Clay Minerals (1991) 26, 311-327
T H E U S E OF G L Y C E R O L I N T E R C A L A T E S IN T H E
E X C H A N G E O F CO3 2- W I T H 8 0 4 2 - , N O 3 - O R C L - IN
PYROAURITE-TYPE
COMPOUNDS
H . C. B. H A N S E N
ANO R . M. T A Y L O R *
Royal Veterinary and Agricultural University, Chemistry Department, Thorvaldsensvej 40, 1871 Frederiksberg C,
Denmark and *CS1RO, Division of Soils, Private Bag 2, Glen Osmond, South Australia 5064
(Received 12 July 1990; revised 26 October I990)
ABSTRACT: Strongly held carbonates anions in the interlayer region of pyroaurite-type
compounds, [Mlnl xM2nlx(OH)2]x+[(CO3)x/2yH20]x (x = 0-154).44), can be relatively easily
exchanged by other anions such as NO3 , C1 or SO42 dissolvedin heated glycerol. Some Fe(III) of
pyroaurite is reduced to Fe(II) by the glycerol treatment. If pyroaurite is treated with glycerol in the
absence of dissolved anions or with glycerol vapour at 120~ glycerol becomes intercalated in the
interlayers. In all examples the interlayer C032- is partially or completely lost as COz. In aqueous
salt solutions a glycerol intercalated form allows other anions to substitute in the interlayer.
Expansion in the c-axisdirection followingglycerol treatment depends on (1) the method of glycerol
treatment, (2) the trivalent metal in the octahedral sheet, and (3) the interlayer anions. Heated
glycerol causes a different expansion to that produced by its vapour phase at 120~ The variation in
d(003) follows the series: hydrotalcitevap.> pyroauritevap.> hydrotalciteliquia> pyroauriteliquia>
SO42--pyroauritevap.> SO42--hydrotalcitevap.,where liquid refers to glycerolused at 160-180~ and
yap. refers to glycerol vapour at 120~ Carbonate-free, glycerol-intercalatedpyroaurite compounds
may serve as re-usable anion absorbers. Glycerol intercalates of pyroaurite-type compounds may
also be useful to study and identify stacking sequences and interlayer compositionsin these minerals.
Minerals in the pyroaurite-sj6grenite group generally consist of hexagonal, platy, clay-sized
crystals. Structurally the minerals are c o m p o s e d of trioctahedral, brucite-like, hydroxide
sheets in which trivalent metal cations partially substitute for divalent ones, resulting in a
net positive octahedral charge which is c o m p e n s a t e d by interlayer anions, generally
associated with variable amounts of water. H - b o n d s and electrostatic forces give cohesion
to the compounds ( A l l m a n n , 1968, 1969, 1970; A l l m a n n & Lohse, 1966; Ingram & Taylor,
1967).
The minerals described have the general formula M l n l _xM2mx(OH)2]X+[Axm y H 2 0 ] x - ,
with x taking values b e t w e e n 0-15 and 0.44 (Kruissink et al., 1981; Thevenot et al., 1989)
where M1 and M2 are divalent and trivalent metal cations, respectively, and A is an
interlayer anion of charge - n . The octahedral charge, and consequently the amount of
a d s o r b e d anions, are controlled by the MIII/M II ratio in the octahedral sheet. Crystalline
pyroaurite, M g - F e ( I I I ) hydroxy carbonate, is r h o m b o h e d r a l but can be indexed in the
hexagonal system with ao = 3.11 A and Co = 3 z 7-80 ~ ( A l l m a n n , 1968). Both ao and Co
d e p e n d on the nature and relative proportions of M1 and M2 cations (Brindley & Kikkawa,
1979; Pausch et al., 1986). The structure, charge and size of the interlayer anion also affect
the Co value ( A l l m a n n , 1968) which can often give a useful indication of the anion present.
W h e n there is no interlayer carbonate, the anions present are readily exchanged with
9 1991 The Mineralogical Society
312
H. C. B. Hansen and R. M. Taylor
other anions or with C O 3 2 . In contrast, the extreme preference shown by the structure for
the carbonate anion generally hinders further access and exchange. Non-carbonate forms
have been produced in several ways: (i) synthesis under rigorous exclusion of CO2 (Miyata,
1975; Mascolo & Marino, 1980); (ii) treatment of the C O l - form with dilute mineral acid
solutions (Bish, 1978); (iii) heating to 450-900~ followed by rehydration and anion
adsorption (Sato et al., 1986); and (iv) anion exchange by dispersing the compound in an
unbuffered solution containing the exchanging anion. Synthesis of the CO32 form is
relatively straightforward and generally leads to more crystalline products (Miyata, 1983)
than when CO2 is rigorously excluded. During synthesis, washing and drying this exclusion
is difficult and time consuming. Procedures (ii) and (iii) often result in partial or complete
destruction of the compound, whereas (iv) is extremely slow (Mendiboure & Sch611horn,
1986).
Whereas certain swelling phyllosilicate clay minerals intercalate one or two layers of
glycerol or ethylene glycol (Bradley, 1945; MacEwan, 1948) when in contact with the
respective vapours of these compounds, only the sulphate forms of pyroaurite-type
compounds have previously been observed to form intercalation complexes with these
liquids (Bish, 1978; Bish & Livingstone, 1981; Drits et al., 1987).
This paper describes the formation of SO42-, N O 3- or C1- intercalated pyroaurite-type
compounds using a reaction which involves exchange of the CO32 anion via a glycerol
intercalation stage. The formation of stable glycerol intercalates in synthetic and natural
pyroaurites is also discussed.
EXPERIMENTAL
All the compounds considered have rhombohedral symmetry. Where C O l - is the
interlayer anion, existing mineral names have been used. For non-carbonate forms the
mineral names are prefixed by the type of interlayer anion, after Drits et al. (1987).
Synthesis
Two different techniques of synthesis were employed to ensure that the results obtained
were independent of the preparative method. In preliminary anion exchange experiments,
Mg-Fe(III) and Mg-Mn(III) hydroxy-carbonates (pyroaurite and desautelsite, respectively)
were synthesized using the oxidation of Fe(II) or Mn(II) in a Mg-rich solution at constant
pH. Techniques for the synthesis of pyroaurite and reevesite (a Ni(II)-Fe(IIl) hydroxy
carbonate) and desautelsite have been described by Hansen & Taylor (1990 and 1991,
respectively). Co(II)-Fe(III) and Zn(II)-Fe(III) hydroxy carbonates were synthesized in
much the same way as the reevesite. During the formation of these compounds 0.5 Mand 1 M
NaHCO3 solutions were used as titrants to maintain the pH values at 6-60 and 6.50,
respectively. Pyroaurite and hydrotalcite (Mg-A1 hydroxy-carbonate) used in the subsequent series of experiments were prepared as follows.
Pyroaurite. 500 ml of aqueous FeC12 solution (0.16 M) containing HC1 (0.12 M) was
pumped (0.4 ml rain -1) into 500 ml of a well-stirred 0.80 M Mg(NO3)2 solution. By using a
Radiometer Titralab unit operated in the pH-stat mode, the pH was held constant at 8-50 +
0.01 by the automatic addition of an NaOH (2 M) + Na2CO3 (0.15 M) solution and the
temperature was maintained at 35~ Air was bubbled through the Mg solution at a rate
high enough to avoid the formation of green Fe(II)-Fe(III) hydroxy compounds in solution
Glycerol intercalates of pyroaurite-type compounds
313
or in suspension. At the end of the reaction, the precipitated product was separated by
centrifugation, washed three times with water and then stirred and refluxed for 24 h as a
suspension in 800 ml of 0.0125 M Na2CO3 solution. The material was again separated by
centrifugation, washed twice with both water and acetone and dried at 60~ for 24 h. Two
natural pyroaurites used in the intercalation experiments were found in the Sultanate of
Oman; referred to as golden and silver after their respective colours, they have been
described by Taylor et al. (1991).
Hydrotalcite. Using a similar procedure, a 500 ml aqueous solution, 0.5 M in Mg(NO3)2
and 0.16 M in Al(NO3)3, was pumped at a fixed rate into 500 ml of water controlled at 25~
and maintained at pH 9.00 _+ 0.01.
Anion exchange and glycerol intercalation
To exchange interlayer CO32- with SO42 , NO3- o r E l - , ~300 mg of the synthetic or
natural hydroxy-carbonate compound was suspended in 20 ml of 85% glycerol containing
5 mmole of either Na2SO4, NaNO3 or NaC1 (added either as a salt or aqueous solution).
After dispersing ultrasonically for 30 s, the suspension was heated and stirred until all water
had evaporated. The temperature was then maintained at 160-180~ for a further 15 min. In
some experiments, solvents other than glycerol and/or different heating ranges were used.
The suspension was then cooled and diluted with 10 ml of hot, boiled (CO2-free) water. The
product was separated by centrifugation or filtration and shaken for a few minutes with
10 ml of a hot 0.5 M solution of the respective salt. Finally the material was washed twice
with CO2-free water and then with acetone and dried at 60~ for 1 h.
A SO42 -exchanged pyroaurite was also made by shaking the glycerol-intercalated
sample (see below) with a 0.5 M aqueous solution of Na2SO4 for 6 h at room temperature.
The product was separated, washed and dried as described above.
Glycerol intercalation of hydrotalcite and pyroaurite was achieved in the vapour phase by
subjecting the compound, dispersed over the surface of a watch glass, to a glycerol
atmosphere in a dessicator at 120~ for 3-5 days. Oriented films (1 mg/cm2) required only
3~5 h at this temperature for maximum intercalation at which stage the X-ray diffraction
(XRD) spacings became constant. Oriented films of natural pyroaurites were intercalated
with ethylene glycol by exposing them to vapour either at 60~ for 26 h (Wilson, 1987) or at
room temperature for 1 month. The different anion exchange and intercalation procedures
which have been used in this investigation are represented schematically in Fig. 1.
Analysis of products
Generally, all anion-exchanged samples were analysed by XRD and infrared spectroscopy (IR), and qualitative tests were performed for the presence of SO42-, C1- and CO2
in acid digests.
Chemical analysis. Duplicate chemical analyses were made to determine the composition
of the original and some anion-exchanged products. CO32 was determined by absorbing
the COz liberated during acid digestion of the sample into a Ba(OH)2-BaC12 solution and
then back titrating the excess hydroxide with acid (Larsen, 1949). Metal concentrations in
the acid digests were determined by atomic absorption spectroscopy (AAS). To estimate
amounts of interlayer anions in some of the compounds, 50 mg of the respective samples
were shaken twice with 10 ml of a 10 mra Na2CO3 aqueous solution for 6 h and washed twice
with 5 ml water. This exchanged CO32- for the original SO42-, CI-, and N O 3 - interlayer
H. C. B. Hansen and R. M. Taylor
314
r
NaCI, Na2S04, NaNO3
glycerol liquid (160 - 180~
Water washing
[
Glycerol vapour (120*C)
r
Glycerol liquid
(160-180*C)
No water washing
I c,-, so ,-or NO3 PTC's I I Glycer~intercalatedPTC I
I
I
Na2CO3
Water (25~
Na2SO4
Water (25~
FI6.1. Flow diagram showing some of the methods used to exchange anions in the pyroaurite-type
compounds (PTC).
anions which were then determined in the extract by ion chromatography. The hydroxyl
content of the CO32- forms were calculated by difference, assuming overall charge
neutrality. In all anion-exchanged samples it was assumed that each octahedral site was
occupied (OH : (M1 + M2) = 2 : 1).
The CO2 liberated when different compounds were heated with water-free glycerol was
determined using an apparatus similar to that described by Larsen (1949). In the reaction
flask 20 ml of water-free glycerol with --300 mg of the compound to be analysed was heated
to 170~ on an oil bath. In some experiments, 5 mmole of NaC1 was also added. The CO2
evolved was pumped by a peristaltic pump into the Ba(OH)2/BaCI2 absorber. After 20 min
circulation the excess hydroxide was back titrated with HC1. The method of Ftirst (1948)
was used to test for the presence of glycerol.
X-ray diffraction. Using Philips PW1710 and PWl800 diffractometers, the samples,
prepared either as non-oriented powders, oriented films or smeared glycerol pastes on glass
slides, were examined by XRD with Co-Kcr radiation. Samples were scanned at 1~ or 3~ 20
min -1 and the outputs collected as digital files from which diffractograms were produced
(Raven & Self, 1988). Indices used in the text refer to a hexagonal cell.
Infrared spectroscopy. IR spectra were recorded from 4000-400 cm 1 using sample disks
composed of 1 mg sample per 300 mg KBr pressed under 10 t. Either a Digilab FTS 15/90
Fourier Transform Spectrometer or a Perkin Elmer 580 grating spectrophotometer was
employed.
Nuclear magnetic resonance (NMR). 1H/13C NMR spectra of extracts from glycerolintercalated pyroaurites were recorded on a Bruker AC 250P Puls/FT spectrometer. 100 mg
of the glycerol-intercalated pyroaurites were shaken with 5-0 ml 0.1 M K 2 S O 4 for 12 h. The
supernatant was subsequently freeze-dried, and the spectra of the dried product dissolved
in 1 ml of D20 were recorded. Tetramethyl silane was used as an internal standard.
RESULTS
Preliminary experiments
Synthetic pyroaurite, reevesite, desautelsite, Zn(II)-Fe(III) and Co(II)-Fe(III) hydroxy
carbonates, all with M1 : M2 ratios close to 2, were examined by XRD and IR before and
Glycerol intercalates of pyroaurite-type compounds
315
after treatment at 160-180~ in glycerol containing dissolved Na2SO 4. The results of the
treatment, summarized in Table 1, show that in all samples d(003) increased by 1.0-1.6/~ to
8.7-9.2/~. The IR spectra of the products all show sharp and strong SO42- absorption peaks
at c. 620 and 1120 cm 1 (with a shoulder towards lower wavenumbers). The intense 1360
cm-1 and other absorptions due to CO32- were not detected. The weak peaks at 2875 and
2935 cm-1 (C-H stretching) and probably that at 1060 c m x are ascribed to the glycerol.
Pyroaurite structure or hydroxyl vibrations are considered to be responsible for the other
absorption peaks. It is apparent that in the interlayer, CO32- has been exchanged by SO42and a small amount of glycerol has been retained.
Fig. 2 shows the changes in XRD traces following different anion exchange treatments of
a synthetic desautelsite (A) containing a trace of rhodochrosite (MnCO3) as a residual
impurity. Heating the original CO32- form with pure glycerol at 160-180~ followed by
water + acetone wash, did not alter the basal spacings (B). In contrast, the same treatment
for 5 min in a glycerol solution containing Na2SO4 changed the basal spacing for part of the
material (C). Extending the heating to 15 min resulted in complete expansion of d(003) to
9.17 A (D). When the anion exchanged product was shaken in a 0.1 ~ Na2CO3 solution for
5 min, the basal spacing collapsed to almost the same value as for the untreated material
(E). In contrast, the MnCO3 impurity peaks remained unchanged through all treatments.
Anion exchange of a hydrotalcite and a pyroaurite
The chemical compositions and XRD traces for the hydrotalcite and pyroaurite used for
the anion exchange reactions are given in Table 2 and Fig. 3, respectively. Both compounds
have nearly the same M1 : M2 ratio and are of comparable crystallinity in the c-axis direction
(as measured by width at half height (WHH) of the 003 peak). However, only pyroaurite
contains a metal cation, Fe(III), which is reducible with hot glycerol. This explains why,
when heated in glycerol, pyroaurite changed colour from light yellow to green, and why an
acid digest gave a positive test for Fe(II). Subsequent washing in oxygen-containing water
or salt solution to exchange the anion restored the original colour.
Table 2 gives the changes in chemical compositions, XRD and IR features following
Table 1. XRD and IR characteristics of five different pyroaurite type compounds treated with glycerol + Na2SO4 at
160-180~ for 15 min.
Metal
M1, M2
XRD l
d(003)so42- (•)
6(003)
Mg, Fe
8.71
1.01
Ni, Fe 515(m)
9.17
1.59
Zn, Fe
9.09
1.28
Co, Fe
9-17
1.59
Mg, Mn
9.17
1-50
IR peaks2 (3000-500 crn 1)
(cm-l)
2940(vw),2880(vw), 1630(w), 1485(vw), 1390(vw), l125(vs),
625(s), 595(s)
2945(vs),2885(vw), 1630(w), 1465(vw), 1390(vw), lll5(vs),
1050(w), 675(s), 620(s)
2931(w),2875(w), 1630(w), 1475(vw), 1125(vs), 1060(w),
625(s), 600(vw)
2875(w),2930(w), 1630(w), 1460(vw), l l20(vs), 1055(m),
665(s), 625(s), 512(m)
2875(w),2935(w), 1640(m), 1495(s), 1125(vs), 865(m), 630(s),
600(s)
(1) 6(003) = d(OO3)so42- - d(003)co32 .
(2) Intensity of absorptions: vw: very weak; w: weak; m: medium; s: strong; ,~s: very strong.
H. C. B. Hansen and R. M. Taylor
316
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Glycerol intercalates of pyroaurite-type compounds
317
Table 2. Chemical composition, XRD and IR characteristics of original (CO32- interlayered) and anion exchanged
hydrotalcites and pyroaurites.
Compound
XRD
d(003) WHH(003)2
(A)
(~
CO32- Hydr. 7-76
8042- Hydr. 8-73
NO3- Hydr.
9.10
0.59
0.81
0-95
CI Hydr.
CO32-Pyr.
SO42- I Pyrfl
NO 3 Pyr.
CI Pyr.
8042- II Pyr.
7.96
7.78
8-69
9.27
9.74
8.65
0.63
0-51
0-90
0.72
0.77
1.12
IR 1
CO32-(S)
5042-(S);Gly(vw)
NO3 (s); Gly(m-w);
CO32-(m)
CO32 (s); Gly(vw)
CO32 (s)
SO42 (s); Gly(vw)
NO3-(s); Gly(m-W)
CO32 (w); Gly(m-w)
SO42-(s);COl (w)
Chemical composition4
Residual
charge per
formula
unit
Mgs.70AI2.27(OH)10)(CO3)1.11
Mgs.81AI2.19(OH)I6)(CO3)o.14(SO4)0.63
Mgs.72A12.28(OH)16)(CO3)oqs(NO3)bso
Mgs.85Al~.ts(OH)16)(CO3)0.47(C1)b le
Mg5.73FelII2.24(OH)16)(CO3) 1-09
Mgs.98Fem2.o2(OH)16)(CO3)o.18(SO4)o.6~
Mg5.97Fen12.03(OH)16)(CO3)0.t8(NO3)1.13
Mg6.01FeIll1.99(OH)16)(CO3)0.30(C1)1.04
Mg5.71FeU12.29(OH)16)(CO3)0.17(804)0.66
+0.65
+0.42
+0"09
+0-44
+0.60
+0.35
+0.63
(1) 5042-: 620, 985, 1110, 1160 CIII-1;CO32-:680, 1360 cm-1; NO3-: 840, 1385, 1730, 2430 cm-1;
(Gly)cerol: 930, 1085, 1115, 2885, 2940 cm-l.
Intensities of absorptions: vw: very weak; w: weak; m: medium; s: strong.
(2) WHH(003) = Width at half height of the 003 diffraction peak.
(3) I: Using the standard anion exchange method. II: Anion exchange of a glycerol-intercalated pyroaurite by
shaking for 6 h with aqueous 0-1 r~ Na2SO4 solution.
(4) The content of SO42 , C[- and NO3 is determined as the amount of interlayer anion which is exchangeable
with a 10 mM Na2CO3 solution for 6 h.
exchange of the interlayer CO3 2- for other anions using the glycerol-salt solution method.
The products of such t r e a t m e n t are referred to as an anion-I c o m p o u n d , e.g. SO42--I
hydrotalcite. Also included in Table 2 are the data for a glycerol-intercalated pyroaurite
following t r e a t m e n t with a 0.5 M aqueous Na2SO4 solution (SO42--II pyroaurite). A f t e r
t r e a t m e n t the solution was alkaline ( p H 9) and contained glycerol. In experiments where
SO42-, NO3 o r CI c o m p o u n d s were leached with a 10 mM Na2CO3 solution, X R D and I R
confirmed that CO32 actually replaced these other anions.
To understand more fully the mechanism underlying the anion exchange reaction, further
experiments using hydrotalcite were carried out under different conditions. Refluxing
hydrotalcite with an aqueous 0.1 M Na2SO4 solution for 5 h or with a 1 : 5 glycerol : aqueous
NaeSO4 solution (0-1 M Na2SO4) for 2 h did not change the basal spacing. O t h e r treatments,
such as refluxing with ethanol-, methanol-, or dimethyt sulfoxide-Na2804 solution, or
heating hydrotalcite dispersed in a long chain alkane (paraffin) oil to 180~ for 15 min
followed by washing with hexane, also failed to change the X R D spacings. H o w e v e r ,
refluxing with a 9 : 1 glycerol : aqueous Na2SO4 solution (0-1 M in Na2SO4) for 2 h caused
partial exchange o f CO32- by SO42-. Hydrotalcite heated in a water-free solution of
Na2SO4 in glycerol (0.1 M Na2SO4) for 1 h at 90-100~ also partially exchanged CO32 for
SO42 . The extent of the exchange increased m a r k e d l y with t e m p e r a t u r e .
The d a t a given in Table 3 shows that CO2 was liberated m o r e readily from pyroaurite
than from hydrotalcite when these compounds were h e a t e d in glycerol. The rate of
liberation from both minerals increased when NaC1 was present. The first three rows of
Table 3 show that whether or not NaC1 was present, the CO2 evolved was not due to
glycerol decomposition.
318
H. C. B. Hansen and R. M. Taylor
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Glycerol intercalates of pyroaurite-type compounds
319
Glycerol intercalation of hydrotalcite and pyroaurite
Heating synthetic hydrotalcite and pyroaurite for 15 min in salt-free glycerol at 160180~ caused the d(003) spacing to increase to - 9 . 6 - 9 . 7 / k . Oriented glycerol smears of the
treated samples maintained these spacings indefinitely if kept under low water vapour
pressure (e.g. in a dessicator over silica gel) but collapsed if stored in air. Hydrotalcite
almost completely collapsed to its original spacing after 2 days. Water dilution (1 : 1) of the
glycerol supernatants from these experiments gave pH values of 9-10.
Synthetic hydrotalcite, pyroaurite and their respective SO42 -exchanged derivatives
swelled to different degrees when treated with glycerol vapour at 120~ or above. Figs.
4A(1) and B(1) show XRD traces of oriented specimens of hydrotalcite and pyroaurite
treated with glycerol vapour whereas A(2) and B(2) are the traces of their respective
glycerol vapour-treated, SO42- interlayer derivatives. Sulphate-exchanged hydrotalcite
and pyroaurite swell less in glycerol vapour than their respective carbonate forms. There is a
difference of 4-8 A in the d(003) spacings of hydrotalcite depending on whether it was
treated in liquid glycerol at 160-180~ ( - 9 - 6 / k , see above) or in glycerol vapour at 120~
(-14-4 A, Fig. 4). By contrast, the vapour treated pyroaurite shows little further expansion
(--10 .~) compared to that produced in heated liquid glycerol (9.5 A). However, a d(003)
basal spacing intermediate between 9.7 and 10/k has not been observed for the vapourtreated hydrotalcite.
The pyroaurite intercalated with glycerol through the vapour phase was reasonably stable
because shaking with cold water for 20 h did not change the XRD spacings. By contrast, the
glycerolated hydrotalcite was sensitive to water vapour. No CO2 was detected by chemical
analysis in the acid digests of the glycerol vapour-treated pyroaurite and hydrotalcite.
The I R absorption peaks at 930, 995, 1050, 1085, 1115, 1405, 2885 and 2940 cm -1 in the
spectrum of glycerol intercalated pyroaurite, Fig. 5, are ascribable to glycerol. The position
of the OH-stretching absorption at 3420 cm I is unshifted from the untreated pyroaurite.
Table 3. Liberation of CO2 on heating (170~ different compounds with glycerol.
Compoundl
m
m
NaeCO3
Hydrotalcite
Hydrotalcite
Pyroaurite
Pyroaurite
Treatmentl
G rcerol, 170~
GI icerol, NaCI, 170~
GI icerol, NaOH, 170~
GI rcerol, 170~
G1rcerol, 170~
G1rcerol, NaC1, 170~
G1rcerol, 170~
G1Icerol, NaC1, 170~
Rate of CO2
liberation2
0
0
0
+++
+
++
++
+++
(1) The following quantities were used: 0.3 g of N a 2 C O 3 o r
pyroaurite type compound, 15 ml water free glycerol,
5 mmoleof NaCI or NaOH,
(2) 0: No CO2 liberated; +, ++ and +++ indicates liberation
of 0-50, 50--100 and 100-150 ~mole CO2 per 20 rain,
respectively.
320
H. C. B. Hansen and R. M. Taylor
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Glycerol intercalates of pyroaurite-type compounds
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Wavenumber (cm)
400
Fro. 5. IR spectrum of a pyroaurite intercalated with glycerol through the vapour phase.
* absorptions due to intercalatedglycerol.
The frequency of this absorption is relatively low compared with the data on pyroaurite
given by Hernandez-Moreno et al. (1985). The 1610-1630 cm -1 peak is due to H - O - H
bending and its increased intensity relative to that in the untreated pyroaurite indicates that
the glycerol intercalate is highly hydroscopic. The 580 cm-1 absorption probably arises from
a lattice vibration of the pyroaurite (Hernandez-Moreno et al., 1985). In accordance with
the chemical analysis data, peaks due to CO32- at 680 cm -1 and 1360 cm -1 are absent.
Subsequent to the removal of all interlayer C O 3 2 with glycerol vapour, two sharp, medium
intensity peaks at 1325 and 1385 cm -1 appear. No NO3- was detected in the pyroaurite
(absorption at ~1385 cm -1) and since both peaks disappeared on exchanging the glycerolintercalated pyroaurite with 5 0 4 2 , they are considered to be due to CH2 and/or
OH-bending vibrations of glycerol or one of its alteration products in the interlayer.
Fig. 6 shows the effects on XRD of intercalating an oriented sample of natural golden
pyroaurite with glycerol or glycol vapours for comparison with the changes obtained with
the synthetic materials. Fig. 6(1) shows the untreated golden sample, (2) after treatment
with glycerol vapour at 120~ and (3) after treatment with ethylene glycol vapour at
60~ for 26 h. IR and chemical analysis revealed that this pyroaurite sample contained
SO42-, C1 and CO32- interlayer anions (Taylor et al., 1991) and has the composition
Mg6.10FeInl.81A10.09(OH)16(CO3)0.66C10.07(SO4)0.26. The glycerol vapour treatment at
120~ produced two sets of basal spacings: (i) 14.46, 7.33, 4.88, 3-65; and (ii) 9-48, 4-74 and
3-16 A, indicating that the pyroaurite, containing both CO32- and SO42-, consists of two
kinds of interlayers which differ in their interaction with glycerol. A similar natural sample,
silver pyroaurite, containing only traces of SO42 (Taylor et al., 1991), gave only the 14.4 ]~
set of spacings after glycerol vapour treatment. Treatment with ethylene glycol at 60~ also
caused the golden pyroaurite to swell, increasing the basal d(003) to 13.14 A. Intermediate
"complexes" are suggested by the further reflections at 11.55, 9.27, 6.96, 4.61, 4-25, 3.65
and 2.65 A. Saturation with ethylene glycol was achieved either by retention over dried
ethylene glycol in a desiccator at room temperature for one month, or by exposure to the
vapour at 60~ for 26 h. The room temperature treatment, however, only caused the 003
and 006 peaks to broaden.
322
H. C. B. Hansen and R. M. Taylor
DISCUSSION
Anion exchange
The six different pyroaurite-type compounds used in this study exchanged interlayer
CO32- with SO42-, NO3 or C1- when heated to 160-180~ in a glycerol solution of the
respective Na or K salt of the anion. This exchange is reversible, e.g., the SO42- anions can
be further replaced by CO32 in the interlayer when the sample is placed in a carbonate
solution (Fig. 2E). The broadening of the XRD peaks following anion interchange (Table 2
and Fig. 2) may be due to retention of glycerol in the interlayers as seen from IR data.
Glycerol retention could give rise to an increase of disorder in layer stacking of noncarbonate pyroaurite-type compounds, or alternatively, structural alteration might have
occurred during the heating in glycerol. Differential 003 line broadening following anion
exchange (Table 2) suggests that variable amounts of interlayer glycerol might be retained
with these anions. Reported d(003) ranges for the different anionic forms of hydrotalcite,
irrespective of the Mg:A1 ratio, are: 8042- 8.6-8-7/~; NO3 8.3-8.8/~ and C1 7.9/~
(Miyata, 1975, 1983; Miyata & Okada, 1977; Calvalcanti et al., 1987). Table 2 shows that
003 spacings of the NO 3--intercalated hydrotalcite or pyroaurite and the C1 -pyroaurite are
somewhat higher than those quoted in the literature. These variations could arise from
13.14
25
t11.55
6.96
4.25
.62
'-Q
•r
r
0
~ 3
L46
4.
4~1~~
15
(2)
7.88"~
(I
10
3.96
3.65
~ 3.16
3.91
(1)
5
6.00
16.00
Co-Ko radiation
24.00
32.00
Angle (deg) 20
FIG. 6. XRD traces of an oriented sample of natural golden pyroaurite after various treatments:
(1) untreated; (2) treated with glycerolvapour at 120~ for 6 h; (3) treated with ethyleneglycol
vapour at 60~ for 26 h.
Glycerol intercalates of pyroaurite-type compounds
323
differences in relative humidities (Bish, 1978) or in the M1 : M2 ratio of the octahedral sheet
(Miyata, 1983). However, the increased basal spacing was observed only where IR
indicated a relatively high interlayer glycerol content.
The assumption in Table 2 that the hydroxide layers are trioctahedral (OH : (M1 + M2) = 2)
applies well to the untreated CO32- forms, but may not be strictly correct for the anion
exchanged samples. The derived formulae bear a net positive charge. This may arise from
negative interlayer anions not considered in the calculation, e.g., glycerolate anions.
However, the samples anion-exchanged by the glycerol salt solution method show slight
decreases in their A1 or Fe contents compared to the original untreated compounds.
Fe(II,III) or AI(III) may leave the structure bonded to glycerol and thereby create
vacancies in the trioctahedral layer, reducing the net positive charge of that layer.
Glycerol intercalation of a CO32--pyroaurite and subsequent treatment with aqueous
Na2SO4 (SO42--II pyroaurite) does not affect the Fe : Mg ratio or result in a greater degree
of exchange than produced by heating the sample in a glycerol-Na2SO4 solution (SO42 -I).
CO32- detected in anion-exchanged compounds may partly be due to re-adsorption of
atmospheric CO2 during washing or drying (the SO42- or N O 3- forms contains much less
CO32- than the C1 form). A SO42 substituted form, as free as possible from interlayer
CO32- and adsorbed glycerol, is therefore best prepared by the (SO42--I) exchange
method.
Mechanisms
CO32--intercalated pyroaurite-type compounds swell in contact with glycerol at low
water contents, irrespective of the presence of a salt. After intercalation with glycerol, the
rate of which increases with temperature, other anions can be readily introduced. The
possibility of formation and participation of alkyl carbonate compounds (Faurholdt &
Hansen, 1942) during these reactions has been considered, but no confirmatory evidence
could be obtained to support their presence. Other hypotheses have therefore been
proposed to explain observed exchange reactions.
(i) A decrease in the positive charge of the octahedral sheet resulting from any reduction
of the component trivalent transition metals by glycerol. Under those conditions
CO32 will be liberated from the interlayer and also protons, generated by the
oxidation of glycerol, will shift the CO2-H20 equilibria to favour CO2 liberation. The
reversible colour change from yellow to green due to reduction of Fe(III) on heating
pyroaurite with glycerol is in accord with published observations on reactions between
Fe(III) oxides/hydroxides and glycerol (Radoslovich et al., 1970; Fuls et al., 1970).
Heating in glycerol at 160-180~ for more than 24 h did not, however, destroy
pyroaurite structure although such treatment converts most iron oxides/hydroxides
into alkoxides of glycerol (Fuls et al., 1970; Radoslovich et al., 1970). Anion exchange
can not, however, result solely from this reduction-oxidation process. Hydrotalcite,
with no reducible octahedral sheet cation, swells on glycerol intercalation to a greater
extent than pyroaurite, and undergoes anion exchange on subsequent treatment with a
salt solution.
(ii) Interlayer swelling between the brucite-like sheets following penetration by glycerol
into the interlayer decreases the electrostatic and H-bond forces acting on the CO3 zThis anion is then exchangeable with anions present in high concentrations outside the
interlayer in formation of anion-I type exchange compounds.
324
H. C. B. Hansen and R. M. Taylor
(iii) Theoretically, O H - , or negatively charged glycerolate anions, could form under
dehydrating conditions in reactions involving the interlayer CO32 , e.g. when the
materials are treated with liquid glycerol or at high temperature with its vapour. Such
reactions can be represented by the two equilibria:
CO3 2- +
H20
glycerol 160 ~
<
) C O 2 ( g ) q- 2 O H
2(CH2OH-CHOH-CH2OH) + CO32
,~
(1)
)
CO2(g) q- 2 ( C H 2 O H - C H O H - C H 2 0 ) - + H 2 0
(2)
Either or both of these reactions could occur. With the decomposition of the CO32 and its
liberation as CO2, it is suggested that the O H and/or the more basic glycerolate anions are
retained in the interlayer as the charge compensating anion.
Although the precise nature of the reaction between a pyroaurite-type compound,
intercalated with glycerol through its vapour phase, and an aqueous Na2SO4 solution is not
yet known, it is assumed to involve exchange of the interlayer glycerolate or O H - with
SO42- anions. If the glycerolate anion was involved, its release into the external solution
would be followed by its subsequent hydrolysis as shown:
CH2OH-CHOH-CH20
+ H20 <
)CH2OH-CHOH-CH2OH + OH-
(3)
Accompanying the exchange of the charged species, the glycerol intercalant was also
removed from the interlayer and the pH of the supernatant solution increased. This
indicates either that hydroxyl anions were present as the exchangeable counter anions, or
that reaction (3) has taken place. The yellowish colour of the alkaline, Fe-free extract
following exchange probably results from trace amounts of glycerol oxidation and/or
degradation products, although such compounds were not detected by either I R of the
intercalate (Fig. 5), or by 13C/1H NMR analysis of the extract. Only unaltered or "free"
glycerol could be identified in this extract.
Table 3 shows that the presence of an anion like CI- in the glycerol suspension increases
the rate at which CO2 is liberated from hydrotalcite or pyroaurite. This can not be attributed
to an effect of the C1- anion on the rate of diffusion of CO2 from the interlayer and it is
therefore presumed that the reaction mentioned in hypothesis (ii) together with the
reactions (1) and (2) taking place in solution proceed at a faster rate than reactions (1) and
(2) occurring in the interlayer.
The precise nature of glycerol incorporation into the interlayers of these materials is not
presently known. However, solvation of the interlayer anion and hydrogen bond formation
with the octahedral sheet O H are considered to be the main factors involved. Most of the I R
absorption peaks for the glycerol-intercalated sample, before exchange with the SO42
solution, occur at the same frequencies as those of "free" glycerol. A C - O stretching
frequency of 1115 cm -1 for intercalated pyroaurite compared with 1105 cm -1 for glycerol
may simply indicate different H-bonding of the interlayer glycerol. As discussed earlier, the
existence of covalent bonds between glycerol and metal cations of the octahedral sheet ( C O-Fe, C - O - A I and C - O - M g bonds) cannot be ruled out, and their possible contribution to
the stabilization of the interlayer glycerol is unknown.
Glycerol intercalates of pyroaurite-type compounds
325
Glycerol intercalation applied to mineral identification
In 2:1 phyllosilicate clay minerals, where a negative charge largely originates in the
embedded octahedral sheet, glycerol can be coordinated around interlayer cations between
the silicate layers to form one or two layer "complexes". However, in pyroaurites, a positive
charge originates in the brucite-like sheets and acts only over a short distance. If the
trioctahedral layer thickness of pyroaurite and hydrotalcite is taken as 4.9 ~ , the 003
spacings of the different intercalates (Fig. 4) allow the height of the interlayer to be
calculated. For hydrotalcite, pyroaurite and their SO42 -exchanged forms, values of 9-5,
5-1, 4.2 and 4-6 ~ , respectively, are obtained after glycerol vapour treatment. The 4-24.6/~ values correspond to the height of vermiculite mono-layer glycerol "complexes",
whereas the 9.5 ~ distance is somewhat greater than that found for two-layer glycerol
"complexes" with smectites (MacEwan & Wilson, 1980). The glycerol molecule with its
carbon chain in the c-axis direction (i. e. orthogonal to the octahedral sheet) is too short to
give a basal spacing of 9.5 A. This suggests that in pyroaurite, SO42--pyroaurite and
SOa2--hydrotalcite, the glycerol intercalate is a one layered complex whereas in pure
hydrotalcite, the intercalate is double layered.
In hydrotalcite the amount of absorbed glycerol appears to depend on how the
intercalation was achieved. For example, treatment with liquid glycerol gives a d(003) of
9-7 ~ , whereas glycerol vapour expands this spacing to 14-4 /~ (one and two layer
"complexes", respectively). Therefore, to use the glycerol intercalates effectively for
identification and characterization of pyroaurite-type minerals, a consistent intercalation
procedure must be followed.
Glycerol-intercalated SO42- and CO32- forms of hydrotalcite and pyroaurite exhibit
different d(003) spacings, suggesting that such intercalation may be helpful in distinguishing
between the different cationic compositions of the octahedral sheets as well as the interlayer
anionic species. The effect of interlayer anion on spacing is clearly demonstrated in the
variations shown by the natural pyroaurites following their glycerolation, although Fe(III)
was the dominant trivalent octahedral cation in both samples. The silver pyroaurite behaves
like CO32--hydrotalcite, whereas the golden sample (Fig. 6), shows two sets of basal
spacings. The CO32 interlayered crystallites in both samples behave as hydrotalcite,
whereas the $042- containing layers behave like SO42--pyroaurite, Fig. 6(2). XRD of the
untreated material gave no clear indication in these samples of the existence of domains
containing only SO42 as their interlayer anion, nor of regular or random interstratification
of C032 and $042- interlayers. The swelling behaviour of the golden coloured pyroaurite
is consistent with both CO32 and SO42- anions co-occupying interlayers in some
crystallites, with the larger tetrahedral SO42- anion determining the interlayer spacing.
Such a mixed occupancy of the interlayer is also consistent with the proposed genesis of
these natural samples (Taylor et al., 1991).
Further studies are needed to clarify the potential use of glycerol intercalates for the
interpretation of layer sequences or layer compositions in the pyroaurite-type minerals.
However, the results presented show that glycerol offers more advantages than ethylene
glycol in such applications.
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