GEOPHYSICALRESEARCHLETTERS, VOL. 14, NO. 12, PAGES 1231-1233,
ON THE NATURE
OF PRESSURE-INDUCED
IN SILICATE
MELTS
COORDINATION
AND
DECEMBER1987
CHANGES
GLASSES
Edward M. StolperandThomasJ. Ahrens
Division of GeologicalandPlanetarySciences,CaliforniaInstituteof Technology
glassare obtainedin situ at high pressureand room temperature,the
followingis observed[WilliamsandJeanloz,1987]: Up to 20 GPa, only
tetrahedrallycoordinatedsiliconis observed.With increasingpressure,
bandsattributedto octahedrallycoordinatedsilicon grow, while those
attributedto tetrahedrally
coordinated
silicondecrease.This transformation is continuousand reversible. After pressure-quenching
to 1 arm
from conditions under which octahedrally coordinated silicon was
observed,only bandsattributedto tetrahedrallycoordinatedsiliconwere
Abstract. Progressivedecreasesin the Si-O-Si angles between
comer-sharedsilicate tetrahedrain glassesand melts with increasing
pressurecanleadto arrangements
of oxygenatomsthatcanbe described
in termsof edge-or face-shared
octahedra.This mechanism
of compression can accountfor the gradual,continuousincreasesin melt and glass
densitiesfrom valuesat low pressurethatindicatedominantlytetrahedral
coordinationof Si to valuesat severaltensof GPa that suggesthigher
coordination.It alsocanexplainthe unquenchable
natureof octahedrally
coordinatedSi in glasses,the absenceof spectroscopically
detectable
octahedrallycoordinatedSi in glassesuntil they are highly compressed,
the gradualand reversibletransformation
from tetrahedralto octahedral
coordinationin glassesonce the transformation
is detectablespectroscopically,and the fact that this transformation
takesplace in glassat
room temperature. It may also have relevanceto pressure-induced
observed.
The Proposal
What mustbe soughtis a mechanismof compression
that (1) begins
with only tetrahedralcoordination
at low pressureandleadscontinuously
to octahedralcoordinationat high pressure;(2) does not generate
spectroscopically
observableoctahedrallycoordinated
siliconuntil after
substantialcompression;(3) produces, with increasing pressure, a
transformations
from crystallineto glassyphases,the difficulty in
retrieving some metastablehigh pressurecrystalline phases at low
pressure,andthe observeddifferencesbetweenthe pressures
requiredfor
phase transformationsin shock wave experimentson glasses and
crystals.
continuous transformation
of tetrahedral to octahedral coordination
as
detectedby infraredspectroscopy
at pressures
abovethe first appearance
of octahedrallycoordinatedsilicon;(4) proceedsat room temperature;
and (5) is reversibleandusuallyunquenchable.
Introduction
Figure1 illustrates
thekindof mechanism
we propose
cansatisfy
It is generallyacceptedthat siliconis tetrahedrallycoordinatedby
oxygenionsin silicatemeltsandglassesat 1 atm totalpressureandthat
adjacentsilicatetetrahedra
formpolymersby comer-sharing
[e.g.,Mozzi
andWarren, 1969; Taylor andBrown, 1979;OkunoandMarumo, 1982].
By analogywith the structuresof crystallinesilicatesat pressuresof
severaltensof GPa and basedon shockwave studiesof the Hugoniot
statesof moltensilicates[Rigdenet al., 1984, 1987, andin preparation],
it is likely that siliconis more highly coordinated
by oxygensat high
pressures.In thisnotewe proposea mechanism
by whichthe coordination of $i (and other cations found in tetrahedralcoordinationat low
pressures)
may increasein glassesandmeltswith increasingpressure.
Severalobservations
constrainpossiblemechanisms
of compression
in silicateglassesand melts: (1) Shockwave studiesof moltensilicates
in the systemanorthite-diopside
demonstrate
that the compression
from
severaltenthsto severaltensof GPa is gradualand continuouswithout
any abruptdensitychanges[Rigdenet al., 1984, 1987, andin preparation] andsuggest
thatthe mechanisms
of compression
may be continuousoverthispressure
range. It is widelybelieved[e.g.,Navrotskyet al.,
1985; Ohtani et al., 1985; Hemley et al., 1986] that the principal
mechanism
of compression
in amorphous
silicatesat low pressures
is
increasedpackingefficiencyof linked aluminosilicate
tetrahedraby
decreasesin the (Si,Al)-O-(Si,Al) anglesbetweenadjacenttetrahedra.
(2) Glasses(or melts)held at elevatedpressures
andthenbroughtrapidly
to room temperature
andpressure("pressure-quenched")
and examined
by infrared or Raman spectroscopyshow no features attributedto
octahedrallycoordinatedsilicon[Arndt andSttffier, 1969; Arndt, 1983;
Grimsditch,1984; Hemley et al., 1986; Williams and Jeanloz,1986].
This is even true for "permanently"densiftedglasses,which are
presumed
to havepreserved
on quenching
partof the structural
changes
they experiencedat high pressures.Albitic glassesquenchedfrom high
temperatures
andpressures
greaterthan6 GPa do havefeaturesin their
NMR spectrathat have been assignedto octahedrallycoordinated
aluminum[Ohtaniet al., 1985]. (3) When infraredspectraof anorthitic
theseconstraints.A chainof silicatetetrahedrawith an Si-O-Si angleof
180ø(Figurela) is compressed
bydecreasing
thisangle.Figures
la-le
show variousstagesin this continuouscompression.A comparisonof
Figures le and If, both of which have the sameplacementof oxygen
ions(Si-O-Si= 1000in Figurele) illustrates
ourbasicpoint: This
configurationof oxygenatomscanbe viewedeitheras a highlydistorted
and compressedchain of tetrahedraor as a chain of face-sharedoctahedra. Figure 1 thusillustratesa possiblemechanismby which a low
pressure,low density arrangementof oxygen tetrahedracan transform
continuouslyinto a closelypackedarrangementof oxygen octahedra.
Although we have not illustratedit, the chain of tetrahedrashown in
Figure l a could alsohave beencompressed
into a chainof edge-shared
pentahedra.
Althoughthe end productof compression
of the chain of tetrahedra
shownin Figure l a is a chainof face-sharedoctahedra(which basedon
Pauling's rules is not expectedto be an energeticallyfavorableconfiguration),edge-shared
octahedralarrangements
can also be produced
by distortionand compressionof groupsof corner-shared
tetrahedra.
Figure2a showstwo highly distorted,linkedringsof silicatetetrahedra;
Figure2b showsthat the centralportionof this configurationof oxygen
atomscanalsobe viewedastwo edge-shared
octahedra.BothFigures2a
and 2b are actuallyportionsof the crystalstructureof stishovit½;
the
arrangement
of oxygenatomsin crystallinestishovite,whichis usually
viewedas linked chainsof edge-shared
octahedra,couldalsobe viewed
asa networkof irregular,corner-shared
tetrahedra.
In transformingfrom Figure le to If or 2a to 2b, thereis no movementof oxygenatoms. Therewill, however,be minormovementof Si
atoms.In thepicturesshowingtetrahedral
coordination,
eachSi is only
chemicallybondedto four oxygenatoms,althoughtwo other oxygens
arenearby. If the Si bondsto thesetwo additionaloxygens,it would be
expectedto moveacrossoneof the facesof theoxygentetrahedron
so as
to be more centrallylocatedwithin the octahedrondefinedby the six
nearest oxygen atoms.
Discussion
Copyright1987by theAmericanGeophysical
Union
Papernumber7L6598.
0094-827 6/87/007L-6598503.00
Our proposal that the transformationof Si from tetrahedralto
octahedral
coordination
in meltsandglasseswith increasing
pressureis
1231
1232
Stolper
andAhrens:
Pressure-Induced
Coordination
Changes
\
/
_oo o
decreasingSi-O-Si anglesbetweenadjacenttetrahedracan remainthe
sameall thewayfrom 1 atmto severaltensof GPa. Oncetheconfigura-
tion has reachedone compatiblewith close,octahedral
packingof
oxygenatoms,themeltor glasswill be expected
to stiffenconsiderably.
Thishasbeenobserved
in shockstudies[Wackefie,1962;Boslough
et
al, 1986;Rigdenet al., 1987]. Theproposed
mechanism
of compression
may alsohelpto explainwhy undershockcompression
the densityof
silicaglassincreases
smoothlyandapproaches
thatof stishovite
at lower
pressurethandoesthe densityof shockcompressed
crystallinequartz
[Schmitt,1987], because
the glass,unconstrained
by crystalstructure,
canmorereadilydistortinto the proposed
highdensityarrangement
of
oxygenpolyhedra.
(2) It is not clearthatwhena packingof oxygensconsistent
with
octahedralcoordinationof Si has been achieved,the material will
necessarilygive up its tetrahedralbondingenvironment. Thus, for
example,it is conceivablethat a glasscouldremain as a tetrahedral
network(e.g., Figure 2a) to very high distortionsand compressions.
Fig. 1. Progressive
distortionof a chainof SiO4tetrahedra.Dashedlines
are"hidden"edgesof polyhedra.(a) Si-O-Siangle= 180ø. (b) Si-O-Si
angle= 160ø. (c) Si-O-Siangle= 140ø. (d) Si-O-Siangle= 120ø.
(e) Si-O-Siangle= 100ø. (f) Sameaschainin (e), exceptcoordination
polyhedraredrawnas a chain of face-sharedoctahedra.The numbered
circlesshowthepositions
of the sameoxygenatomsin (e) and(f). Si
positions (crosses)have been moved to be near the centers of the
octahedra
in (f). Because
of the finitelengthof thechainin (e), oneSi
andoneO (at lowerleft) arenotreproduced
in (f).
Suchtetrahedra,however,wouldbe highlydistorted,and we think it
likely that at somepoint,the Si atomswouldbeginto bondwith the
oxygenatomsthat are beingpressedin on them and a changeover
to
octahedral
bonding
(e.g.,Figure2b) wouldtakeplace. Giventhelikely
inhomogeneous
natureof compression
in real systems
(i.e., a chainsuch
as that shownin Figure1 wouldprobablycompress
irregularly),we
would expectthe changein bondingenvironment
to take place at
differentpressures
at differentpositionsin the glassor melt structure.
Thiscanexplaintherangeof pressures
overwhichbothoctahedrally
and
tetrahedrally
coordinated
Si are observed
in anorthiticglass[Williams
StolperandAhrens:Pressure-Induced
Coordination
Changes
and Jeanloz,1987]. It is clear, however,that in the early stagesof
compression(e.g., Figures lb, lc) nothingbut tetrahedralunits are
presentin the glassor melt. This wouldexplainwhy vibrationalspectra
of denseglasses,
whetherobtainedat highpressure
or at low pressureon
permanentalydensifted glasses, have revealed only tetrahedrally
computer-generated
graphics.This work was supported
by NSF Grants
EAR-8407784andEAR-8618545. CaltechDivisionof Geologicaland
PlanetarySciencesContribution4471.
References
coordinated silicon.
(3) To first order,we would expectthe compression
of glassesand
meltsby theproposed
mechanism
to be elastic;thatis, whenwe squeeze
on the chain in Figure la, the Si-O-Si angledecreases,
and when we
releasethe pressure,it will springback. In the rangeof compression
over which the bonding environmentschange from tetrahedralto
octahedral,we would still expect the transformationto be nearly
reversible.
We thus rationalize the observation that the transformation
from tetrahedralto octahedralcoordinationof Si in anorthiticglassis
reversible [Williams and Jeanloz, 1987]. In addition, when viewed in
this light, it is not surprisingthat the transformation
from low to high
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thatwe haveproposed
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tetrahedramay be bonded through metal cations to other silicate
polymers,andthesebondswouldprobablyhaveto breakas the silicate
chain compresses
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packing from the expandedlow pressuretetrahedralnetwork to a
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oxygenpackingwouldtakeplaceessentially
continuously
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to octahedral
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wouldplay
an importantrole in the kineticsof suchpressure-induced
tetrahedralto
octahedralconversions.This may explainthe fact that major pressure
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Acknowledgments.
We thankJ. Bass,G. E. Brown,Jr.,R. Jeanloz,
and Q. williams for suggestions
that helpedus to developthe ideas
presented
in thispaper,andR. Hemley,J. Holloway,andC. Meadefor
comments
andreviews. We alsothankR. Myersfor assistance
with the
• 2 33
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silicates:Is glassyoctahedralsiliconunquenchable?
(abstract),Final
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Meeting), 312, 1987.
EdwardM. Stolperand ThomasJ. Ahrens,Division of Geological
and PlanetarySciences,CaliforniaInstituteof Technology,Pasadena,
CA
91125
(ReceivedMay 15, 1987;
acceptedSeptember25, 1987.)