American Mineralogist, Volume 72, pages l195-1203, 1987
X-ray diffraction and 2esimagic-angle-spinningNMRof opals:
Incoherent long- and short-rangeorder in opal-CT
B.H.W.S.nB Jor.qc
Applied Physics,SP-DV-025,Corning GlassWorks, Corning,New York 14831,U.S.A.
J. vaN Hoex. W. S. Ynnvr,lN
Department of Physical Chemistry, University of Nijmegen, Toernooiveld, 6525 ED, Nijmegen, The Netherlands
D. V. M.cssoN
Gemmological Institute of America, 1660 Stewart Street,Santa Monica, California 90404, U.S.A.
Dedicated
to Dr. J.G.Y.deJongon his 78thbirthday
AssrRAct
X-ray diffraction and 2eSivras Nvrn experiments have been carried out to assessthe
structure of amorphous and quasi-crystallinenatural and synthetic opals.We collected'?eSi
MASNMRspectrafor nine opals,which, accordingto their X-ray signature,belong to either
category A (amorphous) or CT (cristobalite-tridymite). A,ll Nrvrnspectra of natural opals
have chemical shifts of about - I l2 ppm, characteristicof amorphous silica, and resemble
in peak shapeamorphous silica most and, for different reasons,silica gel and quartz least.
The full width at half maximum (FWHM) of the Nr'anspectraincreasesin the order opalCT, opal-A, and amorphous silica. None of the opal NMR spectra can be mimicked by
combining those of the crystalline-silicapolymorphs. ComparisonsbetweenX-ray and'zeSi
MASNMRspectraindicate that long-rangeorder suggestiveof cristobalite-tridymite intergrowth is not strongly coupled to short-rangeordering. There is a distinct diference in'eSi
chemical shift betweenthe eight natural opals and the synthetic one. Synthetic opal has a
maximum chemical-shiftpeak intensity at - 107.0ppm, closeto that for quartz (-107.4
ppm), in contrastto natural opals,whosemaximum chemical-shift peak intensity is around
-ll2 ppm, closeto the averagechemical-shiftvalue of tridymite (-lll.0). Becauseof
this quartzJike character,synthetic opal has a higher density and refractive index in comparison to natural samples.It is an important characteristicof Nvn spectroscopythat it
can distinguish betweenamorphous substanceswith similar X-ray patterns.
lNrnooucrroru
Nakata, 1974; Kastner, 1979:-Kano, 1983; Graetschet
Nucleationis the principal processvariablein the tec- al., 1985, 1987).
BecauseX-ray diffraction of opal can only distinguish
togenesis-i.e., the creation of structure-of crystalline
compounds from amorphous starting materials. A facet three stages,it seemedlikely to us that additional stages
in this processis the interaction betweenshort- and long- in the solid-statetransition betweenamorphous and crysrange ordering. We shall address in the following this talline opal could show up as variations in local Si environments. Such variations can be characterizedby'zeSi
facet using opals as example.
Opal consists of 1500- to 3000-A spheres of silica, MASNMR,and thus a road seemedto open up toward a
mono- or polydispersedepending on the number of nu- refined stratigraphicmarker in sedimentarybasins.In adcleation events, and occurs in a wide variety of natural dition, study of opals in their various transition stages
settings(Frondel,1962;Iler,l979; Gauthier, 1985;Sand- might shedsomelight on the natureof amorphoussolids
ers, 1985;Deelman, 1986).X-ray diffraction experiments and the relation between amorphous-crystallinepairs, a
on natural opal from sedimentary basins reveal the oc- subject that currently is receiving increasing attention
currence of solid-statetransitions from amorphous to (Phillips,1980;Matsonetal., 1983;Grimmer etal, 1984;
crystalline, the various stagesbeing designated opal-A de Jong et a1., 198l, 1984a, 1984b; Gerstein and Nicol,
(amorphous),opal-C (cristobalite),and opal-CT (cristo- 1985;Engelhardtet al., 1985;Gladden et al., 1986).
In this paper we show that in the opal-A to opal-CT
balite-tridymite). The final stageof burial metamorphism
is indicated by the presenceof quarlz in opals. Thus opal transition, long-rangeorder as determined by X-ray difand its various transition stagesfunction as stratigraphic fraction occurs first followed by short-rangeorder, as demarkersin sedimentarybasins(Fl6rke, 1956;Murata and termined by 'esi MASNMR.In addition, we show that all
I 195
I 12-1195$02.00
0003-o04x/87/1
I 196
DEJONG ET AL.: MORPHOGENESIS OF OPAL-CT
opal-A and opal-CT samples used in this study show local Si environment distributions closely akin to those
found in amorphous silica. Next, we show that the ,eSi
MASNMR spectra of opals cannot be constructed by simple
superposition of the spectra of cristobalite and tridymite.
Hereafter, we show that silica in opal is completely polymerized and that there is a fairly narrow range of SiG-Si angles between those of amorphous silica and tridymite. Finally, we suggest reasons why long-range ordering and short-range ordering are not developed synchronously with one another.
ExpnnrlrnNTAl
DETATLS
Eight opals from different localities and one syntheticone were
characterizedby X-ray diffraction, chemical analysis, nsn, and
proton and 'zeSimagic-angle-spinningNrran.HrO measurements
on the sampleswere carried out in three temperaturestepsusing
a modified Karl-Fischer assayingmethod.
'?eSi
rrr.ns
Nun spectrawere collectedon a home-built spectrometer with a 4.2-T wide-bore Oxford magnet at 35 756 kHz and
on a cxp 300 Bruker spectrometerat 59 600 kHz. Normal rr-rwrn
techniquesin conjunction with u,q,swere used to obtain all spectra. Most spectrawere collectedwithout proton decouplingonce
it was observedthat such decouplingdid not afect the':eSiline
width nor line position in opals. Cylindrical rotors (8-mm internal diameter)were usedfor collection of the "Si rrteswun spectra
with typical spinning speedsof 2.5 kHz. A 90' pulse, varying
recycledelay between I s and I h (30 s for the spectradiscussed
here), 1000 data points, and 20-kHz spectral line width were
used in all cases.Line broadening due to exponential multiplication of the free-induction decay is l0 Hz. The "Si chemical
shifts reported in this paper are referencedby sample exchange
to TMS. Two spectrawere run with benitoite (-94 ppm) as an
internal standard (de Jong et al., 1984a).
Proton uls wr'rRspectrawere collectedin similar fashion. Due
to the much shorter proton I, relaxation time, recycledelaysof
I s were used. The proton spectra were referencedby sample
exchangeto benzene.The negative sign indicates that the protons in opals are more shielded than those in benzene.Proton
and 'zeSiMAs NMR spectra of one sample were measured after
heat treatment for 24 h at 300 qC under vacuum (0.005-mm
Hg). esn spectraon opals were collected on a Varian E-123 (xband) spectrometerat room temperature.
Density and refractive-index measurementswere carried out
using a picnometer and refractometer,respectively.
Rrsur,rs
Results of our X-ray diffraction and MASNun experiments are collected in Table I and illustrated in Figures
1,2,3, and 4. The opalsin Table I are designatedeither
opal-A or opal-CT depending on their amorphous or
quasi-crystalline character as determined by X-ray diffraction (Fig. 3). The do,, spacing of cristobalite and
tridymite are 4.05 and 4.10 A, respectively.Larger d
spacingsare thought to be a measure of the tridymitecristobalitetransition in opal-CT (Kano, 1983).The do,,
s p a c i n g i ns a m p l e s2 , 3 , a n d 7 i s 4 . 0 7 , 4 .1 3 ,a n d 4 . l l A ,
respectively,suggestingthat sample 2 is most and sample
3 least like cristobalite. One sample, number 6, has an
opal-A pattern with some quartz. The amorphous haloes
of natural and synthetic opal-A have an intensity maxim u m a t a 2 0 o f 2 2 "( d ^ " : 4 . 0 4 A ) a n d 2 1 . 5 "( d ^ , 8 : 4 . 1 3
A), respectively,slightly diferent and narrower than that
of vitreoussilicaat a20 of 2l'(d^"":4.23 A).
Proceduresfor manufacturing synthetic opals have been
developed by K. Inamori and P. Gilson. X-ray patterns
of these synthetic opals, designated Inamori or Gilson
opal, show the presenceof ZrOr, particularly in the Gilson opal as noted by other workers(Simonton et al., 1986;
Gauthier, 1986). The calculated crystallite size of opalCT, basedon line broadeningof the (01l) diffraction line,
varies between 60 and 90 A, bracketing the c dimension
of tridymite PO (81.864A;(Ktug and Alexander,1954;
Fagherazziet al., l98l; Konnert and Appleman, 1978;
Vieillard, 1986;Nukui and Fl6rke, 1987).
The '?eSi
chemical shifts of all natural opals fall around
- I 12 ppm (Fig. l), between the chemical shifts for cristobalite and tridymite. The full width at half maximum
(FWHM) of 'zeSiNun peaks varies between 5.8 and 9.8
ppm for natural opals, but is substantially broader, I I
ppm, for the one synthetic sample.The'zeSichemical shift
of this sampleis lessnegative,- 107 ppm, and closerto
the chemical shift of qtJarrz,than those of the natural
samples.
There are large variations in fr relaxation times for
silica species.7",for quartz is ofthe order of5 h, whereas
7, for opals varies between 0.25 and l0 s, presumably
becauseof the presenceof either paramagneticimpurities
or water in the latter (Gladden et al., 1986). The width
of the components building up a 2eSiresonanceline in
opal is 50 Hz (l ppm) as revealed by 7", determinations
obtained using the Hahn spin-echo technique (Hahn,
1950).The meaning of this result is that 50 Hz is the
lower resolutionlimit betweena collection of silica species
and hence that the width of the '?eSiMASNMR lines in
opals is due to a distribution ofsilica speciesrather than
to variations in relaxation efects.
Proton chemical shifts do not show large variations
between samples.Proton decoupling does not afect the
'zeSispectralJine width nor its position. The water content, characteristic for physisorbed (20-105 'C) and
chemisorbed(105-500 "C) water, varies betweensamples
without discernible trend.
The esn spectra were collected to determine if paramagnetic centers affect the opal NMR line width. Some
samples (notably 3, 6, and 8) show strong ESRsignals,
indicative of substantial concentrations of paramagnetic
centers,whereasothers(samplesl, 4, and 5) do not. There
is no significant difference in Nrurnline width between
sampleswith high or low concentrationsof paramagnetic
centers(Table 1).
DrscussroN
Since the pioneering study of Lippmaa et al. (1980),
2eSiuas NMRspectroscopyhas becomea well-established
technique to probe the structure of crystalline aluminosilicates, particularly zeolites (Klinowski, 1984; Oldfield
and Kirkpatrick, 1984;Kirkpatrick et al., 1985).On the
tl97
oE JONG ET AL.: MORPHOGENESIS OF OPAL-CT
other hand, 2eSiuas Nvrn experiments on silicate glasses
are still fairly rare, though the number of such studies is
rapidly increasing(Grimmer et al., 1984; de Jong et al.,
1983,1984a,1984b;Murdochet al., 1985;Dupreeet al.,
1984,1986;Weedinget al., 1985;Fujiu and Ogino,1984;
Gersteinand Nicol, 1985;Engelhardtet al., 1985;Gladden et al., 1986; Kirkpatrick et al., 1986; Aujla et al.,
1986;Yang et al., 1986;de Jongand Veeman,1986;Devine et al., 1987; Turner et al., 1987; Schneideret al.,
1987). Still unique is the Nvrn study on molten silicates
ofStebbinser al. (1985).
We start our discussionwith the 'zeSiMASNMRresults
for opals and compare these results with those for silica
gel and glass. Next we discuss differencesbetween the
X-ray diffraction and MASNMRresults.
oPAL A
-ilt,s
oPAL CT
-|24
Comparisonbetween2eSiv.ls uun spectraof opals,
silica gel, and silica glass
The 'zeSiMASNMRspectraof opals cover the chemical
shift range of -99 and -123 ppm (Fig. l). This range
indicates that within Nun detection limits, all Si atoms
are coordinated to four oxygens in a three-dimensional
array of corner-sharingtetrahedra. Thus the uncommon
configurations postulated by various authors to exist in
amorphous silica, such as edge-sharing tetrahedra or
threefold-coordinated Si (chemical shift less negative than
-t30
-ilo
-90
60 ppm), and fivefold- or sixfold-coordinated Si (chem(ppm)
SHIFT
CHEMICAL
ical shift more negativethan 130 ppm), if present,do not
'?eSi
Fig.
1.
ves
r.n"rn
spectra
of amorphous(opal-A)andquasioccur in sufficient concentrations in opal to be detected
(O'Keeffeand Gibbs, 1984; Garofalini, 1984;Weyl and crystalline(opal-CT)gem-qualityopals.
Marboe, 1959).The absenceof silanol groups,i.e., Qo,
Q,, Qr, and Q, tetrahedra with protons attached to the that most water in thesecompounds is either physisorbed
nonbridging oxygens,is confirmed by the lack of line nar- or chemisorbed,albeit that the secondtemperature step,
rowing of the Si spectrum on proton decoupling, in ac- 500 'C, is somewhat high for chemisorbed water, as
cordancewith the HrO chemical analyses,which indicate pointed out by Knauth and Epstein, 1982.A fairly strong
(FWHM),
TneLe1. Protonand2esichemical
number
shifts,fullwidthat halfmaximum
of Si free-induction
decays(FlD),andweightpercentwaterin opalsin temperaturesteps
HrO (wt%)
Sample
1
2
3
4
5
6
7
I
9t
10t
111
12
Description
Opal-A
Opal-CT
Opal-CT
Opal-A
Opal-A
Opal-A +
Opal-CT
Opal-A
Opal-A +
Opal-A +
Opal-A +
Opal-A+
qtz
qtz
qtz
qtz
4Si shift. 'H shift.- FWHM
(ppm)
(ppm)
(ppm)
-111.8
-111.9
-1'12.4
-1115
-111.5
-111.4
- 1 11 . 8
-111.5
- 11 0 . 6
-111.9
-111.4
- 10 7 . 0
-1.8
-1.7
-1.7
-1 I
-1.9
-1.7
-1.8
-1.9
n.d.
n.d.
n.d.
9.8
6.2
5.8
8.6
8.0
9.8
7.2
8.6
9.8
10.6
10.1
11.0
FID
5 009
5340
8 502
5 650
13592
2772
7 940
10 658
11 1 8 8
6274
82
2500
105- 50020105rc 500'c 1000ec
n.d.
1.74
0.58
n.d.
2.04
2.04
1.08
2.68
2.04
n.d.
2.04
n.d.
n.d.
7.0
4.4
n.d.
4.6
2.9
7.5
4.1
2.9
n.d.
2.9
n.d.
n.d.
0.01
3.8
n.d.
0.27
0.24
0.23
0.01
0.24
n.d.
0.24
n.d.
'Relative to TMS, proton shifts relativeto TMS: benzene,7.3 ppm; H2O,4.8-4.7 ppm; (H) in
opal, 5.5 ppm
-t Relativeto benzene.
tThree other experiments were carried out on sample 6: in sample 9, protons were decoupled;
in sample 10, protons were removed atlet 24-h heat treatment at 300 rc; in sample 11, experiment
was carriedout on Brukercxp 300 at 7.1 T.
+ Inamorisynthetic opal
l 198
DEJONG ET AL.: MORPHOGENESIS OF OPAL-CT
I
4tl
@tov
or..lARTz
OPAL CT
I
433
(4 328)
tl
-JL_
-l;J
stLtca
GLASS
I
CRISTOBALITE
t
tl
OPALA + OUARTZ
426
-JLOPALA
V I T R E O US I L I C A
-85-rO5-r25-85.rO5-t25
(opm)
CHEMTCAL
SHTFT
Fig.2. 2eSi
u.e.s
r.nanspectraof silicagel(MacietandSindorf,
1980),silicaglass(Murdochet al., 1985),quartz,tridymite(Smith
andBlackwell,1983),andopal.
interaction between water and silica is suggestedby the
observation that proton chemical shifts of opal are about
l.l ppm more positive than for water (Table 1). This
conclusion concerning the absenceof silanol groups in
opals differs from the one reachedby Graetsch et al. ( I 985)
basedon rn data of opal-C.
Line broadening of the 2eSiues NMR spectra of opals
may be causedby dipolar coupling betweenprotons and
'zeSi
nuclei. To test for this possibility we carried out three
additional rurvrnexperiments on one sample (no. 6), in
which the protons were decoupled (sample 9, Table l),
the spectrum was measuredafter removal of water (sample l0), and the spectrumwas measuredat 7.1 T rather
than the standard4.2 T (sample I l). The resultsin Table
I indicate that the FWHM of the 2rSi spectra were not
affectedin these three experiments.Hence, the causefor
the broad FWHM of opal-A spectrahas to be found in a
distribution of chemical shifts.
Ofall the silica spectraconsideredhere, the opal spectral shaperesemblesthat of silica glassmost and that of
either silica gel or quartz least (Fig. 2). The differences
between the spectra of silica glass and opal are that the
peak maximum for opal is more negative[about -ll2
vs. - I I1.5 ppm (Gladdenet al., 1986)or - 110.9ppm
(Murdoch et al., 1985)l and that the FWHM for opal is
smaller(5.6 to 9.8, vs. 11.6ppm).
The distribution of mean Si-O-Si angles per tetrahedron, (Si-O-Si), for natural opals, calculated using the
Itl
40
30
?o
ro
20 (degrees)
Fig.3. X-ray diffractionpatternsof opal-CT,opal-A + qt:*rrtz,
opal-A,and vitreoussilica.The numbersin parentheses
arethe
d spacings
of tridymite.
method of Thomas et al. (1983) and illustrated in Figures
4 and 5, indicates a narrower range for the opals than for
silica glass.For opal-A, (Si-O-Si) varies between 133'
and 168" with a maximum around 151". For opal-CT,
(Si-O-Si) varies between I 38' and I 70' with a maximum
around 152o,whereas for silica glass, the range varies
between 122' and 170" with a maximum around 148".
The (Si-G-Si) variation of the silica glassof Dupree and
Pettifer (1984) indicates their sample to be most likely a
gel. It should be noted that the calculation of (Si-O-Si)
angles assumesthat 2eSichemical shifts of crystals and
glassescorrespond to one another, an assertion that, as
we have demonstratedelsewhere(de Jong et al., 1984b),
is not necessarilycorrect. It necessitatesin addition that
spectralJine width is caused by a distribution of silica
speciesrather than by relaxation effects.That silica distribution turns out to be the causefor line broadeninghas
been demonstratedwith our spin-echo experiments.
Compared to crystalline SiO, phases, the calculated
range of (Si-O-Si) values in opals corresponds most
closely to that of tridymite MC, i.e., tridymite with 12
nonequivalent Si positions (Baur, 1977). For tridymite
MC, the averagechemical shift is - I I I ppm (Smith and
Blackwell, 1983)with a correspondine(Si-O-Si) angle of
150o,whereas for tridymite OP, i.e., tridymite with 80
tt99
os JONG ET AL.: MORPHOGENESIS OF OPAL-CT
I DUPREE
A PETTIFER
2 MURDOCH
etol
OPAL A
(*9)
.{
z
5
o
I
(degrees)
TOTANGLE
Fig.4. Comparison
between(SiSi) distributionof opal-A
andamorphoussilicausingthe formulaof Thomaset al. ( I 983).
The sampleof Dupreeand Pettifer(1984)is most likely silica
gel.
nonequivalentSi positionsand (Si-O-Si) angleof 148.3'
(Konnert and Appleman, 1978),no NMR spectrum has
been measured to date. The maximum and minimum
(Si-O-Si) anglesobservedfrom X-ray diffraction in tridymite MC are 157.5'and 146.7',respectively,obviously
a rangeofvalues substantiallynarrower than the one observed in opals. The single Si site in cristobalite has a
chemical shift of - 108.5 ppm correspondingto a (Si-OSi) of 146.4' (Dollase,1965).Thus in terms of chemical
shifts and variation in (Si-O-Si) distribution, local Si
environments in opal most resemblethose in tridymite.
The major differenceis that for opals, the correlation between (Si-O-Si) and 2eSiNrunchemical shift indicatesthe
presenceof tetrahedra with (Si-O-Si) anglesas small as
133o,whereastridymite contains no such tetrahedra.
It needs to be pointed out that there is a diference
betweenangular distributions obtained from Nun chemical shifts and from X-ray difraction. The latter technique in its application to amorphous solids assignsangular distributions basedupon experimentallydetermined
Si-O, Si-Si, and O-O distances,resulting in a distribution
of individual Si-O-Si angles(Taylor and Brown, 1979).
On the other hand, NMRmeasuresa distribution of mean
Si-O-Si angles, (Si-O-Si), per tetrahedron. These two
distributions are not necessarilythe same, as can be illustrated for tridymite MC. In this crystal the minimum
i-O-Si ANGLE(degr@s)
Fig. 5. Variationin (Si-G-Si) distributionbetweenopal-A
andopal-CTusingthe formulaof Thomaset al. (1983).
and maximum (Si-O-Si) anglesare 146.7' and 157.5o,
respectively, whereas the minimum and maximum SiO-Si angles are 143.4 and 179.1', respectively(Baur,
r977\.
In addition to the empirical formula derived by Thomas et al. (1983), three other semi-empirical formulas have
been constructed to relate chemical shift to (Si-O-Si)
angle (Radeglia and Engelhardt, 1985; Ramdas and Klinowski, 1984; Smith and Blackwell, 1983).Thesethree
relationships addressdifferent facets of variation in the
diamagnetic contribution to the chemical shift, focusing
attention on electron-density variation on Si (Ramdas and
Klinowski, 1984), on oxygen (Radeglia and Engelhardt,
1985),or in the Si-O bond (Smith and Blackwell,1983).
The linear least-squaresfits to the data points, correlation
coemcients,and chemical-shift cutoff values are collected
in Table 2. We have calculatedthe angular variation for
opal sample 9 using thesefour formulas and illustrate the
result in Figure 6. Calculated (Si-G-Si) angular variations using the different formulas are slight except for the
relationshipof Thomas et al. (1983), which, becauseit
includes in the fitting procedurethe chemical shift of zunyite (-128.2 ppm), shifts the Si-O-Si distribution to
smaller angles.We have used this last procedurenot only
becauseit has the best correlation coefficient (Table 2)
but also becauseit enablescalculation of (Si-O-Si) for
(Si-O-Si)angles,A,from4Si chemicalTneLe
2. Equations
of variousleast-squares
fittedlinesusedto calculate
shiftvalues,D
Formula(ppm)
6=
6:
6:
d:
- 1)
0.63514 - 241.448(cos(0)/(cos(d)
149.947 - 270.532(sin(0)12)
-170.337- 50.9861(sec(a))
-19.8215- 0.608526((d))
Cutotf value of
Correlation chemicalshift
(ppm)
coetficient
0.908
0.910
0.904
0.911
120.1
120.6
11 9 . 4
129.4
Reference*and comments
(1)
(2)
(3)
(4)
s hybridizationof oxygen atoms
non-bondedSi-Si interactions
Si-O overlap integrals
empirical
- 1, Radegliaand Engelhardt,1985; 2, Ramdasand Klinowski,1984; 3, Smith and Blackwell,1983; 4, Thomaset al., 1983.
1200
DEJONG ET AL.: MORPHOGENESIS OF OPAL-CT
TeeLe3. Density and refractive index of natural and syntheticopals
OPAL A
{r 9)
Sample
R
R
R
R
R
R
R
I
z
J
f
q
o
12715'
11399.
1377911397.
13154-13153..
13213t
Density (g/cms) Refractiveindex
2.129
2.121
2 122
2.122
2.215
2 212
2 209
1.447
1.448
1.443
1.448
1.460
1.464
1.460
- Naturalsamoles.
*. Inamorisyntheticsamples.
t Gilsonsyntheticsampleto
r50
160
t70
t80
tridymite? The reasonfor this is that the X-ray diffraction
Fig6 (si-o-si)
.",J;jT:;:ff;i"
rherourrormuras
collectedin Table2. The singleline represents
the distribution patterns of opal-CT (Fig. 3) are usually interpreted as
according
to Thomaset al., 1983.
being due to disordered cristobalite (Fl6rke, 1956; Kastner, 1979; Graetschet al., 1987).Thus from the X-ray
the tail of the opal NMR spectrum that has a chemical evidence one might have expectedNvrn spectraof opals
shift more negativethan - 121 ppm.
with chemical shifts around -108 ppm, corresponding
The 'zeSiMAs NMRspectrum of a natural opal-CT and to a (Si-O-Si) distribution with a maximum frequency
the previously published spectraof silica gel, vitreous sil- around 146'and a narrower variation in anglesthan acica, quartz, tridymite, and cristobalite (Smith and Black- tually observed.
well, 1983; Murdoch et al., 1985; Maciel and Sindorf,
The question ariseswhether the three peaksin an opal1980)are shown in Figure 2. Comparison of thesespectra CT X-ray diffraction pattern were indeed characteristic
clearly indicates that the opal-CT spectrum cannot be of disorderedcristobalite or whether alternative interpreconstructed by superposition of those of tridymite and tations are possible.Cristobalite and tridymite both concristobalite.
sist of six-membered rings of silica tetrahedra, the ring
The one synthetic opal measured in this study has a conformation varying between chairs and boats in tridchemical shift of - 107 ppm, closeto the value for quartz ymite versuschairsonly in cristobalite(Taylor and Brown,
(- 107.1 ppm). By analogywith the crystalline silicas, we 1979).The oxygenatoms in crystobaliteconsistof a threeexpected the natural opals, with their chemical shifts layer sequenceequivalent to the sequenceofcubic closest
around - I 12 ppm, to be lower in density and refractive packing, whereas those in tridymite consists of a twoindex than the synthetic ones. Triplicate measurements layer sequenceequivalent to hexagonal closest packing
of refractive indices and densities of four natural opals (Graetschet al., 1987).
and three synthetic ones (two samples manufactured by
More important as a clue to interpret the opal-CT X-ray
Inamori and one by Gilson) (Table 3) confirm this hy- difraction pattern are the characteristic radial distribupothesis. Thus, the less-negativechemical shift, i.e., the tion maxima for vitreous silica as a function of Si-O-Si
more quartz-like constitution, of synthetic opals coin- angle (Taylor and Brown, 1979).Thesedata indicate that
cides with higher density and higher refractive index. A amorphous SiO, with straight Si-O-Si anglesshould have
similar phenomenonis observedin the pressure-induced maxima around 1.6, 2.3, and 3.2 A plus threeadditional
160/odensification of suprasil, which causesa chemical maxima between 3.3 and 4.5 A Gig. 7A). Si-O-Si bendshift of 2.5 ppm to less-negativevalues (Devine et al., ing causesthird- and fourth-nearest-neighbordistancesto
1987).The same l6olodensificationis also observedin coincidewith maxima around 2.6,3.5, and 4 A Fig. 7B).
neutron irradiation of vitreous silica (Maurer, 1960; Pri- Comparison of these distances with those observed in
mak, 1958), suggestingthat, if pressure and irradiation opal-CT X-ray patternsindicatesthat the 4.3-, 4.1-, and
densificationof vitreous silica lead to the sameend result, 2.5-A spacings are characteristic oxygen-{xygen disno anomalous Si coordinations occur in either process tancesin angle-constrainedsilicas. It is therefore equivwithin Nrrandetection limits. The chemical-shift varia- ocal to interpret the opal-CT diffraction pattern as that
tions for amorphous silica speciespoint toward an im- ofdisordered cristobalite, ordered cristobalite with a tridportant feature of Nrrrnspectroscopy,namely, that it can ymite stackingsequence,or a well-ordered, possibly triddistinguish between amorphous substancesthat yield ymiteJike oxygenarray, with cristobalite stacking faults.
similar X-ray diffraction patterns.
Any of thesestackingsequences,
and possiblymany more,
may give rise to long-range unidirectional ordering of
Distribution of local Si environmentsin opal and
close-packedoxygen atoms, while maintaining local Si
long-rangeordering in opal-CT
environmentsbetweenthoseencounteredin tridymite and
Why did we emphasize in the above discussion the amorphous silica.
similarity or dissimilarity between opal and cristobaliteThe pronounced character of the (l0l) X-ray diffrac-
DEJONG ET AL.: MORPHOGENESIS OF OPAL-CT
t20l
o
o
O
o
o
-80
in SiO, at Si-O-Si angleof
Fig. 7. (A) Distancefrequencies
in SiO' at Si-O-Si angleof 140'
180".(B) Distancefrequencies
(TaylorandBrown,1979).
-too
-t20
C H E M I CSAH
L I F T( P P m )
spectrumof opal-Aandquartztaken
r'resNIvrR
Fig.8. (A)'zeSi
with 30-stime intervalbetweenpulses'(B) 2eSitr'tlsNMRspectrum of opal-CTtakenwith l-h time intervalbetweenpulses.
amount of quartz can be detected in opal-A, two experiments were caried out. In the first experiment (Fig' 8A)
a 2eSiues NMR spectrum of quariz was compared with
that of sample 6, a quartz-containingopal-A (experimental conditions: 59.6 MHz, 30-s delay, similar spin concentrations). One free-induction decay (FID) of quartz
yields a signal to noise (S/N) ratio of three; 120 FIDs of
Sensitivity of 2eSiu.r,s Nrrn vis-i-vis X-ray diffraction in
quartz should give a S/N ratio of 33 (3 x y'T20). Hence
detectingcrystalline phases
if 1/33 or 30/oof sample6 consistsof quartz, a peak should
The precedingsection has raised some questionsabout appearat -107.4 ppm with a S/N ratio of l, provided
the common interpretation of X-ray diffraction patterns T, of qtartz in opal is similar to Z, in quartz alone. The
of opal-CT as indicating the coexistenceof microcrystal- 5o/oquartz found in the X-ray diffraction pattern of samline and amorphous silica. We address in this section ple 6 does not give rise to a discernible quartz peak in
factors pertinent to detectingsmall amounts of crystalline the teSiMASNMRspectrum of this sample.
In a second experiment the recycle delay of opal-CT
phasesin amorphous silica.
In a two-phasesystem,as opal-CT is accordingto X-ray (sample 3) was changedfrom 30 s to I h in order to test
diffraction, detectability of a phase by means of NIrrn for the possibility that cristobalite in this samplehas considerablylonger Z, than opal itself. The spectrumdid not
spectroscopydependson the relative concentrationofthe
phases,whether the peaksoverlap, and on the spin-lattice change except for a very small peak, just above backrelaxation time, 2,. I, values for opal and quartz are ground at -107.4 ppm, characteristicfor quartz (Fig. 8B).
about l0 s and 5 h, respectively.Thus, considering only We conclude from theseexperimentsthat, provided that
I, variations, for a sample of opal-A containing quartz, the I, of cristobalite is not pathologically long, the local
a 30-s delay between pulses enhances the amorphous Si environments, sampled by wrvrnon the scaleof 5-6 A,
reflect an arrangementcomparableto that of amorphous
component by a factor of 600 relative to quartz.
To determine the conditions under which a small silica. On the other hand and as already pointed out,
tion peak of opal-CT, sampling long-rangeordering of at
least 50 A, and the broad opal-CT 2eSives NMR spectrum, resemblingamorphous silica much more than tridymite or cristobalite, suggestthat in silica species,solidstatetransitions are initiated by an ordering ofthe oxygen
array followed by that of the Si sites.
1202
DEJONG ET AL.: MORPHOGENESIS OF OPAL-CT
long-rangeorder of at least 50 A, when sampledby X-ray
diffraction, showsa transition toward a crystalline array.
In this context it is ofinterest to note that oxygen interactions determine geometry in silica species,as demonstrated by de Jong and Brown (1980) and used by
Thattachari and Tiller (1982) in modeling the structure
of amorphous silica. The data in this study on opals suggest that long-rangeordering ofthe oxygen array has already taken place in opal-CT, whereasthe Si atoms have
not yet found their equilibrium positions. It is a matter
of speculation if such long-rangeorder preceding settlement of local Si environments is the rule rather than the
exception in solid-phasetransitions from amorphous to
crystalline silicates.
Sulruany AND CoNCLUSToNS
Opal-A and opal-CT yield relatively broad 2eSiuns
NMRspectrathat cannot be interpreted as being due to a
superposition of the spectra of crystalline silicas. The
X-ray diffraction pattern ofopal-CT can therefore not be
interpreted as being caused by line broadening due to
cristabolite-tridymite microcrystallites for which Nun
spectroscopy,sampling local environments in the order
of 5 to 6 A, would show a combination of tridymite and
cristobalite peaks.The differencebetween the X-ray and
MASNMRsignaturesof opal-A with quartz indicates the
greatersensitivity of the former techniqueto the presence
of crystalline phasesin this particular system in contrast
to, for instance, the detection of corundum in aluminosilicateglasses(de Jong et al., 1983).Resultspresented
here also indicate that long-rangeordering in the transition from amorphous to quasi-crystallineopals precedes
short-rangeordering.
Proton Nvn does not indicate a significant amount of
silanol groups in opals. Our work suggeststhe presence
ofwater physisorbedon the substrate,akin to the type of
water present in zeolites. Synthetic opals are, according
to their'?eSichemical shift, intrinsically more quartzJike
than natural ones. As a consequence,such opals can
readily be distinguishedfrom natural onesby their higher
refractive index and density. Our results indicate, thus,
that NrvrndistinguishesbetweenX-ray-comparable amorphous phases.
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Kirkpatrick have helped considerably in straighteningout the interpretation of the observedphenomena
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