A multinuclear magnetic resonance study of the structure of hydrous albite glasses

Geochimica et Carmorhimica Ada Copyright 8 1989 Fkrgamon Ras

Vol.53. pp. 2925-2935 plc.F’rintcd in U.S.A.

0016-7037/89/$3.00+ .oO

A multinuclear magnetic resonance study of the structure of hydrous albite glasses S. C. KOHN, ' R. DUPREE,* and M. E. SMITH* ‘Department of Geology, University of Manchester, Manchester, M I3 9PL. U.K. *Department of Physics, University of Warwick, Coventry. CV4 7AL, U.K. (Received March 28, 1989; accepted in revisedform September 6, 1989)

Abstract-The structures of a series of hydrous albite glasses quenched from melts at high pressures and temperatures have been studied using 2gSi, 23Na, 27A1,and ‘H nuclear magnetic resonance. Changes in the isotropic chemical shift, the chemical shift dispersion, and the mean nuclear quadrupole coupling constant for 23Na as a function of dissolved water concentration were deduced from spectra obtained at two different magnetic fields. Major changes in the sodium environment occur, but the spectra for *‘Si and 27Al, and hence their structural environments, are similar throughout the range of water concentrations studied (O-67 mol%). No previous model is consistent with the results of this study. The data suggest the existence of the following structural features: i) exchange of H + for Na+ as a charge-balancing cation; ii) formation of Na( OH) complexes; iii) incorporation of molecular water; iv) no octahedrally coordinated aluminium; v) no Al-OH or Si-OH. These features can be summarised in terms of the equilibrium NaAlS&Og + Hz0 = HAlSi308 + Na( OH). In contrast to all previous interpretations, we see no evidence for depolymerisation of the aluminosilicate framework, although an increase in the symmetry of the aluminium environments and decrease in the chemical shift dispersion of the sodium environments suggests a more ‘ordered’ structure than in the dry glass. If the structures of hydrous albite melts are the same as those of the glasses studied here the current understanding of the effect of dissolved water on the physical properties of felsic melts must be reassessed. INTRODUCTION

Early studies of volatile dissolution in silicate melts were limited to studies of solubility ( HAMILTONet al., 1964) and the effects on phase relations (e.g., TUTTLE and BOWEN, 1958). BURNHAM( 1974, 1975, 1979) formulated a general model for the mechanism of dissolution of water based largely on solubility data for albite. In Bumham’s model water dissolves in albite melt according to the following reactions.

IN STUDIESOF VOLATILEdissolution

in melts albite (NaAISiJOs) has been the most frequently used composition. It is a major component of felsic rocks so it can reasonably be treated as a simple model for such systems, and it has even been used as a model for all natural magmas (BURNHAM, 1974, 1975). Furthermore, it melts at experimentally accessible temperatures ( 1118°C under anhydrous conditions at one bar pressure), and its melts can be quenched to a clear, bubble-free glass even with several weight percent of water in solution. Despite having been the subject of many studies, the water solubility mechanisms are still poorly understood. High-resolution solid state nuclear magnetic resonance (NMR) has already made valuable contributions to the study of local order in anhydrous glasses (KIRKPATRICKet al., 1986; DUPREE and HOLLAND, 1988). In this paper we have used NMR to study glasses quenched from hydrous melts to provide a new insight into the long-standing problem of water dissolution mechanisms. X-ray diffraction (TAYLOR and BROWN, 1979) has been used to show that anhydrous albite glass has a framework structure based on linked [ SiOd] and [ AlO tetrahedra. According to a model based on these results the tetrahedra are joined to form six membered rings such as those found in tridymite, with Na+ cations in interstices between the rings. This “stuffed tridymite” model contrasts with the structure of crystalline albite which contains four membered rings of tetrahedra. A wealth of evidence exists to show that all the aluminium is tetrahedrally coordinated as part of the glass network (e.g., SEIFERT et al., 1982; MCI&OWN et al., 1985; OESTRIKEet al., 1987).

i) Below 50 mol% NaAISi,OB + H20 + HAISi,O,( OH)(ONa);

( 1)

i.e., tetrahedrally coordinated Al is now charge balanced by H+ instead of Na+. One Si-0-Si bond has been ruptured forming one Si-OH and one Si-O- locally charge balanced by Na+. Using NMR terminology one QJ-OH silicon and one Q3-O- silicon have been generated. ii) Above 50 mol% HA1Si307( OH)( ONa) + Hz0 HAlSi,O,(OH),(ONa),

etc.;

(2)

i.e., all further dissolved water breaks Si-0-Si bonds to form additional Qt4_x)-( OH), units. The relationship between molar water solubility in albite melt and water fugacity led OXTOBY and HAMILTON ( 1978) to suggest that water dissolves by one mechanism below 50 mol% and by a different mechanism above. They further suggested that one mole of water is associated with one mole of NazO or six moles of SiOz. On the basis of the effect of water on phase relations in the granite system, it has been suggested (MANNING et al., 1980: PICHAVANT,1987) that dissolution of water in feldspar melts results in a change of aluminium coordination from 4-fold in the dry melt to 6-fold in the hydrous melt. Changes 2925

2926

S. C. Kohn, R. Dupree, and M. E. Smith

in the physical properties of melts can also provide evidence for changes in structure accompanying water dissolution. For albite, as for other melts with framework structures, viscosity is dramatically reduced by the introduction of water (DINGWELL and MYSEN, 1985; DINGWELL, 1987). For melts of related compositions, it has been shown that water dissolution increases electrical conductivity ( LEBEDEVand KHITAROV, 1964) and cation diffusion rates ( JAMBONet al., 1978). All these observations have been interpreted in terms of reduced polymerisation in the melts. In the last ten years spectroscopic measurements on hydrous glasses have been widely used to deduce structural features of hydrous melts, the assumption being made that the structures of hydrous glasses are similar to the melts from which they are quenched. The available evidence ( AINES et al., 1983; STOLPERet al., 1983; MYSEN and VIRGO, 1986a) suggests that this is a reasonable assumption and one which will also be made here. MYSEN et al. ( 1980) used Raman spectroscopy to study hydrous albite glasses. They proposed a complex reaction mechanism involving the following species: i) S&OH units (Q:,-OH) attached to the framework and produced by the rupturing of Si-0-Si linkages; ii) chain units without attached -OH but containing Si and Al; these would therefore contain Qz -( 0-h silicons; iii) residual framework units unaffected by water dissolution, i.e., Q4 silicons; iv) OH- anions locally charge balanced by Na+. They also stated that no molecular water was present and that A13+ ions are expelled from tetrahedral coordination, changing their structural role from a network former to a network modifier. However, these results were controversial. FREUND ( 1982) disputed some of their band assignments and suggested that their results did not rule out the possible existence of Hz or Al-OH units. MCMILLAN et al. ( 1983) repeated the Raman measurements on another hydrous albite glass and observed a weak peak corresponding to molecular water. STOLPER ( 1982a,b) changed many of the accepted ideas on water dissolution mechanisms in this system by his careful infrared measurements and interpretations. His results strongly suggested the existence of both B-OH and molecular water. Most recent discussions of water solubility mechanisms have concentrated on the relative concentrations of the two species. MYSENand VIRGO ( 1986b) published a further study of hydrous aluminosilicates, including albite. In contrast to their earlier work they observed the presence of molecular water and further suggested that for hydrous albite glass Si-OH and Al [ 41 -OH bonds are absent and that hydroxyl is associated with network modifying Al 3+or Na+. They again observed vibrations which they interpreted as due to the presence of anhydrous depolymerised silicate units containing Qz( -0-h silicons. Aspects of the structure of hydrous glasses have also been inferred from calorimetric studies. CLEMENSand NAVROTSKY ( 1987) measured heats of mixing in the system NaAlSi30s-Hz0 and observed a minimum at a mole fraction of Hz0 of around 0.25. This was interpreted ( NAVROTSKY, 1987 ) in terms of a solubility mechanism involving both molecular water and hydroxyl species. It was suggested that the minimum arose from competition between an endothermic heat of dissolution of molecular water and an exothermic heat of dissociation. OKUNO et al. ( 1987) have performed

X-ray radial distribution analysis of hydrous albite glass. They found that the hydrous glass has a similar structure to the dry glass in that it still consists of TO4 tetrahedra and that all the aluminium is in four-fold coordination. Their radial distribution functions show that hydrous albite glass has a more “ordered” structure than the dry glass and that the change between dry and hydrous glasses is greater for albite than for the other compositions studied (Abj3w7 and An,,Qzs5). They proposed that the change is due to modifications of the linkages between tetrahedra and suggested that clustering takes place. No previous multinuclear NMR study of glasses quenched from melts in the system NaAlSisOs-Hz0 exists. YANG and KUUCPATRICK ( 1989) have obtained NMR spectra of hydrous glasses in their study of the dissolution of albite glass in aqueous solutions. However, their results are not applicable to hydrous albite melts since their reaction products are quenched from well below the liquidus and have compositions which are considerably different from their starting material in that they contain potassium and are generally deficient in total alkalis. EXPERIMENTAL

METHODS

The dry albite glass is the same as that used by OXTOBYand HAMILTON( 1978) and was prepared by melting at - 15OO’Cin a large volume furnace. This glass (in the form of a crushed powder) was used as a starting material for all the hydrous glasses except samples AWB and AWA. These were synthesized from a gel of albite composition and also contain 0.14 wt% MnO which was added to reduce 29Sirelaxation times. The synthesis conditions and water concentrations of all samples are listed in Table 1. The water concentrations quoted for the watersaturated samples (AB7, AB4B, AB4A, ABSD, and ABSG) are based on previous measurements of water solubility ( OXTOBYand HAMILTON, 1978; BLAMART et al., 1986,unpubl. ahstr.) and were checked by comparing the amount of water added to the capsule with the amount remaining in a fluid phase after the run; for the water-undersaturated samples (AB6, ABSB, ABSF), the concentration of water quoted is simply derived from the amount of water added to the capsule. Measurements of weight losses at 1000°C were performed on fragments of each of the glasses as a further check, the water concentrations are believed to be correct to within 10% of the values quoted. All NMR experiments were performed on clear, bubble-free glasses with the exception of ABSG which was a slightly translucent glass. When glasses ABSD and ABSF were recovered from their plat-

Water concentrations and synthesis condftions of all Table 1. samples studied

mater

concentrations

SamDle

(wt X)

(mol I)

Apparatus

P/kb

T/Y

-1500

Run Duration hrs

0

0

LVF

0.001

AM

0.85

IO

IHPV

0.5

1270

4

A67

2.6

29

IHPV

0.5

1270

4

ABOB

4.3

40

IHPV

1.0

1300

7

AB4A

6.3

50

IHPV

2.0

1300

6

A656

8.2

57

PC

10.0

1100

3

AB5D

9.5

60

IHPV

5.0

900

3

ABSG

9.5

60

IHPV

5.0

900

3

67

PC

15.0

1100

3

ABS

AB5F

12.2

LVF IHPV

= =

PC

=

Large-volunh?one-atnosphere furnace Internally heated pressure vessel with argon as pressure medium Solid media piston cylinder apparatus

2921

NMR study of hydrous albite glass inum capsules, they were found to be mainly clear and bubble-free but with part of each sample being translucent. These translucent parts were removed and NMR experiments were only performed on the clear parts of these samples. Sample ABSB was examined using an SEM and found to be free of bubbles larger than - 100 nm (the instrumental resolution). The slightly translucent glass, ABSG, contained a few bubbles ranging in size from 300 to 10000 nm. None of the samples studied possess water~on~ning fluid inclusions as the static ‘H NMR spectra (see later) do not contain a narrow component. Infrared spectra between 500 and 5000 cm-’ were recorded for ABSB and are very similar to those reported by STOLPER (1982a) for an albite glass containing 6.85 wt% HzO. NMR experiments were performed using a Bruker MSL-360 spectrometer (BO = 8.45 T) operating at 71.535 MHz (z9Si), 93.83 MHz (“Al). 95.22 MHz (“Na), and 360.13 MHz (‘H). Some ad~tional 23Na and “Al spectra were collected using a Bruker CXP-200 {I& = 4.7 T) spectrometer operating at 52.90 MHz and 52. I 1 MHz, respectively. All one-pulse spectra were acquired using short (
broadening to improve the signal-to-noise ratio. Some 23Naand *‘Al spectra were accumulated using high-power proton decoupling. ‘H‘?Sicross-polansation (CP) experiments were carried out with contact times between 0.5 and 20 ms, and cross-polarisation experiments to z3Na and *‘Al were also attempted. Some ‘H spectra were obtained under static conditions using a probe with a negligible proton background signal and a dead-time of 1 ~1s. This enabled ‘H spectra to

0

-2M

Fe.3

100

0

-1m PRI

-Em

FIG. I. “Si MASNMR spectra of afbite glasses. (a) and (b) are single-pulse spectra obtained using a pulselength of 2 ps (=x/6) and a recycle delay of 75 s, (a) AB5, dry and (b) ABSD, hydrous. (c)

and (d) are cross-polarisation spectra obtained with contact times of 500 ps, 50 ms of proton decoupling following the pulse and a recycle deiay of 5 s, (c) AB6, 10 molW H20 and (d) ABSB, 57 mol% HzO. Spinning speed for all spectra was -3.5 kHz, and the number of scans was - 1000 (a and b). -12,~(c),and-15~(d).

lx obtained by single-pulsemethods rather than a solid-echotechnique (e.g., ECKERT et al., 1988). Delays of I-60 s were employed. RESULTS 29Si The single pulse mSi spectrum for dry albite glass is shown in Fig. 1a. The width of the 29Si resonance in dry albite glass is greater than that in SiOz glass ( PETTIFER et al., 1988) due to the presence of a range of Q4( n Al) next-nearest neighbour environments. We observe an approximately Gaussian peak with a maximum at -97.8 ppm and a width (FWHM) of 16.2 ppm. This position is similar to previous measurements of -98.7 ppm (OESTRIKE et al., 1987) and -97.9 ppm (MURDOCH et al., 1985 ). Single pulse 29Sispectra of the hydrous albite glasses (Fig. 1b) are very similar to that of dry albite glass. In some spectra there appeared to be a slight increase in intensity at the deshielded side of the peak in the hydrous glasses, but this probably does not exceed the experimental error. Use of cross-polarisation MASNMR (PINES et al., 1973) was less informative than for hydrous silica glass ( FARNAN et al., 1987), though small changes in the appearance of the

pulses compared with 12057 pulses for the spectra shown in Fig. lc and d.). We have also performed single-pulse experiments under static (non-spinning) conditions for AB5 (dry) and ABSB ( 57 mol% H20). This technique is sensitive to the differences in chemical shift anisotropy between Qs and Q4 silicons which gives different peak widths and shapes for the two species ( STEBBINS,1987 1. However, the resonances for the dry and hydrous albite giasses (whose peaks are at the same isotropic chemical shift as the MAS re~nances) have the same shapes and widths (47 t 5 ppm ). again suggesting that the concentration of QI silicons is low. Finally, we have performed a static cross-polarisation experiment on the hydrous glass, ABSB. If any QJ-OH units were present, their larger chemical shift anisotropies would result in the observation of a broader resonance. in fact, the resonance (which is at -90 k 3 ppm) is narrower ( 33 + 5 ppm) than the single pulse spectrum and shows no signs of a shape characteristic of chemical shift anisotropy, providing further evidence for the absence of SiOH units.

spectra as a function of contact time are apparent. Spectra obtained using long contact times (5 and 20 ms) are almost

indistinguishable from single pulse spectra. At shorter contact times (1 ms and 500 ps) the peak maximum moves to - -92.5 ppm with the full width at half maximum (FWHM) remaining approximately constant at 16.5 ppm. The CP spectra for AB6 ( 10 mol% HzO) and ABSB (57 mol% HzO), shown in Fig. Ic and d for a contact time of 500 gs, are identical (the feature at -40 ppm in the spectrum for AB6 is an artefact), but acquisition of the spectrum for the high water concentration sample was much more rapid ( 1467

The quadrupolar nature of the sodium nucleus (nuclear spin, I = %) results in the linewidth and peak position being dependent on a number of factors. Measurements carried out at two magnetic field strengths (here 8.45 and 4.7 T) allow deconvolution of these effects. The 23Na spectra of three representative samples at 8.45 T are shown in Fig. 2a, b, and c and at 4.7 T in Fig. 2d, e, and f. At a magnetic field strength of 8.45 T the peak maximum for dry albite glass is at -2 1.5 ppm (Fig. 2a) similar

S. C. Kohn,

2928

I

L

,....l,...I,,..I....I

200

No

-zoo

-100

0

sy)

zoo

0

-mm

-4oQ

Pm

Ry

FIG. 2. 23NaMASNMR spectra of dry and hydrous albite glasses.(a, b, c) obtained at a magnetic field strength of 8.45 T and with MAS at 7-8 kHz. (d, e, f) obtained at a magnetic field strength of 4.7 T with MAS at -4 kHz. (a) and (d) AB5, dry. (b) and (e) AB7,29 mol% HzO. (c) and ff) ABSB, 57 mol% H20. A pulselength of 1~ (
to that of -20.7 ppm reported by GEISINGER et al. ( 1988) at the same field and -20.3 ppm reported by OESTRIKE et al. ( 1987) at 11.7 1. At both fields the peak maximum ( bpak) is found to he a function of dissolved water concentration (Fig. 3); however, the difference between the trends at the two fields is considerable. At 8.45 T, 13,~ becomes less shielded with increasing water concentration throughout the range of con~n~tio~ studied, whereas at 4.7 T, 6, first becomes less shielded then mom shielded with increasing water concentration. There are also differences in the trends in peak width (FWHM). At 8.45 T, FWHM generally decreases with increasing water concentration, whereas at 4.7

-.v

1

-15

-20 ” ‘peak

. _26 -

l

.

.

El

Z7AI

q

0

n

.

(pwd

.. .

m ‘

LI

-30 D

-35 -40’

1.1.1.1.1-1 20 0 10

30

T it remains approximately constant for the samptes with the lower water con~nt~tions but increases for the highest water concentrations (Table 2). ‘H-‘jNa CP experiments using NaB& and NazB407 - lOH20 (borax) to set the match conditions (HARRIS and NESBIT~, 1988) produced no crosspolarisation for the hydrous glasses. Experiments involving high-power proton decoupling have also been performed. No changes between sit&e pulse and decoupled spectra were observed either for static or MAS conditions, indicating that effective dipolar coupling to the sodium is weak. The 23Na resonance in dry sodium disilicate glass was also examined at both fields in order to provide a comparison with the albite glasses. The peak position changes from -2 ppm at 8.45 T to -5 ppm at 4.7 T, with the width changing from 3700 Hz to 1900 Hz.

40

50

.,.I 60

J 70

Mole % dissolved water FIG. 3. Position of the “Na peak maximum (8,) as a function of dissolved water concentration at magnetic field strengths of 8.45 T (filled symbols) and 4.7 T (open symbols).

The peak maximum of the “Al resonance in dry aibite giass is 50.5 ppm at a field of 8.45 T, This compares with a vatue of 54.8 ppm at 11.7 T (OESTR~WSet al., 1987). With increasing water concentration the *‘Al peak maximum shifts slightly, reaching 53 ppm for ABSG. A much more marked change is that the peak becomes narrower and more symmetric (Fig. 4) with the FWHM changing from 23 ppm for AB5 to 20.5 ppm for AB6 and 14 ppm for ABSG. No peak near 0 ppm corresponding to octahedrally coordinated ahrminium is present. The normahsed aluminium signal intensity from all the albite glasses remains constant and, compared to our a-Alz03 standard, 15-208 more signal was observed.

2929

NMR study of hydrous albite glass

sample

Simulated

Experimental

Linewidths (FWHll)/Hr

Linewidths U'W4)/Hr

ualdcal shift dispersion Jdi/ppm

Simulated contributions ar 8.45Tl"Z

Simulated contributions at 4.7TiHZ

4.71

8.45T

4.1I

8.45T

"9

"b

A85

2380

2730

2390

2730

,260

1127

700

2030

21.3

A86

2495

2930

2‘80

2920

1224

L250

680

2156

23.7

ABI

2410

2450

2400

2450

1500

900

830

,620

17.0

ABhB

2381

2360

2360

2360

1530

820

850

I~180

15.5

An58

2625

,790

2645

1790

2160

220

1200

400

ABSG

2770

1590

2770

1570

,920

0

1070

0

Hence all the 27Al nuclei are observed in these glasses (see DUPREE et al., 1988, for a discussion of the quantification techniques). High-power proton decoupling experiments were again performed with no detectable changes in the resonance, and *‘Al CP experiments, using the alum, NH* Al( S04h - 12H20, to set up the match conditions gave no signal. “Al MASNMR spectra for sample AB6 were run at both 8.45 T and 4.7 T. The peak position shifts from 5 1.5 ppm to 48.8 ppm while the FWHM increases slightly from 2070 Hz to 2180 Hz at the lower field.

"9

%

4.2 0

‘H

The single-pulse static spectra consist of two components: a broad one due to Hz0 and a narrower one due to OH. The existence of two components is more difficult to deduce from the MAS spectra, shown in Fig. 5 for sample ABSB. All MAS spectra consist of one broad resonance centered on 3.5-3.8 ppm with several pairs of spinning sidebands. Resolvable resonances such as those observed in hydrous silica glasses ( KOHN et al., 1989) are not observed, presumably because the OH and Hz0 have similar shifts and an increased residual width. As the total dissolved water concentration of the glass is increased, changes in the MAS spectra broadly in agreement with ECKERT et al. ( 1988) are observed, although the changes were easier to detect in the static spectra. There are several problems associated with quantifying the water speciation from ‘H NMR spectra in these glasses. Firstly, it was noted that the relaxation time for protons in the Si-OH group were very dependent on water concentration. At high total dissolved water concentrations no difference was observed in the spectrum as a result of varying the relaxation delay between Is and 60 s. However, for AB6 ( 10 mol% HzO) at least 60 s recycle delay was required to allow complete relaxation. Secondly, the residual width of the MAS lines increases with increasing water concentration (due to increased homogeneous coupling); quantification based on peak heights using MAS may therefore be in error. Lastly, there were problems in completely eliminating background signals due to the probe and water adsorbed onto the surface

‘H

150

J

100

50

0

-50

PPM

FIG. 4. 27A1MASNMR spectra of albite glasses. AB6. 10 mol% Hz0 (c) ABSB, 57 mol% HrO (d) HrO. 200 1 ps (
(a) ABS, dry (b) ABSG, 60 mol% a site where c, is kHz were used.

FIG. 5. ‘H NMR spectra for ABSB (a) Static (b) MAS. Around 1000 scans were obtained using a 2 ps pulse ( - T/ lo), a 2 s recycle delay. and (for (b)) MAS at -4 kHz.

S. C. Kohn, R. Dupree, and M. E. Smith

2930

of the sample. It should be noted that even nominally hydrous albite glass, AB5, showed a small ‘H signal.

an-

DISCUSSION

29Si The similarity between the single pulse spectra for dry and hydrous albite glasses suggests either that any changes in silicon environment are small or that they only affect a small proportion of silicon atoms-i.e., Si-OH units have a concentration which is below the limit of detection (5%). This is an unexpected result since nearly all previous models for water dissolution involve the formation of these units. Cross-polarisation experiments are more sensitive to the presence of low concentrations of SCOH groups and therefore provide further important evidence. The change in chemical shift of the resonance using cross-polarisation with short contact times indicates that the protons in the hydrous glasses are, on average, nearer silicon atoms with a less negative shift. In principle these could be QJ( nAl)-OH, analogous to the results for hydrous SiOl glass ( FARNANet al., 1987 ) or Q4 silicons with a larger number of aluminiums in the next nearest shell. Figure 6 shows a CP spectrum (A) together with a single pulse spectrum (B) for ABSB scaled such that a subtraction can be performed to show the portion of the spectrum that has been preferentially enhanced by cross polarisation (C). The resonance (C) is centered on -92 ppm, a difference of only 6 ppm from the position of the single pulse resonance. If the resonance is due to QJ( nAl)-OH one would expect a change of around 10 ppm (assuming random breakage of (Si, Al)-0-(Si, Al) bonds), although a resonance at this position could be consistent with the presence of QJ( OAl)-OH (obtained by selective breakage of Si-O-Si bonds). The other possibility is that the protons are associated with Al. A difference of 6 ppm corresponds to an increase of - 1 aluminium in the next-nearest neighbour shell (KIRKPATRICKet al., 1986), i.e., protons are on average nearer silicons with larger numbers of aluminium next-nearest neighbours than silicons

with smaller numbers of aluminium next-nearest neighbours. We will show later that the water appears to be associated mainly with sodium, and, since sodium is thought to locally charge balance [A104]- tetrahedra, these 2gSi data are consistent with this view. The CP results also suggest, therefore, that within the limit of detection (< 1%), no Qj-OH units are present. In glasses with higher water concentrations, the number of silicon atoms close to protons must increase; therefore, the intensity of the CP MAS spectrum for ABSB is much greater than that for AB6 at short contact times. As CP preferentially enhances those silicons close to protons, the fact that the shape of the spectrum is independent of water concentration for a given contact time shows that the relative numbers of silicons observed from different environments remains constant. Both the static single-pulse and CP spectra also suggest that the concentration of Si-OH is negligible. The CP spectra have less-shielded values of chemical shift than the single pulse spectra because Qd(OA1) silicons are not near protons and are therefore not enhanced. The presence of Q,(OAl)-OH can be discounted because the static CP spectrum is symmetric. The ‘selectivity’ and the use of ‘H decoupling in the CP pulse sequence are the causes of the narrowing of the static CP spectrum relative to the non-CP spectrum. “Na For quadrupolar nuclei the chemical shift, its dispersion, and the quadrupole interaction contribute to the observed width and position of the resonances in MAS spectra. Interpretation of NMR chemical shift data for nuclei such as Na, with small chemical shift ranges, is particularly difficult since the influence of factors such as coordination numbers, bond angles, and bond lengths on Na shifts are poorly understood. For example, it is not known why the 23Na chemical shifts for crystals and glasses in the series Ab-Or show opposite trends (OESTRIKE et al., 1987; PHILLIPS et al., 1988). Measurement at several fields aids the interpretation of the spectra of quadrupolar nuclei. The peak position (6,t ppm ) is related to the Larmor resonance frequency (~0) by Eqn. (3):

-

(_x_)C 4ou;

-20

-70

-20

-20

-100

-110

-120

-120

-140

PPM

FIG. 6. *?Si MASNMR spectra for ABSB. Subtraction of single pulse spectrum (B) from the CP spectrumshown in Fig. Id (A) to show the portion of the resonance that has been preferentially enhanced by cross-polaxisation (C) .

(I(1 + 1) - 9m(m - 1) - 3)( I + $/3) Z2(2Z - 1)2

1 (3)

where b,, = chemical shift at the peak maximum (for a Gaussian peak), 6,, = isotropic chemical shift, c, = quadrupole coupling constant ( =e2qQ/h where eq is the maximum component of the electric field gradient tensor, and Q is the nuclear electric quadrupole moment), m = z-quantum number of the (m, m - 1) transition, t) = asymmetry parameter (0 < n =Z1) ( ABFUGAM,1983; LIPPMAA et al., 1986). For 23Na, Z = %, and for the +% to J/z transition, m = ‘12; therefore, if the n term is ignored (as it can only make a small contribution, and its value is unknown) this equation reduces to

2931

NMR study of hydrous albite glass A plot of the peak position against 1/uf allows both the isotropic chemical shift 61, (from the intercept 1/vi = 0) and the mean quadrupole coupling constant S, (from the gradient) to be deduced. From our data it can be seen clearly that Lii, becomes less shielded and & increases with increasing water concentration (Fig. 7a,b). The increase in Cg appears to contradict the interpretation of YANG and KIRKPATRICK ( 1989) in their study of the dissolution of albite glass in aqueous solutions. They suggest that water incorporation resulted in a decrease in Cg, although differences in the compositions and the method of obtaining the hydrous glasses make comparisons unwise. The linewidths of the extreme compdsitions, AB5 and ABSG, show opposite trends in going from 8.45 T to 4.7 T (Table 2), indicating that the linewidth has more than one contribution. The actual lineshape in these glasses is the convolution of a range of isotropic chemical shift values MI* (i.e., different a’) and a range of second-order quadrupolar lineshapes (i.e., different cs). We have simulated our 23Na spectra by taking a single second-order quadrupolar lineshape corresponding to ?q (as deduced from Eqn. 4) and convolving it with a single Gaussian broadening function. The width of this function is the sum of two independent contributions,

. . .

-25’

1.8.1.1.1.1.1.,1 0 10 20 30 Mole

40

% dissolved

50

60

70

water

.

.

0.6’

,.,.,.,.,.,.,.,I 0 10 20 Mole

30

40

% dissolved

50

60

70

water

FIG. 7. *jNa NMR parameters deduced from measurements at two magnetic fields. (a) the isotropic chemical shift, b’,and (b) the mean quadrupole coupling constant, c;l, as a function of water concentration (in mole% ) .

IVYand wq where w4 represents the presence of a range of electric field gradients. At each field ( wb + We) is chosen so as to reproduce the linewidth and shape of the 23Naresonance. The individual values of ws and w6are chosen such that the different field dependences of the two components (Eqns. 5 and 6) are taken into account: (wq)2 = [(&hI(~O)ZI(Mh

(5)

(%I2 = (%hl[(~ohl(~o)21

(6)

where [( Bo)I/(Bo)2] = 1.8 for the fields used here. Hence, measurements of the widths at the two fields and use of these relations allows the contributions of the two components of the linewidth to be deduced. It can be seen from Table 2 that there is a significant change in the relative importance of the two contributions in glasses with different dissolved water concentrations. The width of the high-water content glasses can be attributed solely to quadrupolar effects. However glasses with lower water contents have a much larger chemical shift dispersion (A#) contribution, leading to the greater linewidth (in Hz) observed at the higher field. This deduction is somewhat surprising since the linewidths of quadrupolar nuclei are often dominated by second order quadrupolar effects that decrease with increasing field. For comparison, spectra for dry sodium disilicate glass were also measured at 8.45 T and 4.7 T. As in the dry albite glass, the resonance was broader at 8.45 T (by a factor of approximately 1.8) indicating that in both dry glasses chemical shift dispersion is the major contributor to the linewidth. Sodium disilicate glass also had a very similar Cg value to dry albite of 0.7 MHz but a different 6’ of -0.5 ppm. Although the Na environment in these glasses-where sodium is charge balancing either Q3-O- (in sodium disilicate) or [ A104]- (in albite)-is clearly different (as shown by the large difference in a’), the common factors of a fairly small electric field gradient at the nucleus and a large chemical shift dispersion indicate that the range of environments around the sodium are similar. The high-water content albites have sodium environments with larger electric field gradients but no measurable chemical shift dispersion contribution. Furthermore, the range of electric field gradients (as indicated by w,) when normalised by ??, is -4 times smaller than for the dry glass. All this evidence shows that hydrous albite glasses are more ordered than the dry glass as suggested by the X-ray radial distribution function in hydrous albite glass (OKUNO et al., 1987). The maximum disorder as indicated by both A6’ and w,/Ft occurs between 10 and 29 mol% H20. The change of isotropic chemical shift and chemical shift dispersion are clear indications that major changes occur at the sodium site as the water content is increased.

The absence of an observable signal for non-tetrahedral aluminium combined with the fact that there is no change in normalised signal intensity suggests that within the errors of the measurements ( -2%) no change in aluminium coordination number occurs. This is contrary to some previous models for water dissolution in albite melts (e.g., MYSENet

2932

S. C. Kohn, R. Dupree, and M. E. Smith

al., 1980; MYSEN and VIRGO, 1986b; PICHAVANT, 1987) which have su8gested that Al [ 41 is expelled from the network to produce Al [ 61. The presence of Al-OH units suggested by REMMELEet al. ( 1986) can be discounted since aluminiums in Qr environments are known to resonate at around 70 ppm (UNSEY et al., 1985). Furthermore, the existence of Q3 species is likely to cause an increase rather than a decrease in the width ofthe resonance since the increased electric field gradient of such a site would increase the quadrupolar broadening of the lines. Even if a small degree of depolymerisation occurs, the absence of Al-OH would,be consistent with the observation that in anhydrous glasses with NBO/T > 0 Al is p~ferentially partitioned into the most polymerised sites ( MYSEN et al., 198 1; OESTRIKE and KIRKPATRICK, 1988). The changes in shape and width of the “Al resonances as a function of water concentration suggest that in the hydrous glasses there is a decrease in the number of aluminiums which experience the largest electric field gradients, resulting in a reduction in the mean electric field gradient and a decrease in the distribution of electric field gradients at the Al nucleus. The change in peak position between 4.7 T and 8.45 T indicates that for AB6 the isotropic chemical shift is 52.3 ppm and the mean nuclear quadrupole coupling constant 1.3 MHz. The small increase in linewidth in going to the lower field (by a factor of 1.05) suggests that both second-order quadrupole effects and chemical shift dispersion make significant contributions. Simulations show that at 8.45 T, chemical shift dispersion contributes 1225 Hz (AS’ = 13 ppm) to the linewidth, and the distribution ofquadrupolar coupling constants contributes 825 Hz. At 4.7 T the ~ont~butions are 680 Hz and 1485 Hz, respectively. ‘H

The spectra suggest that water is present both as hydroxyl groups and molecular water. The changes observed as a function of water concentration are consistent with the model of STOLPER(1982a,b) where hydroxyl dominates at low water concentrations and molecular water becomes the major species at high total dissolved water concentrations. However, the water speciation trends observed here are not completely clear due to the problems with quantification of ‘H spectra described earlier and possibly differences in the synthesis conditions between samples. Comparison of Nii4R studies

ofcrystalline and glassy albite

29Si MAS NMR of ordered low albite (KIRKPATRICK et al., 1985; SHERRIPP~~~ HARTMAN, 1985; YANG et al., 1986) showed three resonances which were ascribed to T2m (2Al) -93 ppm, T20( 1Al) -97.1 ppm, andTlm( lA1) -105 ppm with the aluminium strictly partitioned into the Tlo sites. Disordering of the Si/Al distribution by heating causes a marked broadening of the silicon resonance due to different numbers of next-nearest neighbour ~uminiums. Spectral simulation (YANG et al., 1986) showed that the Al still preserved some Tl-site preference. The *‘Al MAS NMR resonance shifted slightly (58.9 ppm to 61.1 ppm) while the FWHM increased from 7.4 ppm to 11.1 ppm, this being

ascribed to disordering of the framework. In ordered crystalline albite the sodium site has an electric field gradient of 2.62 MHz producing a recognisable second order quadrupolar lineshape. On disordering, the linewidth decreases by * 30% probably due to a decrease in c,, which according to YANG et al. ( 1986) is due to the spatial distribution about the sodium site becoming more uniform. The presentation of results for 27Al and 23Na at a single field by YANG et al. ( 1986) meant that much potential information was lost. A very thorough study of the ordered albite-microcline series (PHILLIPSet al., 1988) involving measurements at two fields allowed the nuclear quadrupole coupling constants and isotropic chemical shifts for crystalline albite to be obtained, giving values for *?A1of c, = 3.29 MHz, 8’ = 63.4 ppm and for 23Na c, = 2.6 MHz, and b’ = -8.5 ppm. Previous MAS NMR studies of anhydrous albite glass (OESTRIKEet al., 1987) gave similar widths to the disordered crystal, indicating that a similar range of silicon species were present. We have found that the *‘Si resonance for ABS (dry albite glass) can be simulated equally well either by a wide distribution of Q4(nAl) environments each of which has a restricted (Si,Al)-0-( Si,Al) bond angle distribution or a narrower range of Q4 (nA1) environments with a broader bond angle distribution. In the study of QESTRIKEet al. ( 1987) the &-value for “Al in the glass was inferred to be less than in the crystal, but no quanti~cation of c?~or d’ for 23Na was attempted. Our results for “Al give Cs u 1.7 MHz and 6’ = 52.4 ppm (N.B. for AB6 rather than AB5) and for 23Na ?q N 0.7 MHz and S’ = -20 ppm. This highlights some interesting differences between the crystalline and glassy structures. 29Si and *‘Al are more shielded in the glass by 3 ppm and 10 ppm, respectively. This could indicate that the (Si,Al)0-( Si, Al) bond angles are increased and would be consistent with an increase in ‘ring-size’ as suggested on the basis of Xray radial distribution function evidence (TAYLOR and BROWN, 1979 ) . The loss of the periodic crystallographic lattice appears to lead to the system becoming structurally more relaxed as evidenced by the decreased electric field gradient at aluminium sites in the glass. The sodium data are also consistent with an increase in ring size with sodium ions present in larger cavities with a more uniform distribution of surrounding oxygens and, hence, with a smaller electric field gradient. Also, if a coordination bond strength argument can be applied to the isotropic shift of 23Na (as for 29Si and 27Al) the more negative isotropic shift in the glass indicates increased shielding which is consistent with an increased coordination number for sodium in the glass, i.e., a larger cavity. h4odeZfor the structure o~hydrous albite gfasses. In this section we summa&e the features ofthe glass structure determined from our NMR data and propose a consistent model for the structure of hydrous albite glasses. The applicability of this model to hydrous albite melts and its compatability with their known physical properties is then discussed. The interpretation of our results in terms of a structural model will take account of the following: i) little or no formation of B-OH, Si-O-, Al-OH, or octahedrally coordinated Al implying only small changes in the overall “framework”

2933

NMR study of hydrous albite glass

structure, ii) protons nearer to those silicons which have the most aluminium next-nearest neighbours, iii) coexistence of O-H and molecular H20, iv) gradual change in isotropic chemical shift of Na with increasing water concentration, v) decrease in the contribution of chemical shift dispersion to the Na linewidth, and vi) initially no change in the nuclear quadrupole coupling constant for Na but a large increase above -30 mol%. The last three points indicate that water is associated with Na. The absence of significant concentrations of Si-OH in the hydrous glasses was unexpected. According to the widely accepted model of STOLPER ( 1982a,b), our high-water-concentration glasses should contain around 2 wt% water dissolved as -OH in Si-OH units. This amount corresponds to about 23 mol% -OH expressed as H20. Assuming that there is no reaction with Al (as clearly shown by our 27A1data) and remembering that each mole of water can react with one bridging oxygen to form two Si-OH groups, Stolper’s model would predict that around 20% of the silicons would be in Q, units with the rest in Q4 units. In contrast, both our MAS and static 29Si CP data show that
(7)

We would envisage that the equilibrium constant for this reaction varies with total dissolved water concentration (based on the -OH/H20 ratios measured by STOLPER, 1982a.b). The difference in vibrational frequencies of O-H and O-D groups by a factor of - 1.35 in hydrous rhyolite glasses ( ECKERT et al., 1987) is consistent with the hydroxyl groups behaving as diatomic vibrators rather than as groups attached to the silicate framework. This equilibrium can explain the observation of signals due to both molecular water and hydroxyl. Although our results provide no direct evidence for the site of molecular water incorporation we suggest the situation illustrated in Fig. 8 as a possibility. Since the proton charge balancing [ AlO.,] 5- is likely to be nearer to an oxygen than to the central aluminium, this might also be considered to be a bridging OH group in contrast to a terminal Qs-OH group [cf., zeolites ( ENGELHARDT and MICHEL, 1987)]. Relatively rapid exchange between the two structures in Fig. 8 could account for the weak dipolar coupling between 23Na and ‘H demonstrated by the high power decoupling experiments. The negative ‘H-23Na and ‘H-27A1 CP results may also indicate motion, although they could also be due to the difficulty of determining the match conditions for coupling of quadrupolar nuclei to protons once the second order quadrupolar width becomes large. An interesting comparison can be made between our measurements of Fg for 23Na (related to the average electric field gradient at the sodium site) and Stolper’s

FIG. 8. Proposedmechanism for exchange between molecular water and hydroxyl groups in hydrous albite glasses. The labels 6- and 6+ indicate the presence of polar O-H bonds in the water molecule.

-OH/H20 equilibrium. Stolper’s measurements suggest that the concentration of molecular water as a dissolved species only starts to become significant above around 30 mol%: i.e., it appears that the high field gradients of sodium environments in hydrous albite glasses observed here are related to the coordination of sodium ions by a water molecule. Further support for this interpretation is provided by the plot of 6’ for 23Na vs. dissolved water concentration in mol% (Fig. 7a); there appears to be a change in gradient at -30 mol%. The reduced chemical shift dispersion and the reduction in the range of electric field gradients (wJ?z) for Na and the decrease in the average electric field gradient at aluminium nuclei suggest that hydrous aibite glasses are more “relaxed” with more local order than dry albite glass. This agrees with the differences in X-ray distribution functions observed by OKUNO et al. ( 1987). As the limit of detection for silicons using the single-pulse technique is not as low as for silicons near protons using cross-polarisation ( -5% compared with < 1%) we cannot completely eliminate the possibility that a small degree of depolymerisation involving Qr-O- rather than Q3-OH takes place. This could involve reactions such as: 2NaAISi308 + HrO Z 2Na’ + Si20:- + 2HA1Si206

(8)

2NaAlSi30s f Hz0 2 2Na+ + SiO:- + HAlSi206 + HAlSi30B

(9)

where framework units with higher Al( Al + Si) ratios and containing network-forming Al charge balanced by H+ are produced. We are confident that the structural features of hydrous albite glasses described above are correct. It is unlikely that all our samples are completely different from those studied by previous workers as we used a range of temperatures and pressures and two different synthesis methods (and hence quench-rates). We therefore conclude that much of the previous spectroscopic data on hydrous albite glasses has been misinterpreted. The major problem with studies of this type is that it is still very difficult to show that the structure of hydrous glass determined at room temperature is identical to the melt from which it was quenched. There is some evidence (e.g., AINES et al., 1983; STOLPER et al., 1983) to suggest that hydrous glasses and melts are similar, but much more work is needed in this area. Direct spectroscopic studies of hydrous melts

S. C. Kohn, R. Dupree, and M. E. Smith

2934

would be ideal but technically very difficult; further studies of the effect of quench rate and, hence, fictive temperature on hydrous glass structure could also be informative. It seems probable that some structural features of melts will be more readily quenched into the corresponding glass structure than others (i.e., the glass transition temperature for certain types of exchange will be different from the bulk glass transition temperature). In particular, structural rearrangement between Hz0 and OH in the melt (see Fig. 8) could take place very rapidly compared with even the fastest achievable quenchrates. Since multinuclear magnetic resonance can provide a much more complete picture of the structure of the giass than any other technique by providing info~ation about the spatial relationship between atoms of different types, it can help provide clues as to which features are most likely to have arisen during the quench. This is in contrast to previous infrared measurements which have concentrated solely on water speciation. If we assume that the fully polymer&d nature of hydrous albite glasses also character&s hydrous melts, can the known effects of water on melt properties be rationalised? The form of the water solubility curve as a function of water fugacity and the thermochemistry of water dissolution can be explained in terms of the OH/H20 equilibrium as proposed by STOLPER( 1982a,bf. In order to explain the effects on viscosity, electrical conductivity, and diffision rates the microscopic mechanisms for these processes must be understood. In all three cases large changes could be possible without the formation of any non-bridging oxygens. Taking the case of viscosity for example, i) bridging OH groups, ii) the change from Na+ to H + as a charge balancing cation, or iii) the presence of water in “holes” in the structure, could weaken (Si,Al)-0-t Si,AI) bonds or provide a lower activation energy pathway for breaking and reforming bonds, although much more work on the mechanisms of these processes in dry and hydrous melts is required. For anhydrous melts, NMR studies (LJU et al., 1987, 1988) and detailed analysis of vi~osity-tem~rature relationships (DJNGWELLpers. commun.) are providing a promising start. Similar arguments could be applied to explain the increases in electrical conductivity and diffusion rates of Hz0 and cat-

ions which accompany water dissoiution. The high mobility of the proton and its ability to exchange with Na” could play an important role. CONCLUSIONS We have studied hydrous albite glasses quenched from above the liquidus using a variety of nuclear magnetic resonance techniques, including magic a&e spinning, cross polarisation, and proton decoupling. For quadrupofar nuclei, such as 23Na, it is important that spectra are obtained at two or more different magnetic field strengths. In this study, this approach allowed changes in the isotropic chemical shift, the nuclear quadrupole coupling constant, and the dispersion of these parameters as a function of water ~on~ntration to be deduced. Many of the previous models for the mechanism of water dissolution are inconsistent with our data. In particular we do not observe the presence of Al-OH, octahedrally coordinated aluminium, or S&OH. We suggest that the major

dissolution mechanism involves exchange of NaC for H+ as a charge-balancing cation for [ AlO,]- tetrahedra, formation of Na( OH) complexes, and incorporation of molecular water. There is no observable depolymerisation of the framework structure. Multinuclear magnetic resonance studies of glasses are of great importance to the understanding of melts since they provide info~ation about the spatial ~lationship between atoms and, hence, provide clues about which structural rearrangements are most likely during the transition from melt to glass. This contrasts with previous infrared spectroscopic work which provides information solely on water speciation. Acknow~ed~ents-We would like to thank Dr. I. Fax& for a&stance during the early stages of this work, Prof. R. J. Kirkpatrick and B. L. Phillips for helpful~diseussions,Clan: Grey for running some of the low-field spectra, and S. A. T. Redfem for obtaining infrared spectra. We acknowledge the support of NERC and SERC. Editorial handling: P. C. Hess

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?Si, “AI