American Mineralogist, Volume 65, pages 690-710, 1980
Relations betweenthe anionic structure and viscosity of silicate meltsa Raman spectroscopicstudY
BJ9RN O. MYseN, Devn
Vnco AND CHRIsToPHERM' Scanrn'
GeophysicalLaboratory, CarnegieI nstitution of lAashington
D. C.20008
Washington,
Abstract
Raman spectroscopyof quenched silicate melts has been used to determine their anionic
structure as a function of bulk composition and pressure.The structural results have been integrated with published data on physical and chemical properties of the melts.
The structure of silicate melts on the joins NarO-SiOr, CaO-SiO2,CaO-MgO-SiO2, and
probably other analogous binary joins, can b€ described in terms of anionic structural units
ih"t, oo the average,have NBO/Si : 4, 3, 2, l, and 0 (NBO/Si: nonbridging oxygensper silicon). Distinct compositional regions involving only three of these structural units have been
delineated.Thesecompositionalranges(0--20, -20-50, and >50 moleVometal oxide) coincide with compositional ranges defined by thermal expansivity data and data on the activation energy of viscous flow.
Melts on the join NaAlOr-SiO, have a three-dimensional array of SiOo and AlOo tetrahedra. Two three-dimensional units, which di-ffer in Si/(Si + Al), coexist' Both structural
units becomemore aluminous as the bulk Si/(Si + Al) of the systemdecreases.The structure
of thesemelts remains essentiallyunaffected as the pressureis increasedto 38 kbar. At I atm,
the viscosity of melts on this join decreaseswith increasing NaAlOz/SiOr. This decreaseis
related to the increased proportion of weaker Al-O relative to Si-O bonds in the three-dimensional structure. The decreaseof the melt viscosity with increasing pressureis related to
an increasein the number of nearestoxygen neighbors around the metal cations' We suggest
that each Al-O bond in the structure is weakenedas the result of this process,which results in
the lowering of the melt viscosity.
The Raman spectraof melts in the systemNarO-AlzOr-SiO, with AllNa > I indicate that
nonbridging oxygensare present in the melts. Some aluminum is, therefore, no longer in tetrahedraicolrdination in the melt. This structural change is responsible for the observeddecreasein viscosity and activation energy of viscous flow as Al/Na is increasedabove l.
Inasmuch as theseprocessestake place at pressures
ranging from I atm to those correspondingto the upMost of the dynamic processesof magma formaper mantle, it is also necessaryto understand the
tion and evolution take place in the earth's crust and
of the viscosity of silicate melts.
dependon the pressuredependence
upper mantle. Most of theseprocesses
Such information has recently become available
viscosity of the magma. It has been shown (e.g.,
(Kushiro, 1976, l978a,b,c;Fujii and Kushiro, 1977;
Marsh, 1975; Arndt, 1977) that the percentageof
Kushiro et aI., 1976;Scarfe et al., 1979).Scarfe et al.
melting necessaryfor separation of partial melts and
noted that the viscosity of highly polymerized silicate
residual crystals depends on the melt viscosity. The
melts decreaseswith increasing pressure under isotemperature gradient in a magma chamber required
thermal conditions. Less polymeraed melts (e.g.,
to induce convection also dependson the viscosity of
melts of meta- and disilicate compositions) show a
the magma (e.g., Shaw, 1965;Bartlett, 1969). Furviscosity increase with increasing pressure. Hydrous
thermore, the capacity of a magma to carry material
melts also show an increasein their viscosity with inin suspensionis directly relatedto the melt viscosity.
creasingpressure(Shaw, 1963;Kushiro, 1978c).
I On leave from the University of Alb€rta, Edmonton, Canada.
The viscousbehavior of simple binary and ternary
Introduction
0003-004x/80/0708-0690$02.00
690
MYSEN ET AL; STRUCTURE OF SILICATE MELTS
melts in systemssuch as CaO-AlrOr-SiOr, MgO_
Table l. Composition of starting materials (mole Vo)
AlrO3-SiO2,NarO-AlrOr-SiOr, and KrO-AlrO3-SiO,
has been used to model the anionic structureof siliSysten
Na^G-S10
-22
cate melts (e.9., Bockris et al., 1955; MacKenzie.
NS3
NS5
NS9
NS2
NS20
1960; Riebling, 1964, 1966; Bockris and Reddy, s i 0 ? 6 6 . 7
75.0
80.0
90.0
95.0
Na20
25.0
33.3
20.0
10.0
s.0
1970).
Sysr@ cao-sio2
In simple binary metal oxide-silicate systems,the
CaSi2O5
wL50
wL25
viscosity (a) and activation energy (Ea) of viscous
Wo
Sw70
sw40
sio.
41.7.
45.8
50.0
55.0
60.0
66.7
flow of the melts have beenusedto model their struc- c a o ' 5 8 . 3
54-2
50.0
45.0
40.0
33.3
ture. In thesemodels ("discretepolyion model," see
Sysrem CaO-Mgo-Si02 (CaluC = 1)
Bockris and Reddy, 1970,p. 614 for summary),three
DM95
DM85
DM58
SDTO
DM4O
Di
SD4O
sio2
34.2
35.8
40.4
43.4
50.0
55.0
60.0
distinct compositionalranges(molar metal oxide-sil- l{90
32,9
29,8
28.3
32.1
25.O
22.5
20.0
CaO
2
9
,
a
3
2
.
9
3
2
,
7
2
8
.
3
2
5
.
0
2
2
.
5
20.0
ica ratio <20, 2V50, and >50) have been defined
within which specific anionic structural units coexist.
Sy6ten Na2O-Al203-Si02
This model has subsequentlybeen refined with the
Jd
Ab
AS 50 Nd1si7o16
SAN 3
SA 10
aid of thermal expansivity and partial molar volume s i o 2 5 0 , 0 6 6 . 6
87.5
90.8
93.9
75,O
80.0
A12O125,0
16.7
r2.5
10.0
6.3
6.2
6.1
data (seeRobinson, 1969,for review of data).
Nato- 25.0
76.1
12.5
10.0
6.3
3.0
Rheologicaldata have also been used to conclude
that melts on joins such as NaAlOr-SiOr, Cay''JrOo_
SiOr, and MgAlrOo-SiO, have a three-dimensional joins CaO-SiO,, MgO-CaO-SiO, (CalMg : 1;,
network structure(Riebling, 1964,1966).Subsequent NarO-SiOr, and AlrOr-SiOr. With the possibleexRDF (RDF: radial distribution function) data by ception of AlrOr-SiOr, published spectroscopicdata
Taylor and Brown (1979) have confirmed this con- (e.g., Brawer and White, 1975, 1977; Verweij,
ilusion. Rheologicaldata in the systemNarO-AlrOr- l979a,b; Furukawa and White, l98O; Yirgo et al.,
SiO, have also been used to infer that nonbridging 1980)indicate that in such melts, the ratio of nonoxygensexist in melts where Na/Al is di-fferentfrom bridging oxygen to silicon (NBO/SD is a simple
I (Riebling, 1966).
function of the ratio of metal cations to silicon (M/
Rather than relying on propertiesof melts to infer Si). The systemAlrO3-SiO, was chosento determine
their structure,the structure may be determineddi- whether Al3* is a network former or a network modirectly. A model thus generatedcan be integrated fier in the absenceoflocal charge-balanceofthe tetwith data on physical and chemical properties. In an rahedral cation with a mono- or divalent metal. Day
att€mpt to reach this goal, the anionic structures of a and Rindone (1962a,b) suggestedon the basis of Xbroad range of silicate melts have been determined ray data that aluminum is a network modi-fierwhenwith Raman spectroscopy at both I atm and high ever the molar metal oxide-aluminum oxide ratio is
pressure.
lessthan l. On the other hand, Lacy (1963)has sugIn this paper, the structural interpretation of the gested that Al3* probably is not in AlOu octahedra
Raman data is presentedfirst. These data are com- becausesuch an arrangement is energetically unfapared with the structural models of silicate melts de- vorable. Lacy thereforesuggestedthe AlO.-tricluster
rived with other techniques. The structural model is concept with Al'* in tetrahedral coordination. Melts
then integrated with data on physical and chemical along a join from Al,O, (10 wt 7o)SiO, (90 *Vo) to
properties (mainly rheological data) and fnally ap- AlrO3 (10 wtTo),Na,O (6 vrr%o),
SiO, (84 wt%o)were
plied to more complex magma compositions.
therefore studied, in order to relate the Na/Al of the
melt to the structural role of Al'* of the melt.
Starting compositions
The third group of melts was on the join NaAlOrOn the basis of the physical propertiesof silicate SiOr. The viscosity of melts on this join decreases
melts, a study of their anionic structure can conve- rapidly as a function of decreasing Si/(Si + Al)
niently be divided into three compositional groups. (Riebling, 1966), despite observationsthat melts
Theseare binary metal oxide-silicate melts, alumino- along this join all seemto have a three-dimensional
silicate melts with Al3* charge-balancedwith metal network structure (e.g., Taylor and Brown, 1979;
cations, and aluminosilicatemelts where Al3* is not Navrotsky et al.,1980). The viscosityof melts on this
charge-balanced.The compositions are shown in join decreaseswith increasing pressure (Kushiro,
Table 1. The first group includesbinary melts on the 1976,1978a).This observationhas led to suggestions
Ne
MYSEN ET AL.: STRUCTURE OF SILICATE MELTS
that Al3* may undergo coordination changeswith increasingpressure(Kushiro, 1978a),although Sharma
et al. (1979) indicated that Al'* remains in tetrahedral coordination even at high pressure.Scarfe et al.
(1979) also noted that melts on this join are significantly more compressiblethan Al-free melts with a
significant number of nonbridging cations. Selected
compositions on this join were therefore subjected to
pressuresup to 38 kbar.
Experimental technique
The startingmaterialswere preparedfrom spectroscopically pure (Johnson and Matthey) SiOr, AlrOr,
MgO, and CaCOr, and reagentgradeNanCOr.These
oxides (in batches of less than 200 mg) were ground
under alcohol for more than I hour and dried at
about 400oC. All quenched melts were stored at
ll0'C prior to acquisition of the Raman data. The
quenched melts were made at selectedtemperatures
and pressureswithin the rangeswhere viscositiesand
densities had been determined. One-atm samples
were made by quenching the melts in liquid nitrogen
in.a vertical quenchfurnace.The quenchingrate was
approximately 500'C/sec over the first 50G-1000'C
cooling. fligh-pressure sampleswere prepared in a
solid-media, high-pressure apparatus (Boyd and
England, 1960)with furnaceassembliesof 0.5" diameter. The piston-out technique with 4 percent friction
correction (catibrated againstthe quartz-coesite transition) was used. The temperatureswere monitored
with Pt-Ft90RhlO thermocouples. The uncertainty
of the temperatureis 6-l0oC, dependingon the temperature of the experiment (Mao er al., l97l). Thle
uncertainty of the pressureis +l kbar (Eggler, 1977\.
The quenching rate in the solid-media, high-pressure
apparatus was approximately 250"C/sec over the
first l000oC.
All samples(10-30 mg) both at I atm and at high
pressure,were prepared in sealedPt capsules.In the
one-atm experiments,alkali metals tend to vefutiliz6
(Kracek, 1930;Robinson, 1969;Seifert et al., 1979a)
when exposedto air or gasessuch as CO, and CO.
This problem is circumvented by sealing the capsules. High-pressure sampleswere sealed in Pt capsulesin order to avoid accessof water and other contaminants from the furnace assenbly to the sample.
The Raman spectra were taken on small chips of
quenchednelt (about 0.5-1.0-mm cubes)that were
free of bubbles. The spectrawere recorded with a Jobin-Yvon optical system, holographic grating,
double monochromator (HG25) and a photon-counting detection system.The spectrawere recorded at 3
cm-'lsec. The sampleswere excited with the 488.0
nm line of an Ar* laser using a laser power of 200400 mW at the sample with 90' scattering geometry.
Polarized spectrawere obtained with the focused exsiling beam parallel to the horizontal spectrometer
slit and with the electric vector of the exciting radiation in a vertical orientation. A sheetof polarizer disk
in front of an optical scrambler was used to record
separately the parallel and perpendicular components of the scatteredradiation.
As a matter of routine, replicate spectra from the
same chips, from different chips of the same experimental run product, and from duplicate experiments
were also taken.
The Raman spectra are sensitive to variations of
sample composition, which may result from inappropriate sample preparation. Seifert et al. (1979a)
showed, for example, that sodium loss from melts on
the join NarO-SiO, results in changesof the highfrequency envelopeas Na loss results in a decreaseof
NBO/Si of the melt. The spectrado not indicate such
effects.
As discussedbelow, whenever Na/Al differs from
I for melts on the join NarO-AlrOr-SiO2, Raman
bands due to nonbridging oxygenswill appear. Thus
any heterogeneityof Na/AVSi of the sampledue to
inappropriate sample preparation will result in the
appearance of bands in the Raman spectra due to
nonbridging oxygens.Inasmuch as such bands were
never observed,we conclude that the quenchedmelts
were homogeneous.
Incomplete melting of the very viscous melts on
the join NaAlOr-SiO, will result in Raman spectra
with sharp bands, provided that the crystallites are
larger than 20-50A (Brawer, 1975).In this respect,
the Raman technique is more sensitive than optical
X-ray, or electron microprobe methods. The absence
of such sharp bands in any of the spectra indicates
that all sampleswere completely molten.
Melt vs.quenchedmelt (glass)
In the structural study of silicate melts, quenched
melts (glass) have been used. Most of the physical
properties discussedlater were obtained on molten
silicates. In order to relate the structural determinations of quenchedmelts to the structure of molten silicates, it is necessaryto document that the structural
features under consideration are not significantly affected by quenching.Riebling (1968) and Taylor et
aL (1980)found that the anionic units (silicate polymers) in melts with three-dimensional network structure. such as melt of NaAlSirO. composition, remain
MYSEN ET AL.: STRUCTURE OF SILICATE MELTS
the same as the melt is quenched to a glass. Direct
experimental proof of structural similarity between
melts and glasseson the join NarO-SiO, was provided by Sweetand White (1969)and Sharmael a/.
(1978).In thosestudies,infrared and Raman spectra
of melts of NarSiOr, NarSirOr,and NarSirOTcompo-
693
sition were comparable to the spectra of quenched
melts. It was concluded, therefore, that the anionic
structural units in these melts are the same before
and after quenching.
On the basis of the information given above, we
conclude that the structural features of silicate melts
NS5
200
300
400
500
600
700
800
900
1000 1100
1200 1300 200
300
400
500
600
Wavenumber.
cm-1
200
300
400
500
600
700
800
700
800
900
1000
1100
12m
Wavenumber,
cm-1
900
1000
r'r00
1200
Wavenumber, cm'1
200
300
400
500
600
700
800
900
1000
1100
1200
900
1000
1l0O
1200
Wavenumber,
cm'l
NS2
ltttttttttl
200
300
irco
400
5o0
500
600
600
7oo
700
800
800
wavenumber,
Wavenumber,
cm-l
cm-1
900
900
l0o0
1000
1100
1100
12OO
1200
300
400
500
600
700
800
Wavenumber,
cm-l
Fig. l. Raman sp,ectraof quenched melts on the join Na2O-SiO2 at I atm. SeeTable I for explanation of symbols.
MYSEN ET AL.: STRUCTURE OF SILICATE MELTS
694
discernible with Raman spectroscopictechniques are
quenchable, and the results given in this report are
applicable therefore to molten silicates.
Experimental results
I-atm results
Binary joins. The most comprehensiveset of structural data on silicate melts is from melts on binary
( C a + M g ) i s=i 1 5 0
Dr
1
( C a + M g ) / S= i1 0 0
(Ca + Mg)/Sr = 082
CaSi205
Ca/Si = 0 50
(Ca + Mg)/Si = 067
100 200 300 400 500 600 700 800 900 10001100 1200
cm-1
Wavenumber,
Fig. 2. Raman sp€ctra of quenched melts on the join CaOSiO, at I atm. SeeTable I for explanation of symbols. M, D, C,
and S refer to monomer, dimer, chain, and sheet species,
respectively.
rrlttll
300
400
600 700 800 900 1000 1100 1200
W a v e n u m b e rc.m ' 1
Fig. 3. Raman spectra of quenched melts on the join CaOMgO-SiO2 1CalMS : l) at I atm. See Table I for explanation of
500
symbols. See Fig. 2 caption.
MYSEN ET AL.: STRUCTURE OF SILICATE MELTS
Table 2. Raman data of quenched melts on metal oxide-silicate joins
"c
3, kbar
1650
1650
1650
1650
1650
1650
1650
1650
1650
1650
1650
1650
1650
1650
1500
1500
1500
1550
1650
1200
1200
1200
1425
1550
1650
1650
1685
1300
1300
0 . 0 01
0.001
0.00I
0.001
0.001
0.0010.001
0.001
0.001
0 . 0 01
0.001
0.00I
0 . 0 01
0.00r
0 . 0 01
0.001
0 . 0 0I
0.001
0.001
0.001
15
25
0.001
0.001
10
20
20
0.001
15
I,
DM 95
DM 85
DM 58
DM 40
DM 25
Di
sD 70
sD 40
wL 50
wL 25
Wo
sln 70
sw 40
CaSir0q
NS 2NS3
NS5
NS9
NS 20
NS2
NS2
NS2
D1
Di
Di
Di
Di
NS3
NS3
(bd),
Abbreviations:
(sh),
broad;
Uncertalntles:
w,
380w,p,(bd)
3 8 0 w , p ,( b d )
343w,p,(bd)
340w,p,(bd)
340w,p,Gd)
340w,p, (bd)
530n,p,
527n,p,
5I3m,p'
500n,p,
(bd)
(bd)
(bd)
(bd)
500w, (sh)
5 47 w ,( s h )
)c /wr (sn/
3 5 2 m ,p
350n, p
3 4 5 m , p ,( b d )
480w, (sh)
490s,p
495s,p
490s , p
4 5 0 m ,p
440s,p
430s,p
700a,p
700w'p
686m,p
660m,p
655m,p
6 30m,p
62k,p
610s,p
670s,p
647s,p
620s,p
6I3s,p
586s,p
5 93 s , p
770w, (sh)
848s,p
847s,p
860s,p
863s,p
860s,p
867u,p
873w,p
873w
850s,p
851s,p
867s,p
869w,p
863w
861w
9 77 w , p
9 78m,p
9 73n,p
973n,p
9 73s ,p
967s,p
959s,p
955s,p
960s,p
980s,p
962s,p
963s,p
957s,p
958s,p
942n'p
945w' p
950w ' p
945w
903n,p
903n,p
910s,p
908m,p
913w,p
900s,p
907s,p
5 4 0 s, p
564s,p
564s,p
790n
7908
760w
940n' p
9 3 7 8 ,p
948n, p
980s,p
983s,p
9 7 5 s, p
975s,p
978s,p
9 5 0 w ,p
950w'p
763w
880n,p
880n,p
873(sh)
8 7 3( s h )
8 7 3( s h )
633s,p
632s,p
629s,p
629s,p
636s,p
538s,p
544s,p
very
weak;
shoutder.
Strong
.n
l{avender,
w,
week;
Compositional
to
weak
bands,
m,
nediun
nora!ions
+
-5
cn-1.
Eo weak;
defioed
773n
77\r
m,
are
Very
weak
mediw;
in
Table
bands,
metal oxide-silicatejoins. It is convenient,therefore,
to discussthesedata first (Figs. l-3; Table 2). Raman
and infrared data on suchjoins have been published
by Brawer and White (1975,1977),Sweetand White
(1969),Furukawa et al. (1978),Furukawa and White
(1980), Verweij and Konijnendijk (1976) and Verweij (1979a,b).A surnmary of these data was provided by Ytugoet d/. (1980).
In the present study, the joins NarO-SiOr, CaOSiOr, and CaO-MgO-SiO, have been emphasized.
Inasmuch as the alkaline earth-silicatejoins are intersected by a liquid immiscibility gap rlear l,ol2+/
(Sio*+M'z*): 0.3, our studieswere limited to melts
with a larger M2*/(Si4*+M'z*).The NarO-SiO, join
wasextendedto pure SiOr, asthere is no stableliquid
immiscibility on this join.
(a) NarO-SiOr: The Raman spectra of six
quenchedmelts on the join NarO-SiO, are shown in
Figure I (seealso Table 2). The high-frequencyenvelopebetween800 and 1200cm-' was deconvoluted
into individual bands with Gaussian line-shape by
the following lsshnique. First it was assumedthat the
background in this spectroscopicrange was horizontal. This assumption is an approximation, as the
Rayleigh tail has an exponentialform. Extrapolation
of the Rayleigh tail from lessthan 500 cm-' to the region of interest with exponential equations indicates
lessthan 5Vodeviation from a horizontal background
at wavenumbersgreater than about 800 cn-'. Sec-
re,
medium
!o
scrong;
cn-l.
Shoulders,
s,
strong;
p,
l060D,p
106 3o, p
106 3n, p
1059w
1050n
105 3n
1050h
1060D
I060m
I050D
1060n
10608
10508
1070m
1050m,p
105ls, p
1055s ,p
105 7s ,p
1095s,p
1105s,p
109 3s ,p
1087s,p
1090s,p
1095s,P
1095s,P
1103s,p
1068 (sh)
10 70 (sh )
10 70 (sh)
10 70 (sh)
1066 (sh)
1107s'P
1105s 'p
1170w(?)
1I78n
1166D
11E0o
***(?)
***(?)
***(?)
***
***
polarized;
1.
-10
15-20
cn-l.
ond, all bands were consideredsymmetric. Third,
bands were fitted to the high-frequency envelope
only when their existencewas indicated by separate
peaks or shouldersor abrupt deviationsfrom Gaussian line shape of portions of the high-frequency envelope.
With theseconsiderationsin mind, the spectraof
quenchedmelts on the join NarO-SiO, will be discussed(Fig. l; see also Table 2). The Raman spectrum of one of the endmembers,SiO2,has been discussedin detail elsewhere(e.9.,Bell and Dean, 1970,
1972;Bateset al., 1974;,Stolen and Walrafen, 1976;
Galeener and Lucovsky, l976a,b; Lucovsky,
l979a,b). The Raman spectrumconsistsof a strong,
broad band near 430 cm-' and a lessihtense,sharp
band near 480 cm-' (Fig. l). There are weak bandsat
60f.,795,1059,and I188cm-'. All but the 795,1059,
and 1188bands are polarized.The 430 and 795 cm-'
bands representoxygen bending motions with some
contribution from the silicon atom in the 795 cm-'
(Lucovsky, 1979a).The 480 cm-' band is a rocking
band that stems from rocking of bridging oxygen
bonds in a fully polymerized, three-dimensional network (Bateset al., 1974;Bell and Dean, 1972).The
band near 600 cm-' most likely is due to bending
motions of oxygen bonds in defect structures in
quenchedSiO, melt (Lucovsky, l979a,b).This defect
is generatedby breaking one Si-O bridging bond in
the continous three-dimensional network. The defect
MYSEN ET AL.: STRUCTURE OF SILICAT:E MELTS
density in quenched,anhydrous SiO, melt is approximately l0'e cm-3 (Stolen and Walrafen, 19761.
Lttcovsky, 1979a),which inplies that about 0.017oof the
ideally bridging oxygen bonds have been broken.
The 1059and ll88 cm-' bands are due to antisymmetric stretch vibrations in a three-dimensional
array of SiOo tetrahedra. If this array is homogeneous,only one antisymmetric stretch band would be
expected (Bell and Dean, 1972). Galeener and Lucovsky (1976a,b)suggestedthat the two bands result
from LO and TO spttting. Inasmuch as LO and TO
splitting require long-range order, this interpretation
i5 unlikely in view of the disorderednature of silicate
melts. We suggestinstead that the two antisymmetric
stretch bands are due to two distinct three-dimensional structural units in the quenched melt. The
need to consider models of quenched SiO, melt on
the basis of more than one type of three-dimensional
network has also been suggestedfrom high-resolution electron-microscopicstudies (Gaskell, 1975;
Gaskell and Mistry, 1979; Bando and Ishizuka,
The 1100 and the associated570 cm-' bands are
the two strongest bands in crystalline NarSirO'
whether the c or the B forms are considered(Furukawa and White, 1980).The 950 cm-' band is the
strongest band in crystalline metasilicates(Etchepare,1972;Brawerand White, 1975).Furukawaand
White (1980) also noted that the ll00 cm-' band is
strongly polarized.The band is IR inactive (White et
al., 1979).On the basisof suchconsiderations,Furukawa and White (1980)concludedthat the ll00 cm-'
-O-Si-O"
stretching.Verband is due to symmetric
weij (1979a,b)arrived at a similar conclusion. The
570 cm-' band is due to symrnetric stretching of
bridging oxygen bonds within the same structural
unit as that giving rise to the 1100cm-' band. The
1050cm-'band is due to antisymmetricstretchingof
bridging oxygen bonds in any structure where oxygen bridges exist. The 950 cm-' band is assignedto
-O-Si-Ostretching.
symmetric
-O-Si-Ostretch vibrations nay be derived
The
from a chain or a ring structure (NBO/Si : 2) and
r97e).
With 5 mole7oNa,O addedto SiO, melt (NS20), a
new, sharp band appearsnear ll00 cm-' (Fig. l).
This band becomesmore intense relative to all the
other bands in the high-frequency envelope as the
Na*/Sio* of the quenchedmelts is further increased.
Another very weak band appearsnear 950 cm-' in
NS9 melt (Na*,/Sio* : 0.22) and grows in intensity
with increasingNa*/Sio*. Brawer and White (1975)
and Furukawa and White (1980)extendedtheir studies of melt structure on the join Na"O-SiO, to sodium metasilicate(NS). tn the compositional range
from NS2 to NS, the intensity of the 950 cm-' continued to increase and is the dominant feature of the
spectrum of quenched sodium metasilicate melt. In
the same compositional range the intensity of the
1100cm-' band showeda continuousdecrease.Neither the 1100nor the 950 cm-' band showedsi$dficant frequency changes in the compositional range
betweenNS20 (Na*/Sio* : 0.1l) and NS2 (Na*/Sio*
: l) in the presentstudy and betweenNa*/Sia* : 0.2
and 2 in the study by Brawer and White (1975).Both
the 950 and 1100cm-' bands also remain polarized
in the entire compositional range.
The band near 490 cm-r in the spectrum of
quenched NS20 melt becomesmore intense and
shifts toward 570 cm-, with increasing Na/Si. In
quenchedNS2 melt (Na*/Sio* : 1.0) the band is at
570 cm-', where it is the secondmost intenseband in
the spectrum.The 490-570cm-' band remainspolarized in the entire compositional range.
WL25
Ca/Si 1.18
trlll
800
900
1000
1100
1200
cm'i
Wavenumber,
Fig. 4. Quenched WL25 melt deconvolution using Gaussian
curyes. Seetext for detailed explanation ofprocedures.
MYSEN ET AL.: STRUCT:URE OF SILICATE MELTS
Table 3. Raman frequencies of the stretch vibrations of specifc
Si-rich anionic structural units in silicate melts
Frequgncy
(cm-I)
Characteristlcs
ot
vibrational
node
4
850-880
symetric
stretch
3
900-920
symetric
stretch
sr^0.4-
2
950-980
symetrlc
slretch
s t ^ o -2 -
1
1050-1100
symelric
streEch
sio2
0
1060,1190
antisymeEric
SEructural
unit
NBO/Si
sio 44
s7207-
stretch
-O-Si-Oo
the
vibration from a structural unit that
has NBO/S| - | (e.g.,sheet).Further discussionof
thesespecfficstructural assignmentswill be provided
after discussionof the spectraof quenchedmelts on
the joins CaO-SiO, and CaO-MgO-SiO,.
The Raman bands characteristicof three-dimensional network structural units in the melt (430, 800,
and ll88 cm-' bands) diminish in intensity in the
compositional range between SiO, and NS3. The
430, 800,and 1188cm-'bands are definitelypresent
in the spectra of melts with as much as 25 mole%o
Na,O (Na*/Sio* : 0.67). In the sodium disilicate
quenchedmelt (33 mole%o
NarO), the band near 800
cm-' remains.There is no clear band at frequencies
below about 570 cm-'. The Rayleigh tail flattens out
between400 and 500 cm-'. however.which indicates
a weak band in this spectral region. We conclude,
therefore,that three-dimensionalnetwork structural
units (3D) definitely exist in melts in the compositional rangebetweenSiO, and Na*/Sia* :0.67 (NS3
melt). This compositionalregionmay extendasfar as
Na*/Sin* : 1.0(NS2 melt).
(b) CaO-SiO, and MgO-CaO-SiO, (Ca/Mg: 11,
The Raman spectraof quenchedmelts on the joins
697
CaO-SiO, and MgO-CaO-SiO, are shown in Figures2 and 3 (seealso Table 2). The dominant feature
of each spectrum is the intense, slightly asymmetric
band in the region betweenabout 600 and 670 cm-'
combined with an intense,high-frequencyenvelope
in the region between 800 and 1100cm-'. All these
bands are strongly polarized. The Raman spectraof
quenchedmelts on the join CaO-MgO-SiO, are similar to those on the join CaO-SiO, (Figs. 2 and 3).
The high-frequencyenvelopeof the spectrais somewhat less resolved in the system MgO-CaO-SiOr,
however,presumablybecauseof more local disorder
in the former system(Brawer, 1975).All the bands,
with the exception of that in the 600-670 cm-' region, remain at the samefrequencyas M'*,/Sio* varies (Figs. 2 and 3).
There are at least two bands in these spectra that
do not occurin the compositionalrangeof the NarOSiO, systemshown in Figure I (compareFig. I with
Figs. 2 and 3; seealso Table 2). In melts with M/Si
between1.9(DM 95) and 1.18(DM 25 and WL 25),
there is a very strongsharpband near 870cm-' and a
less intense band near 900 cm-'. Their frequencies
are independentof M/Si of the quenchedmelts (Tables I and 2). The 870 cm-' band remains in all
spectraof melt compositionsfrom DM 95 to CaSirO'
(CalSi: 0.5).The 900 cm-' band disappearsat melt
compositionsbetweenDM 25 (M/Si : l.l8) and Di
(M/Si : l). The intensitiesof both bandsdecreaseas
the M/Si decreases.The intensity of the 900 cm-'
band probably passesthrough a 641imum relative to
the other bands in the high-frequency envelope in
melt compositionswith M/Si near 1.5 (DM 58 and
WL 50; seeTable l). Both bandsremain polarizedin
the entire compositionalrange.
Table 4. Raman data of quenched melts on the join Na2O-Al2O3-SiO2
T 'c
Conp.
P, kbar
wavenumber,
_l
cm -
trNe
Jd
Ab
AS 50
N a A l S i 7 0 16
Si02
SA 10
SAN 3
Jd
Jd
Ab
Ab
1550
1450
1500
l-650
1650
1750
1650
1650
1500
1550
1500
1550
Abbreviations
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
20
38
20
38
440s,p
440s,p
430s,p
4 3 0 s ,p
431s,p
-
and uncertainties:
480s,p
468s,p
4 7 0 s ,p
485(sh)
480(sh)
483s,p
4 8 3 s ,p
483(sh)
479s,p
477s,p
468s,p
467s,p
see Table
561m,p
567n,p
566w,p
595w,p
587w,p
5 9 5 w ,p
60Om,p
590m,p
5 6 6 w ,p
561w,p
573w,p
S80(sh)
2.
790w
l6l (sh)
780(sh)
193m
l93m
790n
794m 950w,p
793m 943w,p
7 8 2( s h )
790(sh)
7 9 4( s h )
795(sh)
9 4 5 w ,p
9 5 9 m p,
9 8 3 w ,p
9 8 7 n 'p
1 0 23 m ,p
1059w
1052m
1045m
9 5 5 mp'
960n'p
9 8 0 m ,p
9 7 0 n ,p
T2
1117mw,p
1120w,p
1014s
1063s
1090s
1110s
1150n
1188n
1190m
IL77n
1064s
1060s
1088s
1080s
rzlrl.
3.6
2.2
2.8
2.2
1.1
< 1
3.5
5.4
3.0
5.0
MYSEN ET AL,: STRUCTURE OF SILICATE MELTS
The 870 cm-' band is by far the most intenseband
in crystalline orthosilicates(Furukawa et al., 1978;
Verweij and Konijnendijk, 1976). Furukawa et al.
(1978), Verweij and Konijnendijk (1976), and Verweij (1979a,b) assignedthis band to symmetric
stretch vibrations of nonbridging oxygen bonds in
separateSiOl- tetrahedral (symbol:Si-O'-). This assignmentis retained here.
The 9@ cm-' band is the major band in crystalline
pyrosilicates(Lazarev, 1972; Sharma and Yoder,
1979).In view ofthe factsthat its frequencyis higher
than that of the symmetric Si-O'?-"stretchband (870
that it is polarized,and that its frequencycoin"--'), with the main band in crystalline pyrosilicates,
cides
this band is interpreted as due to the presenceof dimer structural units (Siroi-) in the melts. Its frequency is independentof NBO/Si of the melt. We
conclude, therefore, that the NBO/Si of this structural unit does not change with composition of the
melt. Further support for this interpretation is found
in the observation that the maximum intensity is
found in spectrafrom melts with NBO/Si near 3 and
the fact that it is associatedwith another band near
700 cm-'. According toLazarev (1972),the 700 cm-'
band in crystalline pyrosilicates is a symmetric
stretch vibration of bridging oxygen bonds. Its frequency is characteristicof dimers.
Structural units with NBO/Si - 2 occur in all
melts discussedso far (Figs. l-3). Providedthat there
are structural units with more than three Sio* cations,
a Raman band near 1050cm-' (antisymmetricSi-Oo
stretch band) is expected (Furukawa and White,
1980).This band could be discernedin melts on the
Na,O-SiO, join with Na*/Sio* lessthan about I (disilicate). White et al. (1979) commented that this
band also occurs in melts as depolymerized as metasilicates. It is expected that this band would also be
present in quenched melts on the alkaline earth-silicate joins (Figs. 2 and 3). In alkaline earth silicate
melts with M'*/Sio* < I, there is also a band due to
-O-Si-Oo
stretchingin the immediate visymmetric
cinity of 1050cm-'. The high-frequencyenvelopesof
these spectra do not indicate the presenceof more
than one band, except that it does not seempossible
to fit any segmentof the high-frequencylimb of the
envelopesto a single Gaussian equation. In melts
N a A l 0 2- S i O 2
S i / ( S i+ A l ) = 0 . 8
NaAlSi04
S i / S i+ A l ) = 0 . 5
Wavenumber,
cm-1
Wavenumber,
cm-1
S i / ( S+i A l ) - 0 . 8 7 5
Al) = 9.67
tttrll
800
900
Wavenumber,
cm-1
1000
1100
1200
1300
Wavenumber,
cm-1
+ Al) = 0.75
Si02
900
1000 1100
1200
1300
Wavenumber,
cm'1
800
900
1000
1100
1200
Wavenumber,cm'1
Fig. 5. Raman spectra of quenched melts on the join NaAlOrSiO2
at I atm. See text for explanation
of symbols.
699
MYSEN ET AL.: STRUCTURE OF SILICATE MELTS
-Othat are less polymerized than metasilicate, only
Si-O-, Si-O'-, and vibrations from dirners occur in
the high-frequency envelope. The high-frequency envelope of the spectrum from WL 25 (M/Si : 1.18)
indicates that the 970 cm-' is quite broad, however,
thus suggesting the presence of a possible fourth
band (Figs. 2 and3).In Figure 4, WL 25 was used in
an attempt to fit a band near 1050 cm-' using the
technique described above. In Table 2, the presence
-O-Si-Oo
of this band together with the symmetric
stretch band has been only inferred. The reported
frequency of this band is, therefore, more uncertain
than those of other bands. A summary of band positions and assignments is given in Table 3.
NaAlO2 - Si02
t.{
E
o
'{.
1100
_+o
@
*
\
tooo
-o-
i-+
-.----;
10
09
08
07
06
05
04
03
02
01
00
SiflSl + Al)
Fig. 6. Frequencies of the two Raman bands in the highfrequency envelope of melts on the join NaAlO2-SiO2.
Aluminosilicates.
(a) The join NaAlO,-SiO,: Raman data of
quenched melts of six compositions between
NaAlSiOo (Ne) and SiO, are given in Table 4. The
high-frequency envelopes of the spectra are shown in
Figure 5. The Raman data show an overall similarity
in the compositional range under consideration.
There is a broad, strong, polarized band below 500
cm-'. For melt compositions between NaAlSLO,o
(AS50) and SiO,, this band is sptt into one band
near MO cm-' and one between 480 and 490 cm-'.
The latter band develops from a shoulder to a separate band with increasing Si/(Si + Al). In addition to
these bands, all spectra show weak bands near 600
and 800 cm-'. These bands are polarized. There is,
therefore, no principal difference between the spectra
of Al-bearing melts and SiO, on this join in the range
between 400 and 800 cm-'.
The high-frequency envelope (Fig. 5) consists of
two bands that occur as distinct peaks in melts with
Si/(Si + Al) greater than about 0.8 and as two shoulders with melts that have lower Si/(Si + Al). Their
frequencies are lowered as a continuous function of
decreasing Si/(Si + Al) (Fig. 6). It also appears that
the highest frequency band becomes more intense
relative to the lower-frequency band as the Si/(Si +
Al) of the melt decreases(Table 4).
The frequencies and the polarization characteristics of all the bands below the high-frequency envelope are nearly identical to those of quenched SiO,
melt. We suggest, therefore, that these bands are due
to the same vibrations in all the melts studied on the
join NaAlOr-SiOr. On this basis, we conclude that
the melts on this join consist of an array of three-dimensional SiOo and AlOn tetrahedra. Taylor and
Brown (1979), on the basis of RDF X-ray data, also
concluded that sodium aluminum silicates with Na/
Al : I most likely have a three-dimensionalnetwork
structure,in agreementwith our conclusion.
Virgo et al. (1979) studied the frequency shifts of
the two high-frequencybandsin systemswherethe T
cation was Al3*, Fe'*, or Ga'*. The metal cations
were Na* and Ca'*. In all the compositions they
studied,the frequenciesof the two stretchbands decreasedas a function ofdecreasingSi/(Si + T). Brawer and White (1977) made similar observationsin
the system CaO-AlrO.-SiOr. Brawer and White
(1977) and Virgo et al. (1979)suggestedthat the decreaseof the frequency was due to an increasedextent of coupling of the Si-Oo and T-Oo antisymmetric stretch vibrations. Becauseof the smaller
force constantsof bonds such as Al-Oo compared
with Si-Oo, the frequency of a coupled band would
decreasewith increasingAl content of the melt.
The existenceof two stretchbands in melts on the
join NaAlOr-SiO, as well as in the other joins studied by Yirgo et al. (1979)has been suggestedby them
to stem from two types of three-dimensionalstructures in the melts. These structures di-ffer in Si/
(Si + T) (Virgo et al.,1979).Theseauthorsalsonoted
that of thesetwo bands,the highest-frequencyband
was more sensitiveto compositionalchangesthan the
lower-frequency band. They concluded, therefore,
that the three-dimensionalstructure resulting in the
highest-frequencyband had the largestproportion of
T cation. This conclusion is also adopted here. All
the quenchedmelts on the join NaAlOr-SiO, have a
three-dimensionalnetwork structure, therefore, and
the tetrahedra become increasingly aluminous with
increasingAl content of the melt. The aluminum is,
however,partitioned betweentwo apparentlydistinct
tetrahedral units. This conclusion is also in agreement with data from Flood and Knapp (1968).They
MYSEN ET AL: STRUCTURE OF SILICATE MELTS
calculated liquidus temperatures in the system
NaAlSirOr-CaAlrSirO, with the assumptionthat the
structural units in the melts mixed ideally. The agreement between.calculatedand experimentalliquidus
data was best when they usedtwo three-dimensional
units in the melt. These two structural units differed
in Si/(Si + Al).
(b) Melts with Na/Al < I: lt has been suggested
(e.g., Day and Rindone, l962a,b; Riebling, 1964,
1966)that if Al were addedto melts on the NaAlO,SiO, join, the extra Al may not be tetrahedrally
coordinated, and is, instead, a network modifier.
900
Lacy (1963),on the other hand, suggestedthat A1O"
octahedrawould not be geometricallyand energetically stable,and suggestedthat AlO. triclusterswere
formed instead.No dbw nonbridging oxygenswould
be formed in this process.Inasmuch as AlOu octahedra occur in minerals, it is not clear why Lacy
(1963) concludedthat AlOu octahedraprobably did
not occur in silicate melts. To our knowledge, no silicate mineral is known where Al is in tetrahedral
coordination withotrt some form of charge balance.
The melt compositions SAl0, SAN3 and
NaAlSirO,. were preparedto evaluatethe proposed
roles of Al3* in silicatemelts.Thesecompositionsdiffer in Na/Al ratio (Table l). The Raman spectraof
si02 thesemelts are shown in Figure 7 (seealso Table 4).
The portion of the spectrabelow about 800 cm-' is
not significantly different from that of quenched SiO,
melt. We conclude,therefore,that a large proportion
1000 1100 1200 1300 1400
of the melts has a three-dimensionalnetwork strucWavenumber,
cm-1
ture. In SAl0 quenchedmelt there are, however,two
additional bands in the high-frequency envelope
S A 10 (near l1@ and near950cm-'; seeFig. 7). Both bands
(si/Ar- 7.6) are polarized and becomelessintenseas 3 wtToNarO
(SAN3) is addedto SAl0. In quenchedSAl0 melt,
the two strongestbands, at 1050 and ll90 cm-',
probably are the two antisymmetric Si-Oo stretch
bands that are also found in quenched SiO, melt.
1000 1100 1200 1300
The frequencies and polarization characteristics of
Wavenumber.
cm-1
the two bands near 950 and 1100 cm-' are nearly
SAN3 identical to those found in binary melts on the join
( N a A l / I A l 0.49) Na,O-SiO, (Fig. 1) and codld be due to the samevi-O-Si-Oo
symbrations in the melt (-O-Si-O- and
metric stretching).
Provided that the 950 and I100 cm-' bands are. in
fact, due to silicate stretch vibrations in structural
units with nonbridging oxygens,it must be concluded
that in SAl0 quenchedmelt there is no evidencefor
1000 1100 1200 1300
Si(Al) coupling, as the frequenciesof the four bands
Wavenumber.
cm-1
in the high-frequency envelopeare similar to those in
analogous Al-free silicate melts. If this conclusion is
correct, the Al in quenchedSAl0 melt is a network
NaAlSi7016
modifier.
(NaAl/IAl = 1)
Consider now the possibility that the two bands
are stretch vibrations involving Al in tetrahedral
coordination. With the assumption that both bands
are coupled in the same manner as the antisymmetric
stretch bands of melts on the join NaAlOr-SiOr, the
800
900
1000
1100 1200 1300
Wavenumber,
cm'1
Fig. 7. Ramanspectraof quenchedmeltswith Na/Al equalto
and less than l. See Table I for explanation ofsymbols.
data for SAl0 are inconsistentwith the data in Figure 6. If the band near 1100cm-' is a coupled stretch
vibration, the secondband would be near 1000cm-',
whereas in fact the second band occurs near 950
cm-'. If the 950 cm-' band was an Si(Al)-coupledvi-
701
MYSEN ET AL.: STRUCTURE OF SILICATE MELTS
bration in a three-dimensionalnetwork structure,the
associatedband would fall near 1000 cm-', where
there is no band. We conclude,therefore,that at least
one of the two additional bands is an Si-O stretch
band indicative of nonbridging oxygensin the melt,
and that some,perhapsall, the Al in SAl0 is a network modifier.
ln composition SAN3, approximately one half of
the Al3* is balanced with Na*. In that composition
the frequencies of the two Si-Oo antisymmetric
stretchbandsare loweredslightly relativeto their frequenciesin pure SiOr, thus indicating some Si(Al)
coupling. The Si/(Si + Al) of the three-dimensional
network cannot be significantly lessthan 0.95 (compare Figs. 6 and 7).
The intensityof the band near ll00 cm-'in SAN3
melt is diminished relative to the overall intensity of
the high-frequencyenvelopecomparedwith the data
for SAl0 melt. This decreaseis consistentwith the
fact that less Al'* may be a network modifier in
SAN3 melt than in SAl0 melt. The spectraof SAl0
and SAN3 melts are quite sirnilar, however, which
leadsto the conclusionthat the structuresof the two
melts are similar.
The NaAlSi,O,u quenchedmelt is fully polymerized, as discussedunder NaAlOr-SiO, spectraabove
(Fig. s).
The effectof pressureon melt stntcture
Binaryjoins. The Raman spectraof NS, NS2, NS3
and Di melts quenchedfrom pressuresup to 20 kbar
are identical to the one-atm spectra within experimental uncertainty (Table 2). We conclude, therefore, that there is no discerniblepressuredependence
of the anionic structure of thesemelts.
The join NaAlOr-SiO, Two melt compositions,
NaAlSi,O, (Ab) and NaAlSi,Ou (Jd), were subjected
to up to 38 kbar pressure.The detailed Raman spectroscopic data resulting from these experiments are
shown in Table 4, with deconvolutedhigh-frequency
envelopesin Figure 8. The overall appearanceofthe
spectra is not affectedby pressure.If, for example,
some Al'* were to leave the network to form AlOu
clustersor octahedra,bands due to nonbridging oxygen are expected(seeFig. 7).
The only observablespectroscopicchangein samples quenchedat high ptessureis an increasein the
intensity of the I, band relative to the I, band in the
high-frequency envelope (Table 4). This change nay
indicate that the overall proportion of the most aluminous, three-dinensional network structure in the
melt has increased relative to the less aluminous
structural unit. The same trend was observedat I
atm as the Si/(Si + Al) of melts on the join NaAlO,SiO, decreased.On that join, the Si/(Si + Al) of the
two structural units also displayed a decrease'an
effectthat is not observedasthe pressureis increased
on a given composition (Figs. 7 and 8; seealso Table 4).
Discussion
Structuralmodelsof silicatemelts
The most important observation made from the
above assignmentof the Raman spectra is that there
is a unique set of coexistinganionic structural units
for specificrangesofthe ratio ofnonbridging oxygen
to silicon. In Table 3, the anionic units are definedon
the basis of averageNBO/Si. There are severalaspects of theseconclusionsthat warrant further comment. In comparisonto some models of melt structure (e.g., Masson, 1977),the present model is
strikingly simple. This simplicity should not be surprising in view of the fact that when comparing the
crystal structures of silicate minerals, only a few
structural arrangements can be found (see Dent
Glasser, 1979,for review). In fact, in silicate crystal
chemistry,the metal cations are consideredvery important in controlling the type of polymers that will
occur. Polymer theory as applied to silicates (e.9.,
Masson, 1977)does not take metal cations into account.
Previousmodelsof melt structuresinclude features
such as trimers, tetfamers,pentamers,etc., in addition to rings and branched chains. The experirnental
basis for most of these models has been chromatographic data derived from trimethylsilyl derivatives
of the silicatepolymers(TMS derivatives).When applying that method to glassstructural determinations,
certain inconsistenciesare found. First, when comparing the resultsfrom TMS derivativesto structural
data derived from Raman spectroscopyof the same
materials, the results differ (compare, for example,
the results of Lentz, 1964 with Brawer and White,
1975for the systemNarO-SiOr, and the data for the
system PbO-SiO, by Smart and Glasser, 1978 and
Furukawa et al., 1978\.Second,the results obtained
with chromatographic characteruation of TMS derivativesare internally inconsistent.Furthermore,the
yield of the derivation techniqueis never lNVo. Fot
example,Lentz (1964)published resultsfor the proportion of structural units in 2NarO'SiO, glass,
which produced monomers(43.5Vo),dimers (l9.8%o),
Theseproportrimers (l0.0%o),and tetramers(9.6Vo).
MYSEN ET AL.: STRUCTURE OF SILICATE MELTS
tions total to 87.9Eo.The bulk NBO/Si of this total is
2.84.lf it is assumedthat the remainderof the material is that of a pentamer(NBO/Si :2.4), addition of
this proportion to that total NBO/Si yields bulk
NBO/Si : 2.79 for 2Na,O.SiO, glass.If the unreactedremainderincluded polymersthat were more
polymerizedthan that of a pentamer,the bulk NBO/
Si would be even smaller. For lead orthosilicate
(Smart and Glasser,1978),the result of a similar calculation is NBO/Si : 2.76- Inasmuch as orthosilicates have an overall NBO/Si : 4, ttre results
from TMS derivativesindicate that the silicate polymers condensedduring preparationof the derivative.
Similar results are obtained when calculating bulk
NBO/Si for more silica-rich glasses.The existenceof
such problemshas been further documentedby Kuroda and Kato (1979), who showed that the chromatographicresults dependedon the type of silylating agent used to form the derivatives.
Another feature of polymer models for silicate
melt structureis a positive correlationbetweenorder
and proportion of silicate polymers and bulk NBO/
Si of the melt. In Raman spectroscopicstudies,such
an evolution would result in a successiveincreaseof
the frequencyof Si-O stretchbandsasthe number of
Sia* cations in the polymers increases(e.g.,Lazatev,
NaAlSi206
800
800
NaAlSi30g
900 1000 1100 1200
Wavenumber.
cm-1
800
900
800
1000 i100
900
1000
1100
1200
Wavenumber.
cm-1
1200
Wavenumber,
cp-1
900
1000
1100 1200
Wavenumber,
cm-1
rl
800
a
900 1000 1100 1200
Wavenumber.
cm-1
800
b
900
1000
1100 1200
Wavenumbet,
c6-1
Fig. 8. Raman spectra of quenched NaAlSi2Oe (a) and NaAlSi3Os @) melts as a function of pressure.
MYSEN ET AL.: S?:RUCTURE OF SILICAT:E MELTS
1972;Brawerand White,1975; Furukawa and White,
1980;seealso Table 3). On this basisit would be expected that if anionic structural units with degree of
polymerization between dimers and chains were
formed in melts on the various binary silicatejoins
suchasthosediscussedabove(seeFigs. 2 and 3), new
bands would occur or bands such as that near 9@
cm-' would shift to higher frequencyas the M/Si of
the melt is decreased.There is no such spectroscopic
evidencein our data or in any relatedpublished data
(Brawer and White, 1975, 19'77;Furukawa et al.,
1978; Verweij and Konijnendijk, 1976; Verweij,
l979a,b; Furukawa and White, 1980).We conclude,
therefore,that structural units with NBO/Si between
that of a dimer and that of a chain (3 and 2, respectively) do not exist in significant amounts in silicate
melts.
The structural unit with NBO/Si : 2 has been referred to as a chain structure. Ring structureshave,
however,the sameNBO/Si. The idea of ring structures in silicatemelts has experimentalsupport in experimental studies of TMS derivatives of silicate
melts (e.g.,Masson, 1977).Ring structureshave also
been suggestedin melts on binary alkali metal-silicatejoins on the basisof the viscousbehaviorof such
melts (e.g.,Tomlinson et al., 1958;Mackenzie, 1960,
p.205; Bockrisand Reddy,1970,p.612).In view of
the discussionabove, it is not clear whether TMS
data give an accuratedescriptionof the structure of
the melt.
Mass balanceconsiderationsrequire that the melts
with M, C, and S-bandsneed at least one structural
unit with NBO/Si lessthan 2. We suggestthat the Sband in Figures 2 ar^d3 reflectsvibrations in such a
structural unit. Consider, for example, the spectrum
of Wo (CaSiO,) in Figure 2. The Wo composition
has bulk NBO/Si : 2. TtLestrong 870 cm-' and an
even stronger970 cm-' band are due to the presence
of structural units in the melt with 4 and 2 NBO/Si,
respectively.In order to maintain mass balance of
oxygen and silicon, there must be structural units in
this melt that have NBO/Si lessthan 2. Careful analysis by in particular Verweij (1979a,b) and Furukawa and White (1980) and also by Yirgo et al.
(1980)has led to the conclusionthat the band in the
frequencyrange between 1070and ll0o cm-' in the
MO-SiO, and M,O-SiO, melts most likely is a
stretch vibration from a unit that has NBO/Si : l.
One might suggest that such a vjbrational mode
could stem from end units in a linear structure. Such
an interpretation does not, however, provide for
structural units that satisfy the mass balance of the
melt. It might also be suggestedthat the presenceof
branched chains or multiple chains in the melt constitutesthe structural uni(s) with NBO/Si lessthan
2. The end result of the evolution of such branching
is, of course,an infinite sheet.The frequency of the
band in question (denoted S in Figs. 2 and 3) does
not shift with changesin M/Si. Its intensity increases
with decreasingM/Si, however. On this basis, we
concludethat the NBO/Si of this structural unit does
not vary with changesof bulk NBO/Si of the melt.
The possibility of a branchedchain is consideredunlikely for two reasons.First, there is no experimental
evidencefor the presenceof such structuresin sitcate
melts (branched chains with NBO/Si less than 2).
Second, according to a survey by Dent Glasser
(1979),branchedchainsdo not occur in minerals.Instead.chainsand sheetsare formed. There is no clear
reason,therefore,why one would expectsucha structure in a melt.
c\l
Ia
=-(5
j
o
g
LU
-o
10
20
30
40
50
60
70
Mole% oxide
Fig. 9. Activation energy of viscous flow of binary melts on
alkali metal-silicate and alkaline earth-silicate joins. The
activation energy (E,) is recalculated to a mnstant number of SiOz
units (l). Data from Bockris and Lciwe (1954), Bockris e/ a/.
(1955),MacKenzie(1960,p. 201)and sum.maryof data by Bockris
and Reddy (1970,p. 604).
MYSEN ET AL.: STRUCTURE OF SILICATE MELTS
An option that doesexist to explain the 1070cm-'
is multiple chains. It cannot be establishedfrom the
Raman data whether the S-band (Figs. 2 and 3) is
due to an infinite sheetor a finite sheet(e.9.,multiple
chains).In view of the data summarizedabove,it is
most likely that the structural unit has an average
NBO/Si near I (infinite sheet), and the structural
units representedby this Si-O stretch band in the
Raman spectrawill be referredto as a sheet.
Becausethe relative integrated intensities of the
bands assignedto the diferent anionic speciesmay
not be linearly relatedto NBO/Si, only a generaldiscussionof their relative abundancesin thesesilicate
melts is possibleat this time. We suggestthat monomers are the most abundant speciesin the melts near
the orthosilicate composition and that they become
increasingly unstable with decreasingNBO/Si but
are still presentin the CaSirO, sheetcomposition.In
contrast,the dimer unit has a restrictedrange of stability, with an apparent maximum in its abundance
between the ortho- and metasilicate compositions.
The instability of the dimer speciesappearsto coincide with the appearanceof the sheetunit, and with
increasingNBO/Si the sheet and chain speciesincreasein abundancerelative to the monomer units.
From the above discussion,we propose that the
equilibria relating the coexisting anionic species
(units) in silicate melts can be expressedwith the following equations:
For4>NBO/Si>2:
2Si,Ot- a 2SiOX-+ Si,O:-
(l)
for2> NBO/Si> l:
3Si,O:- e 2SiO;- + 2Si,O3-
Q)
andfor I >NBO/Si>0.1:
2Si2O?-e Si,O:- + 2SiO,
(3)
This model of the anionic structure of silicate
melts delineatesthree distinct compositionalregions.
Inside each region a combination of three anionic
structural units occurs.The Raman spectraprovide
little information about the metal cation-oxygen
polyhedra.
Melt structureandphysicalproperties
Binary joins. Data on thermal expansivity (e.g.,
Tomlin5sn et al., 1958;Bockris and Kojonen, 1960;
Robinson, 1969) indicate no compositional dependenceof this parameterin the compositionalregions
with less than about 10-15 mole 7o metal oxide.
Those authors concluded, therefore, that the melts
are essentiallymade up of three-dimensionalSiO,
units where Na* cations,for example,are trapped in
"holes" in the melt. Thermal expansivity increases
rapidly as the molar metal oxide content is increased
beyond about lOVo,which leads to the conclusion
that nondirectional anionic, metal-oxygenbonds are
responsible for the melt expansivity (Tomlinson et
al.,1958).Bockrisand Reddy (1970,p. 615) pointed
out, however,that partial molar volume data for SiO,
in the same melts indicate that discrete SiO, "icebergs" must remain in the structure to at least 33
mole Vometal oxide. These observationsare consistent with the Raman spectroscopic data. The data
(Fig. l) were interpretedto indicate that binary metal
oxide-silicate melts break down into at least three
structural units at about l0 mole Vo metal oxide.
Thesestructural units are of three-dimensionaltype,
of sheet type (NBO/Si : l), and of chain type
(NBO/Si : 2). We conclude,therefore,that the interpretation of the Raman data leadsto a structural
model that accordswith thermal expansivityand partial molar volume data.
The viscositydata of silicatemelts on binary metal
oxide-silicatejoins (e.9.,Bockriset a(.,1955;Bockris
and L<iwe,1954;Tomlinson et al., 1958;MacKenzie,
1960,p. 201) are particularly relevant to the anionic
structure of the melts. Bockris and Reddy (1970,p.
604),in a summary of thesedata, concludedthat the
activation energyof viscousflow, E4, show three distinct regions. The E4 decreasesrapidly as metal
oxide is addedto SiO, until about 20 mole 7ooxide is
dissolvedin the melt, followed by a plateau that extends to about 50 mole Vometal oxide. A further increasein the metal content results in the activation
energy falling. To a first approximation, the contributions to E4 are from breakageof Si-O bridging
bonds and perhapsfrom M-O bonds so asto form an
entity in the melt that can move from one location to
another in the melt (flow unit) during viscousflow. In
fact, Bockris and Reddy (1970, p. 600) concluded
that the value of E4 was primarily a function of the
heat of dissociationof Si-O bridging oxygen bonds
(-104 kcallmole). A secondcontribution may derive
from the energy required to form the new volume
("hole") into which the flow unit moves(Glasstoneet
al.,1941,p. a8l). The latter contributionmay be more
important for an understanding of the absolute melt
viscosity values (Lacy, 1968)than for the activation
energy of viscousflow (Bockris and Reddy, 1970,p.
600). In fact, Bockris and Reddy (1970,p. 600) concluded that the value of E4 was primarily a function
of the heat of dissociationof Si-O bridging oxygen
bonds (-104 kcallmole).
705
MYSEN ET AL.: STRUCTURE OF SILICATE MELTS
If it is assumedthat the energy of dissociationof
Si-O bonds does not vary significantly with type of
metal cation, the argumentsby Bockris and Reddy
(1970,p. 600) and MacKenzie(1960,p. 201)may be
usedto calculatethe averagenumber of nonbridging
oxygensper silicon that must be broken to generate
the flow unit. This value may then be added to the
original NBO/Si of the melt to calculatethe average
number of nonbridging oxygen per silicon in the flow
unit [(NBO/Si)a]. In order to carry out this calculation, the E4 must be converted from kcal/
mole(gram-formulaweight) to kcallSi or kcallSiO,.
The resultsof such calculationsare shown in Figure
9. The data basefor Figure 9 is limited in that only a
few oxideshave been studiedover the compositional
range of the suggestedthree compositional regions
delineated by Bockris and Reddy (1970, p. 604).
Note that in the systemsthat cover the compositional
rangefrom pure SiO, to >50 mole Vometal oxide,the
E4 curvesdefine three compositionalregions similar
to those summarizedby Bockris and Reddy. These
compositional regions correspondto the regions of
structural similarity described in equations 1-3,
within experimental uncertainty.
It appears from the data in Figure 9 that for a
given mole Vo metal oxide components, the Ea (in
kcallSiOr) increaseswith increasing metal cation
size. For metal cations of a fixed size,the activation
energyof viscousflow increasesas the valenceof the
cation increases.If in fact the dissociationenergy of
the bridging Si-O bonds is not afected significantly
by the type of metal cation, and there is no significant contribution from M-O bonds to the value of
the activation energy, the data in Figure 9 imply that
the averageNBO/Si of the flow units during viscous
flow dependson the type ofcation (Fig. l0). The average (NBO/Si)4 shown in Figure l0 is derived on
the basisthat the energyofdissociation ofthe bridging oxygen bonds is the only contribution to the
value of E4. Metal-oxygen bonds and steric considerations are not included. We conclude, therefore,
that the values of (NBO/Si)4 in Figure l0 are minimum values.
Another consequenceof the above observationsis
that the constancyof E4 over a given range of M/Si
requires that the average size of the flow unit decreaseswith increasing M/Si (Fig. l0). This conclusion differs from that of Bockris and coworkers
and MacKenzie and coworkers(Bockris et al., 1955,
1956;MacKenzie,1960,p.205; Bockrisand Reddy,
1970,p.614).Thoseauthorsproposeda "polyanionic
structure" model where in a given compositional
@
'i
o
s
o
o
(NBO/Si)n
Fig. 10. Calculated minimg6 number of nonbridging oxygens
per silicon in flow units on the joins Na2O-SiO2 and BaO-SiO2.
See text for further explanation.
range the consequenceof constant E4 is that the
types of structural units in the melt and the types of
flow units must be the same.Our conclusionsare that
the types of structural units in the melt are similar
over the individual compositional ranges,but the size
of the flow unit may vary. The data in Figure 10 indicate that the averagesize of the flow unit decreases
with increasing M/Si, an observation that accords
with the observation that the absolute viscosity of the
melts decreasesas the M/Si decreases(seeBottinga
and Weill, 1972,for surnmaryof data).
Aluminosilicate melts with charge-balancedAI'*.
Viscosity, thermal expansion, and thermochemical
data (Riebling, 1966;Navrotsky et al., 1980) have
been used to suggestthat melt compositions along
the join NaAlOr-SiO2 are structurally similar. Structural data by Taylor and Brown (1979)agreewith the
present data in that these melts have a three-dirnensional network structure. The Raman data shown
here also indicate, however, that there may be two
anionic three-dimensional structural units in the
'106
MYSEN ET AL.: STRUCTURE OF SILICATE MELTS
melts. These two units differ in their Si/(Si + Al),
whereasthe value of this ratio in both units decreases
with decreasingbulk Si/(Si + AD.
The activation energyof viscousflow in SiO, melt
is about twice that of NaAlSirO, melt (Riebling,
1966).Inasmuch as these two melts appear to have
the same structural features, it has frequently been
suggestedthat the decreaseof 4 and E4 with increasing Al content is due to the increased proportion of
weaker Al-O relative to Si-O bonds (e.g.,Taylor and
Rindone, 1970). The energy of dissociation of the
bridging Al-O bonds is likely to depend on the type
of metal cation needed for local charge balance of
Al'* in tetrahedral coordination. Both q and E4 are
loweredwith decreasingradius of the charge-balancing cation and with increasing electrical charge (Rossinet a|.,1964;Hofmaier, 1968;ftisfling, 1964,1966;
see also Bottinga and Weill, 1972 and Cukierman
and Uhlmann, 1973).For example,the viscosity of
NaAlSiOo melt is nearly an order of magnitude
greaterthan that of CaAlrSirO*melt (F"t'SiO, : 9.5
in both melts). The viscosity of MgAlOr-SiO, and
BaAlOr-SiO, melts at .Fd'SiO, : 0.75 is near l0'
poise for the former and -10' poise for the latter
melt. We suggest,therefore,that the strength of the
Al-O bridging bonds (heat of dissociation)in threedimensional aluminosilicate melts decreasesas the
size of the charge-balancingcation decreasesand as
its electrical charge increases.The absolute values of
the energy of dissociation of these bonds as a function of suchvariablesis not known. It is not possible,
therefore, to estimatethe size of the flow units on this
basis. The Raman data may be used to suggesta
model for at least some aspects of viscouS flow in
NaAlOr-SiO, melt and probably also other MAIOrSiO, and MAlrOo-SiO, melts.On the basisof the Raman data we suggestthat the most Si-rich three-dimensional structural units may representthe actual
flow units, whereasthe majority of the Al-O bonds
that will be broken to form the flow unit is derived
from the most Al-rich structural unit in the melt.
Aluminosilicatemeltswith Na/Al < 1. The viscosity
and activation energy of viscousflow decreasesrapidly as Al'* is added to melts on the join NaAlO,SiO, (Riebling, 1966).Lacy (1968) commentedthat
there is a positive correlation betweenNBO/Si of a
melt and its viscosity. This conclusion accords with
the Raman data, which indicate that nonbridging oxygensare formed as Na*,/Al3* of the melts is lowered
below I (Fig.7).
Efect ofpressure.The pressuredependenceofthe
viscosity of four melt compositions in the binary
joins discussedabove was determined by Kushiro
(1976) and Scarfeet al. (1979).Of thesemelts, those
of Di, NS, and NS2 compositionhave (d\/dP). positive, whereasNS3 quenchedmelt has negativepressure dependenceof the viscosity(Table 5). Unfortunately, no data are availableon the activation energy
of viscousflow of thesemelts. [t is dfficult, therefore,
to assesswhether the flow mechanismsof thesemelts
changewith pressure.There is no signifisant change
of the anionic structure of thesemelts with increasing
pressure,although the proportion of the least polymeitzed unit (monomer) seems to decreasewith increasingpressure.Intuitively, such an evolution may
result in increasedsteric hindrance of viscousflow.
Scarfe et al. (1979\ noted that the compressibility
of melts with a negativepressuredependenceof vis-
Table 5. Changes(percentagerelative to values at I atm) ofphysical properties between I atm and high pressure
Compositlon
(pressure
and tenperature)
NBO/T
Viscoslty
Density
Reference
C a M C S i 2 o 6( 0 . 0 0 1 - 1 5 k b a r , 1 6 4 0 o C )
2
+267
+8.3
Scarfe et
al.
(f979)
Na2Si03 (0.001-20 kbar, 1300'C)
2
+650
+6.7
Scarfe et
a].
(1979)
t
+311
+6.3
Scarfe et
a1.
(1979)
*"2tt205
(0.001- 15 kbar, l2oooc)
Kzo'Mgo.5slo2 (5-20 kbar, 1300"c)
0.8
-64
N a Z S 1 3 o 7( 0 . 0 0 1 - 2 0 k b a r , 1 1 7 5 ' C )
NaAlSi2o5 (0.001-20 kbar, 1350.C)
N a A I S 1 2 0 8( 0 . 0 0 1 - 2 0 k b a r , 1 4 0 0 ' C )
0.67
-65
0
0
-90
-84
Kushiro
(1976)
*rr")
Kushiro
(1978a)
(Kilauea, 1921)
ollvine tholeiite
(0.001-20 kbar, 1400eC)
(45-3954-8-1-9)
Abyssal rholeiire
(0.001-12 kbar, 1300"C)
Crater Lake andesite
(0.001-20 kbar, 1350oC)
o,76
-51
+I2.2
Fujlt
and Kushiro
(1977)
+7.1
Fujii
and Kushiro
(1977)
0.66
o.27
-51
Kushiro
(1977)
Kushiro
(1976)
Kushiro
et
al.
(1976)
MYSEN ET AL.: STRUCTURE OF SILICATE MELTS
cous flow is much greaterthan those of melts with a
positive pressuredependence(seealso Table 5). The
Raman spectra,however,demonstratethat the melts
do not show significant changes of their anionic
structureas a function of pressure(Table 2). In contrast to the observationat I atm, where the decrease
of 4 and E4 may be related to increasingNaAlOr/
SiOr, there is no spectroscopicevidence for any
change of Si/(Si + A1) of the structural units as a
function of pressure. The thermal expansivity of
thesemelts is negligible(Riebling, 1964,1966).Bockis et al. (1956)and Bockrisand Reddy(1970,p. 615)
suggestedthat thermal expansionwas primarily due
to expansionof M-O polyhedra, and that such expansion could only take place when the melt consisted predominantly of large, discrete polyanions.
Evidently this reasoningdoesnot apply to compression of melts, becausethe largest compressibilityis
found in melts with the smallestthermal expansivity.
An explanationof the large compressibilitymay be
a collapseof the three-dimensionalnetwork structure
around the metal cation. This collapse may result in
an increasednumber ofnearest oxygenneighborsof
the metal cation. Consequently,Brown (1978) suggestedthat the efective valenceofeach oxygen may
ofsuch an effectwould be a
decrease.A consequence
decreaseofthe strengthofthe Al-O bonds,as the effect would be the sameas a decreasein the sizeof the
metal cation at I atm. The result of this collapseis,
therefore,a decreasedviscosityand activation energy
of viscousflow of the melts along the join NaAlOrSiO, as a function of pressure.It is also likely that
other aluminosilicatemelts would show a decreasein
4 and E4 with increasingpressure.
Applications
Melt structurescan be describedin terms of threedimensional network units, sheets,chains, dimers,
and monomers.When the proportions of structural
units in natural magmaare calculated,Al'*, Tio*, and
Ps*must be consideredpart of the network structure.
Ferric iron is a network former only when sufficient
alkalies are presentfor local chargebalance (Mysen
et al., l98Da;Seifert et al.,1979b). The result of calculations of the proportions of structural units in natural magma where their pressuredependenceof viscosity is known (Table 5) is shown in Table 6. In
these calculations, it was assumedthat alkali aluminates (MAlOr) were formed in preferenceto other
complexessuch as MFeO'. Any M cation left over
after the formation of MAIO. would form MFeOr.
Any alkali excessover that neededto form MFeO' is
Table 6. Compositions of rocks for which the prcssure
dependenceof their viscosity is known
lio
2
Al203
Fe2O3
Feo
MnO
MgO
Cao
Na2O
Kzo
Pzo5
NBo/T
1.
2.
Kilauea
1962).
Abyssal
0.73
1 7. 9 0
L.64
2.29
IJ.
5 9. 4 6
50.0
49.L6
sio2
1 5 .0
JJ
1.31
9.7L
0.16
10.41
10.93
2.L5
0.51
0.16
ll. 2*
0.22
8.40
1 0 .6 0
2.70
0.13
0.76
0.66
tholeiite
1921 olivine
tholelite
5.18*
0. 10
3.7r
6. 4 5
4.23
o.27
(Yoder and Tilley'
(Fujii
45-395-8-r-9
and Kushiro,
r977).
a
Crater Lake andesite
ALl iron as Feo.
(Kushiro
et
al.'
1976).
considereda network modifier. Any Fe'* left over after all the alkali metals have been consumedis also
considereda network modifier. If there is insufficient
alkali metal present to transform the aluminum to
MAlOr, the alkaline earths are used to form MAlrOn
complexes.The proportions of tetrahedral cations
thus calculated are assignedto structural units in the
melt. Table 6 showsthat basalthas NBO/T near that
of NarSirO, composition.The andesitemelt is even
more polymenzed. In view of the mechanismsfor
viscousflow discussedabove, the observednegative
pressuredependenceof their viscosityis expected.
For melts with constantNBO/T, the valuesof viscosity and activation energy of viscous flow depend
on the proportion and strength of T-Oo bonds (where
T cationsdo not include Sio*).For example,we suggest that the melt viscosity will decreasewith increasing Na*/K*, Ca'*/Mg'*, and Ca'*/Na* of the melt.
It is also likely that with Fe3* as a network former,
the viscosity of the melt will decreasewith increasing
Fe'*,/Al3*.The latter efect may be observedif the redox ratio (Fe'*/Fe'*) of a peralkaline magrna [(Fe'*
+ A1'*) < (Na* + K*)l were allowed to increase.
It has been observedthat the viscosityof hydrous
andesite and granite melt tends to increase with increasing pressure, even though their anhydrous
equivalentsshow a negativepressuredependenceof
viscosity(Shaw, 1963;Kushiro,1978c;Kushiro e/ a/.,
1976).Inasmuchas solution of HrO in such melts results in a significant increaseof NBO/T (Mysen et
a/., 1980b),it is likely that the mechanismsof viscous
708
MYSEN ET AL.: STRUCTURE OF SILICATE MELTS
flow of such melts resemble that of binary metal
oxide-silicate melt, where, in fact, the viscosity increaseswith increasingpressure.
Basanitesand picriteshave NBO/T betweenI and
2. We suggest,therefore,that magmasof such compositions will show a viscosity increasewith increasing pressure.The pressureeffect is likely to be greater
for picrite than for basanitemelt becausethe NBO/T
is greater in picrite than in basanite.
Acknowledgments
Critical reviews by I. Kushiro, E. Takahashi, and H. S. Yodcr,
Jr. are appreciated.Reviews by G. E. Brown, G. Waychunas, S. A.
Brawer, and H. S. Waff of an early version of this manuscript
greatly improved the paper.
Scarfe acknowledgessupport from the Carnegie Institution of
Washingtonand CanadianNsERcgrant A 8394.
This research was partially supported by NSF grant EAR
7911313and partially by the CarnegieInstitution of Wl5hington.
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Manuscript received, September 21, 1979;
accepted for publicalion, March 21, 1980.