American Mineralogist, Volume 7 5, pages 1009-1019, 1990

Hydrothermal reaction of a rhyolitic-composition glass: A solid-state NMR study

Wl.Nc-HoNc Ar-nx YING'* R. Jmans KrnxplrnrcrDepartment of Geology, University of Illinois, 1301 West Green St., Urbana, Illinois 61801, U.S.A.

Arsrru,cr

This paper presents a study of the structure and chemical composition of the solidproducts of the hydrothermal reaction of a synthetic Fe-free rhyolitic glass (Na'O-KrO-MgO-CaO-AlrOr-SiOr) a| 250'C and nominally buffered pH of initially 1.0, 3.0, and 9.1and of the mechanisms of this reaction using high-resolution solid-state NMR spectros-copy in combination with electron microprobe analysis, SEM, XRD, and IR spectroscopy.During this reaction molecular water is incorporated in the rhyolitic glass and substantiallychanges the glass's structure, as for a highly polymerized glass of nearly albite composition(Yang and Kirkpatrick, 1989). The structural changes due to hydration include an in-creased average Si-O-T (T : Si,Al) bond angle per tetrahedron and formation of hydrationshells around the Na atoms and presumably around the other large cations. The three-dimensional aluminosilicate framework in the residual hydrated glass is not substantiallydepolymerized, the Si/Al ratio remains the same, and the dissolution of the aluminosilicatenetwork occurs only at or near the glass-solution interface. The structural changes and theamount of dissolution are smaller than for the glass we previously examined, probablydue to a higher Si/Al ratio and the presenc€ of divalent cations as components of therhyolitic composition. Additionally, the secondary minerals produced at high pH are claysrather than zeolites.

INrnooucrroN

Interaction between rhyolitic glass and aqueous fluidsat low temperatures is a common process on and nearthe Earth's surface. The chemical changes in such glassesdue to this process and the kinetics and mechanisms ofthis reaction have been previously studied (Jezek and No-ble, 1978; Nesbitt and Young, 1984; White and Claassen,1980 and references therein). There is also a large bodyof literature concerning leaching of glasses, especially po-tential nuclear waste storage materials (see Lutze, 1988,for a review). In many cases however, the structuralchanges in the glass due to aqueous attack and the natureand distribution of the solid reaction products are notwell known. Understanding these factors is essential toclarifying the kinetics and mechanisms of such reactions.

In a previous paper (Yang and Kirkpatrick, 1989) wehave shown that the dissolution of a nearly fully poly-merized synthetic sodium aluminosilicate glass with acomposition close to albite (NAS glass) at 250 { in buf-fered aqueous solutions occurs by (l) incorporation ofmolecular water into bulk glass, (2) exchange of protonsfor Na, (3) a small amount of depolymerization in theinterior of glass due to OH groups, and (4) dissolutionof the aluminosilicate framework at the glass-water in-terface. The crystalline reaction products of these exper-iments become more polymerized with increasing pH,

* Present address: EM Division, Geo-environmental Services,Inc., 130 West Wieuca Rd., Suite 108, Atlanta, Georgia 30342,U.S.A.

and the average Si-O-T (T : Si,Al) bond angle per tet-rahedron in the glass increases owing to the hydration'These conclusions are based on high-resolution solid-statenuclear magrretic resonance (NMR) spectroscopy in con-junction with X-ray diffraction (XRD), scanning electronmicroscopy (SEM), infrared spectroscopy (IR), and elec-tron microprobe analysis.

In the study described here, we use the same techniquesto analyze the solid products after a similar reaction be-tween a synthetic, Fe-free rhyolitic glass (componentsNa,O-KrO-MgO-CaO-AlrO3-SiOr) and hydrothermalfluids at 250 "C and nominally buffered pH of about 1.0,

3.0, and 9.1. Like the NAS glass, the rhyolitic glass isnearly fully polymerized (NBO/T : 0.07) but is closer toactual rock compositions. To avoid paramagnetic broad-ening of the NMR peaks, the glass was synthesized withno Fe. Ca and Mg were used to bring the total divalentcation content to about 6.5 molo/0. The overall composi-tion (Table l) is similar to those of many natural rhyo-lites, except that it is deficient by -l-2 wto/o in totalalkalis and similarly enriched in divalent cations (Car-

michael et al., 1974).Although the rhyolitic glass has a more complicated

composition than the NAS glass, multinuclear (2esi, 274'1,'z3Na) magic-angle-spinning (MAS) and 'zeSi and 27Al

cross-polarization magic-angle-spinning (CP-MAS) NMR

spectroscopy is still a very sensitive probe of variationsin structure and composition of the glass and the fine-grained, often amorphous, reaction products. The struc-tural and compositional changes in the rhyolitic glass due

0003404x/90/09 l 0-l 009$02.00 1009

l 0 l 0

TraLe 1. Chemical analyses of the synthetic rhyolitic-compo-sition glass used in this study

YANG AND KIRKPATRICK: HYDROTHERMAL REACTION OF GLASS

Component

Assynthesizedcomposition

(wt%)

Analyzedaverage(wt"/o)-

Analyzedaverage(mol%).

. Average of five electron microprobe analyses." Nonbridging O per tetrahedral cation.

to hydration are similar to those of the NAS glass. Therates of hydration and formation of secondary phases,however, are much smaller.

Mnrnons

The base glass was prepared by fusing reagent gradeSiOr, AlrO3, MgO, CaCOr, &CO., and Na2CO3 mixturesin a Pt crucible at 1573'C three times for about 40 mineach time. The sample was quenched by immersing thebottom of the crucible in water and ground with an agatemortar and pestle between fusions. Composition and ho-mogeneity were analyzed by using the methods describedby Yang and Kirkpatrick (1989). Analyses could be takento within about 5 pm of the edges of glass fragments. Thenominal and analyzed compositions agree within analyt-ical uncertainty (Table l). The glass was ground and drysieved to 5-40 pm for the hydrothermal experiments.

The hydrothermal experiments were carried out at 250'C under autoclave conditions using the methods de-scribed by Yang and Kirkpatrick (1989). Solution pH wasnominally buffered at 1.0 by 0.2 M KCI and 0.2 M HCl,at 3.0 by 0.1 M potassium hydrogen phthalate and 0.1M HCl, and at 9.1 by 0.1 M (trihydroxylmethyl) ami-nomethane and 0. I M HCI and changed to about 1.4,5.2, and,9.4-9.8 (as measured at room temperature), re-spectively, during the experiments. Experiment times wereup to 120 d. NMR, IR, XRD, and SEM dara were ob-tained using the same methods as in our previous work.Some clay phases were separated from the experimentalproducts using a sonifier and centrifuge and examined bypowder XRD before and after expansion with ethyleneglycol in an oriented mount on a ceramic tile using aSiemens powder diftactometer operate d at 2 20/min wirthCuKa radiation.

Rrsur,rs AND SpECTRAL TNTERpR_ETATToNS

Base glass

The 2eSi, 274,1, and ,3Na MAS NMR spectra of thebase glass (dashed lines in Fig. l) consist ofsingle broadpeaks. The,eSi peak maximum is at - 101.6 ppm and the

2'Al peak maximum is at 53.8 ppm at a magneticfield strength (Ho) of ll.7 T. Both are in the ranges forfully polymerized framework aluminosilicates (Kirkpat-rick et al., 1985; Oestrike et al., 1987). There is no 27Al

peak due to octahedrally coordinated aluminum (Alt6l)which would occur at about 0 ppm. The'3Na peak max-imum is at -18.4 ppm at Ho: l l .7 T, comparable tothe value for the NAS glass. The broad peaks and thepeak positions indicate that this glass, like glasses withsimilar compositions, has an essentially three-dimension-al tetrahedral framework structure with a highly disor-dered Al-Si distribution and wide ranges of Si-O-T (T :Si,Al) bond angles (see, e.g., Hochella and Brown, 1984;Oestrike et al., 1987). Based on its composition and lackof Al[6], the method of Mysen et al. (1985), yields anaverage of 0.07 nonbridging O per tetrahedral cation(NBO/T), indicative of a nearly fully polymerized struc-ture and consistent with the NMR data.

Hydrothermal reaction at pH 1.0 and 3.0

The reaction products ofthe rhyolitic glass are gener-ally similar to those for the NAS glass, but the extent ofthe chemical and structural change is less. Even after 60d at pH 1.0 (final pH : 1.4 at room temperature), powderXRD shows only a very broad hump with a maximumat about 24 20, indicative of an amorphous aluminosil-icate, and an almost unobservable peak at 12.3'20 cor-responding to the 7.20 A basal spacing of kaolinite. Forthe NAS glass, secondary phases were detected within 7d. The presence of secondary kaolinite in the runs withthe rhyolite glass is confirmed by the "Al MAS and "Siand 27Al CP-MAS NMR data (Figs. 18, lE, and 2). The'??Al MAS NMR spectra contain a small 4,l[6] peak atabout 4 ppm corresponding to the ,4.l[6] in kaolinite (Kin-sey et al., 1985), in addition to that for Al[4] in residualglass. The relative intensity of the kaolinite 4,l[6] peakincreases slowly with increasing experiment time. Therelative intensity of this peak is much larger in the ,?Al

CP-MAS NMR spectra (Figs. 24, 2C), consistent withthe high concentration of OH- groups in the kaolinite andour previous results for the NAS glass. The CP-MAStechnique preferentially enhances the sigrral intensity fromnuclei close to protons. The'zeSi MAS NMR spectrumof the 60-day sample (Fig. lD) contains a small shoulderat about -91 ppm, corresponding to the Q.(0AD Si sitesin kaolinite (Magi et al., l98l; Barron et al., 1983). Therelative intensity of this signal is much larger in the,eSiCP-MAS NMR spectra (-91.7 ppm peak in Figs. 2B,2D).

SEM micrographs of the samples with pH I (e.g., Fig.3A.) show angular to slightly rounded fragments -40 pmin size with an etched surface on which kaolinite (smallwhite particles) appears to have precipitated. The sizeand morphology of the larger fragments indicate that theyare residual glass, and thus that the amorphous phasedetected by XRD and NMR is residual hydrated glassand not a new phase formed by dissolution and precipi-tation.

NaroK.oMgoCaOAlr03sio,

Total

3.403.502.503.10

14.1073.40

100.004.42

3.433.552.683.37

14.6372.44

100.10

3.592.442.693.899.30

78.10100.00

4.200.07

Si/Al (atomic)NBOiT-'

YANG AND KIRKPATRICK: HYDROTHERMAL REACTION OF GI-ASS l 0 l I

R H Y O L I T I C G L A S S

" s i M A S S "A r MASS

Ar (4)

" N a M A S S

2 8 D A Y Sp H 1 . 0 -

D A Y S

1 . 0 -

-50 -100 -1 50 1 00 0 0 -80

PPM (FROM IM NACI ]PPM (FROM TMS) PPM (FROM 1M AICI . ]

Fig. |. DSi (A, D), ,A1 (B, E), and ,rNa (C, D MAS NMR spectra of rhyolitic glass after 28-d (A, B, C) and 60-d (D, E, F)

hydrothermal reaction in butrered solution at pH 1.0 and 250 € with a solution/solid ratio of20 mllg (final pH : 1.4). The dashedcurves are the corresponding spectra of unreacted rhyolitic glass for comparison. The "Si spectra were collected at a magnetic field

strength of 8.45 T, the 27Al and '?3Na spectra at 11.7 T.

6 0pH

The 'zeSi resonance of the residual glass becomes sys-tematically narrower with increasing reaction time, from17.0 ppm for the base glass to 15.3 ppm after 60 d. Si-multaneously, the 2eSi peak maximum becomes morenegative (shielded), from -101.6 ppm for the base glassto - 104.9 ppm after 60 d (Figs. lB, lD). The positionof the "Al[4] peak maximum does not change, but thepeak becomes niurower, decreasing from 17.4 ppm forthe base glass to 13.4 ppm after 60 d (Figs. lB, lE). Thenarrowing occurs on both sides of the peak, suggestingthat it is caused at least in part by a decrease in the rangeof chemical shifts. A decrease in the average quadrupolecoupling constant may also contribute to this peak nar-rowing. The 2rNa peak maximum becomes signifrcantlyless negative with reaction, from - 18.4 ppm for the baseglass to -13.8 ppm after 28 d and -15.1 ppm after 60d. This change is not monotonic with increasing experi-ment time and is much less than the change from - 19.0ppm to -7.7 ppm observed for the NAS glass after 24 d.Unlike the reacted NAS glass, the 23Na peak breadth forthe rhyolitic glass does not become narrower with in-creasing reaction.

The IR spectra of the residual glass (not shown) are

similar to those of reacted NAS glass and contain a broadX-O-H (X : H or a cation) stretching band at about 3500cm ' and an H-O-H bending band at about 1650 cm '

(see, e.g., Newman et al., 1986). These results indicatethat molecular HrO or hydronium ion are important pro-ton-bearing species (e.g., Ernsberger, 197 7 ; Bartholomewet al., 1980).

In both the 'zeSi and 2'1J CP-MAS NMR spectra, theintensity ofthe peaks due to hydrated glass increases rel-ative to those due to kaolinite as the reaction proceeds(Fig. 2). There are also significant differences between the27Al and 2esi MAS and CP-MAS NMR peak maxima (Fig.2). The "Si CP-MAS resonance is less shielded thanthe corresponding MAS resonance, the difference increas-ing from l.l ppm at 28 d to 1.8 ppm at 60 d. The Al[4]'??Al CP-MAS resonance due to hydrated glass is about2.4 ppm less shielded than that of the corresponding MASresonance after 60 d.

The compositional changes in the glass due to reactionare also similar to those in the NAS glass. With increasingreaction time at pH I the Si/Al ratio remains constantwithin analytical uncertainty, but the K content increasesand Ca, Mg, and Na contents increase (Table 2), although

t0t2 YANG AND KIRKPATRICK: HYDROTHERMAL REACTION OF GLASS

RHYOLITIC GLASS

"Ar - 'H CPIMASS

Al (61

100 0

PPM (FROM 1M A lC l3 l

' ns i - tH cPIMASS

l l- 50

I-150-100

PPM (FROM TMS]

CORRESPONDING MASS SPECTRA

SB: SPINNING SIDEBANDFig.2. 27Al-'H (A, C) (800 ps contact time, ll.7 T) and "$i-t|{ (8, D) (20 ms contact time, 8.45 T) CP-MAS NMR spectra

of the rhyolitic glass after 28-d (A, B) and 60-d (C, D) hydrothermal reaction in buffered solution at pH 1.0 and 250 "C with asolution/solid ratio of 20 mUg (final pH : 1.4). The dashed curves are the corresponding MAS NMR spectra of the same reactedsamples for comparison.

YANG AND KIRKPATRICK: HYDROTHERMAL REACTION OF GLASS 1 0 1 3

TABLE 2. Comparison of chemical analyses of synthetic rhyoliticglass before and after hydrothermal reaction at 250 "Cand for different pH values and experiment times

Un-component reacted pH 1.0-1.4- pH 1.0-1.4' pH 9-1-9'8-

(vt"/.) glass 28d 60 d 60 d

NarO 3.43K,O 3.55MgO 2.68CaO 3.37Al,o3 14.63sio, 72.44

TOTAL 100.10HrO-" 0.0Si/Al (atomic) 4.2O

- Range given is total range for all electron microprobe analyses'*- Difierence between 100.10% and analyzed total.

NAS glass, which reacted substantially at high pH afteronly 7 d. For the rhyolite glass, powder XRD of non-oriented samples shows a very broad peak with a maxi-mum at about24'2d, corresponding to the residual glass,and a broad peak with a m€rximum at about 650 20,which corresponds to a typical d(001) of smectite.

Powder XRD patterns of separated and oriented sam-ples of this layer silicate contain low-angle peaks withd(001) values of l5.ll A (60-day sample) and 14.42 A(120-day sample). These d(001) spacings expand to 17.7A and 17.0 A, respectively, after treatment with ethyleneglycol. This clay is, thus, an expandable smectite.

The presence of smectite is confirmed by the NMRdata (Figs. 4 and 5) and SEM observation (Fig. 3B). It isrepresented by the "Si NMR peak at -94.5 ppm (Figs.4A,4D Weiss et al., 1987) and the '??Al NMR peak forAl[6] at about 4 ppm (Figs. 4B, 4E). The 2'Al signal forthe clay, including a shoulder for Al[4] at about *68.2ppm (Kinsey et al., 1985), is preferentially enhanced inthe'??Al CP-MAS NMR spectra due to its large OH con-tent (Figs. 5A, 5C). The 'z?Al shoulder at 68.2 ppm in-dicates some Al for Si substitution in the tetrahedral sheetof the smectite (Kinsey et al., 1985). The 'zeSi and 27Al

peak maxima for this clay are more shielded than thoseof the smectite formed by reaction of the NAS glass withbuffered solutions with pH 3.0-5.5. This difference isprobably due to a significant amount of Mg[6] in the clayformed from the rhyolitic gilass (Weiss et al., 1987). Thus,this clay is probably a trioctahedral smectite, such as sap-onite, containing some ,{l[6]. Both XRD and NMR in-dicate that the fraction of the smectite increases with in-creasing experiment time. There is no NMR, XRD, orSEM evidence for the presence of zeolites in these sam-ples, as was observed for the NAS glass reacted at pH9 . 1 .

The NMR spectra and electron microprobe data of thepH 9.1 samples (Figs. 4 and 5) also indicate structuraland compositional changes in the residual glass due tohydrothermal reaction. The "Si peak maximum be-comes slightly more negative, changing from - 101.6 ppmfor unreacted glass to - I 03.4 ppm after 60 d and - I 03.8

3.18J.74 0.13J.08 1.24J.653.39J.93 1.94-6.24 2.594.142.06-2.65 1.63-2.89 1.78-2.462.46-3.28 1.86J.52 1.95J.10

12.59-14.61 11.06-14.29 11.10-14.4370.38-71.82 65.75-71.89 67.84-71.1695.88-98.16 90.01-95.80 88.83-96.96

1.94.2 4.3-10.1 3.1-11.34.114.U 3.90-4.81 4.1rt-4.86

Fig. 3. (A) SEM micrograph of rhyolitic glass after 28-d hy-drothermal reaction in butrered solution at pH 1.0 and 250 .Cwith a solution/solid ratio of 20 mllg (final pH : 1.4). (B) SEMmicrograph of the same glass after 60-d hydrothermal reactionin buffered solution at pH 9.1 and 250 lC with solution/solidratio of 20 ml./g (final pH : 9.8).

more slowly than the Na content of the NAS glass de-creases. The increased K content is probably due to Kuptake from the KCI-HCI buffered solution. The maxi-mum proton content of the reacted glass, as estimated bylow totals in the microprobe analyses (Jezek and Noble,1978) is about 10-11 wto/o as HrO, comparable to themaximum content for the reacted NAS glass. The distri-bution of the protons within the glass is difficult to de-termine because of the small grain size.

The reaction between rhyolitic glass and pH 3.0 buf-fered solution was extremely slow, and the spectral changesare similar to those at pH 1.0 (data not shown). Thisresult is consistent with the results for the NAS glass.

Hydrothermal reaction at pH 9.1

The reaction ofthe rhyolitic glass at pH 9.1 occurredslowly also, and even after 120 d there were many resid-ual glass fragments readily observed by SEM (e.g., Fig.3B). This observation contrasts with the results of the

1014 YANG AND KIRKPATRICK: HYDROTHERMAL REACTION OF GLASS

R H Y O L I T I C G L A S S

t t s i M A S S "A r MASS , , N a M A S S

60 DAYSp H 9 . 1 -

1 2 0 D A Y Sp H 9 . 1 - 9 . 4

t ' , r l r r r r l

-50 - 100

P P M ( F R O M T M S ) P P M ( F R O M 1 M A t C t 3 l

Fig. 4. 4Si (A, D), 'Al (B, E), and,3Na (C, F) MAS NMR spectra of rhyolitic glass after 60-d (A, B, C) and 120-d (D, E, Dhydrothermal reactions in a buffered solution with initial pH of 9.1 at 250 'C with a solution/solid ratio of 20 rnUg (final pH :9.8). The dashed curves are the corresponding NMR spectra ofunreacted rhyolitic glass for comparison. Spectral condition as inFigure 1.

- 1 5 0 1 0 0 0

ppm after 120 d. This change is slightly smaller than atlow pH for the same run time. The 2esi, 2tAl, and 23NaNMR peaks for the residual rhyolitic glass at pH 9.1 be-come systematically narrower with increasing experimenttime. The maximum HrO content at pH 9.1 is about l0wto/o (Table 2), comparable to the pH I results. The IRspectra ofthe experimental products (not shown) are sim-ilar to those of the pH I samples and contain X-O-H (X: H or a cation) stretching and H-O-H bending bands.

The "Al and 2esi CP-MAS NMR peak positions forthe residual glass after 120 d, at pH 9. l, however, are onlyabout 0.3 ppm less shielded than the corresponding MASpeaks, a significantly smaller difference than at pH l. The

"Si CP-MAS peak maximum for the 120-d experimentbecomes more shielded with increasing Hartmann-Hahncontact time (Hartmann and Hahn, 1962; Fig. 6). TheHartmann-Hahn contact time is the time during whichthe spin systems of 'H and the second nuclide (e.g., ,?Al

or'zeSi) are simultaneously excited and can exchange nu-clear spin (see Yannoni, 1982, for an introduction to theCP-MAS method).

The 23Na peak maximum becomes significantly lessnegative, from - 18.4 ppm for the base glass to -9.1 ppm

0 -80

P P M ( F R O M 1 M N a C I )

after 60 d and to -8.4 ppm after 120 d. The peak widthalso decreases from 18.6 ppm for unreacted glass to 14.8ppm after 60 d and 15.2 ppm after 120 d. These changesare significantly larger than for the experiments with pH1.0- 1.4.

As at pH 1.0, the Si/Al ratio of the glass remains con-stant and the same as that of the base glass. kss Na-,Ca-, and MgJeaching and K-uptake occur under alkalineconditions than under acid conditions after similar ex-periment times (Table 2).

DrscussroN

Proton speciation

The results described here demonstrate that the struc-tural and compositional changes in the rhyolitic glass dueto proton uptake are similar to those in the NAS glass.The NMR, XDR, and SEM data (Fie. 3) show that hy-drated glass is the major reaction product at all pH val-ues. The size, morphology, and etched or ripplelike fea-tures on the surfaces of the reacted fragments indicatethat they are residual glass rather than a secondary amor-phous phase formed by dissolution and precipitation. Both

A r ( 4 1

RHYOLITIC GLASS

' h t - 'H CP/MASS

At t6 l 6O DAYSpH 9 .1 - 9 .8

A A t (4 )

Q O

At i4 )

Q3

1 2O DAYSpH 9 .1 - 9 .4

PPM (FROM 1 M AICIS ]

l ' l . ' l

r r r r l

-50 -100 -150

PPM (FROM TMS)

___ CORRESPONDING MASS SPECTRA

SB: SPINNING SIDEBAND

Fig. 5. ,7Al-rH (A, C) (800 ps contact time, I1.7 T) and ,,Si-rH (8, D) (20 ms contact time for B and 2 ms contact time forD, 8.45 T) CP-MAS NMR spectra of rhyolitic glass after 60-d (A, B) and 120-d (C, D) hydrothermal reaction in a buffered solutioninitially at pH 9.1 and at 250 "C q/ith a solution/solid ratio of20 mllg (final pH : 9.8). The dashed curves are the correspondingMAS NMR spectra of the same reacted samples for comparison.

'ns i - 'H cpl r r lnss

1 0 1 6

R H Y O L I T I C G L A S S

1 2 0 D A Y S o H : 9 - 1 - 9 4

1 4 1 8 2 2 2 6 3 0

Fig. 6 chemica, ,"*i;.#:#rffi:' .P-MASNMR spectra (8.45 T) of hydrated rhyolitic glass after 120-dhydrothermal reaction in pH 9.1 solution vs. the Hartmann-Hahn contact time (Hartmann and Hahn. 1962) used in theCP-MAS experiment. The increased shielding with increasingcontact time is consistent with the presence of Si-O-H linkages,because these linkages have shorter Si-H distances and moredeshielded chemical shifts than Q4Si atoms in tetrahedra withHrO molecules. The short contact time thus preferentially en-hances the Si sites having shorter Si-H distances.

27Al and "Si in this residual glass readily cross-polarize,consistent with the electron microprobe results that in-dicate proton contents of up to about l0 wto6 as HrO.

The relatively shielded '7 AlI4l and 2eSi MAS NMRchemical shifts of the residual glass (Figs. I and 4) indi-cate that it remains highly polymerized after hydrationand that not many protons are present as OH- groups.The large H-O-H bending band at 1650 cm ' in the IRspectra (not shown) also indicates the presence of molec-ular HrO or hydronium ion. Using the microprobe anal-yses of Table 2 and the methods of Yang and Kirkpatrick(1989), the maximum number of OH groups per tetra-hedron required for charge balance is zero within analyt-ical uncertainty at both pH I and 9. Clearly, the domi-nant proton-bearing species in the hydrated glass at thesepH values is molecular HrO, as it is for the NAS glass.

The increasing deshielding at ,eSi and rTAl in the CP-MAS NMR spectra relative to the corresponding MASNMR spectra with increasing experiment time, especiallyat pH l, however, suggests that a small number of OHgroups on Si tetrahedra are present and that their con-centration increases with increasing experiment time. Sitetrahedra with one attached OH- group (QLOH sites)typically resonate at chemical shifts about l0 ppm lessshielded than those with no OH groups (e.g., Brinker etal., 1988; Farnan et al., 1987). This signal from the Qr-OH sites is prefcrentially enhanced under CP-MAS con-ditions, causing the observed decreased shielding.

The increased shielding of the peak maxima in the ,eSi

YANG AND KIRKPATRICK: HYDROTHERMAL REACTION OF GLASS

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Eu - 1 0 4

I - , ' 0 .ur@

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CP-MAS spectra with increasing Hartmann-Hahn con-tact time (Fig. 6) is also consistent with the presence ofsome OH goups attached to Si tetrahedra. Farnan et al.(1987) have shown that for hydrous SiO, glasses, the rel-ative intensity of the QLOH sites decreases with increas-ing contact time, because longer contact times allow Sinuclei farther from the protons to be polarized. For ourglasses, separate '?eSi peaks cannot be resolved becauseofSi/Al and bond-angle disorder, and the effect ofsucha relative decrease in intensity for Si on Qr-Q11 sites wouldbe an increase in the average shielding, as observed. Thenumber of OH groups necessary to cause the chemicalshift differences between the MAS and CP-MAS spectracould be quite low, because OH- groups could be muchmore efficient at cross-polaizing Si than HrO molecules,especially if the HrO molecules are in rapid motion.

Natural hydrated rhyolitic glasses (perlite, pitchstone,etc.) typically contain 2-10 wto/o meteoritic water (Rossand Smith, 1955; Friedman and Smith, 1958; Jezek andNoble, 1978 and references therein). Obsidian can evenabsorb HrO from the atmosphere to form a hydrated lay-er (Friedman and Long, 1976). The results of this studysuggest that the protons in natural rhyolitic glass areprobably present mostly as molecular HrO. HrO-bearingalbitic, rhyolitic, and basaltic glass€s quenched from abovethe liquidus contain coexisting molecular HrO and OHgroups with the relative amount of HrO molecules in-creasing with increasing HrO content (Stolper, 1982).Farnan et al. (1987) found a similar result for SiO, glass-es.

It is also possible that the residual glass is heteroge-neous, with volumes with lower Si/Si + Al ratios (lessshielded'?eSi chemical shifts) giving rise to more intensesignals in the CP spectra. We see no evidence to supportthis possibility.

Structural variations due to hydration

The changes in the 23Na, 27,{1, and 'zeSi NMR peakmaxima and widths (Figs. I and 4) indicate considerablestructural change in the rhyolitic glass after reaction. Asfor the NAS glass, these variations appear to be relatedto leaching of large cations from the glass (in this caseNa*, Mg2*, Ca'?*) and uptake of proton-containing speciesand to a lesser extent K* from the buffered solution. TheSi/Al ratio remains essentially constant (Table 2).

The '?eSi peak maxima of the residual glasses becomemore shielded with increasing experiment time. Becausethe Si/Al ratio remains constant, this increased shieldingat'zeSi is most likely due to an increased average Si-O-T(T : Si,AD bond angle per tetrahedron (Radeglia andEngelhardt, 1985 and references therein). Based on thecorrelation of Smith (1984), the average Si-O-T bond an-gle per tetrahedron increases from about 145" for the un-reacted glass to about 148" for the most shielded hydratedsample. These bond-angle changes are probably causedby a decrease in the cation-induced polarization of theSi-O bonds due to a hydration shell of molecular H,O

around the large cations and to cationJeaching (Yang andKirkpatrick, 1989).

Another significant change in the NMR spectra is thatthe 'zeSi and 2'Al peak widths of the residual glass be-come narrower with increasing hydration (Figs. I and 4).This decrease in peak width indicates that as the Si-O-Tbond angles increase owing to hydration, the range ofaverage Si-O-T bond angle per tetrahedron decreases. Thedecreased shielding and possibly more isotropic environ-ment at 'z3Na with increasing HrO content are probablydue to the formation of a shell of HrO molecules aroundthe Na cations. The average number of HrO moleculesper large cation estimated from microprobe analyses andthe method of Yang and Kirkpatrick (1989) are 0.54(+-0.2) after 28 d and 2.53 (+-0.2) after 60 d at pH1.0-1.4, and2.16 (t -0.3) after 60 d at pH 9.1-9.8. Thishydration reduces the interaction between the Na-cationsand the O of the aluminosilicate framework. However,at pH I the 23Na peak width does not narrow monoton-ically with increasing HrO content or as much as for theNAS glass. These differences are probably due to the pres-ence of some unhydrated Na atoms, possibly because ofcompetition for the HrO molecules by other larger cat-ions, such as Ca2* and Mg2*, which have larger hydrationenergies. This reason and the smaller average number ofHrO molecules per large cation probably account for thesmaller changes in the 'z3Na NMR parameters of therhyolitic glass relative to the NAS glass.

The 23Na peaks for the hydrated rhyolite glass at pH9.1-9.8 after 60 d are narrower and more shifted com-pared to those of the base glass than those for the glassat pH I .0- 1.4 after 60 d, despite a similar average num-ber of HrO molecules per large cation. This difference isprobably due to the smaller amount of cation-exchangeunder alkaline conditions (Table 2). At pH 1.0-1.4 mostof the Na-atoms accessible to molecular HrO are likelyto be leached and, thus, a larger fraction of the Na atomsare unhydrated or have incomplete hydration shells. AtpH 9.1, less Na is leached (Table 2), and it remains inthe glass to be hydrated. Thus, the 23Na NMR spectraof the pH I .0 glasses are more like those of the base glassthan those of the glasses at pH 9.1. This slower leachingat higher pH is similar to the results for many borosilicatenuclear waste glasses (see Lutze, 1988, for a review).

Although the changes in peak position and width in the23Na, "Al, and 'eSi NMR peaks of the rhyolitic glassdue to hydration are similar to those for the NAS glass,the extent of the variation for the rhyolitic glass is muchsmaller. Cation exchange and incorporation of HrO mol-ecules over tens of micrometers without disruption of thealuminosilicate network of the glass must occur by solid-state diffusion. Thus, the slower reaction rate for therhyolitic glass is due to lower exchange diftrsion coeffi-cients. This slower diffi.rsion is probably caused by a high-er Si/Al ratio and the presence of divalent cations, whichhave smaller diffirsion coefficients in framework alumi-nosilicate glasses (Winchell, 1969).

1017

The mechanism of glass dissolution

The SEM evidence for residual glass fragments and dis-solution features on the glass surface, the microprobe ev-idence of substantial cation exchange at apparently con-stant Si/Al, and the NMR evidence of internal structuralchanges in the glass indicate that the dissolution of therhyolite glass is incongruent and involves diffusion-con-trolled cation exchange in the interior of the glass anddisruption of the aluminosilicate network at the glass-

water interface. The rate ofcation exchange is greater atpH 1.0 than at pH 9.1 (Table 2). This difference may berelated to the ratio of the activities of alkali cations toprotons in solution, with the low ratio of low pH causingmore rapid cation exchange. White and Claassen (1980)have shown that mass-transfer rate constants for Nat,Mg2*, and Ca2* during dissolution of a rhyolitic glass de-crease with increasing pH, consistent with this idea.

The XRD, SEM, and NMR data indicate that the rateof formation of secondary phases at pH 9. I is more rapidthan at pH 1.0. This higher rate reflects more rapid dis-ruption of the aluminosilicate framework at pH 9.1. Thehigh OH- concentration in alkaline solutions acceleratesthe aqueous attack on the aluminosilicate network nearor on the glass surface by nucleophilic attack ofSi tetra-hedra (Iler, 1976; Bunker et al., 1988).

The dissolution mechanism of the rhyolitic glass thusconsists of the following simultaneous steps: incorpora-tion of molecular HrO into the glass, exchange of protons

and large cations between the glass and fluid, a smallamount of depolymerization of the aluminosilicateframework in the interior of the glass, and disruption ofthe aluminosilicate framework at the glass-H'O interface'Cation exchange is relatively less important at high pHvalues.

Secondary phases

The secondary crystalline phases formed from therhyolitic glass are kaolinite at pH 1.0-1.4 and smectite,probably saponite, at pH 9.1-9.8. As shown in the SEMmicrographs (Fig. 3), these clay phases precipitate on thesurface ofthe residual glass and thus the reaction occursby dissolution and precipitation. The lack of secondaryzeolites at high pH is probably due to the presence of Mgleached from the glass. Hawkins and Roy (1963) haveshown that smectite rather than a zeolite is the chief re-action product formed from Mg-bearing solutions. Thus,we expect that the products of natural rhyolite-HrO re-action should depend on the MgO content of the initialglass and the relative efrciency with which Mg is re-moved from the local system, as well as the local physical

and chemical conditions.

Coxcr,usroxsThe dissolution ofa synthetic rhyolitic glass at 250'C

under autoclave conditions and initial buffered pH valuesof 1.0, 3.5, and 9.1 occurs incongruently, with incorpo-

YANG AND K]RKPATRICK: HYDROTHERMAL REACTION OF GLASS

1 0 1 8

ration of molecular HrO, a small amount of depolymer-ization of the aluminosilicate framework probably due toOH groups, cation exchange throughout the glass frag-ments, and dissolution of the aluminosilicate frameworkat the glass-HrO interface. These results are similar tothose for the sodium aluminosilicate glass examined byYang and Kirkpatrick (1989). The proton-bearing spe-cies, almost entirely molecular HrO, are eventually in-corporated throughout the glass and change its structure.The structural changes caused by this hydration includelarger average Si-O-T (T : Si,AD bond angles per Si-tetrahedron and narrower ranges of ,eSi, 27A1, and 23Na

chemical environments. At both pH 1.0 and 9. l, the three-dimensional framework of the glass is not substantiallydepolymerized and its Si/Al ratio remains the same. Theless shielded 23Na environments in the hydrated glassesare likely caused by the formation of a shell of HrO mol-ecules around the Na atoms in the residual glass, reducingtheir bonding interaction to the bridging O. The Na at-oms in the residual glass at pH 9.1-9.8 are more hydratedthan those at pH I .0- 1.4 after the same experiment time,probably owing to more rapid leaching of hydrated Naunder acid conditions.

The rates of structural change and dissolution of therhyolitic glass due to aqueous attack are slower than forthe sodium aluminosilicate glass. This difference is prob-ably related to the presence of divalent cations in therhyolitic glass and its higher Si/Al ratio. Divalent cationshave smaller diffusion coefficients than monovalent cat-ions and also apparently cause reduction in the mass-transfer rate of Na* and K* (Winchell, 1969; White andClaassen,1980).

The secondary phases precipitated from solution arekaolinite at pH I .0- I .4 and an expandable 2: I layer phyl-losilicate, probably saponite, at pH 9. l-9.8. Unlike theresults for the sodium aluminosilicate glass, no zeolitesform at pH 9.1-9.8, probably because ofthe presence ofMg in the system.

AcxNowr,nocMENTs

This research was supported by NSF grants EAR 84-08427 and EAR87 -06729. We thank Ian M. Steele and Cameron Begg for assistance withthe olectron rnicroprobe analyses, and Brian Phillips and Charles A. Weiss,Jr. for much useful discussion. Reviews by Julia A. Peck and a secondreviewer substantially improved an eadier version.

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