Clay Minerals (1965) 6, 17.
POTASSIUM
RETENTION
IN
MONTMORILLONITES
A. H. W E I R
Rothamsted Experimental Station, Harpenden, Herts.
(Received 28 January 1965)
ABSTRACT:
The amounts of potassium retained, after exchange with normal
ammonium acetate solution at pH 7, were measured with six montmorillonitebeidellite minerals, whose interlayer charge ranged 0-72-1.00 M+/SisO ~ and where
tetrahedral substitution of A1 for Si ranged 0.14--0.93 M+/SisO ~. The amount of
potassium retained increased with increasing interlayer charge, but there was no
correlation between the amount of potassium retained and the tetrahedral charge.
The retention of potassium by expanding minerals is important to agriculture in
connection with the fixation of potassium by soil clays. There have been several
attempts to find how fixation is related to the composition and charge distribution
of expanding 2:1 lay silicates. Wear & White (1951) suggested that the amount
of potassium retained in these minerals after exchange varied directly with the
proportion of negative charge caused by tetrahedral substitution of A1 for Si. Barshad
(1954) considered that the total charge controls expansion and hence the removal
of potassium, and is thus more important than the type of substitution. However,
Weaver (1958) and Mackenzie (1963) considered that potassium fixation is related
to the amount of tetrahedral charge. Because of the difficulty of obtaining pure
mineral samples of suitable composition, the relative importance of total charge and
type of substitution has been tested only for a small range of composition. This
paper gives the results obtained with six montmorillonites of different charge and
isomorphous substitution.
MATERIALS
The samples studied were beidellite (U.S.N.M. R4762) from the Black Jack Mine,
Idaho, obtained from the U.S. National Museum; beidellite* from Unterrupsroth,
Germany, obtained from Professor Dr U. Hofmann; montmorillonite from Wyoming,
U.S.A., obtained from F. W. Berk and Co; montmorillonite from Redhill, England,
obtained from the Fullers Earth Union; montmorillonite from Camp Berteau,
* Beidellite is used here as defined by Weir & Greene-Kelly (1962). A very similar mineral
from Unterrupsroth is also referred to as a beidellite by Weiss, Koch & Hofmann (1955).
B
A. H. Weir
18
M o r o c c o , o b t a i n e d f r o m M. J. M6ring; m o n t m o r i l l o n i t e f r o m S k y r v e d a l e n , N o r w a y ,
o b t a i n e d from P r o f e s s o r I. Rosenqvist.
T h e B l a c k J a c k M i n e beidellite a n d S k y r v e d a l e n m o n t m o r i l l o n i t e c o n t a i n e d
q u a r t z i m p u r i t i e s which were r e m o v e d b y careful selection. T h e U n t e r r u p s r o t h
beidellite c o n t a i n e d stilbite; this a n d the C a m p Berteau m o n t m o r i l l o n i t e were purified
b y N a - s a t u r a t i n g , dispersing a n d collecting the less t h a n 2 t~ equivalent spherical
d i a m e t e r fraction. T h e R e d h i l l a n d W y o m i n g m o n t m o r i l l o u i t e s c o n t a i n e d fine i r o n
oxide, q u a r t z a n d felspar, a n d were purified b y dispersing a n d collecting the less
t h a n 0-2 tL e q u i v a l e n t s p h e r i c a l d i a m e t e r fractions. P u r i t y was c h e c k e d b y X - r a y
diffraction a n d electron a n d light m i c r o s c o p y .
CHEMICAL
ANALYSIS
T a b l e 1 gives c h e m i c a l analyses of the six s a m p l e s expressed on a n ignited weight
basis; s a m p l e analyses 2, 3, 5 a n d 6 were r e p o r t e d b y W e i r (1960) l a b y R o s e n q v i s t
(1959) a n d 4 is new. A l l the analyses except l a relate to N a - s a t u r a t e d materials.
TABLE 1. Chemical analyses on ignited weight basis
SiO2
A1203
Fe2Oa
FeO
MgO
CaO
NasO
K~O
la
lb
lc
66.11
25.48
0"52
.
4.07
3 '75
0.07
-100"00
----
---.
--0-15
6"48
.
.
--4 "34
0"17
2
3
4
5
6
59.30
36.11
0"50
64.0
29.0
0'21
66.32
22.42
3"30
3.03
-3 "98
0'05
100"27
4.07
0-03
3 "68
0"06
99'88
64.7
18.6
8"12
0"25
4.32
-3' 30
0"04
99"33
64.8
24.6
4"25
0.21
2.83
-3"15
0"04
99"88
.
0-10
0"02
3 "98
0"11
100"12
.
la. Montmorillonite, Skyrvedalen, Norway (Rosenqvist, 1959). Analysis recalculated on ignited
weight basis.
lb. Montmorillonite, Skyrvedalen, Norway, hand-picked, Na-saturated sample. Analyst, G. Pruden.
lc. Montmorillonite, Skyrvedalen, Norway, hand-picked, K-saturated sample. Analyst, G. Pruden.
2. BeideUite, Black Jack Mine, U.S.A., hand-picked, Na-saturated sample.
3. Beidellite, Unterrupsroth, Germany, purified < 2/z e.s.d, fraction, Na-saturated sample.
4. Montmorillonite, Camp Berteau, France, purified, < 2 /~ e.s.d, fraction, Na-saturated sample.
Analyst, G. Pruden.
5. Montmorillonite, Redhill, England, purified < 0"2/~ e.s.d, fraction, Na-saturated sample.
6. Montmorillonite, Wyoming, U.S.A., purified < 0.2/~ e.s.d, fraction, Na-saturated sample.
19
Potassium retention in montmorillonites
TABLE 2. Structural formulae calculated from the analyses in Table 1
1. Untreated montmorillonite, Skyrvedalen
+ 1.00
- 0.72
- 0.26
Nao.o2 Cao.~7 Mgo.o2 (A13.25 Fe3.o~ Mgo.60 (Si7.~4 Alo.26) 033
= 3"99 atoms
2.
Na-saturated beidellite, Black Jack Mine
+0"93
+0-10
- 1"03
Nao.21 Ko.o2 (A18.28 Feo.o4 Mgo.o2) (Si~.2, Alt.o3) 023
~. = 4"04 atoms
3. Na-saturated beideUite, Unterrupsroth
+0.91
-0.41
--0.51
Nao.9o Ko.ol (A13.43 Feo.o2 Mg3.53) (Sir.42 Alo.3x) 023
~. = 4"04 atoms
4. Na-saturated montmorillonite, Camp Berteau
+0-85
-0-72
--0.14
Nao.s4 Ko.ox (A13.2g Feo.2g Mgo.72) (SiT.s6 Alo.x~) 033
~. = 4"00 atoms
Na-saturated montmoriUonite, Redhill
+0.79
-0"63
-0-15
Na0.78 Ko.01 (A12.5x Fe+~4 Fe0.0s3+Mg0.Ts) (SiT.s5 A10.15) 032
~. = 4"06 atoms
6. Na-saturated montmorillonite, Wyoming
+ 0"72
- 0"43
- 0-31
2+ Mgo.~o) (SiT.e9 Alo.a0 033
Nao.7~ (A13.13F~a+
~o.$8Feo.o3
= 4"03 atoms
TABLE 3. Potassium retained in the minerals after exchange with normal ammonium
acetate, m-eq/100 g ignited weight basis
Interlayer cations Potassium retained
Total
equivalent to the
in the sample
interlayer
tetrahedral
dried at
cations
charge
20 ~ C for 1 week
Montmorillonite, Skyrvedalen
Beidellite, Black Jack Mine
Beidellite, Unterrupsroth
Montmorillonite,
Camp Berteau
Montmorillonite, Redhill
Montmorillonite, Wyoming
Potassium retained
in the sample
dried at
105 ~ C for 24 hr
143
131
129
121
37
131
72
20
39
25
19-5
15
40
23
20
12-5
107
103
20
41
2"2
7-5
2'5
7"6
A. H. Weir
20
Analysis l a is for naturally occurring material in which Ca 2+, Mg 2+ and Na + can
occur as interlayer cations. Mg can also occur in octahedral positions within the
layer. T o allocate the Mg content found by analysis to the appropriate positions
in the structural formula, the cation exchange capacity was determined by analyses
of Na-saturated and of K-saturated material (Table 1, columns lb and lc); this
showed that the exchange capacity was 143 m-eq/100 g and allowed the structural
formula of the Skyrvedalen mineral given in Table 2 to be derived.
The amount of potassium retained was determined by the following method. The
minerals were washed ten times with normal potassium chloride solution to saturate
with potassium, and the excess salt was removed with ten washings with 90% ethanol.
40
(~_Sky
"~ 30
g
~
{3"
?
v
E 2C
BJH
Unt
"o
r
40 cB
t. 10
~-wy
(~.Red
0--
100
I
I
I
1
I
110
120
130
140
150
Intertayer charge (m-eq/lOOg(ignited weight))
FIG. 1. Plot of interlayer charge to potassium retained in the mineral after
exchange with normal ammonium acetate pH 7, The samples were dried at 20 ~ C
( + ) , and at 105 ~ C ( 9
Sky = montmorillonite, Skyrvedalen; BJM = beidellite,
Black Jack Mine; Unt = beidellite, Unterrupsroth; CB = montmorillonite, Camp
Berteau; Red = montmorillonite, Redhill; Wy = montmoriUonite, Wyoming.
Each sample was then divided into three parts. One part was ignited and analysed
spectrochemically for potassium to establish that the clays were fully saturated.
The second and third parts were dried at 20 ~ C for 1 week and at 105 ~ C for 24 hr
respectively; they were then washed ten times, over a period of several days, with
normal ammonium acetate to remove exchangeable potassium and the excess ammonium acetate was removed by several washes with 90% ethanol. Finally, the
samples were ignited and the amount of fixed potassium determined.
21
Potassium retention in montmorillonites
40
30
o
o
"~8JM
17"
~ 2c
.c
.e lC
Unt
Jr CB
O
~l)Wy
..~Red
I
1
I
I
I
20
40
60
80
100
120
Interlayer cations equivalent to the tetrahedral charge
(m-eqll00g (ignited w e i g h t ) )
I
130
FIG. 2. Plot of tetrahedral charge to potassium retained in the mineral after
exchange with normal ammonium acetate pH 7. The samples were dried at 20* C
( + ) , and at 105 ~ C ( O ) . Sky = montmorillonite, Skyrvedalen; BJM = beideUite,
Black Jack Mine; Unt = beidellite, Unterrupsroth; CB = montmorillonite, Camp
Berteau; Red = montmorillonite, Redhill; Wy = montmorillonite, Wyoming.
RESULTS
Table 2 gives structural formulae calculated from the analyses in Table 1 and shows
that all the minerals are dioctahedral. The negative charge on the layers covers a
range from 0-72 to 1.00 per unit cell; the proportion of total layer charge originating
from tetrahedral substitution ranges from 16 to 100%.
Table 3 gives the amount of potassium retained after the two 'fixing' treatments,
the total charge per layer, expressed in m-eq/100 g ignited weight, and the charge
originating from substitution in the tetrahedral layer. Fig. 1 and 2 show the relationship between potassium retention, total charge, and tetrahedral charge. The amount
of potassium retained increases as the layer charge increases, but there is no relation
between the amount of potassium retained and the tetrahedral charge.
The minerals used originated either hydrothErmally or from bentonite deposits.
Weaver (1958) suggested that montmorillonites formed hydrothermaUy or from
volcanic glass are less liable to collapse when treated with potassium than montmoriUonites formed from the alteration of micas. However, as Foster (1956) showed
that the charge on dioctahedral micas can originate from either tetrahedral or
octahedral substitutions, a tetrahedral origin of charge cannot be assumed for expanding minerals derived from micas. The difficulty of obtaining pure montmorillonites derived from micas has so far prevented the investigation of their properties.
22
A. H. Weir
ACKNOWLEDGMENTS
I thank Professor I. Rosenqvist, M. J. M6ring, and Professor Dr U. Hofmarm for the samples
from Skyrvedalen, Camp Berteau, and Unterrupsroth respectively.
REFERENCES
BARSHAD 1. (1954) Soil Sci. 78, 57.
FOSTER M.D. (1956) U.S. Geol. Surv. Bull. 1036-D.
MACKENZIER.C. (1963) 1st Int. Clay Con/., Proc. (I. Th. Rosenqvist & P. Graff-Petersen, editors),
vol. 1, p. 183. Pergamon Press, Oxford.
ROSENQVISTI. TrI. (1959) Saertrykk av Norsk Geologisk Tiddsskri# 39, 4.
WEAR J.I. & WHITE J.L. (1951) Soil Sci. 71, 1.
WEAVER C.E. (1958) Am. Miner. 43, 839.
WEiR A.H. (1960) Ph.D. thesis, p. 73. London University.
WEIR A.H. & GREENE-KELLYR. (1962) Am. Miner, 47, 137.
WEISS A., Koch G. & HOFMANNU. (1955) Bet. K.D.G. 32, 12.