Clay Minerals (1986) 21, 949-955
C A T I O N E X C H A N G E IN S Y N T H E T I C M A N G A N A T E S :
I. A L K Y L A M M O N I U M
EXCHANGE IN A SYNTHETIC
PHYLLOMANGANATE
E. P A T E R S O N ,
J. L. B U N C H
AND D. R. C L A R K
Department of Mineral Soils, Macaulay Institute Jbr Soil Research, Craigiebuckler, Aberdeen AB9 2QJ,
Scotland
(Received 30 May 1986; revised 15 September 1986)
ABSTRACT: On treatment with dodecylammonium chloride, a synthetic Na-saturated
phyllomanganate undergoes a cation-exchange reaction in which the structure expands from a
collapsed basal spacing of 7 ,~ or a hydrated one of 10 N to ~ 25-5 •. The exchange reaction is
strongly influenced by the saturating cation in the original manganate and by the degree of
hydration of the interlayer zone in which the exchangeable cations are located. Of the 12 cations
tested, from Groups IA, IIA and lIB of the Periodic Table, only Co-manganate undergoes
alkylammonium exchange when fully-dehydrated by evacuation. The results are discussed in the
context of existing knowledge of the synthetic phyllomanganate and, by extrapolation, the
potential value of the technique in the characterization of natural manganates has been assessed.
One of the major difficulties in mineralogical studies of both terrestrial and marine
manganese nodules is that the identification of specific minerals by X-ray diffraction, the
primary investigative technique in mineralogy, often rests on the presence of only one or two
reflections. This has given rise to considerable confusion and controversy over the relative
importance of phyllomanganates, such as the birnessite group, and tectomanganates, such as
the cryptomelane/hollandite group, in soils, sediments and marine deposits. This controversy
is most clearly seen in the so-called 'todorokite question', where a protracted discussion has
taken place over whether todorokite is a valid mineral and also its relationship to other
manganate minerals (Burns et al., 1983 ; Giovanoli, 1985a). It is only with the increasing use
of high-resolution transmission electron microscopy (Turner & Buseck, 1981) that some
clarification has taken place. However, the conflict between the inherent heterogeneity of
many manganese oxide deposits and the high selectivity of transmission electron microscopy
means that X R D could still play a valuable role in establishing the dominant phases present.
Clearly, any simple chemical treatment that can increase the specificity of XRD, such as
those developed for clay mineralogy (Brown & Brindley, 1980), will be of value.
In the elucidation of the properties of phyllomanganates, extensive use has been made by
the Berne group of a synthetic phase studied first by Wadsley (1950) and a second phase (the
dehydration product of the first) studied by Feitknecht & Marti (1945), Buser et al. (1954) and
later Giovanoli et al. (1970). Wadsley's compound, Na-manganese(II,llI) manganate(IV), or
synthetic buserite, has been shown to undergo substantial lattice expansion on treatment
with dodecylammonium cations, a reaction that was suggested to be of potential value in the
characterization of manganates such as todorokite (Paterson, 1981). This suggestion has been
substantiated by the work of Arrhenius & Tsai (1981) on marine manganates. However, in a
recent paper, Giovanoli (1985b) has correctly drawn attention to the fact that the original
9 The Macaulay Institute for Soil Research 1986
950
E. Paterson et al.
reaction uses, as a starting material, the moist, freshly prepared Na-phyllomanganate of
Wadsley (1950) and has reported that lattice expansion is not observed for some other cation
forms of synthetic buserite.
The present study was undertaken to clarify specific aspects of the exchange reaction
between alkylammonium cations and a synthetic phyllomanganate. In this way it was hoped
that the potential role of alkylammonium exchange, as a specific chemical treatment used in
conjunction with XRD for the characterization of naturally occurring phyllomanganates,
could be assessed. Alkylammonium exchange has been widely used by Lagaly and co-workers
to characterize the layer-charge density in swelling phyllosilicates (e.g. Lagaly, 1982). In the
present paper, the influence of various properties of the manganate, such as the saturating
cation and the hydration state, on the exchange reaction with the dodecylammonium cation
has been studied. In part II of this investigation (Paterson et al., 1986) [following paper] the
structure of the expanded phase itself, for a wider range of alkylammonium cations, is
considered.
EXPERIMENTAL
Materials and methods
Na-phyllomanganate was synthesised by oxidation of freshly-precipitated Mn(OH)2 at
high pH using the method given by Giovanoli et al. (1970). After oxidation, the black
precipitate was allowed to settle and the supernatant poured off. The residue was then
washed with deionized water until the pH was ~ 7 and peptization commenced. The
synthetic product (Na~Mn~40,7 .xH2 O) was stored in aqueous suspension until required.
A portion of suspension containing ~ 250 mg of phyllomanganate was treated with 50 ml
of a 0-5 M aqueous solution of the appropriate metal chloride. The suspension was allowed to
stand for 16 h then shaken again before centrifuging and discarding the clear supernatant.
The manganate was washed four times with 50 ml portions of 0.1 M solutions of the saturating
cation followed by 50 ml portions of deionized water until the supernatant was chloride-free.
The cation-forms prepared for examination were Na, K, Cs, NH~, Mg, Ca, Sr, Ba, Mn, Co,
Ni and Cu.
The moist residue after washing was divided into four parts. The first of these was smeared
on a glass slide and examined while moist by XRD. The second portion was maintained moist
for treatment with dodecylammonium chloride. The third was air-dried at ~ 50~ r.h. before
exchange with dodecylammonium chloride, and the fourth was dried at room temperature
over P205 in a vacuum desiccator before exchange.
A 0.1 M solution of dodecylammonium chloride was prepared by dissolution of a weighed
quantity of dodecylamine in the stoichiometric amount of 1 M HC1. Dissolution was aided by
constant stirring and heating to 50~ in a Pyrex beaker loosely covered with a watch glass.
The solution was made up to volume using pre-heated water and adjusted to pH 6.5 by
addition of small amounts of the amine. A 50 ml portion of 0.1 M dodecylammonium chloride,
preheated to 70~ was added to approximately 25 mg of the various cation-forms of the
phyllomanganate, each of which had been subjected to the three different drying procedures
given above, and placed in an oven at 70~ for 16 h. The sample was then centrifuged whilst
still hot and the clear supernat.ant discarded. The residue was washed with warm deionized
water, then air-dried on a glass slide for XRD.
X-ray diffraction was carried out on a Phitips 1130/90 diffractometer using Fe-filtered CoKc~radiation. The different cation forms of the manganate were prepared for examination by
Alkylammonium exchange in synthetic phyllomanganate
951
smearing ~ 25 mg of the moist sample on a glass slide, First, they were examined moist, then
after air drying, and finally after evacuation over P205 in a vacuum desiccator. Rehydration
was limited by wrapping the slides in clear plastic film. The alkylammonium manganates
were prepared as oriented aggregates by air drying 2 ml of aqueous suspension on a glass
slide. All the samples were examined over the range 3-15~ at a scan speed of 2~
RESULTS
AND DISCUSSION
The formation of an alkylammonium derivative from moist Na-manganate (Paterson, 1981),
as manifested by a strong X R D reflection at 25.4 A. with a series of higher orders, was
confirmed in the present study (Fig. la,b). On the other hand, the failure to undergo cation
25"4 R
7"1,~
(d)
(b)
}.9~,
7.1~
(c)
~
Co K #,.
~
Co Ka.
I
t
I
I
I
I
15
10
5
15
10
5
FIG. 1. Effect of alkylammonium exchange on the XRD traces of Na-manganate before (a) and
after (b) treatment, K-manganate before (c) and after (d) treatment.
952
E. Paterson et al.
exchange in the interlayer regions can also be clearly seen in the X R D traces for the Kmanganate (Fig. lc,d). Thus X R D can be used as a convenient indicator of whether cationexchange in the interlayer has occurred, as well as providing information on the structure of
the inorganic starting materials.
The basal spacings of the inorganic cation-forms are summarized in Table 1 (in the text the
values are rounded to the nearest integer). The results are consistent with those obtained in a
previous study of the main group cations (Tejedor-Tejedor & Paterson, 1979) and with results
for the transition metal cations (Giovanoli et al., 1975).
The basic structural unit in buserite is a layer of manganese(IV) ions, octahedrally coordinated to oxygen and/or hydroxyl ions. The layers have a permanent negative charge
caused by vacancies and/or diadochic substitution of Mn(II) or Mn(III) for Mn(IV).
Successive layers are separated by an interlayer region containing exchangeable cations,
water molecules and perhaps some Mn(II) ions. The two distinct basal spacings at 10 and 7 A
arise from a two-layer and one-layer hydrate, respectively. Small highly-polarizing cations,
such as Mg 2+, stabilize the two-layer hydrate whereas large cations, such as Ba 2+, favour the
formation of the one-layer hydrate. A similar pattern is observed for the monovalent cations.
Although hydration energies have frequently been invoked to account for similar trends in
phyllosilicates, other factors may be involved including a role for interlayer water as a
'bridge' between the counterion and the source of negative charge (Farmer & Russell, 1972).
In the present case, the presence of only a single, octahedral manganate layer in the structure
means that the lattice charge can only be delocalized over a few surface oxygen ions and,
where the cation is small, the two-layer hydrate will be stabilized.
As expected for relatively small divalent cations, saturation with the transition metal
cations stabilizes the 10 A phase in the presence of excess water but, on air-drying,
differences are observed within the group, with the most stable two-layer hydrate being
TABLE 1. Summary of basal spacings, /~., and alkylammonium exchangeability of various ion-forms of
manganate as a function of water content.
Moist
Cation
form
Na
K
NH 4
Cs
Mg
Ca
Sr
Ba
Mn
Co
Ni
Cu
-
Ionic
radius, ,~
0.95
1.33
1.48
1.69
0"65
0.99
1.13
1.35
0-80
0.72
0.69
0.62
S p a c i n g Alk.-exch.
9.9
7.1
7.1
7.3
9.5
9.9
9.8
7.0
9.7
9.5
9.7
(9.5 + 7.2)*
* indicates ill-defined reflection
ND not determined
not carried out
-
Air-dry
YES
NO
NO
NO
YES
YES
YES
NO
NO
YES
No
No
Vac-dry
S p a c i n g Alk.-exch.
7.1
7.1
7.1
7.3
9.6
9.9
7.0
7.0
9.6*
9.5
9.7
7.1
YEs
---YES
YES
NO
--YES
---
Spacing
Alk.-exch.
7.1
ND
ND
ND
7"1
7.1
ND
ND
7.0
7"2
7-2
7.1
No
---NO
NO
---YES
---
Alkylammonium exchange in synthetic phyllomanganate
953
observed for the d 7 and d s ions. The d s and d 9 species show an increased readiness to
dehydrate to the single-layer 7 A form. However, it is not possible to say whether this effect is
causally related to the variation observed in hydration properties of the cations themselves
across the first transition series (Heslop & Jones, 1976) or whether it is due to the amount and
effect of cation uptake on the structure of the manganate layer (see later).
The effect of drying and the saturating cation on the exchange with dodecytammonium are
also summarized in Table 1 along with standard data on the ionic radii of the various cations
used.
For the moist samples, the main group cation-forms fall into two distinct groups. Those
species that give a 10/~ spacing on XRD can undergo alkylammonium exchange and lattice
expansion whereas those with a 7/~ spacing cannot. The situation is more complex for the
transition metal cations. All four species give a 10 & spacing when moist (although the Cuand Mn-samples show some signs of collapse) but only the Co-manganate forms an expanded
phase on treatment with C12-NH~ +.
After air-drying, only four samples form the expanded C12-NH4 + phase, the Mg-, Ca-, Coand Na-manganates. Of these, the first three are 10 ,& species, consistent with the observation
made for the moist samples, but the Na-manganate, although quite clearly a 7 ,/~ species,
undergoes exchange.
Evacuation over P2Os of the 10/~ air-dried Mg-, Ca- and Co-species showed that all three
collapse to 7 ,~ on dehydration. On subsequent treatment with dodecylammonium cation no
lattice expansion was detected for the main group cations, only for the Co-form. Thus, the
only fully-collapsed 7 A species capable of lattice expansion is Co-manganate. Clearly, the
severe dehydration, caused by evacuation, resulted in irreversible changes in the interlayer
constituents for the alkaline earth cations.
In attempting to rationalize the behaviour of the various cation forms towards
alkylammonium exchange the effect of cation saturation on the original manganate must first
be considered. In particular, the behaviour of the transition metal cations is probably greatly
influenced by the likelihood that the manganate layer itself, the primary cause of cation
exchange, has been altered. This alteration is indicated by the release of Mn to solution
during sorption of transition metal cations on disordered synthetic buserite/birnessite
(Loganathan & Burau, 1973). Direct replacement of Mn(II) in the lattice by another divalent
cation would probably have little effect but if vacancies in the manganate layer (Giovanoli et
al., 1970) are filled then considerable changes in lattice charge are likely. Further
complications include oxidation of the saturating cation, e.g. Co(II) -~ Co(III) (Murray &
Dillard, 1979) and the concomitant reduction of Mn as well as possible formation of hydroxycation species. Clearly, the complexity of the situation is considerable and there is little to add
to the empirical observations.
The situation for main group cations is, however, more promising. Distinct patterns of
behaviour may be discerned and, since the saturation process does not involve any release of
Mn and the attendant alteration of the manganate layer, the alkylammonium exchange
behaviour may be considered on the basis of the structure of the hydrated cation phase. In all
cases, the two-layer hydrate structure, indicated by a 10 /~ basal spacing, can undergo
alkylammonium exchange. In the two-layer structure the inorganic cations are almost
certainly octahedrally co-ordinated to water molecules and are thus constrained to positions
near the centre of the interlayer. In these positions there will be ready access for exchange
with alkylammonium cations. On the other hand, the 7 & phase (with only one exception,
Na-manganate) does not undergo alkylammonium exchange. In the 7 A phase, octahedral co-
954
E. Paterson et al.
ordination of the saturating cations by interlayer water cannot occur and large cations, such
as K +, Cs + and Ba 2+ (Table 1), can approach both surfaces of the interlayer. This close
approach of counterions to the negative charge, delocalized over only a few surface oxygen
ions on the manganate sheet, causes irreversible collapse of the manganate structure
(Tejedor-Tejedor & Paterson, 1979) and may 'fix' the interlayer cations (Paterson et al., in
preparation). In the air-dry Na-form, the only main group 7 A species capable of
alkylammonium exchange, the interlayer cation is probably too small (Table 1) for close
approach to both sides of the interlayer and exchange with alkylammonium can occur, aided
by the monovalent-monovalent nature of the exchange.
The effect of severe dehydration in vacuo may be to encourage reactions of the type
Mg(H20) 2+ ~ Mg(OH) + + H +
The protons produced in such a reaction could be readily accommodated in the highly
electronegative environment of the manganate lattice, thus moving the equilibrium to the
right and encouraging the formation of hydroxy-cation interlayer species. Both the formation
of these species and the concomitant decrease in layer charge may then render the structure
non-expanding.
CONCLUSIONS
Alkylammonium exchange and lattice expansion in a synthetic phyllomanganate is a
complex function of the saturating cation and the hydration state of the manganate. Thus
caution must be exercised when attempting to use this technique for the characterization of
mineral species. In particular, although a positive reaction indicates the presence of a
phyllomanganate, a negative reaction does not indicate its absence. The reaction should be
carried out on a moist sample of the original material, if possible. In addition, care should he
taken to ensure that any expanded phase present is due to a manganate rather than a
phyllosilicate. Despite these reservations, the simplicity of the technique and the relative ease
with which the expanded phase may be identified by X R D makes it worthy of further testing
on mineral samples. Taken in conjunction with X R D data for the moist (if possible) and airdried samples, a more positive identification should be possible. Failing this, however, the
result of the test could, in itself, be taken as a characteristic of the system.
REFERENCES
ARRHENIUS G.O. & TSAI A. (1981) Structure, phase transformation and prebiotic catalysis in marine
manganate minerals. Scripps Institution of Oceanography ReJerence Series 81-28, 1-19.
BURNSR.G., BURNSV.M. & STOCKMANH. (1983) A review of the todorokite-buserite problem: Implications
to the mineralogy of marine manganese nodules. Am. Miner. 68, 972 980.
BUSER W., GRAF P. & FEITKNECHTW. (1954) Beitrag zur Kenntnis der Mangan(II)manganite und des 6MnO2. Heh'. Chim. Acta 37, 2322-2333.
BROWN G. & BRINDLEYG.W. (1980) Pp. 305-360 in: Crystal Structures of Clay Minerals and their X-ray
Identification (G. W. Brindley and G. Brown, editors). Mineralogical Society, London.
FARMERV.C. & RUSSELLJ.D. (1972) Interlayer complexes in layer silicates. The structure of water in lamellar
ionic solutions. Trans. Far. Soc. 67, 2737-2749.
FEITKNECHTW. & MARTIW. (1945) Uber Manganite und kunstlichen Braunstein. Helvetica Chimica Acta 28,
149 156.
GIOVANOLIR. (1985a) A review of the todorokite buserite problem: Implications to the mineralogy of marine
manganese nodules: discussion. Am. Miner. 70, 202 204.
Alkylammonium exchange in synthetic phyllomanganate
955
GIOVANOLI R. (1985b) Layer structures and tunnel structures in manganates. Chem. Erde 44, 227 244.
GIOVANOLI R., BURKI P., GIUFFREDI M. & STUMMW. (1975) Layer structured manganese oxide hydroxides.
IV: The buserite group; Structure stabilization by Transition Elements. Chimia 29, 517 520.
GIOVANOLI R., STAHLIE. & FEITKNECHT W. (1970) Uber die Oxidhydroxide des vierwertigen Mangans mit
Schichtengitter. 1. Mitteilung: Natriummangan(II,III) manganat(IV). Helv. Chim. Aeta 53, 209-220.
HESLOP R.B. & JONES K. (1976) Inorganic Chemistry, p. 553. Elsevier.
LAGALY G. (1982) Layer charge heterogeneity in vermiculites. Clays Clay Miner. 30, 215-222.
LOGANATHANP. & BURAUR.G. (1973) Sorption of heavy metal ions by a hydrous manganese oxide. Geochim.
Cosmochim. Acta 37, 1277 1293.
MURRAY J.W. & DILLARD J.G. (1979) The oxidation of cobalt(II) adsorbed on manganese dioxide. Geoehim.
Cosmochim. Acta 43, 781-797.
PATERSON E. (1981) Intercalation of synthetic buserite by dodecylammonium chloride. Am. Miner. 66, 424~
427.
PATERSONE., CLARKD.R., RUSSELLJ.D. & SWAEFIELDR. (1986) Cation exchange in synthetic manganates:
II. The structure of an alkylammonium-saturated phyllomanganate. Clay Miner. 21, 957-964.
TEJEDOR-TEJEDOR M.I. & PATERSONE. (1979) Reversibility of lattice collapse in synthetic buserite. Proe. Int.
Clay Con]. OxJord, 501-508.
TURNER S. & BUSECKP.R. (1981) Todorokites: a new family of naturally occurring manganese oxides, Science
212, 1024-1027.
WADSLEYA.D. (1950) A hydrous manganese oxide with exchange properties. J. Am. Chem. Soc. 72, 1781-1784.