Clay Minerals (1990) 25, 235-241
E F F E C T S OF T E T R A H E D R A L
ISOMORPHIC
S U B S T I T U T I O N ON THE IR S P E C T R A OF S Y N T H E T I C
FLUORINE MICAS
K. KITAJIMA
AND N . T A K U S A G A W A
Faculty of Engineering, Shinshu University, Wakasato, Nagano 380, Japan
(Received 14 July 1989; revised20 October 1989)
A B S T R A C T: Effects of tetrahedral isomorphic substitution on the IR spectra of fluorine micas
were demonstrated by comparing the spectrum of taeniolite KMgeLi(Si4Olo)F2with those of the
substituted analogues, in which Ga, AI, B or Ge substituted for Si in tetrahedral sites. The a~ and a]
bands move towards lower frequencies as the content of substituted cation in tetrahedral site
increases, whereas the e~ band moves towards higher frequencies. Linear relationships were found
between the a~ band frequency and the extent of tetrahedral substitutions. The magnitude of shifts
per molar substitution (Av-)for the aTbands is dependent on the species of substituted cation and
decreases in the order Ga > AI > Ge > B, showing an intimatecorrelation with the field strength
ZI'Z2/F2M-0of the substituted cations. This implies that the polarizing power of the substituted
cations has a pronounced effect on the Si-O stretching vibrations.
Isomorphic substitutions in synthetic fluorine micas have been extensively studied (Shell &
Ivey, 1969; Matsushita & Shikanai, 1971; Toraya, 1981). H o w e v e r , systematic studies on
infrared (IR) spectra of fluorine micas with regard to t e t r a h e d r a l isomorphic substitution
have b e e n lacking. The use of suitably substituted synthetic fluorine micas with closely
varied compositions provides a valuable empirical a p p r o a c h to studying the correlation
between I R spectra and compositions in layer-silicates. F o r this purpose, taeniolite
(KMgzLi(SiaO10)F2) (Toraya et al., 1977) is the most suitable species for study as an endm e m b e r mica because its tetrahedral sheet consists of only silica t e t r a h e d r a , and has a
rather simple I R spectrum.
In this p a p e r , the effects of t e t r a h e d r a l isomorphic substitution on the I R spectra of
fluorine micas are d e m o n s t r a t e d by comparing the spectrum of taeniolite with those of the
substituted analogues, in which G a , A1, B or G e substituted for Si in tetrahedral sites.
MATERIALS
AND METHODS
The raw materials used were pure K F , M g O , MgF2, LiF, SIO2, GeO2, AleO3, Ga203, and
B 2 0 3. These were mixed in p r o p o r t i o n s corresponding to the compositions of four series,
KMg2 + xLil - x(ZxSi4 - xO10)F2 ( Z = G a , A1, B) and KMg2Li(GexSi4- xOa0)F2, where x =
0-0-1-0. E a c h mixture was sealed in a platinum tube, h e a t e d at 1400~ for 2 h, then slowly
cooled at a rate of 2-3~ per rain to 800~ The platinum tube was r e m o v e d from the
furnace to cool in air. The crystal aggregates thus o b t a i n e d were investigated by p o w d e r
X-ray diffractometry and optical microscopy. Cell dimensions of micas (1M) were
d e t e r m i n e d by the least-squares p r o c e d u r e using 15 20 values m e a s u r e d by a reflection
technique in the range 24-74 ~ (Cu-K~I = 1.54050/~). I R spectra were o b t a i n e d by the K B r
~) 1990 The Mineralogical Society
K. Kitajima and N. Takusagawa
236
~
10.10 ~-
10.10
9,20
9,2r
9.~o
.ca 930
5.30
o'r
5.20
-
5.20 ~
99.8~0.0
5.30 f
99,8
l
0.2
0.14
i
0,6
0.18
1,0
0.0
0.2t
x
0./r;
O.16
I
0.'i3
LO
x
FIG 1. L a t t i c e c o n s t a n t s o f i s o m o r p h i c s u b s t i t u t e d fluorine m i c a s , K M g 2 + xLil _ x (ZxSi4 - xOt0)F2 a n d
KMgzLi(Ge•
plotted against x value. O: Ga-substituted series (Z = Ga);
9 Al-substituted series (Z = AI); ~: B-substituted series (Z = B); ~: Ge-substituted series.
disk m e t h o d with a Shimazu IR-430 double b e a m s p e c t r o p h o t o m e t e r in the range 4000-330
cm 1. The sample (0-8 mg) < 1 0 / ~ m in size was well mixed with 200 mg of K B r powder.
Samples sedimented on a K R S crystal were also used to determine the effect of p r e f e r r e d
orientation of the mica flakes on the spectra.
RESULTS
AND DISCUSSION
Lattice constant
Fig. 1 shows the lattice constants of the micas p r e p a r e d . Most of the cell dimensions
except c for the AI and B series changed almost linearly with the a m o u n t of tetrahedral
substitution (x value). The magnitude of the difference in b-dimensions b e t w e e n the endm e m b e r micas is in the o r d e r Ga- > A1- > Ge- > B-substituted series, being approximately
in accordance with the o r d e r of ionic size, G a 3+ > A13+ = G e 4+ > B 3+, while that in the cdimension and c. sin/3 is in the o r d e r Ge-, Ga-, B-, Al-substituted series. This anisotropic
chemical expansion results from the fact that the dimensional misfit between tetrahedral
and octahedral sheets caused by cationic substitution is c o m p e n s a t e d through structural
deformation, such as the rotation of t e t r a h e d r a and flattening of octahedral sheets
(Radoslovich & Norrish, 1962). These continuous and systematic changes in cell
dimensions imply that the tetrahedral substitutions in the taeniolite mica structure occur in
the p r o p o r t i o n s as desired. This is also in good agreement with the fact that no coexistent
minerals were observed, with the exception of the highest x value (x = 1-0) in the B-series.
IR spectra of fluorine micas
237
IR spectra
Fig. 2a shows the I R spectra of the Ga series. The intensities of bands at 1118 cm -1 and
709 cm 1 for taeniolite (x = 0.0 in Fig. 2a) were decreased by orientation of mica flakes to
the IR beam as seen in the broken line in Fig. 2a. This indicates that these bands have a
transition moment perpendicular to the layer. The band at 1118 cm -1 is most likely due to
the tetrahedral stretching vibration of silicon-apical oxygen (Si-Oa) (Velde, 1978) or the a]
normal vibration of (Si205)n (Ishii et al., 1967). The band at 709 cm -1 is due to the a2
vibration. The band at 964 cm-1 may be attributed to the e~ vibration of (Si205)n, which has
its transition moment parallel to the layer. The assignment of the band at 462 cm -1 to the e~
vibration or Si-O bending vibration is most plausible. The IR spectrum of taeniolite is
characterized by the separation of the high-frequency Si-Oa stretching band at 1118 cm -1.
Some differences become pronounced with increasing Ga content, the most obvious one
being the marked shift of the Si-Oa band towards lower frequency with increasing Ga
content, becoming more poorly resolved at the higher x values. In contrast, the e~ band
shifts towards higher frequency, indicating that the energy and wavenumber of the e~
vibration increases as the structure becomes trisilicic. The influence of Ga substitution is
also noted in the increased absorption band at 742 cm -1. This band is attributable to the
perpendicularly polarized G a - O a (apical oxygen) vibration, appearing in the region
corresponding to that of G a ( I V ) - O vibration observed for garnet (Saine et al., 1982) and
glass containing tetrahedral Ga (Whichard & Day, 1984). The a 2 band at 709 cm 1 becomes
rather broad and shifts to lower frequency with Ga substitution.
Characteristic changes in the IR spectra with ionic substitutions have been also observed
in the AI series, as shown in Fig. 2b. The changes in the A1 series are analogous to those in
the Ga series described above, i.e. a~ and a~ bands move towards lower frequencies with
increasing A1 contents, whereas the e] band moves towards higher frequencies. However,
the size of the shift for each band in the A1 series is smaller than that of corresponding bands
in the Ga series. The appearance of a new band at 810 cm -~, which\corresponds to A1-Oa
(apical oxygen) vibration (Farmer & Russell, 1964), is also noticed. The splitting of the e]
band at intermediate x values of the A1 series is noted, exhibiting a different behaviour from
the other series. This splitting seems to result from the lifting of degeneracy of a Si-O
vibrational mode due to the symmetry changes of (8i205) n or Si-O framework. The
spectrum of x = 1.0 for the A1 series very closely resembles that of fluorophlogopite
(Farmer, 1974) as the chemical composition of x = 1-0 substantially coincides with that of
fluorophlogopite.
The changes in spectra in the 900-1200 cm -1 region of the B series are small (Fig. 2c),
even when a condensed tetrahedral sheet incorporates both Si and B. However,
characteristic changes with B substitution, such as the shift of a~ and a~ bands to lower
frequencies, are also analogous to those with the Ga and AI series. The gradual appearance
of broad shoulder at 825 cm-1 is probably due to A1 stretching vibration of BO4 tetrahedra
(Ross, 1969). The abrupt appearance of bands at ~ 1270, 760, 620 cm-1 for x = 1.0 of the B
series is certainly caused by unknown coexisting minerals.
Characteristic changes in the IR spectra for the Ge series (Fig. 2d) are also similar to the
other series. However, the position of the e] band is almost unchanged by Ge substitution,
and so-called "two-mode" behaviour (Chang & Mitra, 1968) is distinctly observed for the
germanate vibrational bands at ~850 and 920 cm-1, which are observable as large shoulders
even at lower x values. Another characteristic is that the Si-O bending vibration band at 462
K. Kitajima and N. Takusagawa
238
X =0.0
X=O.O
X=0.2
i
(b
o
E
X=O.4
"i
.r
X =0.6
X=0.2
X =0.4
X=0.6
(- X:O.B
c"
X=0.8
I.--
V
I
1200
I000 800
600
1201 1000 800
400
600
400
W a v e n u r n b e r / c m -1
W a v e n u m b e r / c m -~
(a)
(b)
i
X=0.0
K=0.2
,/x=o.o
(D
(J
E
x=o.2
H
--"
K=0.4
X=0.6
IX=0.8
L
I
I
i
Wavenumber/cm
(c)
I
6oo 40o
1
x =0.4
/
E:
r / X =0.6
r~
X=0.8
t_
I-
'
/
X =1.0
12oo looo 8oo
X :I.0
t-X
X=I.0
i
,/
L
I
I
x =1.0
I
12oo lOOO Boo 60o 4oo
W a v e n u m b e r / c rr~!
(d)
FIG 2. IR spectra of isomorphic substituted fluorine micas, KMgz+ xLi~_x(Z•
and
KMgeLi(GexSi4_xO10)F2: (a) Ga series (Z = Ga); (b) AI series (Z = A1); (c) B series (Z = B);
(d) Ge series. The spectra are of randomly oriented samples in KBr disks, except for (a), where the
broken line indicates the absorption of orientated flakes at normal incidence.
IR spectra of fluorine micas
239
720
7 i::::
U
700
680
0.0
0.2
0.4
X
0.6
0.8
1.0
FIG. 3. The relationship between peak position of the ~ band and tetrahedral substitution (x) for
fluorine micas, KMg2+ xLil •
xOlo)F2and KMg2Li(GexSi4_xOlo)F2.O: Ga series (Z = Ga);
9 A1 series (Z = A1); ~: B series (Z = B); q): Ge series.
cm 1 shifts toward lower frequencies (the overall shift is 12 cm -1) with increasing Ge
content. This probably results from the mass effect of heavy Ge cations.
The magnitude of the band shift
In every spectrum shown in Fig. 2, three bands shift sensitively with increasing
substituted cation content in tetrahedral sites. This behaviour has been referred to as a
"one-mode" shift (Chang & Mitra, 1968), and is especially appropriate to the well-resolved
a~ band, a study of which enables the effect of substitution on absorption band frequencies
to be made. Fig. 3 shows the plots of peak positions of the a 2 band versus the tetrahedral
composition (x value). Linear relationships were found between the a 2 band frequency and
the extent of tetrahedral substitutions. These relationships provide a sensitive and useful
method using only a few milligrams of sample to study the structure of layer-silicates. As
seen in Fig. 3, the magnitude of the shift is dependent on the species of the substitutional
cations and decreases in the order Ga > A1 > Ge > B. This difference arises mainly from
the fact that vibrations of silicate ions in the mica structure are not independent of the rest of
the mica structure, reflecting the different disturbing influences of tetrahedral substituted
cations on the neighbouring Si205 units or Si-O framework.
In order to study the effect of tetrahedral substitutions on Si-O vibrations, the
correlations between the magnitude of shifts per molar substitution (Ag) and the field
strength, zl.zz/r2M o of the substituted cations were examined, where zland z2 are the
charges of the tetrahedral cation and oxygen anion, respectively, and rM - o is the bonding
distance estimated by the sum of ionic radius (Shannon, 1976) of the tetrahedral cation and
oxygen anion. The field strength of Si 4+ is larger than those of the substituted cations. As
seen in Fig. 4, A ~ increased almost linearly with decreasing field strength of the cation. This
implies that the more pronounced effect of the tetrahedral substitution is exerted on the
Si-O stretching vibrations as the difference in polarizing power between Si4+ and the
substitutional cations becomes larger. Especially, the restoring force between Si and Ob
(bridging or basal oxygen), caused by tetrahedral substitution of the cation having
extremely different polarizing power from Si4+ at neighbouring Si sites, results in a larger
increase of the bonding energy whereas the restoring force between Si and Oa (non-bonding
or apical oxygen) results in decreasing bonding energy, as illustrated by the shift of a~ and a 2
240
K. Kitajima and N. Takusagawa
40
3O
"7E
ta
s~ 20
10
2~
2
z;zJ rM_o
3'0
FI6.4. The relationship between the magnitude of the shift of the a2 band and the field strength of
tetrahedral cations for fluorine micas. O: Ga3+; O: AI3+; ~: B3+; ID: Ge4+.
bands to lower frequency. The Si-Oa distance (1.586 &) is distinctly shorter than the Si-Ob
distance (1-683 A ) in taeniolite (Toraya et al., 1977) whereas in fluorophlogopite ( T a k e d a &
Morosin, 1975) in which x = 1.0 in the AI series, the T (tetrahedral c a t i o n ) - O a distance
(1.656 A ) is almost equal to the Y - O b distance (1.646 A ) . This suggests that the m e a n T - O a
distance may become longer, while the mean T - O b distance m a y become shorter, and the
difference in bonding distance between T - O a and T - O b decreases with increasing
tetrahedral substitution. It seems that the degree of separation between the a~ band and the
e~ band corresponds to the disproportion of bonding distance b e t w e e n T - O a and T - O b .
This m a y be supported by the fact that the force constant of Si-Oa is larger than that of
Si-Ob in tetrasilicic mica, in which the Si-Oa distance is also much shorter than the S i - - O b
distance ( T a t e y a m a et al., 1976). The shortening of the T - O a distance may result from the
larger negative charge on apical oxygens rather than on basal oxygens in tetrasilicic micas
and the analogous Ge-substituted micas. The tetrahedral elongation along the c* axis in
trisilicic micas is considered the result of a relatively low potential at the apical oxygen site
( A p p e l o , 1978).
ACKNOWLEDGMENTS
We thank Dr. N. Daimon for support, and Mr K. Tachiuchi for help in performing experiments.
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