REVIEW OF THE RELATIONSHIPS IN THE
MONTMORILLONITE GROUP OF CLAY MINERALS 1
by
CLa_R~NC~ S. R o s s
U.S. Geological Survey, Washington, D.C.
ABSTRACT
Since the publication of the paper on the minerals of the montmorillonitegroup by Ross
and Hendricks (1945) some species have been added to the group, and minor changes in
the acceptable species have become advisable. No radical changes are proposed, but a
review of the members and their grouping will be presented. The acceptable dioctahedral
members of the group appear to be montmorfllonite, nontronite and volehonskoite.
Beidellite should be discontinued. The acceptable trioctahedral members of the group
appear to include saponite, hectorite, sauconlte, stevensite and medmontite. Griffithite
is probably a useful term for an iron-rich saponite. Representative chemical analyses
and formulas have been assembled to illustrate relationships.
INTRODUCTION
The paper b y Ross a n d Hendricks (1945) on the minerals of the montmorillonite group laid the f o u n d a t i o n for our u n d e r s t a n d i n g of t h a t group, a n d
although refinements have been made a n d some new minerals added since
t h a t time, there have been no basic modifications. However, it seems opport u n e to review briefly t h e present status of the various members of the
m o n t m o r i l l o n i t e group.
Ross a n d Hendricks (1945) described montmorillonite, beidellite a n d
volehonskoite as the dioctahedral members, a n d saponite a n d hectorite as
t h e trioctahedral members. None of these n a m e s were n e w ; all h a d been
previously used. Ross (1946) later added sauconite, the zinc member, a n d
discussed the relations of iron-rich griffithite. F a u s t a n d Murata (1953)
added stevensite, a m a g n e s i a n member, a n d Chukhrov a n d Anosov (1950)
described m e d m o n t i t e , a copper member.
DISCUSSION
OF
RELATIONSHIPS
The various m e m b e r s of the m o n t m o r i l l o n i t e group t h a t have been post u l a t e d a n d those which seem acceptable i n the light of present-day knowledge
are listed below.
Dioctahedral members :
montmorillonite (well defined)
nontronite (well defined)
volehonskoite (probably acceptable)
beidellite (name discarded).
1 Publication authorized by the Director, U.S. Geological Survey.
225
226
SEVENTH N A T I O N A L C O N F E R E N C E
O N CLAYS A N D C L A Y M_I-NERALS
Trioctahedral members:
saponito (welldefined)
heetorite (well defined)
saueonite (well defined)
stevensite (well defined)
medmontite (well defined)
griffithite(an iron-richsaponite, but useful term).
TABLE I.--CHEMICAL
A N A L Y S E S OF R E P R E S E N T A T I V E M E M B E R S OF T H E
I~IONTMORILLONITE G R O U P OF M I N E R A L S
SiO2
AlsO3
FesO3
Cr2Os
1%O
ZnO
MgO
MnO
CaO
CuO
(1)
(2)
(3)
50.37
17.11
2.68
40.72
4.96
29.57
-O. 71
45.70
4.85
6.36
15.02
--
0.20
(5)
(6)
42.99
6.26
39.64
9.05
1.83
7.32
55.86
0.13
0.03
-2.57
-7.83
4. ~9
0.74
0.58
1.23
1.98
3.10
2.03
O. 54
-tr.
1.04
--
NiO
KsO
Na20
LisO
TiOs
(4)
0.09
O. 56
22.96
0.11
15.80
--
2.93
none
0.71
0.02
F-
HsOH~O +
Total
12.29
10.93
15.46
6.66
100.05 100.82
16.38
7.37
13.65
6.85
12.31
4.90
9 9 . 9 0 100.29 100.49
--
25.03
none
tr.
(7)
(8)
(9)
57.30
none
0.32
35.95
6.57
2.36
43.88
13.25
3.25
none
~
~
27.47
0.21
0.97
33.70
1.26
0.04
0.62
--
--
0.10
2.68
0.03
0.03
1.05
. . .
none ,
-5.96
none
9.90
6.69
2.24
7.17
102.98
--O=]~
100.47
100.19
0.10
0.44
. .
0.07
-11.34
7.24
--
0.59
0.06
1.58
20.96
0.87
0.44
0.72
-7.02
7.52
9 9 . 6 9 100.14
L o c a l i t y a n d references :
(1) M o n t m o r i l l o n i t e f r o m L e m o n , Miss., R o s s a n d H e n d r i e k s (1945, p. 34, a n a l y s i s
no. 20).
(2) N o n t r o n i t e , Spokane, Wo,sh., R o s s a n d H e n d r i c k s (1945, p. 35, a n a l y s i s no. 59).
(3) A slightly i m p u r e volchonskoito, Lyalevo, B u l g a r i a ( D i m i t r o v , 1942).
(4) Saponite, A b m e e k m i n e , Mich., P a l a e h e a n d V a s s a r (1925, p. 418).
(5) Saponite (iron rich) (described as grlffithitc) Griffith P a s s , Calif., L a r s e n a n d
Steiger (1928, p. 1-12).
(6) Hectorite, H e c t o r , Calif., R o s s a n d H e n d r i c k s (1945, p. 35, a n a l y s i s no. 76).
(7) Stevensite, Springfield, N . J . , F a u s t a n d M u r a t a (1953, p. 974).
(8) Sauconite, New D i s c o v e r y m i n e , Leadville, Colo., R o s s (1946, p. 418).
(9) M e d m o n t i t e , D z h e z k a z g a n , copper deposit, Steppes o f K a z a k h s t a n , C h u k h r o v
a n d A n o s o v (1950, pp. 23-27).
MONTMORILLONITEGROUP OF CLAy MINERALS
227
Montmorillonite, the dominant member of the dioctahedral group, is
well defined, has received much detailed study, and requires no additional
discussion. Nontronite is well defined ; it tends to range from an iron-bearing
montmorfllonite (ferrian montmorillonite) to a member in which Fe 3+ is
the dominant component occupying oetahedral positions in the crystal
lattice.
Volchonskoite, a chromium member of the group, was discussed by Ross
and Hendricks (1945, p. 28) and a number of chemical analyses were presented. All the samples seem to contain impurities and no suitable formula
for a member of the group can be calculated from the available analyses.
One analysis, which seems to be the best available, is given in Table l,
TABLE 2.~TYPICAL F O R ~ S
OF ~/~INERALSOF THE ~ONT~ORIIJ~Oh~'ITEGROUP
Dioctahedral Members
(1) Montmorillonite
(AILaaFea+o.16Fe~+o.mMgo.44)(Si3.87AI0.13)Olo(Oi)2Mg]20.ogCa/2o,10~Ta0,16
(2) Nontronit~
(FoS+l.92Fe2+o.osA1o.o2Mg0.ol)(Si3.a2A10.48)Olo(OH)~Mg/20.osCa/20.1s
(3) Volehonskoite (see comments), best available analysis, no. 3 in Table 2,
Trioctahedral Members
(4) Saponito
(Mg~,71Fe2+0.17Fea+o.loA10.o0)(Sis.4~Alo.5s)O10(OH)2Ca/2o.17Na0.16
(5) Griffithito (iron-rich saponite)
(Mg1.88FeU+o,5~Yea+o.44Alo.04)(Si3.19A]0.81) O16(OH)2Ca/2o.~sNa0.11
(6) Hectorito
(Mg2.s7Li0.30) (Si8.99Alo.ol) Olo(F, OH)~Nao.38
(7) Stevensite
(Mg2.ssMno.o~FeS+o.o~)(Si4)Olo(OH)2(Ca/2Mg/2)o.15
(8) Saueonito
(Zn2.40~/~go.laA/o.22Fe3+o.17)(Si3.47A10.53)O10(OH)2Ca/20.1aNao.o9
(9) Medmontite
(Alo.s9FeS+o.26Cu1,aoMgo,07) (Si3.61A10.39)O10(Ot:[)2Ca]20.4Nao,07Ko.09
column (3), but no corresponding formula is given in Table 2. However,
these analyses obviously represent a material in which a chromium member
of the montmorillonite group is the dominant component. Therefore, volchonskoite is an acceptable member of the group and a suitable analysis m a y
be expected.
Ross and Hendricks (1945, pp. 26, 47) listed beidellite as a member of the
group, but additional studies of the group, especially by Foster (1954) and
Grim and Rowland (1942), have indicated t h a t some of the analyses of highalumina clays were made on impure materials. I n suitable material the
substitution of AI for Si in tetrahedral position is small, the usual amount of
replacement not exceeding t h a t represented b y Sis.so Alo.2o (Foster, 1953,
p. 153). This indicates t h a t no naturally occurring member of the montmorillonite group has been observed in which A1 proxies Si in tetrahedral
positions in amounts justifying a distinct mineral name. Synthetic material
228
S E V E N ~ NATIONAL CONFERENCE ON CLAYS AND CLAYMINERALS
having larger amounts of A1 replacing Si has been reported by R o y and Sand
(1956). Thus, aluminian montmorillonite is the best term for all natural
aluminous members of the group so far observed.
The ions, A18+, Fe3+, and Cr a+ can, no doubt, proxy one another in a
wide range of proportions; intermediate members of the group have been
described and others are to be expected. The term aluminian montmorillonite
as a replacement for the name beidellite has already been mentioned. Some
of the intermediate montmorillonite-nontronite series were described by
Ross and Hendrieks (1945, pp. 55-56) as corresponding to ferrian montmorlllonite. A chromium-bearing member of the montmorillonite group has
been examined but not analyzed in our laboratory. The index of refraction
indicates a chromian montmorillonite with a few percent of Cry03.
The trioctahech'al members are all well defined. Saponite, which m a y be
taken as representative of the group, is widespread in occurrence but seems
to be limited to small deposits. Hectorite is interesting as it represents a
nearly alumina-free clay mineral and because of its high content of lithium
and fluorine. Sauconite, a zinc member, is interesting because mineralogists
long refused to credit their own definitive studies. How could there be a true
zinc clay? However, Genth (1875), who possessed an almost uncanny mineralogical aptitude, recognized sauconite as a true mineral. Stevensite has been
discussed b y Faust and Murata (1953) and their statements need not be
amplified here. Medmontite from the Steppes of Kazakhstan was described
by Chukhrov and Ansov (1950) and is the only copper member that has
been reported. However, the data presented seem to establish it as a valid
member of the group.
Substitutions are characteristic of all members of the montmorfllonite
group, and certain new members are to be expected. Griffithite, a member
with ferrous iron = 7.83 percent and ferric iron = 7.32 percent, was described by Larsen and Steiger (1928). Ross (1946) grouped it as an iron-rich
saponite, a relationship confirmed by Faust (1955). I t is unusual among the
clay minerals as it contains an essential proportion of ferrous iron. Although
it does not represent a distinct species, griffithite probably is a useful term.
Ross and Hendricks (1945, p. 35, analysis 101) cited an analysis of a very
impure saponitic clay containing about 11 percent of NiO. This seemed to
represent an impure nickeloan saponite, and indicated that a nickel member
of the group m a y be expected. Dimitrov (1942) described a member of the
group with A12Oa = 17.16, Fe20~ = 11.00, and NiO = 3:58 percent. This
would make it a nickel-bearing ferrian montmorillonite. The chemistry and
ionic radius are both suitable for the formation of a cobalt member of the
group if the rather rigorous geologic prerequisites are fulfilled.
Chemical analyses of representative members of the montmorillonite
group are given in Table 1. Formulas calculated from the analyses given in
Table 1 are presented in Table 2. Those numbered (1) (Foster, 1951, p. 724),
(2), (4), (5), (6) and (9) were calculated b y Margaret D. Foster who has cooperated in the preparation of this paper. No. (7) was calculated by George
T. Faust, and no. (8) by Ross (1946, p. 418).
M O N T M O R I L L O N I T E G R O U P OF C L A Y MINERALS
229
REFERENCES
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pp. 23-27. (In Russian.)
Dimitrov, Strashimir (1942) ChrenLhaltige Tonminerale u n d Nickel-Asbolan in der
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