Transformation of Chlorite to Smectite Through Regularly Interstratified Intermediates1

A. L. SENKAYI, J. B. DIXON, AND L. R. HossNER2

ABSTRACTTrioctahedral, iron-rich chlorite separated from east Texas

lignite overburden shales by high gradient magnetic separa-tion was characterized by XRD and by IR spectroscopy. Thechlorite was then subjected to chemical oxidation by digestion(ca. 100°C) in a saturated bromine solution to simulate weather-ing. X-ray diffraction indicated that the chlorite was trans-formed to smectite through regularly interstratified chlorite-vermiculite and chlorite-smectite intermediates. The smectiteultimately dissolved. All of the chlorite reacted in 2 weeks.Electron micrographs of partially weathered particles of chloriteshowed thin plates folded at the edges and unreacted cores inthe center. Iron released during the reaction tended to precipi-tate along particle edges. It is postulated that the thin, foldededges and the cores were smectite and chlorite, respectively.Formation of the iron-rich chlorite in the lignite overburdenshales probably resulted from interactions between aluminoussmectite and solutions rich in ferrous Fe. Exposure of the over-burden shales to oxidizing conditions may result in the trans-formation of chlorite to smectite with precipitation of releasedFe.

Additional Index Words: weathering, chlorite-vermiculite,chlorite-smectite, lignite, magnetic separation.

Senkayi, A. L., J. B. Dixon, and L. R. Hossner. 1981. Trans-formation of chlorite to smectite through regularly interstra-tified intermediates. Soil Sci. Soc. Am. J. 45:650-656.

VARIOUS IRON-BEARING MINERALS, particularly sulfides(pyrite) and carbonates (siderite), occur in east

Texas lignite overburden shales (Arora et al., 1978).The Fe in these minerals is in the reduced form. Fer-rous Fe enters silicate structures, resulting in the for-mation of ferriferous silicates such as chlorite or glau-conite (Burst, 1959; Millot, 1970; Velde, 1977). Forma-tion of Fe-rich chlorite in reduced tidal sediments ofCalifornia by incorporation of ferrous Fe into thesmectite structures has been described by Lynn andWhittig (1966). Exposure of the sulfide-bearing sedi-ments to atmospheric weathering conditions resultsin a rapid oxidation of the sulfide minerals, releasingHzSCv In the presence of H2SO4, the diagenetic Fe-rich silicate minerals become unstable. Weatheringof chlorite readily occurred when overburden ma-terials were exposed to simulated acid weathering, andappreciable quantities of Fe and Mg were released.3Lynn and Whittig (1966) observed complete weather-ing of chlorite from acid sulfate soils after 60 years.These investigators postulated that the acid preferen-tially removed the hydroxide sheets of chlorite duringweathering, resulting in the formation of montmorillo-

1 Contribution from the Texas Agric. Exp. Stn., College Sta-tion, Tex. Paper presented before Div. S-9, Soil Sci. Soc. Amer.,7th Aug. 1979, Ft. Collins, Colo. Received 4 Tune 1980. Approved13 Jan. 1981.

3 Research Associate and Professors in Soil Mineralogy andSoil Chemistry, respectively, Dep. of Soil & Crop Sciences, TexasA&M Univ., College Station, TX 77843.

3 A. L. Senkayi, J. B. Dixon, and L. R. Hossner. 1978. Simu-lated weathering of lignite overburden shales from northeastTexas. Agron. Abstr. p. 174.

nite. Recent experimental data obtained by variousinvestigators (Ross, 1975; Ross and Kodama, 1973,1976) have confirmed that it is possible to selectivelyremove interlayer hydroxide sheets of Fe-rich chloriteby acid dissolution if the hydroxide sheets are firstdisturbed either by dehydration during heating or bychemical oxidation of the ferrous Fe. Herbillon andMakumbi (1975) point out that irreversible oxidationof Fe2+ is important for the alteration of chlorite tovermiculite under natural conditions.

MATERIALS AND METHODS'Concentration of Chlorite by High Gradient Magnetic Separa-

tion (HGMS)—Air-dried samples (<2.0 mm) of lignite over-burden shales were treated with normal pH 5.0 NaOAc toremove carbonates and with H?O2 to oxidize organic matterand any sulfides present. A dithionite-citrate-bicarbonate treat-ment (Mehra and Jackson, 1960) was carried out to removeiron oxides. The samples were washed free of excess salts usinga 50:50 mixture of water and 95% alcohol followed by acetonebefore particle size fractionation. Chlorite was concentratedfrom the fine silt fraction (2-5 /«n) by HGMS. The procedurewas slightly modified by carrying out two separations (Fig. 1)so that strongly magnetic chlorite could be .separated fromthe weakly magnetic mica. The sample suspended in pH 10Na2CO3 (1.0 g/liter) was passed through a magnetic filter (ata flow rate of approximately 500 ml/min) held between thepoles of the electromagnet (model 7600 electromagnet energizedby a model 45-30 current regulated power supply manufacturedby Alpha Scientific Inc., Oakland, Calif.) The poles of themagnet were 12.2 cm in diameter, and the gap between thepoles was set at 1.9 cm. The magnetic field strength betweenthe poles was controlled by varying the power source currentwhile keeping the gap between the poles constant. The variousfractions obtained were freeze-dried and characterized by x-raydiffraction (using a Philips diffractometer with a graphite mono-chromator and a Cu-target x-ray tube operated at 35 kV and

FIRSTSEPARATION

SECONDSEPARATION

ORIGINAL

SUSPENSION

MAGNETIC

FILTERFIELD STRENGTH: 1.2T

FIELD

Fig. 1—Flow sheet showing the major steps during separationof chlorite from overburden shales by high gradient mag-netic separation.

650

SENKAYI ET AL.'. TRANSFORMATION OF CHLORITE TO SMECTITE 651

5-2pmMg

500cps

13.7

FRACTION

MAGNETIC

5-2 pm500cps

Fig. 2—X-ray diffraction patterns of various fractions obtainedby HGMS. Data for the original sample and the nonmagneticfraction (Fig. 1) not shown.

14 mA) and infrared (IR) spectroscopy (Perkin-Elmer modelno. 283 spectrophotometer). Elemental analysis of the stronglymagnetic fraction was carried out according to the proceduredescribed by Bernas (1968). The ferrous and ferric Fe con-tents in this sample were determined using the procedure de-scribed by Roth et al. (1968). Light was excluded during theferrous Fe determination. Exposure of the samples to lightresults in an overestimation of the ferrous Fe (C. B. Roth,personal communication). Two reference samples (Mg-chloriteand penninite) were provided by Dr. D. C. Bain of MacaulayInstitute for Soil Research, Aberdeen, England; the prochloriteis a standard sample from Chester, Vermont, obtained fromWard's Natural Science Establishment.

Simulated Weathering of Chlorite—Samples of the Fe-richchlorite (200 mg) electromagnetically separated from ligniteoverburden shales were suspended in 20 ml of saturated brominewater in plastic (40 ml) centrifuge tubes. The tubes were closedwith both plastic covered rubber stoppers and screw caps be-fore being placed in a boiling water bath. Portions of thesamples were periodically withdrawn from the suspensions, satu-rated with Mg or K, and analyzed by XRD, IR spectroscopy andtransmission electron microscopy (TEM). Each time part of the

I

4000 MOO 3200 1200 1000 000 WO

Wavenumber (cm )Fig. 4—Infrared spectra of chlorite from overburden shales

(a). Spectra of other reference chorites have been included forcomparison; (b) Mg-chlorite; (c) prochlorite; (d) penninite.

MgG

Fig. 3—X-ray diffraction patterns showing the effect of heatingon chlorite from overburden shales.

suspension was withdrawn from the tube, the saturated brominewater was renewed and the tubes were sealed again and placedback in the boiling water bath.

To determine the rates of release of cations during chloritealteration, another set of samples (200 mg) was accuratelyweighed and placed in 40-ml plastic centrifuge tubes. A solu-tion of 0.2M NaBr (20 ml) saturated with liquid bromine wasthen added. The tubes were closed tightly and placed in aboiling water bath. The purpose of the NaBr was to saturatethe exchange complex with Na and to standardize the ionicstrength. After predetermined intervals of time, the tubes wereremoved from the water bath and cooled. The pH of thesuspensions was determined both before and after each incu-bation. The supernatant solutions were collected by decantingafter centrifuging at 1,500 rpra for about 5 min. The residueswere washed once with 0.2M NaBr before another portion ofsaturated bromine solution was added for further weathering.The combined wash and supernatants were dilluted to 50 mlusing 0.2M NaBr before being filtered through a 0.2-jum poresize milliporc filter. The solutions were analyzed for Fe, Mg,Al, and Si by atomic absorption and for K by emission spec-troscopy using a Perkin-Elmer model no. 603 spectrophotometer.

RESULTSCharacterization of the Chlorite from

Overburden ShalesX-ray Diffraction—X-ray diffraction data (Fig. 2)

reveal relatively pure chlorite in the strongly magneticfraction. This fraction accounted for approximately20% by weight of the original sample (5-2/xm). Chlor-ite was also observed in the coarse silt as well as thesand and clay fractions. The much greater intensityof the even-order peaks (7.1 and 3.53A) compared tothe odd-order peaks (14.2 and 4.7A) is indicative ofthe relatively high Fe content of this chlorite (Brind-ley, 1961). The number of heavy octahedral atoms,mainly Fe, in the 2:1 mica-like layer (FeMLVI) minusthose in the interlayer, brucite-like (FeBLVI). hydroxidesheet was estimated from the intensity ratio of thethird and fifth-order peaks (looa/Ioos) by employingthe curve developed by Petruk (1964). The degreeof asymmetry (DA), which may be expressed by thefollowing equation, (FeMLVI — FCBLVI) = DA, givesan indication of the relative distribution of Fe in thetwo types of octahedral positions. For symmetrical dis-tribution of Fe in the two octahedral sites, the degreeof asymmetry is equal to 0. The average degree ofasymmetry obtained from several XRD patterns using

652 SOIL SCI. SOC. AM. J., VOL. 45, 1981

Table 1—Elemental composition of chlorite concentrated from________lignite overburden shales by HGMS.________SiO, A1,O, FeO Fe,O, MgO CaO K,O TiO, Cr.O, MnO H,0 L.O.I.t Total

37.3 19.8 12.9 7.14 8.3 0.16 2.0 0.9 0.02 0.2 1.2 9.8 99.7

t Loss on ignition from 110 to 950 °C.

random powder mounts was 0.25, indicating that therewas slightly more Fe in the octahedral sites of the 2:1units than in the interlayers. The presence of the(060) peak at 1.55A (data not shown) indicated thatthe chlorite was trioctahedral. Heating at 600°C for1 hour resulted in dehydration of the brucite-likesheets and loss of the higher order peaks (Fig. 3). Heat-ing also resulted in increased intensity and a slightdisplacement of the first-order (14.2A) peak to 13.7A.

Infrared Spectroscopy—Spectra of the overburdenchlorite and of the three reference chlorite samplesare given in Fig. 4. Penninite (d) is a magnesianchlorite with very little substitution of Al for Si inthe tetrahedral positions (<0.75 formula positions outof 4 are occupied by Al), whereas prochlorite (c) hasabout 50% of the tetrahedral positions occupied byAl (Bailey, 1975). All spectra in Fig. 4 show doubleabsorption peaks in the region 4000 to 3000 cm"1 dueto stretching of OH groups in the hydroxide sheets(Farmer, 1974; Hayashi and Oinuma, 1965). Thesebands occur at the lowest frequency in the spectrum ofthe overburden chlorite (a) which is an indication ofthe higher Fe content of overburden than the referencechlorites (spectra b, c, and d). Penninite shows its char-acteristic peaks at 3630 and 3460 cm-1. The OH bend-ing region (600 to 700 cm"1) of the overburden chloriteshows a doublet at 670 cm"1 and 650 cm"1, whereasthe three magnesian specimens show single peaks be-tween 660 and 670 cm"1. Hayashi and Oinuma (1965)investigated the relationship between the absorption

MgG3.33 SOOcps

3.343.53

14

4 >

10.0.

I

«•¥» 7.1 14,

T

2.

0

Fig. 5—X-ray diffraction patterns of artificially weatheredoverburden chlorite. The numbers refer to days after treat-ment was begun.

bands at 620 to 692 cm"1 and the octahedral composi-tion of chlorite. They observed that the wave numberdecreased with decreasing octahedral Al and increas-ing Mg. A high-octahedral Fe content resulted in evensmaller wave numbers. The same relationships wereobserved for the absorption peaks between 744 and765 cm"1. The presence of a weak peak at 670 cm"1

in the spectrum of the overburden chlorite is an in-dication that some Mg is present in this chlorite, al-though Fe seems to be the dominant cation. Theabsorption bands at 820 and 760 cm"1 are associatedwith tetrahedral Al-O vibrations and increase in in-tensity with increasing Al-for-Si substitution (Farmer,1974). These peaks are strongest in the prochloritespectrum, which has the highest Al substitution, andweakest in the penninite spectrum, with the least sub-stitution of Al in the tetrahedral position. Accordingto Farmer (1974), increasing Fe content reduces theintensity of the 820 cm"1 peak as was observed in thespectrum of the Fe-rich chlorite from lignite overbur-den. All of the magnesian chlorites show peaks at 380cm"1; the overburden chlorite shows a peak at 350cm"1. These peaks are probably related to Mg andFe, respectively, in the interlayer hydroxide sheets.

Elemental Analysis— The elemental composition ofthe overburden chlorite is shown in Table 1. Thedata indicate that the chlorite is quite high in Fe,particularly ferrous Fe (Fe2+/(Mg2+ + Fe2+) = 0.47atomic ratio). According to Foster's classification sys-tem (Bailey, 1975), this chlorite would probably beclassified as a brunsvigite due to the high ferrous Fe-to-Mg ratio and appreciable tetrahedral substitution ofAl and Si. Tetrahedral substitution of Al for Si wasindicated by presence of Al-O bands at 820 and 760cm"1 and the poor resolution of the Si-O stretching

14.0

Fig. 6—X-ray diffraction patterns of chlorite samples weatheredfor 3 days showing the effect of various heat treatments.

SENKAYI ET AL.: TRANSFORMATION OF CHLORITE TO SMECTITE 653

band at 1000 cm-1 (Fig. 4). The reference chloritewith minimum tetrahedral substitution of Al for Si(d) showed the greatest details in the Si-O stretchingregion.

Transformation of Chlorite to SmectiteThe major mineralogical changes observed during

the transformation of overburden chlorite are indi-cated in Fig. 5 and 6. Both chlorite-vermiculite andchlorite-smectite regularly interstratified minerals wereformed within < 2 days (Fig. 5). Chlorite-vermiculitewas indicated by the presence of peaks at 27.6 (001)and 9.3A (003), whereas chlorite-smectite was indi-cated by the 31.5 (001), 15.8 (002), and 8.0A (004)peaks in Mg-saturated and glycerol-solvated samples(Sawhney, 1977). Progressive alteration of the regular-ly interstratified chlorite-vermiculite and chlorite-smectite to smectite was indicated by a gradual in-crease in the intensity of the 17.7A peak as the in-tensities of the 27.6 and 31.5A peaks decreased. Thechlorite-smectite apparently reacted more readily thanthe chlorite-vermiculite. This was inferred from therelative changes in the intensities of the 27.6 and 31.5Apeaks with time (Fig. 5). The transformation ofchlorite to smectite may be represented as follows:chlorite -> chlorite-vermiculite -» chlorite-smectite-> smectite -» dissolution. There is no direct evidence,however, to confirm that the chlorite-vermiculite wasfirst altered to chlorite-smectite before being trans-formed to smectite. After 2 weeks, all the chlorite haddisappeared. Preferential alteration of chlorite tosmectite resulted in accumulation of the less weather-able mica in the weathered samples, as indicated inFig. 5. Figure 6 shows XRD patterns of a sampleweathered for 3 days. Saturation with K and dryingat room temperature caused the vermiculite compo-nent of the chlorite-vermiculite interstratified mineralto collapse from 14 to 10A so that the chlorite-vermi-culite (001) peak moved from 27.6A in the Mg andglycerol-solvated sample to 23.2A. The peak at 11.2Ais due to partially collapsed smectite. Heating at

Table 2—Release of cations from chlorite by saturatedbromine solutions.

Cumulativetime, hours

12345686

109129151188275

Suspension pHInitial

4.94.95.35.25.05.05.15.25.2

Final

1.81.91.81.31.91.91.71.81.2

Fe

15.348.153.868.972.375.178.780.683.8

Cations releasedMg

19.539.554.673.574.077.079.582.088.0

Al

- % -17.035.542.555.056.558.060.563.069.5

K

4.08.0

15.520.023.025.029.031.535.5

Si

5.513.521.029.031.534.036.539.045.0

100°C for 1 hour greatly reduced the intensity of thispeak and at the same time increased the intensity ofthe 10A peak. The increased intensity of the 10Apeak on heating to 100°C was due to further collapseof smectite. A weak peak also appears at 12.6A in theXRD pattern of the sample heated at 100°C (Fig. 6).This peak gradually increased in intensity with fur-ther heating, obtaining maximum intensity after heat-ing at 550°C. The position of the peak, however, grad-ually shifted to 12.3A after heating at 300°C andfinally to 11.9A on further heating to 550° C. Thispeak was identified as the second-order peak of regu-larly interstratified chlorite-vermiculite due to rein-forcement on heating. Reinforcement of the second-order peak of regularly interstratified chlorite-vermi-culite is attributed to dehydroxylation of the inter-layer hydroxide sheets of the chlorite component onheating (Sawhney, 1977). The (001) chlorite-vermi-culite peak is not visible in Fig. 6 due to its weak in-tensity as compared to the (002) peak.

Kinetics of Cation Release—The pH of the suspen-sions measured before and after successive incubationsand the percentages of cations released are shown inTable 2. Approximately 80 to 90% of Fe and Mg werereleased in 275 hours. The large amounts of Si re-leased (45%) after 275 hours suggest that appreciabledestruction of the silicate structures occurred.

-0.8-

-1.6

-2.4

_os o-

-0.8-

-1.6

-2.4

0 40 80 120 160 200 240 280 0 40 80 120 160 2OO 240 280

0 40 8O 120 160 200 240 280 O 40 80 I2O 160 200 240 280

HoursKg- 7—Relative rates of cation extraction from overburden chlorite by saturated bromine solutions.

654 SOIL SCI. SOC. AM. J., VOL. 45, 1981

Extraction of cations from similar structural sitesin layer silicates obeys a pseudo first-order rate law,provided that a large excess of the extracting solution(in this case, bromine water) is present (Brindley andYouell, 1951; Osthaus, 1954, 1956; Ross, 1969; Thomp-son and Hower, 1975; Thompson et al., 1977). Assum-ing that a pseudo first-order rate law applies to thisreaction, a plot of In y versus time (t) was made (y =fraction of the cations remaining in the solid phase;t — cumulative time in hours the sample was exposedto the extracting solution). Two straight lines wereobtained for each cation (Fig. 7), indicating that ineach case there was an initial higher rate of extraction(represented by the steeply sloping portion) followedby a slower rate. Each of the straight lines in Fig. 7can be described by an equation of this form:

In y = at + b, [1]where a and b are the slope and intercept, respectively.Values of the linear correlation coefficients (r2) arealso indicated in Fig. 7. The greater slopes of thelines for Mg and Fe (Fig. 7) suggest that both ofthese cations were released at much faster rates thanAl. The high values of r2 also indicated that therelease of cations during the transformation of chloriteto smectite closely followed the pseudo first-orderkinetics model, particularly in the initial stage of thereaction. It is reasonable to assume that the initialfaster rates of release corresponded to removal of ca-tions from the alternating interlayer hydroxide sheetsof chlorite. Evidence for this was the rapid initialformation of regularly interstratified minerals. Theslower rates probably represented release of cationsfrom the remaining interlayer positions of the regular-ly interstratified minerals or release of cations from

co

'</>litE(0c(0

4000 3600

WavenumberFig. 8 — Infrared spectra of artificially weathered overburden

chlorite. The numbers refer to days after treatment was begun.

octahedral positions of the 2:1 units during dissolu-tion.

Infrared spectra of partially altered chlorite samples(Fig. 8) revealed that the peaks due to hydroxyl stretch-ing and bending in the interlayer sheets (3560, 3430,670, and 650 cm"1) were drastically affected by thereaction. There was a gradual decrease in the intensityof these peaks as the interlayer regions were removed.It appears that most of the interlayer regions wereremoved after about 4 days (Fig. 8). This agrees withthe kinetics data which also indicated a decrease inthe rates of release of cations after about 4 days. Thex-ray data, however, indicated that after 4 days, therewas still a considerable amount of chlorite in thesample (Fig. 5). This apparent conflict can be ex-plained by assuming that, although x-ray data showedsome chlorite in the sample, the regularity of atomicordering was greatly disturbed so that no definite IRpeaks could be seen at this time. A gradual increase inthe intensities of the peaks at 3620 cm"1 (A12OH)was due to a preferential increase in the concentrationof the more aluminous minerals (mica, smectite, andkaolinite) in the weathered samples.

DISCUSSIONIt is generally agreed that oxidation of ferrous Fe

is the most important factor involved in both experi-mental transformation of chlorite to vermiculite andweathering of chlorite under natural conditions (Ross,1975; Ross and Kodama, 1976; Herbillon and Ma-kumbi, 1975; Coffman and Fanning, 1975). However,Ross and Kodama (1976) found that structurally simi-lar chlorite reacted differently under the same con-ditions when treated with saturated bromine water.Iron-rich chlorite (diabantite) was altered directly tovermiculite within a few weeks (Ross, 1975), whereasa chlorite with an intermediate-Fe content (bruns-vigite) could only be altered to a regularly interstrati-fied chlorite-vermiculite with no further alteration tovermiculite, even after 4 months (Ross and Kodama,1976). In the present study, a chlorite with an inter-mediate-Fe content similar to the brunsvigite investi-gated by Ross and Kodama (1976) was readily alteredto smectite through regularly interstratified chlorite-vermiculite and chlorite-smectite intermediates bysaturated bromine water. It may be necessary to con-sider both the content of ferrous Fe in the chloriteand its distribution between the hydroxide and the 2:1units to explain the differences in the reaction ratesand the various products obtained under apparentlysimilar experimental conditions. If the Fe occursmainly in the interlayer hydroxide sheets, it is likelyto be oxidized and removed quite rapidly as was thecase in this study. On the other hand, if most of the Feis located in the octahedral positions of the 2:1 units,the reaction will be slower. Secondly, oxidation ofany ferrous Fe within the 2:1 units will result in fur-ther reduction of the charge and formation of smec-tite instead of vermiculite. Since smectite was the finalproduct in this study, it appears that most of the Fewas initially removed from the interlayers of chlorite,and that oxidation of ferrous Fe in the 2:1 units re-sulted in a lowering of the charge and formation ofsmectite. Hence, although both the overburden chlor-ite and the chlorite investigated by Ross and Kodama(1976) had approximately equal quantities of FeO

SENKAYI ET AL.: TRANSFORMATION OF CHLORITE TO SMECTITE 655

Fig. 9—Transmission electron micrographs showing partially altered chlorite particles with unreacted cores and thin edges (a and b)and precipitation of iron at the edges (e and d). Each bar represents 0.2/im.

(about 13%), there was more Fe in the octahedral posi-tions of the 2:1 units in the chlorite investigated byRoss and Kodama. This explains the fact that thischlorite could only be altered to a regularly inter-stratified chlorite-vermiculite with no further altera-tion to vermiculite. The authigenic overburden chlor-ite, however, had a lot more Fe in the interlayer hy-droxide sheets; it therefore was readily transformedinto regularly interstratified minerals and eventuallyto smectite.

Hypobromous acid (HOBr) probably played themajor role in the oxidation of ferrous Fe during re-action of chlorite with saturated bromine water. Thereaction between bromine and water to produce hy-pobromous acid can be represented as follows:

Br2(aq) + H2O = HOBr + Br~ + H+. [2]

The acidity produced by the above reaction selectivelydissolved the interlayer hydroxide sheets from chloritefollowing the oxidation of ferrous Fe. Appreciabledissolution of the 2:1 components also occurred be-cause of the relatively low pH values (Table 2). Thisresulted in a gradual decrease in the size of the chlorite

particles. Reaction of the chlorite particles with bro-mine water probably occurred from the edges of theparticles inward towards the center. Electron micro-graphs (Fig. 9) of partially altered particles of chloriteshow thin plates which are folded at the edges andunreacted cores in the center. It is postulated that thethin, folded edges and the cores are smectite and un-altered chlorite, respectively. The Fe released duringthe reaction tended to precipitate at the thin edges ofthe particles (Fig. 9). The relatively rapid reactionrates suggest that weathering of chlorite is likely tofollow soon after exposure of the lignite overburdenshales to oxidizing atmospheric conditions, particular-ly when H2SO4 is produced by oxidation of pyrite.

ACKNOWLEDGMENTThe authors would like to acknowledge the Center for Energy

and Mineral Resources, Texas A&M University, for support andfinancial assistance in carrying out this research.

656 SOIL SCI. SOC. AM. J., VOL. 45, 1981