceofdissolvingfeldspars(ChouandWol- allowed to vary widely during the experiment. These 0003{04xl91/0102-021l$02.00 2rr 2t2 CASEY ET AL.: DISSOLUTION RATES OF PLAGIOCLASE TABLE1. Plagioclasecompositions by Holdren and Speyer(1987). Once prepared,the pow- ders were treated identically. All of the feldsparsexcept the Grass Valley sio, 66.83 62.89 63.74 54.71 52.34 49.45 44.97 anorthite Tio, 0.00 0.00 0.01 0.05 0.07 0.03 0.01 were acquired as large crystals. These crystals were ground Alros 19.26 21.95 23.02 27.93 29.17 32.13 34.53 and wet-sievedto isolatethe 25-7 5 rrm sizefraction. This Fe2O3 0.04 0.05 0.09 0.28 0.31 0.40 0.31 powder FeO 0.04 0.06 0.09 0.25 0.29 0.36 o.28 was repeatedlywashed in distilled HrO and spec- Mgo 0.01 0.01 0.04 0.23 0.14 0.14 0.02 trophotometric-grade acetonein order to remove ultra- MnO 0.01 0.01 0.01 0.02 0.02 0.00 0.01 fine particles that cling to the larger grains. Washing re- CaO 0.16 4.15 4.47 9.45 12.31 15.49 18.52 Nio 0.12 0.05 0.08 0.10 0.00 0.24 0.10 moves most, but not all, ultrafine particles,as determined Naro 11.51 9.05 7.93 5.23 4.30 2.73 0.89 by scanningelectron microscopy. t&o 0.11 0.35 1.43 0.93 0.30 0.12 0.o2 The anorthite was separatedfrom an anorthosite from Note.' Separate grains within each powder were analyzed, averaged, Grass Valley, California. This material was crushed in a and normalized to 100%. Molar Fe concentrations are divided equally steelmortar and pestle,followed by sieving to isolate the between terric and ferrous oxidation states. The average stoichiometry and the total number ol analyses are includedas footnotes. In all cases, 25-75 prn size fraction. This powder was washedrepeat- the analytical precision analysis is less than the compositionalvariation edly in distilled HrO and acetoneto remove ultrafine par- within a singlemineral. 1 : Keystonealbite: (NaoeTGoloao o2Alossi3 @Os oo), 337 analyses.2 : Mitchellcounty oligoclase:(Nao,ulG*CaoroAl,,ESirsO6@), ticles. The resulting powder consistedof a mixture of sep- 317 analyses.3 : Bancroft albite: (Nao6slc osoao.,Al, roSi. roO"*),266 anal- arate grains of mafic minerals and feldspar. These minerals yses.4 : Saranac Lake andesine:(Nao 16Ko6Oao6Alr $Sir soos@),294anal- were sepaxatedwith a Franz Isodynamic Separator,which yses.5 : PuebloPark bytownite:(NaosKo@Cao60A11$Sireosoo),48 anal- yses. 6 : Crystal Bay bytownite: (Naor4GolcaoT6AllT4sirreoooo),314 produced a nearly pure feldspar powder. Final purifica- analyses.7 : Grass Valleyanorthite: (NaoosG@Cao@Alreosi2@06@), 46 tion was accomplishedusing high-density liquids. Anor- analyses. thite grains were then washedwith distilled HrO and ac- etone. variations can lead to precipitation of secondaryminerals Feldspar compositionsand sample purity onto the dissolving surface(see review by Helgesonet al., The feldsparcompositions were measuredwith an elec- 1984) or to important changesin mineral surfacechem- tron microprobe and averagecompositions are reported istry during the experiment. in Table l The total range of measuredcompositions is The goal paper of this is to define a relation between shown on ternary diagrams in Figure l. The diameter of the dissolution rates and composition of plagioclasefeld- the electron beam used to analyze these sampleswas 16 spars.To achieve this goal it is necessaryto supplement rlm. the existing data with additional experimentsso that rates Each feldspar exhibits some variation in composition are available plagioclase for a wide range of composi- as measuredwith the electron probe, and the traditional tions. The investigation is restricted to dissolution in name of some samples can be misleading. Specifically, strongly acidic solutions where the character of the re- the Pueblo Park bytownite and the Bancroft albite are action is relatively well understood (e.g.,Chou and Wol- labradorite and oligoclase, respectively. Much of this last, 1985;Holdren and Speyer,1985; Casey et al., 1988, variation in composition is attributable to microscopic 1989) precipitation and the of secondaryminerals can be mineralogic heterogeneities.Zoning, exsolution lamellae, easily suppressed. and accessoryphases are apparent from microscopic ex- amination of material (see Marnnllrs AND TEcHNTeuES the Appendix). Although all but one of the compositions are consistent with the per- Preparation of samples missible range of solid solution (Fig. l), most of these All samplesexcept the Pueblo Park bytownite were ac- materials contain more than a single feldspar phase(Ap- quired from Wards Natural Science Establishment, pendix). The Mitchell County andesine, for example, Rochester,New York. The samplesand their trace min- contains exsolved potassium feldspar (Appendix), which eralogy are described in the Appendix l.' Two samples introducesconsiderable scatter in the ternary plot of com- of the Mitchell County oligoclaseand SaranacLake an- position (Fig. l). desine were prepared independently. Granular samples were independentlyprepared from crystalsin 1983-1984 Specific surface areas (seeHoldren and Speyer, 1987) and as part ofthis study Specificsurface area of each sample was measuredpri- in 1988. The procedures used by each researcherwere or to dissolution using a Micromeritics Digisorb 2600 similar, except that a ball mill was used to crush the ma- device. After degassing overnight at 200 oC, varying terial in this study, rather than the disk grinder employed mixtures of N and He were adsorbed onto the powder surface.The areaswere calculatedfrom the sorption iso- therm and the BET theory of area measurement (Bru- ' A copy of Appendix I may be ordered as Document AM- nauer 1938). The precision of this 9l-449 from the BusinessOfrce, Mineralogical Society of Amer- et al., and accuracy ica, 1130 SeventeenthStreet NW, Suite 330, Washington, DC technique is generally better than 50/0,as determined by 20036, U.S.A.