American Mineralogist, Volume 74, pages 1147-1 151, 1989
Crystallization of silica gel in alkaline solutions at 100 to 180 'C:
Characterizationof SiOr-Y by comparisonwith magadiite
Hlluro
Muursnr
Department of Chemistry, Fukuoka University of Education, Akama, Munakata, Fukuoka 8 I I -41, Japan
AssrRAcr
Sior-f, an intermediate product of the transformation of silica gel to quartz, was synthesized from KOH or NaOH solution containing SiO, and NaCl with a molar ratio of
SiOr:HrO:Na*:OH-equalto l:100:2-4:0.5at 100-180'C. In almost all reactionprocesses,
SiOr-)z appearedas the first crystal phaseand then was transformed into SiOr-Xr, cristobalite, and f,nally quartz. The characterizationsbased on the chemical and physical data
for SiOr-/ showed that Sior-y is magadiite (NarSi,oOrr.xHrO,x: 9-ll), which was
discovered at Lake Magadi (an alkali lake), Kenya, and has been considered to be an
important precursor of inorganic bedded cherts.
INrnooucrroN
et al. (1973).Lagaly et al. (1975) have sinceshown that
Hydrothermal crystallization of amorphous silica was natural and synthesized magadiite is a sodium silicate
investigatedin the 6inary systemSiOr-HrO at high tem- with the compositionNarSi,oOrn'11HrO.
We also obtained the product identified as SiOr-/in
perature and high pressure,and the transformatlon via
some metastable phases(cristobalite, keatite) to quartz Naollt KgH, and NarCO, solutions containing NaCl at
was observed6eat, teS+; Carr and Fyfe, 1958).When 100-180'C at the autogenouspressureof an autoclaveat
alkali-metal or alkaline-earth salt was added in aqueous a given temperature.In this paper, the transformation of
media, the transformation to quartz occurred at lower amorphous to crystal phasesunder hydrothermal conditemperatureswith various intermediate phasesthat al- tions was further examined on the basis of the run duration,temperature,andconcentrationofalkali-metaland
ways contain some alkali ions and ro u.. not crystal
alkaline-earth salts. The preparation condition of SiOr-Y
phasesin the one-componentSiO, system.
Heydemann (1964) observedthat amorphous silica in was determined from the experimental observations,and
KOH solution crystallizesvia SiOr-Xand cristobalite to its synthesiswas attempted. The nature of SiOr-I was
qurirrzat 100-250 "C, and characterizedthe nature and determined and compared with that of the other silicates.
properties of SiOr-X. Mitsyuk et al. (1973, 1976) de- Theresults showedthat SiOr-Yis indeedmagadiite.The
scribedmany intermediate phasesthat appearat 150-2g0 details-ofthechemicalpropertiesand the X-ray difraction
.C when alkali-metal or alkaline-earth salt is added
to an data of SiOr-Xr, from which it was determined to be a
alkaline medium. Sior-y, which is one of the interme- quasi- kenyaite,will be reportedin a subsequentpaper.
diate crystal phases,was formed in 0.5-2o/oKOH soluExpnnrnanNTAl- DETAILS
tions containing NaCl in the temperaturerange 150-180
'C at 0.5-4.0 kbar. This is a sodium silicate that has
Hydrothermal experiments on the transformation of
the
compositionNarO'2lSiOr'l2HrO-NarO.29SiOr.l4HrO,
silica gel were carried out in the temperaturerange 100dependingon the nature of the reaction medium. This 180 'C in l-2 wto/o(0.18M4.36M, M = mol/L) KOH
compoundwas found to have a high capabilityfor cation solutionscontaining 0.2, 0.5, 1.0, and 2.0M NaCl. We
exchange.Kitahara et al. (1982)and Muraishi and Kita- usedTeflon bottleswith a volume of 15, 50, and 100 cm3
hara ( 1986) observedthe effectsof seedcrystals,some as reactionvessels.Usually a I 5-cm3bottle was used,and
metal oxides, and alkali-metal salts on the crystallization 0.3 g of starting silica (Wako's silica gel Q-22, through
of silica gel in KOH solution at temperaturesbelow 300 200 mesh) was put in the bottle together with 7.5 mL of
oC,under which conditions SiOr-X, SiO2-Xr,and SiOr-I
the solution. The Teflon bottle was closed with a Teflon
mainly formed as intermediate phases.
plug, and it was placed in a stainless-steelautoclave that
On the other hand, Eugster (1967) described new hy- was designedto accommodate the bottle. After the audrous sodium silicates,the mineral magadiite [reportedly toclave was kept at a given temperature for a definite
NaSi'O'r(OH)3'3HrO)l and kenyaite [NaSi,,O,o,(OH).. time, it was quenchedrapidly with water. The reaction
3HrO)] from Lake Magadi, Kenya. It is thought that ma- products were taken out of the autoclaveand washedwith
gadiite precipitates from alkaline lacustrine brines and NaOH solution (pH 10), since SiOr-I has good cation
converts to kenyaite and finally to quartz (chert). Its syn- exchangebelow a pH of 10. Some of the conditions for
thesishas been reported by McCulloch (1952) and Lagaly preparing SiOr-y are describedin Table 2.
0003-{04xl89/09
10-1I 47$02.00
lI47
r 148
MURAISHI: CRYSTALLIZATION OF SILICA GEL IN ALKALINE SOLUTIONS
z
-+-
a
200'c
c
o
s0
(h)
50
(h)
-+-
I 80'c
c -:-.
o
--:-
<r-_
z
:
-:--
1 6 0" c
:
10
(d)
l0
(d)
-:o
z
1400c
a
o
o
02168
'.
0
Reoction
time/ d
s
R e o c t i o nt i m e
Fig. L Effectof NaCl concentration
in l0loKOH solutionin
: 3:l:l.l-11.0at 160'C on
thecomposition
of SiOr:OH-:Na*
the crystallization
sequence
of SiOr-I (4, SiOr-X,(Xr),cristobalite(C),and quartz(Q) as a functionof reactiontime.
Fig. 2. Efect of temperature for a l0/o KOH solution containing NaCl of 2 moUL on the crystallization sequence defined
in Figure I as a function of reaction time.
Magadiite was synthesizedby heating mixtures of the
ratio of 9 mol of SiOr, 2 mol of NaOH, and 75 mol of
HrO at 100 "C for 4 weeks,according to the method describedby Lagaly et al. (1975).As it is known that magadiite as well as SiOr-I exhibits a cation-exchangecapability, synthetic magadiite was also treated with the
same method describedabove for SiOr-f.
The reaction products were identified by the use of a
Rigaku xcc-20 X-ray difractometer. Chemical composition of the products was determined by conventional
gravimetric methods for silica and by atomic absorption
for Na and K. The water content was measured bv a
thermogravimetric method.
lower temperatures(e.g.,below 120 "C) in the l0loKOHNaCl or 0.5MNarCO, solutions.When NaOH is usedas
an alkali sourceinstead of KOH (i.e., using NaOH-NaCl
solutions),a well-crystallizedSiOr-I is synthesized.When
KOH is used instead of NaCl (i.e., using KOH-KCI solutions), SiOr-X, is directly formed and transformed into
cristobalite, which then goesto quartz. This differenceis
understandablefrom the fact that SiOr-I is a sodium
silicate and SiOr-X, is formed as a combination of both
sodium and/or potassium silicatesas describedbelow.
Figure 2 shows the effect of temperature on the crystallization of silica gel for a loloKOH solution containing
2MNaCl. It has been reportedthat SiOr-yis formed in
a temperaturerangeof 150-180'C under high pressure
(Mitsyuk et al., 1973). In our investigation,the period
over which SiOr-Ywas stablewas found to increasewith
a decreasein the temperature, and it was synthesizedeven
at 100 'C. Comparatively well-crystallizedSiOr-I was
obtained after a reaction time of I to 3 months at 100
'C. Sior-y was stable for several months and slowly
transformed into quartz. Thus, a lower temperature was
advantageousfor the formation of SiO'-Icompared with
SiOr-Xr. In addition, the amount of solid phase in the
alkaline solutions was followed through the reaction
term. The solid phasedecreasedin the initial stageof the
reaction and then increasedwith reaction time up to constant amounts. These results suggestthat the transformation processof silica gel to SiOr-I may be a dissolution-precipitation process.
The appearanceof SiOr-Ivaried as a function of NaCl
Rnsur,rs AND DrscussloN
Preparation conditions of SiOr-Y and rnagadiite
It was observedthat SiOr- y, SiOr-Xr, and cristobalite,
as intermediate crystal phasesappearingin the order listed, form in the 100-180 "C experimentsin lo/o(0.18M)
KOH solutions containing various amounts ofNaCl. The
final product is quartz. The results of the reaction sequenceof 160'C experimentsare shown in Figure l. In
this figure (and in Fig. 2 also), the schematicline breadth
for each phasewas determined on the basis of the intensity of a main X-ray diffraction peak. Increasingthe NaCl
concentration induces the formation of SiOr-Y and cristobalite. Further addition of NaCl seemsto result in preventing SiOr-X, from appearing, in which situation
SiOr-Y is directly followed by cristobalite. The same experimental results were also observed for the reaction at
MURAISHI: CRYSTALLIZATION OF SILICA GEL IN ALKALINE SOLUTIONS
tr49
ct
z
I
6
E
Y,Xz
o
(J
o
z
Y,Xz
o
o
o
X2
o
u
c
o
X2
x2
(J
x2
XXT
100
120
1t0
160
Temperqture ('C )
x2
100
200
300
M o l o rr o t i o H z OO
/ H-
400
Fig. 4. Chemical composition of the solutions required for
the formation of SiOr-Iat 100-150'C and magadiiteat 100'C.
The amount of silica in the solutions is SiO,/OH- : 2-6.
X
180
Fig.3. TherangeofSiOr-Yappearance
asa functionofNaCl
concentration
and temperaturefor a l0loKOH solution.(The
crystalsindicatedwith smalllettersare minor phasesas determinedby X-ray diffractionpatterns.)SiOIX (X); other abbreviationsasin Figure1.
concentration and reaction temperature for a 10/oKOH
solution,as shownin Figure 3. SiOr-Yappearsseparately
in the dotted region and is more stable under conditions
of lower temperatureand higher NaCl concentrations.In
the other region, Sior-f forms together with the other
intermediatephasesor doesnot form at all.
Figure 4 shows the composition of the solutions in
which SiOr-Iand magadiitewere prepared.For the preparation of SiOr-I, a relatively large amount of water was
used, and the concentration of Na* in the solution was
made to vary independently againstthat of OH-. This is
different from the procedure to synthesize magadiite,
which was done at 100-120 oC in suspensionstypically
containing I mol of NaOH,2 to 6 mol of SiOr, and l0
to 60 mol of HrO (Lagaly et al., 1973).
The resultsof the conversion of crystalline phaseswith
time (Figs. I afi,2) show the following reaction path:
amorphous silica - SiOr-I. * SiOr-& - cristobalite qtafiz. Sometimesit was observedthat the crystal growth
of SiOr-Iand SiOr-Xr,however,did not proceedconsecutively, but simultaneously, and that cristobalite was
formed prior to the appearanceof SiOr-Xr. If theseresults
haveany fundamentalsignificance,the reactionpath ought
to be expressedby the processincluding the parallel reactions in which Sior-y and SiOr-X, are independently
rather than consecutivelyformed.
X-ray diffraction and chemical compositionof products
The X-ray diffraction peaksof the SiOr-Yand synthetic
magadiite prepared by us are given in Table I and compared with those of Sior-y from Mitsyuk et al. (1973\
and natural magadiitefrom Brindley (1969).As seenin
Table l, there is little difference between the X-ray diffraction peaksof natural magadiite and those of the synthesizedmaterial. The slight differencemay be causedby
a variety of impurities included in natural magadiite.The
patterns of the two kinds of synthesizedsilicates-magadiite and SiOr-I-are similar to each other except for
the differencesin intensity, 1, resulting from their differencesin crystallinity.
Thermal changesin SiOr- Y and magadiite were determined by meansof differential thermal analysis(ore) and
thermal gravimetric analysis (rce). The DrA-rcA curves
for both silicatesdrying in air are shown in Figure 5, and
the resultsactually overlap each other. It was found from
the rc.l curves that the dehydration occurred mainly in
four steps. The dehydration at temperaturesbelow 100
'C may be assignedto adsorbedHrO and some water of
crystallization, which are easily released. When these
sampleswere stored in a desiccatorusing silica gel for a
desiccant,the ignition loss below 100'C was no longer
observed.At about 130 and 190'C, some 500/oto 90o/o
of the total ignition loss was released.The bulk of the
containedwater was lost by heatingto 550'C. The water
content in the preparations varied slightly with the conditions ofsynthesisand storage.On the other hand, orn
curveshave a small broad endothermic peak at about 680
'C and an exothermic peak at about 750
"C. The former
was assignedto the destruction of the lattice of SiOr-I
(or magadiite) and to the formation of quartz, and the
latter was assignedto its reconstitution into tridimite.
Table 2 summarizes the molar ratio of Na and Si in
some of the SiOr-I and magadiite samplesprepared under various conditions. From these results, the chemical
formula of SiOr-Iwas determinedto be NarSi,oOrn.xHrO.
It was deduced from ignition loss that the amount of
water of crystallization,r, equaled 8.5-ll. It is worth
noting that Sior-y was usually free from K*. This characteristic of SiOr-y distinguishesit from SiOr-Xr, which
can contain as structural atoms both Na and K in anv
proportion.
MURAISHI: CRYSTALLIZATION OF SILICA GEL IN ALKALINE SOLUTIONS
l 150
o
U
I
0
200
400
500
( .C )
Temperoture
800
Fig. 5. Thermal behavior of synthesizedSiO,-Y (solid lines)
and magadiite (dashed lines). (a) WeightJoss curves; (b) orn
diagram.
Properties
Sior-y and magadiite were suspended in water after
drying in air and grinding into fine powders through 200
mesh to determine ion-exchange properties. The suspensions were titrated continuously with 0.lMHCl solutions
using an automatic titrator with a rate of addition of 1.0
mllh. The results are shown in Figure 6. The titration
curves of both silicates showed that half of the alkali in
the crystal is neutralized at approximately pH 7, and al-
10
5
0 . 1M
0 H C t ( m t)
0
U
Fig.6. Typicaltitrationcurvesof SiO,-Y andmagadiitesusp"nrionrwith 0.10MHCl. Suspension,
0.5 g of silicateper 50
mL of water;rate of additionof HCl, 1.0 mllh; temperature,
25.0"C.
most all of it at pH 4.4 + 0. l. This is closely analogous
to that of crystalline sodium polysilicateNarO'8SiOr'
gHrO investigatedby Iler (1964).The result proves that
these sodium silicates [abbreviatedas Na-(silicate)] have
high exchangeabilityin acidic solution and high selectivity for H* and then suggeststhat the following reaction
goesto completion:
Na-(silicate)+ H+ - Na* * H-(silicate),
where H-(silicate) is crystalline silicic acid from sodium
silicate. The pH value of 4.4, which is the pH of the
Trele 1 . X-ray diffractionpatternsof SiO,-Y and magadiite
Magadiite
sior-y
d(A)
15.50
7.76
6.26
5.17
d(A)
100
20
30
10
3.877
3 637
10
30
J. IOU
90
16
15.64
7.76
oc
15
15.56
7.77
100
I
s.20
23
5.20
5.03
4.48
23
13
20
5.18
5.00
4.46
18
14
18
13
10
23
33
100
4.01
c
(4 44),
4.23
3.450
3.304
d(A)
d(A)
d(A)
15.78
7.80
4.48
90
80
qn
+.zJ
3.641
3.550
3 453
3 308
J. ICJ
2.831
10
2.864
2 827
2.578
(2.36)
2.289
2.012
1.937
1.833
10
2.585
10
10
10
30
IJ
a
21
J.CCU
100
oc
3.445
3.308
81
3.151
9
2.876
2.823
z,\tJo
b
2.287
5
1.830
4.O2
3.920
3.648
2.603
2.353
2.273
1.826
oc
74
n
5
18
3.626
3.544
3.434
3.298
3.197
3.145
2.989
2.866
2.82',1
2 639
2 599
2.353
15.64
7.90
7.26
5.22
b5
12
8
20
4.48
13
8
4.O2
10
13
19
3.663
3.569
3.460
3.314
23
23
100
10
3.159
2.998
2.880
2.831
72
I
8
8
11
12
2.600
2.350
I
IJ
40
11
60
b
2.067
12
1.822
30
oc
1.833
Nofe; Columnsare as follows:(1) 155 'C, in KOH + NaClsolution(Mitsyuket al., 1976).(2) 150 rc, 5 days, in 1% KOH + 1.5M NaClsolution(this
'C,
work). (3) 100 'C, I weeks, in 1oh KOH + 1M NaCl solution (this work). (4) Natural magadiite(Trinity County, California)(Brindley,1969). (5) 100
8 weeks, in the solutionwith molar ratio of SiOr:NaOH:HrO: 9:2:75 (thiswork).
1151
MURAISHI: CRYSTALLIZATION OF SILICA GEL IN ALKALINE SOLUTIONS
TABLE
2. Reactionconditionsand analysisof products
Atomic ratio of oroduct
Molar ratio of solution
Run'
rfc)
Y-1
Y-2
Y-3
Y-4
Y-5
M-1
140
150
140
150
140
100
Time (d)
5
bU
OH
Na*
0.25
0.32
0.64
0.32
0.341
o.221
1.36
1.80
1.36
5.41
1.70
0.22
sio,
H.o
Na
Si
50
100
100
100
1.91
1.94
1.98
2.01
1.99
2.03
14
14
14
14
14
14
5U
8
0.03
0.02
0.03
0.01
'Y, SiO,-Y;M, magadiite.
t NaOHwas used as an alkalinesource
solution at the equivalencepoint in Figure 6, corresponds
to the concentration of hydronium ion formed by the
reaction of the resulting silicic acid from ion exchange
with water:
H-(silicare) + HrO + H3O* + (silicate) .
The ionization constant plq of the silanol groups on the
crystallinesilicic acid was estimatedto be about 6.9 on
the basisof the conceptof weak acid solutions.This is in
good agreementwith some estimatesbasedon titration
curvesfor amorphoussilica, which are about 6.5 to 7.7
(Schindlarand Kamber, 1968;Strazenkoet al., 1974).
Both SiOr-Iand magadiite,synthesizedby us, have a
refractiveindex of 1.481 + 0.003.This value is the same
as that reported for natural magadiite by Eugster(1967),
who gavean averagevalue of 1.48.
Therefore, from the results previously stated,it is concludedthat SiOr-f is indeed magadiite.
RrrnnnNcns crtpo
Iler, R.K. (1964) Ion exchangepropertiesofa crystalline hydrated silica.
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Krtahara,S, Matuie, T, and Muraishi, H. (1982)Someobservationon
the crystallization of silica gel under hydrothermal conditions at 150
to 300 'C. In S. Somiya, Ed, Proceedingof the First International
Symposium on Hydrothermal Reactions,p. 480-495. Gakujutsu Bunken Fukyu-kai, Tokyo.
Lagaly,G., Beneke,K, and Weiss,A (1973)Organiccomplexesof synthetic magadiite. Proceedingsof the Intemational Clay Conference,
Madrid 1972,p. 663-673.
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oxides, and alkali metal salts on the crystallization of silica gel in the
aqueoussolulionsofpotassiumhydroxideat 160 to 200'C. In S. Sorniya,Ed , Hydrothermalreactions,vol. l, p. l-13. Uchida Rokakuho,
Tokyo
Schindlar, P., and Kamber, H R (1968) Acidity of silanol groups. HelveticaChimica Acta, 51, 178l-1786.
Strazhesko,D.N, Strelko,V.B., Belyakov,V.N., and Rubanic, S.C. (1974)
Mechanism of cation exchangeon silica gels Joumal of Chromatography,102,191-195.
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other conditions AmericanMineralogist,54, 1583-1591
Cam,R.M., and Fyfe, W S. (1958)Someobservationson rhe crystallization of amorphous silica. American Mineralogist, 43, 903-9 I 6.
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fidge zur Mineralogie und Petrologie, 10,242-259
Mmcu 6. 1989
Maxuscnrpr ACcEPTED