American Mineralogist, Volume 79, pages31-42, 1994
Correlations between17ONMR parametersand local structure around oxygen in
high-pressuresilicates: Implications for the structure of silicate melts at high pressure
Xr,tNvu XuE,* JoNlrruN
F. SrnnrrNs
Department of Geology, Stanford University, Stanford, California 94305, U.S.A.
Mlsllrr
K,rNz,q,rr
Department of Inorganic Materials, Tokyo Institute of Technology,Meguro-ku, Tokyo 152, Japan
Ansrru.cr
In order to understand the correlations between '7O NMR parameters and the local
structure around O in high-pressuresilicates,we have obtained '7O MAS and static NMR
spectraon severalcrystalline alkali silicate and silica phases,as well as on ambient pressure
glasses.Our results show that '7O NMR is capableof distinguishing nonbridging O atoms
(NBO), bridgingatoms (BO: t4rsi-O-t4rsi),
linkage,and O atoms
O atoms in the tatSi-O-r6rsi
coordinated to three t6rsi (rno). NBO are characteized by small quadrupolar coupling
constants(e'qQ/h, -2.3 MHz), but their isotropic chemicalshifts (d,) are very sensitive
to the type of network-modifuing cations. The prO (in stishovite), on the other hand, has
by far the largest e'zqQ/hvalue (6.5 MHz) and a larger Dr(109 ppm) than any BO in
silicates.BO and t4rsi-O-r6rsi
both have intermediateerqQ/h values(-4.5-5.7 MHz), and
the D,value of the latter (97 ppm in wadeite-Krsinoe) is larger than those of the former
(-50-65 ppm).
We have also applied '?O NMR to study the structure of alkali silicate glassesquenched
from melts at high pressure.The'?O NMR spectraof a KrSioO,glassquenchedfrom 6
GPa and of NarSioOnglassesquenchedfrom 6 to 10 GPa are consistentwith the presence
(and possiblyalsot4rsi-O-r5rSi)
of O sitesin the torSi-O-t6tSi
linkage,supportingour previous
model developedfrom "Si NMR. In addition, '7O static NMR also revealsthe development of another new type of O site, possibly one that is connectedwith two or three I61Si
or I5rSiatoms, in the NarSioO,glassquenchedfrom l0 GPa. Thus, I6rSiand tstSimay be
largely isolated from one another below l0 GPa, whereasat higher pressure,clustering of
theseSi speciesmay becomesignificantin NarSioOnmelts.
IxlnooucrroN
The high pressuresof the Earth's mantle are known to
have large effectson the physical and chemical properties
of magmas,includingphaseequilibria,density,viscosity,
and diffusivity. In order to gain the fundamental understanding needed to generateaccurate predictive models
of igneous processes,geophysicalinferencesand laboratory measurement of macroscopic properties must be
complemented by studies of atomic-scale structure and
dynamics of melts. Spectroscopicstudiesare playing an
increasinglyimportant role in the understandingof pressure-inducedchangesin the bondingand structureofsilicateliquids and glasses.
The latter are generallyassumed
to contain the structure of the liquid quenched in at the
glasstransition, which is usually many hundredsof degreesto more than a thousanddegreesbelow the liquidus
temperatures. Studies of glassesare thus the traditional
first step in inferring the structure ofliquids, but the possibility of large temperature effectsabove the glasstrant Present address:Earthquake ResearchInstitute, University
ofTokyo, 1-1 Yayoi, Bunkyo-ku,Tokyo 103,Japan.
0003-004x/94l0102-o031$02.00
3l
sition cannot be neglected(e.g.,Brandrissand Stebbins,
I 988; Stebbins,I 99 l). Becausein-situ, high-temperature,
and high-pressurestudies remain very difrcult, further
assumptionsabout the effectsof ambient temperaturedepressurizationof glasssamplesmust also be made. It is
clearthat many effectsofpressureare retainedin quenched
glasses,but it has also becomeapparentthat some structural changescan be rapidly reversible(e.g.,Wolf et al.,
1990). Despite these complications, results on glasses
produced by quenching stable liquids at high pressure
(such as those presentedhere) provide a qualitative starting point for understanding the actual liquids at high Z
and P.
Most spectroscopic
studiesof glasses
havebeenon simplified analoguesof geologicalcompositions.Particularly
for NMR, most easily interpretableresults have come
from binary or ternary compositions, for which structural
effects can be much more clearly distinguished than in
more complex systems.Alkali di- and tetrasilicates,for
example,althoughfar from magmaticin composition,do
have ratios of nonbridging O atoms to tetrahedralcations
(l and 0.5, respectively)that are similar to thoseof mafic
5Z
XUE ET AL.: I'O NMR PARAMETERS IN HIGH-P SLICATES
to intermediate magmasand thus probably capture many ning. The nuclear quadrupole interactions are described
of the general effects of composition, temperature, and by two parameters: the quadrupolar coupling constant
pressurethat are important to igneousprocesses.The re- (e'zqQ/h,where eQ is the nuclear quadrupolar coupling
sults presentedhere are part ofan ongoing study to char- constant, e4 is the principal component of the electric
acleize the structure of such compositionally simple sil- field gradient at the nucleus, and h is Planck's constant)
and the asymmetry parameter, (a), for the electric field
icate glassesquenched from melts at high pressure.
In silicate glassesand melts at ambient pressureof geo- gradient tensor. Although the breadth of the '?O NMR
logically important compositions, almost all Si and Al are spectra yields less resolution for different O sites comcoordinated by four O atoms (lorSiand tolAl). Spectro- pared with those of spin-72nuclei (e.g.,"Si), it has the
scopic studies on silicate glassesquenchedfrom melts at advantagethat in addition to the isotropic chemical shift
pressures<3 GPa have revealedseveraltypes of pres- parameter(6,),two more parameters(ezqQ/h,4) can be
sure-inducedchangesin the tetrahedral structure, such as used lor structuralcharacterization.
O is the most abundantelementin silicatesand oxides.
reduction of intertetrahedral angles and changesin the
second neighbor connectivity of the SiO4 and AlOo tet- The study of O structural environmentsis thus an imrahedra (e.g.,Mysen et al., 1983; Hochella and Brown, portant complementto studiesof cation coordination.It
1985;Dickinsonet a1.,1990;Mysen, 1990).It has been hasbeenshown that '?ONMR is a promisingmethod for
predicted from molecular dynamics simulations that, ar studying the structure and bonding in these materials
even greaterpressure,the dominant structural changefor (Schrammet al., 1983;Schrammand Oldfield,l984; Tursilicate melts could be an increasein the coordination of ner er al., 1985;Timken etal.,1986a,1986b,1987;Walthe network formers (Si and Al), with accompanying ter et al.. 19881Janesand Oldfield, 19861Bastow and
changesin densityand viscosity(e.g.,Angell et al., 1982, Stuart, 1990; Mueller et al., 1992: Faman et al., 1992).
1983, 1987,1988;Matsui et al., 1982;Matsui and Ka- In particular, bridging O atoms linked to different types
wamura, 1980, 1984).In our earlier papers(Xue et al., of network formers (e.g.,Si-O-Si, Si-O-Al, AI-O-P), as
1989, 1991;Stebbinsand McMillan, 1989),we have re- well as nonbridgingO atoms in crystallinesilicates,have
ported 2eSimagic angle spinning nuclearmagneticreso- been found to have distinctly different r7O NMR charresultson alkali silicate acteristics in terms of the 6, and e2qQ/h parameters
nance(MAS NMR) spectroscopic
glasses
quenched
from melts at pressuresup to (Timken et al., 1986a,1986b,1987;Walter et al., 1988;
and silica
12 GPa Thesedata yielded strongevidencefor the pres- Mueller et al., 1992). These results are useful for interence of significant amounts of five- and sixfold-coordi- preting spectra of disordered phases.Kirkpatrick et al.
nated Si (t'rSi and rerSi)in alkali silicate melts at high (1986) have applied '?O MAS NMR to study the orderpressure.More recent"Si NMR studieshave also iden- disorder of cations in alkaline earth silicate glasses
tified the presenceof a small amount of t5lSiin alkali quenched at I bar. More recently, Farnan et al. (1992)
silicate glassesquenched from melts at I bar (Stebbins, were able to quantify the cation disorder in KrSioO, and
KMgorSioOnglassesquenchedat I atm using dynamic
1991;Stebbinsand McMillan, 1993).
questions
angle
spinning(DAS) '7O NMR.
There are, however, many
unansweredby
As discussedabove,,new types of local O environour earlier"Si NMR studies.For example,the "Si NMR
spectrado not have enoughresolutionto yield informa- ments, especiallythose connectedto t5lsiand t6lSi,must
tion about the connectivitiesof t5tSiand t6lSi,i.e.,whether exist in silicatemelts and glassesat high pressure.Therethey are isolated or linked to each other in the silicate fore, in order to apply '?O NMR to study the structure
melt structure, and, if the latter is the case,whether they ofsilicate glassesquenchedfrom high pressure,it is necare connectedby edge sharing or corner sharing. On the essaryfirst to understand the '7O NMR characteristicsof
basisofthe compositionaldependenceofthe abundances thesenew local O environments.In the first part ofthis
of t5lSiand t6rSiand by analogywith the structure of alkali paper, we present 'tO MAS and static NMR spectrafor
germanatemelts and glassesat I bar (cf. Ueno et al., several low-and high-pressurealkali silicate and silica
1983),we have proposeda model that tstSiand t61Si
are crystallinephases,aswell as alkali silicateglassesquenched
of I?O
pressure
formed in silicate melts at high
at the expense from melts at I bar, and discussthe characteristics
of nonbridging O atoms, which are forced to connectwith NMR spectrafor different types of local O sites. We find
changesin O environments
the highly coordinatedSi (Xue et al., 1991).Quantifica- that some pressure-induced
tion of this model relies on answersto the above ques- are readily distinguished.We then report '7O MAS and
tions. Thesequestionsmay be answeredto some extent static NMR data for alkali silicate glassesquenchedfrom
by directly studyingthe O environments,which naturally melts at high pressureand discussthe structural changes
in theseglasseswith pressure.
led us to apply '70 NMR to thesesystems.
The '7O isotope has a natural abundanceof 0.0370/0.
ExpnnrlrnNTAr- PRocEDURES
Becauseits spin quantum number is L the r7O NMR
peaks are typically broadened by second-orderquadruApproximately 10 or 500/o''O-enriched SiO, (cristopolar interactions between the nuclear quadrupole and balite) powders prepared from hydrolysis of SiClo with
the electric field gradient at the nucleus, and the broad- Hr'7O (Cambridge Isotope Laboratories) were used for
ening can only be partially removed by magic angle spin- synthesizingthe starting materials.The alkali silicateglass
XUE ET AL.: '7O NMR PARAMETERS IN HIGH-P SLICATES
33
TaeLe1. Synthesisconditionsfor crystallinephasesand glassesquenchedfrom melts at high pressure
Sample
Starting material'
Synthesisconditions
a-NarSirO.
Na,Si,Os glass, 107o 1?O
e-NarSi.Ou
Na,Si,Os glass, 10% '?O
Wadeite-KrSi4O,
K,Si4Oeglass, 10hnO
Stishovite
SiO, cristobalite, 5Oo/" 17O
K.S|.O"glass,6 GPa
KrSi4Oe glass, 10o/" 17O
Na,SioO"glass,6 GPa
NarSi4Oeglass, 48.6% 1,O
glass,I GPa
Na,Si4Os
Na,Si4Oeglass, 48 6% 1?O
glass,10 GPa
NarSi4Oe
NarSioO, glass, 10olo17O
* Percentage
of17O enrichmentrefers to that of SiO, used in preparingthe starting material.
starting materials were prepared by the melt quench
method describedpreviously (Xue et al., l99l) from alkali carbonate and '7o-enriched SiO, in a clean N, atmosphereto prevent O isotopic exchangewith air. About
0.2 wto/oof GdrO3 (0. I wto/oGdrO3 * 0. I wto/oMnO for
the NarSi.O, sample)was addedto shortenthe spin-lattice relaxationtime.
The synthesisconditions for the crystalline phasesand
for glassesquenchedfrom melts at high pressureare tabulated in Table L Most of the high-pressuresampleswere
synthesizedusing a MAS type multianvil high-pressure
apparatusat Tokyo University. The NarSioOeglasssample quenchedfrom melt at l0 GPa and the KrSioO, glass
sample quenchedfrom melt at 6 GPa were produced using a USSA-2000multianvil apparatusat the University
of Alberta (seeXue eI al., 1991,for a descriptionof the
experimentalprocedure).The wadeite-KrSioO,samplewas
synthesizedusinga piston-cylinderapparatusat the U.S.
GeologicalSurvey, Menlo Park. The identity, purity, and
crystallinityof the crystallinephaseshavebeenconfirmed
by powderX-ray diffractionand 2esior 23NaMAS NMR.
The quenchedglasssampleshave been examinedunder
a polarizingmicroscope(400x ) and were all found to be
homogeneousand crystal free. Alkali loss during synthesis was monitored by careful weighingsand was found to
be insignificant.
The ''O NMR spectrawere collectedon crushedsamples using a Varian VXR-400S spectrometerat a Larmor
frequencyof 54.2MHz. A Doty Scientifichigh-speedMAS
probe with a 5-mm rotor was used for the MAS NMR.
A Varian probe with a horizontal 5-mm rf coil was used
for the static NMR. The liquid 90" pulse lengthsmeasuredon deionizedHrO in both probeswere about 8-10
ps. For the MAS NMR experiments,singlepulsesof 0.5I ps (about a 15-30" tip angle for the central transition
in solids)wereused,and the samplespinningspeedswere
usually9- I I kHz. Small pulseangleswereusedto ensure
quantitative and selective excitation of the central transitions for all O sites. For the static experiments,a
901-z - 90i-z solid-stateecho pulse sequencewith a z
value of 55 ps was applied to overcome the problem of
spectrometer dead time, which is a severe problem for
broad spectra. Delay times of 0.2-30 s between pulses
(or pulse sequences)were used, depending on the spinlattice relaxation time of the sample. Frequencieswere
1 a t m ,8 1 0 ' C , 1 6 h , i n N ,
6 G P a .9 0 0 ' C . t h
2.64 GPa,770 "C, 25 h
9.5 GPa. 860.-900qC. t h
6 GPa. 1950'C. 10 min
6 G P a .1 7 5 0 ' C . 5 m i n
8 GPa, 2000'C, 5 min
10 GPa. 2050 'C. 5 min
externally calibrated to approximately +0.2 ppm against
deionizedHrO.
Values of 6,, e2qQ/h,and 4 for the crystallinephases
have beenderived from computersimulationsof the '7O
MAS NMR spectra based on the algorithm of Miiller
(1982). In addition to the second-orderquadrupolar
broadening,the 17OstaticNMR line shapeof a powdered
sample is in general also affected by chemical shift anisotropy and by the relative orientations between the
chemical shift tensor and the electric field gradient tensor.
For two of the crystalline sampleswe studied (wadeiteK2Si4Oeand stishovite),we were able to obtain the static
NMR spectra.We have simulatedtheselinesusingvalues
of 6,, e'zqQ/h,and 4 derived from the MAS NMR spectra
to obtain the principal component values for the chemical shift tensor. Becauseof the quality of these spectra,
we haverestrictedour simulationsto coincidentprincipal
axes between the chemical shift tensor and the electric
field gradient tensor (seebelow for further discussions).
The coordinate system we adopted is that described in
Narita et al. (1966). The principal componentsof the
chemical shift tensor are described in this paper as 6,,,
6uu,and 0,,, denoting the components along the x, y, and
z principal axes of the electric field gradient tensor (see
Fig. I of Narita et al., 1966).
Connnr-,c.rroNs BETwEENr7O NMR pARAMETERS
AND THE LOCAL
STRUCTURE
AROUND
O
The r7O NMR spectra of crystalline alkali silicates
and silica
Four crystalline alkali silicate and silica phases (aNarSirOr, e-NarSirOr,wadeite-I(rsioOr,and stishovite)
have beenchosenfor this study becausethesephasescontain several different types of O sites. The NMR parametersderived from computersimulationsof the '?O MAS
NMR spectraare reported in Table 2.
a-NarSirOr. There are three distinct O sites in the
structure:Ol, 02, and 03 in the ratio of l:2:2 (Pant and
Cruickshank,1968).Ol and 02 are both bridging O atoms (BO), whereas03 is a nonbridgingO atom (NBO).
The '7O MAS NMR spectrumof this phasecontainsa
narrow doublet and some broader peaks(Fig. la). The
narrow doublet may be attributed to the singleNBO site,
03, as it has been shown by previous studiesthat NBO
34
XUE ET AL.: '7O NMR PARAMETERS IN HIGH-P SILICATES
TABLE2, NMR parametersderivedfrom '7O MAS NMR spectra
cristobalite(sio,r
a-NarSirOu
e-NarSirOu
Wadeite-K2Si4O,
Stishovite(SiO,)
Na,Si,O5glass
Narsi3O?glass
Na,Si4O,glass
KrSi4O,glass
BO
ol (Bo)
02 (BO)
03 (NBO)
NBO
01 (BO)
02 1tot5,-O-",40
rsrO
BO
NBO
BO
NBO
BO
NBO
BO
NBO
d,
e2qQlh
(MHz)
O site
0.125+ 0.005
0.0+ 0.2
o.25! 0.2
0 . 1+ 0 1
5.3 + 0.1
5.7 + 0.3
4.7 + 0.2
2.35+01
0.35+ 0.05
0.20+ 0.05
0.125+ 0.05
0
0
0
0
0
0
0
0
4.45 +
4 90 +
6.5 +
5.0 +
2.3 +
5.0 +
2.3 +
5.0 +
2.3 +
4.9 +
2.3 +
0.05
0.05
0.1
0.3
0.1
0.3
0.1
0.2
0.1
O.2
0.1
(ppm)
Relative
intensity
40+2
55 15
55+2
34+1
45+5
62.5 + 1
97+1
109+2
b5i5
40+2
60+5
39+2
50+4
36+2
52+4
76+2
1
2
2
1
2
1
3
2
5
2
7
2
7
2
Nofe: parameterswithout quoted error have been fixed during simulation.Data for glasses are for samples at 1 bar
. Data trom Spearinget al. (1992).
in generalhave much smaller e'1qQ/hand thus narrower
'?ONMR patternsthan BO (Timkin et al., 1987;Mueller
et al., 1992; Farnan et al., 1992). The broad peaks may
be simulated with two broad quadrupolar doublet patterns corresponding to the two BO sites. The ratios of
thesethree O sites have been fixed during the simulation
to those expectedfrom the crystal structure. One of the
BO has a rather largevalue of e2qQ/h(5.7 MHz), which
may be correlated to the unusually large Si-O-Si angle of
O1 (160.04'forOl vs. 138.93'for02: Pant and Cruickshank, 1968).Positive correlationsbetween"O e'qQ/h
and Si-O-Si angle have been predicted from quantum
mechanical calculations(Tossell and Lazzerelti, 1988;
Tossell,1990)and are also supportedby a recentdynamic anglespinning'7O NMR study of a K2Si4O,glass(Farnan et al., 1992)(seebelow for further discussion).
e-NarSirO..This is a high-pressurephasethat we found
to be stableat pressuresof about 5-6 GPa (Kanzaki et
al., in preparation).Although detailed structure of this
phasehas not yet been solved by single-crystalX-ray diffraction, the '?eSiNMR spectrum contains two distinct
peakscenteredat -81.0 and-82.3 ppm, which from
stoichiometry can be attributed to two Q' sites (Si with
three other second-neighborSi ions), suggestingthat it
has a sheet structure similar to the other lower pressure
NarSirO, polymorphs (Kanzaki et al., in preparation).We
may thus expectthe ratio of BO and NBO in this phase
also to be 3:2. The "Si chemicalshifts of thesepeaksare
about 10 ppm lessnegativethan thoseof Q'sites in other
low-pressurealkali silicatephases,suggesting
that the SiO-Si anglesin the e phaseare small and may be similar
to those in three-memberedrings.
Becauseof the small samplesize (17 mg) and the relatively long spin-latticerelaxationtime of the sample,the
'?O MAS NMR spectrumhas a poor signalto noiseratio,
and only a relatively narrow, noisy peak is observable
(Fig. lb). This peak may be attributed to the NBO in its
structure.Interestingly,it is shifted by about 10 ppm to
a higher frequencythan that of the NBO in a-NarSirO''
The shift is possibly related to the small intertetrahedral
angleswithin the silicatesheetsof e-Na2Si2O5.
Wadeite-KrSioOn.This phasehas been found to be stable over a large pressurerange from about 2 to at least
12 GPa (Kinomura et al., 1974;Kanzaki et al., in preparation). Both totSiand rrsi are present in the structure.
The SiOo tetrahedra and SiOu octahedra share corners
with each other, forming a three-dimensionalnetwork.
There are two types of O sitesin the structure,I4rSi-OlI4rSiand t4lSi-O2-{61Si,
with a ratio of l:2 (Swansonand
Prewitt. 1983).
The'7O MAS NMR spectrumof this phasehas sharp
featuresthat can be well simulated by two O sites with a
ratio of l:2 (Fig. 2a). From the relative intensity,we can
assignthe O site with a largerd, (97 ppm) to 02 and the
other to 01. The Ol site is a normal BO, and its D,and
e'zqQ/hvaluesare both similar to those of normal BO in
other silicates,such as a-NarSirOr, cristobalite (SiOr)
(Timken et al., 1986b;Spearinget al., 1992),and alkaline
earth metasilicates(Timken et al., 1987; Mueller et al.,
1992). The 02 site is unique to high-pressuresilicates
containing mixed fourfold- and sixfold-coordinated Si.
Its e'zqQ/his similar to those of normal BO, but its D,is
significantly larger.
The static NMR spectrum cannot be simulated well
with the same parametersderived from the MAS NMR
spectrumif the chemicalshift anisotropyis ignored(Fig.
2b). This hasbeencommonly observedfor static '?ONMR
spectraof crystallinesilicates(e.g.,Timken et al., 1986a,
1986b;Spearingetal.,1992). A reasonablefit is obtained
by taking this anisotropy into consideration,assuming
that the principal axes of the chemical shift tensor are
coincidentwith those ofthe electricfield gradienttensor
(Fig. 2c). The derived principal components for the
chemicalshift tensorof 01 are 6,": 81,6,,: 81, and 0,,
: 2 5 . 5 p p m . T h o s eo f 0 2 a r e6 , , : l 1 7 , 6 u u : 1 1 7 ,a n d
6., : 57 ppm. Uncertaintiesfor all are about + l0 ppm.
XUE ET AL.: '7O NMR PARAMETERS IN HIGH.P SLICATES
-100
100
1 5 01 0 0
%%M0
50-100
40o 200 poM-aoo 400
35
4o0 200 p0H-200-400
r?OMASNMR specFig.2. (a)Experimental
andsimulated
tra of wadeite-Krsi4or.The experimentalspectrum(top) was
with a l-ps pulselength,a l-s delay,and 14300signal
acquired
The samplespinningspeedwas | 1 kHz. A 4O-Hz
averages.
wasapplied;(b) '?OstaticNMR specGaussian
line broadening
and simulationswithout considering
trum of wadeite-Krsi4on
chemicalshifl anisotropy.The experimental
spectrum(top)was
(seetext),a 1-s
acquiredwith a solid-stateechopulsesequence
A 600delaybetweenpulsesequences,
and8 100signalaverages.
Hz Gaussianline broadeningwasapplied;(c) ''O staticNMR
spectmmof wadeite-KrSioOn
asin b and simulationsthat have
takenaccountof chemicalshift anisotropy.Both the MAS and
staticNMR spectrawereobtainedon an approximately80-mg
sample(l0o/o''O).
150
100
qnn
PPM
-50
-100
Fig. 1. (a) Experimentaland simulated''O MAS NMR spectra of a-NarSirO,. The experimental spectnrm (upper) was acquired on an 84-mg sample(100/o,?O)with a 1-pspulselength,
30-s delay, and 3000 signalaverages.The samplespinning speed
was 10 kHz. A 100-Hzexponentialline broadeningwasapplied;
(b) r?O MAS NMR spectraof e-Na,SirOr.The spectrumwas
acquiredon a l7-mg sample(l0o/o'?O)with a l-ps pulselength,
a lO-s delay, and 6300 signal averages.The sample spinning
speedwas I I kHz. A 260-Hz Gaussianline broadeningwas
applied.
Because the quadrupolar patterns for the two O sites
overlap, we cannot determine the unique relative orientations of the chemical shift tensor and the electric field
gradient tensor for either site.
Stishovite. Stishovite is a high-pressure SiO, polymorph that is stable above approximately 9 GPa. There
is only one crystallographically distinct site each for Si
and O in the structure (Sinclair and Ringwood, 1978).
Each Si ion is coordinated to six O atoms, and the SiOu
octahedra share both corners and edges with neighboring
octahedra. Each O atom is coordinated to three t6lSi
(t3rO)atoms.
The '7O MAS NMR spectrum of stishovite is composed of a doublet that can be well simulated with a single
O site (Fig. 3a). The e,qQ/h value of t3rOin stishovite (6.5
MHz) is by far the largest among O sites in silicates known
so far. Its 6,(109 ppm) is also largerthan thoseof BO in
any silicates,although NBO in strontium metasilicates
and barium metasilicateshave larger d, values (Timken
et al..1987).
For the I7Ostatic NMR spectrum,as for that of wadeite-Krsi4oe, the fit is poor when we simulate only with
the parametersderived from the MAS NMR spectrum,
ignoring the chemical shift anisotropy (FiC. 3b). When
the chemical shift anisotropy is taken into consideration,
again assuming coincident principal axes between the
chemical shift tensor and the electric field gradient tensor,
a reasonablefit can be obtained (Fig. 3b). The derived
principal components for the chemical shift tensor are
: 95, and 0,, : 163 ppm. Uncertaintiesare
6,,: 69, Duu
approximately +7 ppm for all. The quality of this spectrum does not allow us to analyze further the relative
orientationsof the two tensors.
The t7O NMR spectraof alkali silicate glasses
quenchedat I atm
We have also obtaineda '?O MAS NMR spectrumfor
a KrSi4Oeglassand 'iO MAS and staticNMR spectrafor
NarSirOr, NarSi,Or, and NarSioOnglasses,all quenched
from melts at I atm (Figs. 4, 5, and 6). These spectra
resemblethose of alkaline earth silicateglassesprepared
at I atm (Kirkpatrick et al., 1986) and have much less
well definedfeaturesthan thoseof crystallinephasesas a
result of structuraldisorder.
The MAS NMR spectrumof the KrSioO,glassconsists
XUE ET AL.: I7O NMR PARAMETERS IN HIGH-P SILICATES
36
o
1OO
_4.,
-100
-e00
e50
0
-500
PPI,1
Fig.3. (a)Experimental
andsimulated''O MAS NMR spectra ofstishovite.The experimental
spectrum(top)wasacquired
with a l -pspulselength,a 30-sdelay,and I 300signalaverages.
The samplespinningspeedwasabout9 kHz. A 160-HzGaussian line broadeningwasapplied;(b) ''O staticNMR spectrum
and simulationsof stishovite.The experimental
spectrum(top)
was acquiredwith the solid-stateechopulsesequence,
a 30-s
delaybetweenpulsesequences,
and I 900signalaverages
. A 260Hz Gaussianline broadening
wasapplied.The middlespectrum
is simulationswithout consideringchemicalshift anisotropy;
whereas
the bottomonehastakenit into account.BoththeMAS
and static spectrawere obtainedon an approximatelyl7-mg
'?O).
sample(500/o
200 150 100 50
of a narrow peak that is attributable to NBO and a broader doublet that is attributable to BO (Fig. 4). In the MAS
NMR spectraof the sodium silicate glasses,these two
patterns overlap, as was briefly mentioned in Kirkpatrick
(1988), but the narrow component clearly increasesin
relative intensity with increasingNa2O content, suggesting that it is the NBO peak (Fig. 5). Thesespectracan all
be reasonablysimulatedwith a narrow NBO and a broader BO pattern,by constrainingthe ratio of NBO and BO
to that required by stoichiometry and by applying a large
Gaussianline broadeningfunction (500-1900 Hz) to account partially for the disorder (Figs. 4 and 5). The a
values were also fixed to 0 in these simulations owing to
a lack of constraints.Becauseof the simplicity of these
assumptions,the actual uncertaintiesin the mean NMR
parameters (listed in Table 2) are probably larger than
thoseassociatedwith the simulations.The derived mean
e'zqQ/hand 6, values for BO are similar for both the potassiumand the sodium silicateglasses.There is a slight
increasein the mean 6, value of BO in sodium silicate
glasseswith increasingNarO content,consistentwith the
trend observedin crystallinesilicates(e.g.,cristobalitevs.
a-NarSirO,; see Table 2). The mean e2qQ/hvalues of
NBO are also similar for the potassiumand sodium silicate glasses,but the NBO in the KrSioOnglasshas a
much largermean 6,than thoseof sodium silicateglasses,
which is responsiblefor the betterresolutionof the NBO
peak and the BO doublet in the former.
0 -50 -i00 -150-a00
PPM
Fig. 4. Experimental
and simulated'?OMAS NMR spectra
of a KrSi.Onglassquenchedfrom melt at I atm. The experi(l0o/o
mentalspectrum(top) was acquiredon a 96-mg-sample
'?O),with a 0.5-pspulselength,a 0.2-sdelay,and 2000signal
averages.
Thesamplespinningspeedwasl0 kHz.Thespinning
sidebandis markedby a dot. A 50-Hzexponentialline broadeningwasappliedto the spectrum.
The static NMR spectraof the sodium silicate glasses
may be decomposedinto a broad outer doublet and a
narrower inner doublet (Fig. 6). The relative height of the
latter increaseswith increasingNarO content. We may
thus attribute the former to BO that have a large mean
e'?qQ/hvalue and the latter to NBO with a smaller mean
e'zqQ/h,consistentwith the MAS NMR spectra.These
static NMR spectra cannot be simulated well with the
parametersderived from the MAS NMR spectraby assuming an absenceof chemicalshift anisotropy,as is the
casefor the crystallinephases.We have not attemptedto
further simulate these spectra, given the complications
associatedwith the disorderednature of thesematerials.
Characteristic r?O NMR parametersof various
types of O sites
We have plotted in Figure 7 the e'zqQ/hparameter
against6,for the various typesof O sites(NBO, BO, torsi-
XUE ET AL.: I?O NMR PARAMETERS IN HIGH-P SLICATES
NazSizOs
37
A
NazSitO'r
/1
iV
100
0
r00
100
,gn
t\
100
Fig. 5. Experimental
and simulated''O MAS NMR spectra
glasses
quenched
Na,SirOr,
of
NarSi,Or,andNarSi4Oe
from melts
at I atm. Thesespectrawereacquiredon samplesof approxi''O), with a pulseIengthof 0.5-l ps,
mately100-145mg (10o/o
a delayof0.2-l s,and 14000-54000
signalaverages.
The samplespinningspeeds
werel0-11 kHz.A 50-Hzexponential
line
broadeningwasappliedto the spectrumof the Na,SioO,glass,
anda | 00-HzGaussian
linebroadening
wasappliedto thoseof
theNarSirO,andNa.Si.O,glasses.
O-r61Si,
and FrO)in alkali silicateand silica crystalsand
ambient pressureglasses.Becauseof the known variations of theseparameterswith the type of network-modifying cations,we have not includedthe data for alkaline
earth silicates reported by Timken et al. (1987) and
Mueller et al. (1992). The figure clearly shows that the
different types of O sites can be distinguished by one or
both of the two parameters.
NBO are characterized by small erqQ/h values (approximately 2.3 MHz). The d, values of NBO are very
sensitiveto the type of network-modifyingcations,and
a largercation causesa more positive 6,(76 ppm for potassiumtetrasilicatevs.34-45 ppm for sodium silicates),
consistent with previous studies (Timken et al., 1987
Mueller et al., 1992). In addition, out data suggestthat
the D,values of NBO may be sensitive to the intertetrahedral angle of the Si to which the NBO is bonded as
well. The D,value of NBO in the high-pressure
e-NarSirO,
phase,which we suspecthas an unusuallysmall Si-O-Si
angle within the silicate sheets,is shifted by about l0
ppm to a higher frequency, compared with that of
o-NarSirO, (seeTable 2).
BO and t4rsi-O-rrsisites have similar e2qQ/h values
(4.5-5.7 MHz), which are intermediatebetweenthoseof
NBO and t3lo.The two can be distinguishedby 6,,which
is considerablyhigher for the latter (97 ppm for r4rsi-Ot6rSivs. 40-65 ppm for BO). It might be expectedthat O
atoms in the t4lsi-O-rsrsi
linkageswould also have similar
e'zqQ/hvaluesand perhapsD,valuesintermediatebetween
those of normal BO and t4lsi-C)-t6rsi.
It has been shown
previously(Timken et al., 1987;Mueller et al., 1992)Ihat
the 6, and e'1qQ/hvalues of BO are relatively insensitive
-200
400
-600
ppm
Fig. 6. The t'O static NMR spectraof NarSirOr,NarSi,Ot,
and NarSioO, glassesquenchedfrom melts at I atm. AII spectra
were acquired with the solid-stateecho pulse sequenceand a l-s
delay between pulse sequences.The spectrum of the NarSi.O,
with 4000
glasswas obtainedon a 154-mgsample(48.60lo'?O)
signalaverages;those of the NarSirO, and NarSioO,glasseswere
obtainedon samples100-125 mg (100/o''O) with about 50000
signal averages.A 100-Hz exponential line broadening was applied to all.
a
rll
a-ro
N
IT
I
t4tsi_o_t6lsi
d
ds r +
al
(.)
20 30 40 50 60 70 80 90 100 110 120
4 (pprn)
Fig. 7. The parameter e2qQ/h vs. D,for various types of O
sitesin sodium silicate,potassium silicate,and silica crystalsand
ambient pressure glasses.Errors are mainly smaller than the
symbols. Solid symbols denote potassium silicates; open symbols denote sodium silicates:shadedsymbols denote silica.
XUE ET AL.: '7O NMR PARAMETERS IN HIGH-P SILICATES
38
Farnan et al. (1992)
6.0
o-NarSirO.
N
55
;q
Y
cnstobatlte
5'o
s
ot
4.5
wa.0crre
4.0
120
t40
160
Si-O-Siangle(')
180
Fig. 8. The parametere2qQ/hvs. Si-O-Sianglefor bridging
O atoms.Thecurveis derivedby Farnanet al. (1992)from r7O
glass.Thee2qQ/h
dynamicanglespinningNMR dataof a KrSioOn
valueof diopside(4.39 + 0.05 MHz) is from Muelleret al.
(1992);therestarelistedin Table2. Errorbarsaresmallerthan
the symbolwhennot shown.
to the type of network modifiers compared with those of
NBO. The resultswe obtained for ambient pressurepotassium and sodium silicate glassesare consistent with
that conclusion (seeFig. 7). We may expect the same for
O atoms in the IalSi-O-t61Si
or I4lSi-O-taSi
hnkages.
Both quantum mechanical calculations(Tossell and
Lazzerctti,1988;Tossell,1990)and a'7O dynamicangle
spinning NMR study (Farnan et al., 1992)have suggested
that the e'zqQ/hvalue of BO increaseswith an increasing
intertetrahedral angle. Our data are qualitatively consistent with such a conclusion,as shown in Figure 8, where
we have plotted Ihe e,qQ/h value againstthe Si-O-Si angles for a-NarSirOr, wadeite-KrSioOn,
cristobalite(SiOr:
SpearingeLal., 1992),and diopside(CaMgSirOu:Mueller
etal., 1992).However, the e,qQ/h values of thesecrystals
are in general slightly lower than those predicted from
the curve derived by Farnan et al. (1992) from dynamic
anglespinningNMR data of a K,SioOeglass(seeFig. 8).
This offset is not surprising, given that the absolute position of the curve was calibrated indirectly with data for
a single material. There are likely to be other structural
effects that perturb the dominant Si-O-Si angle correlation. It is conceivablethat the e2qQ/hvaluesfor O atoms
in the roiSi-O-l6rsi
and t4rSi-O-tslsi
linkagesalsovary somewhat with the Si-O-Si angle.
Jls IrtQ site in stishovite distinguishesitself by its large
e'qQ/h and 6, values.Its e2qQ/hvalue (6.5 MHz) is the
largestamong O atoms in all known silicates(3.7-5.8
MHz for normal BO and 1.6-3.2 MHz for NBO), and its
6,is also largerthan any normal BO (40-87 ppm) (Timken
et al., 1986a,1986b,1987;Walter et al., 1988;Mueller
etal.,1992; and this study).Recentquantum mechanical
calculations have also suggestedthat threefold-coordinated O atoms (with a mean Si-O bond leneth and mean
Si-O-Si angle similar to those of stishovite)have a significantly higher ezqQ/h value and a lower O NMR
shielding (larger D,) than twofold-coordinated O atoms
(BO) (Tossell,1990).We may expectthat in silicateswhere
additional network-modifying cations also interact with
pro, O atoms coordinatedto three Si atoms, the ezqQ/h
and 6, values for trlO may vary somewhat. In the caseof
normal BO, the d,value of BO in a pure SiO, phase(e.g.,
cristobalite, 40 ppm: Spearinget al., 1992) is lower than
thosein all alkali and alkalineearth silicates(55-87 ppm:
Timken et al., 1987;Mueller et al., 1992 and this study;
also see Fig. 7). This may be explained by the greater
deshieldingofthe O nuclei in the presenceofadditional
network-modifying cations. The observed increasein D,
of BO in sodium silicate glasseswith increasingNarO
content noted above is also consistent with the trend. If
a similar trend holds forl3rO,we may expectevengreater
valuesofd, for tnO sitesin alkali or alkalineearth silicates
than in silica (stishovite).Also by analogywith normal
BO, there may be a variation in the etqQ/h value of I3lO
with the Si-O-Si angle.
It should be noted that there are severalother types of
O sitesthat may appearin high-pressuresilicateglasses
and melts, for which we do not yet know the '?O NMR
characteristics.Examples of these include O ions linked
to two r6rsiby either corner or edge sharing. The former
appears in high-pressure perovskite-type MgSiO, and
CaSiO,phases(Horiuchi et al., 1987;Liu and Ringwood,
1975),whereasthe latter can be seenin the ilmenite-type
MgSiO. phase (Horiuchi et al, 1982). When there is a
high concentrationoft6lSiin high-pressure
silicateglasses
or melts, such O sites may become significant. It is possible that both typesof O sitesmay have even larger'7O
d,valuesthan thoseof O atomsin the r4lsi-O-t61si
linkages,
but their etqQ/h valuescould be either smalleror larger.
Thus these types of O sites could be indistinguishable
from I3lOin '?O NMR spectra.Direct measurementson
high-pressurecrystallinephasescontaining such O sites
would clearly be useful.
Trrn r7O NMR oF ALKALTsrLrcATE GLASSES
QUENCHEDFROM MELTS AT HIGH PRESSURE
KrSi4Oeglasses
As discussedabove, the '?O MAS NMR spectmm of
the KrSioOnglass quenched from melt at I atm can be
simulated with a narrow NBO peak and a broader BO
doublet. In Figure 9, we compare the '7O MAS NMR
spectrumof a KrSioO,glassquenchedfrom melt at 6 GPa
with the above. Both the narrow NBO peakand the broad
BO doublet shift by about 3 ppm to a higher frequency,
from I atm to 6 GPa. The narrow NBO peak also becomes about 200lobroader, and its relative intensity increasesby a similar amount for the glassquenched at 6
GPa. There is no indication of an increasein the width
of the BO peak, which could result in a reduced relative
intensity becauseof instrumental dead time. We have not
attempted detailed quantification of the spectra because
XUE ET AL.: '7O NMR PARAMETERS IN HIGH-P SILICATES
of the uncertainties in fitting broad, overlapping quadrupolar powder patterns.
The shift of both the narrow NBO peak and the BO
doublet to a higher frequency may be explained by an
increasein the mean intertetrahedral angle with pressure
that has been suggested
by our previous Raman and teSi
NMR study (Xue et a1., l99l). There are at least two
possibilitiesfor the causeofthe broadeningofthe narrow
NBO peak.One possibilityis simply that there is a greater disorder in the NBO sites due to an increase in the
population of NBO sitesthat are linked to Si having small
intertetrahedral angles, and there is no significant concentration of new O species.However, there is no reasonable structural model that can account for the apparent growth of NBO relative to BO without the appearance
of a third type of O species.Our earlier "Si NMR study
has revealedthat the sample contains about 40loFrSi,and
(Xue et al., 199l). Becausethe formation of every
2ol0I61Si
ItSi or t6lsi site would createup to five to six O sites that
arelinked to them, we may expectthat the concentrations
of new types of O sitesthat are linked to tsrSior t6lSiare
significant,evenat 6 GPa. A comparisonof the r7OMAS
NMR spectraof theseglasseswith that of wadeite-Krsi4oe
showsthat the high-frequencyhalfofthe doublet for the
linkageoverlapswith the NBO
O atom in the IotSi-O-torSi
peak becausesuch an O site has a higher D,than normal
BO (Fig. 9). Thus the increasedintensity and the apparent
broadening and shift to a higher frequency of the NBO
peak could be causedby the formation of new O sites in
(possiblyslsq la1$i-Q-rstSi)
linkages.At the
the t4rSi-O-t6rSi
concentrationlevel for t5lSiand t6lsi in this sample,we
may expectthat O atoms linked to two or more highly
coordinatedSi atoms are insisnificant.
20 0
100
0
39
- 100
-200
PPm
Fig. 9. The '7O MAS NMR spectraof K'SioOnglasses
quenchedfrom meltsat I atm and 6 GPain comparisonwith
at
The spectrumof the glassquenched
that of wadeite-Krsi4or.
I atm is identicalto that in Fig. 4. That ofthe glassquenched
at 6 GPawasacquiredfrom a sampleof approximatelyl0 mg,
with a 0.5-pspulselength,a 0.2-sdelay,and 196000signal
The samplespinningspeedwasabout6-10 kHz. A
averages.
wasappliedto bothspectra.
line broadening
200-Hzexponential
NarSioOoglasses
The spectrumof wadeiteis identicalto Fig. 2. A 50-Hzexpo'7O
MAS and static NMR spectra nential line broadeningwas applied.Spinningsidebandsare
Comparisonsof the
of NarSioOnglassesquenchedfrom melts at I atm and 6, markedby dots.
8, and l0 GPa are shown in Figuresl0 and I l. As shown
earlier, the '7O MAS NMR spectrum of the NarSioOn
The spectralchanges
glassquenchedat I atm can be simulated with a narrow the line shapesamongtheseglasses.
NBO peak and a broader BO doublet, similar to that of from I atm to 6 GPa cannot be simply explained as an
the KrSioOnglass,exceptthat the resolution betweenthese overall shift of the spectrum to a higher frequency, astwo O sites is reducedbecauseof the smaller mean d, for sociatedwith a decreasein the mean intertetrahedral anNBO in sodium silicates(seeFigs. 5 and 10). The '7O gle, becausethe intensity increaseson both sides of the
static NMR spectrum of this sample is also composedof narrow NBO peak. It is more likely that the number of
a broad doublet attributable to the BO and a narrower O sites that have NMR peakson one or both sidesof the
doublet inside it due to the NBO (seeFigs. 6 and I l).
narrow peak has increased.As in the case of KrSi4Oe
When the '?O MAS NMR spectrum of the glass glasses,we know from 2eSiNMR that NarSioOnglasses
quenched at I atm with that of the glassquenched at 6 quenchedfrom 6 to l0 GPa contain considerableamounts
GPa are compared, the latter showslarge, unresolvedin- of t5rsiand r6lsi(3-7 ar'd 2-50/o,respectively:Xue et al.,
linkage in sodium
tensity increaseson both sides of the narrow NBO peak 1991).An O atom in the tatSi-O-t6tSi
(Fig. l0). Its relative height decreasescorrespondingly. silicates may have a similar '?O NMR pattern to that in
The maximum of the NBO peak also shifts slightly to- wadeite-KrsioOnbecausewe expectthe 6, and e2qQ/hof
ward a higher frequency. The '?O MAS NMR spectraof such O sitesto be insensitive to the type of network modthe glassesquenchedat 6, 8, and l0 GPa are all very ifiers (seeabove). Such an O site would thus yield a dousimilar. Becauseof the baseline problem associatedwith blet in the '7O MAS NMR spectmm that is similar in
spectrometerdead time and the relatively small sample width, but displacedto a higher frequency,comparedwith
sizes(10-20 mg), it is difficult to resolvedifferencesin that of the normal BO. The intensity increaseson either
40
XUE ET AL.: '7O NMR PARAMETERS IN HIGH-P SLICATES
1000
800
600
400
200
0
-200
-400
-600
glasses
Fig. 11. The ''O staticNMR spectraof Na.SioOn
quenched
from meltsat I atm and 6, 8, and l0 GPa.All spectra
wereacquiredwith the solid-stateechopulsesequence
with a
-100
-200
at I atm
ppm
delaytime of I s. The spectrumofthe glassquenched
200
L00
0
quenchedat
is identicalto that in Fig. 6. Thoseof the glasses
glasses
Fig. 10. The I7OMAS NMR spectraof Na,SioOn
I'O
highpressure
wereacquired
on samples
of 20-24mg(48.60lo
quenched
from meltsat I atm and6,8, and l0 GPa.Thespec''O
quenched
glass
glasses
l0o/o
for
for the
at 6 and 8 GPa;
the
trum of the glassquenchedat I atm is identicalto that in Fig. quenched
A
at 10GPa),with 100000-125000
signalaverages.
5. The spectraof the glassesquenchedat high pressurewere
1500-HzGaussianline broadening
wasappliedto all spectra.
I'O
acquired
on samples
of approximately
10-24mg(48.60/o for
'iO for the glass
the glassesquenchedat 6 and 8 GPa, 100/0
quenched
at l0 GPa),with a pulselengthof 0.5-0.6ps,a delay
of0.2-l s, 59000-304000
signalaverages,
and a samplespinningspeed
linebroadening new feature cannot be accounted for by the presenceof
of 10-l I kHz.A2OO-Hz
exponential
linkages,suggestingthe forwas applied to all spectra.Spinningsidebandsare marked O atoms in the tatSi-O-t6tSi
mation of yet another new type of O site. However, we
by dots.
do not seeany featuresdue to such an O site with extreme
NMR parameters in the MAS NMR spectrum of this
side of the narrow NBO peak betweenthe NrSioOnglass sample.That is possiblybecausethe peakis so broad that
quenchedat I atm and that quenchedat 6 GPa are thus the signalwas mostly lost during the spectrometerdead
explainableby the developmentof new O sitesin the t4rSi- time, as these spectrawere measuredwith single pulses.
problem hasbeeneliminated for the staticNMR
O-t6rSi(and possiblyalso t4rSi-O-rsrSi)
linkagesin the sam- The latter
ple quenchedat higher pressure.The slight shift of the spectrum by use of an echo pulse sequence.The large
narrow NBO peak to a higher frequency could indicate e'1qQ/hor large d, of this new type of O site would be
t6tSiatoms(such
that there is also a decreasein the mean intertetrahe- consistentwith one that is linked to three
in
we
cannot
rule out the
as
that
stishovite),
although
dral angle.
The '7O static NMR spectra of NarSioO, glasses possibility that it may also be causedby O atoms linked
t5lSiatoms (seeabove). In either case,it
quenchedat pressuresof I atm to 8 GPa also show a to two r6rsior
would
suggest
that
some of the rrsi (and possibly also
continuous decreasein the relative intensity of the NBO
ttrSi)atoms have started to connect with eachother in the
(Fig.
doublet
I 1),consistentwith the developmentof additional O types that have broader quadrupolar patterns, Na.SioO,melt at 10 GPa. This is probablyto be expected
tsiSi(70lo)and t6rSi(50/o)
as suggestedthat the MAS NMR spectra. In the static from the high concentrations of
(Xue et al., 199l).
in
known
from'7eSi
NMR
the
sample
NMR spectrum of the glassquenchedat l0 GPa, an additional feature appearson the high-frequencyside ofthe
SurrrNr.c.nyAND CONCLUSTONS
spectrum (around 400 ppm). This peak is likely to be the
high-frequency maximum of a quadrupolar pattern for
Our study of model crystalline silicateshas shown that
an O site with an extremely large e2qQ/hor large 6,. This 'iO NMR is uselhlfor studyingO environmentsin silicates
XUE ET AL.: '?O NMR PARAMETERS IN HIGH-P SLICATES
quenchedfrom high pressureand that NBO, BO, t4rsi-Ot6tSi,and t3lOcan be distinguishedby their characteristic
e2qQ/hand 6,.
The '7O NMR spectraof a KrSioOnglassquenchedfrom
the melt at 6 GPa and NarSioO,glassesquenchedfrom
melts at pressuresup to l0 GPa can be interpretedas
(and
indicating the presenceof O sitesin the tarSi-O-t6rSi
possiblyalso t4rSi-O-tsrSi)
linkage.This resultis consistent
with our earliermodel that the formation of t6rsiand ItSi
in alkali silicatemelts (glasses)
is accompaniedby a decreasein the number of NBO that are forced to connect
with these highly coordinated Si atoms, resulting in a
greater polymerization of the structure. In addition, the
I70 static NMR spectraof the NarSioOnglassesalso suggestthat anothernew type ofO site, possiblyone that is
connectedwith two or three t6rsi(orttlSi) atoms,may begin to developat approximately 10 GPa. Thus, 16rSi
and
I5rSimay be largelyisolatedform eachother in the KrSioOn
melt at 6 GPa and the NarSinOnmelt at 6-8 GPa, whereas above l0 GPa, clusteringof theseSi speciesmay begin
in NarSioO,melt.
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High pressureresearchin geophysics,p 5ll-524. Reidel, Tokyo.
for help with synthesizingthe wadeite sample,and ToshitsuguFujii at the
Mueller, K.T., Baltisberger,J.H., Wooten, EW., and Pines,A. (1992)
EarthquakeResearchInstitute, Tokyo University, for permission to use
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the multianvil high-pressureapparatusin his lab. We are also gmteful to
using dynamic-angle spinning NMR Joumal of Physical Chemistry,
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MnNuscnrm RECETvED
Fesrulnv 12, 1993
MeNuscmer AccEprEDSsprrr'lsen 20. 1993