D Y E ADSORPTION A S A METHOD OF I D E N T I F Y I N G CLAYS
By
CHARLES G. DODD *
4. C(jnditions favoral)le to development of characteristic
colors easily controlled ; for example, not altered critically by
moderate changes in ratio between amount of clay mineral
and volume of dye .solution.
ABSTRACT
Dycstuffs and other reagents which exhibit cliiiraoteristic colors
when adsorbed on clay j^ranulcs frequently have been employed as
aids in clay identification. The colors generally are believed to
result from pleochroie effects and acid-base (u* oxidation-reduction
reaction niechanisms. Certain aniline dyes, solutions of which vary
in color according to their hydrogen ion content, may be used to
indicate the relative acidity of clay .surfaces as well as to heighten
the natural jdeochroism of kaolin minerals. When n clay sample
has been pretreated with acid, the c(dors a,ssumed by the adsorbed
dye depend on the characteristic base exch,-inge capacities of the
various cla.v mineral groups present.
Aromatic diamines, amino ]>hen(ds. and other compounds which
can be oxidized to colored semi(piinone cations i)erniit particularly
sensitive and positive identification of members of tlie montmorillonoid family and certain other three-layer clay minerals. Theories
concerning the nature of this apparentl.v catalytic oxidation on
clay surfaces are inadequate to explain the reactions. New concepts
and additional cx]ierimental information are needed.
Inirochiction.
Staining techniques have been em])loyed cfftx-tively in conjunction Avitli petrofjrapliic examinations of clay minerals and as accessory tools to
more complex methods of analysis. Color tests deserve
to be used more widely by clay technologists to complement and supplement the more elaborate methods. They
are p a r t i c u l a r l y a p p r o p r i a t e as aids to petrographic
identification methods because the l a t t e r are inapplicable
to particles the size of those in most clay samples.
Two factors have operated to discourage wider application of staining tests. The semi-empirical n a t u r e of
most of these methods has limited their use by workers
who have preferred to use only those analytical procedures which are based on widely accepted theory.
Others have attempted to employ over-simplified, outdated procedures as universal analytical tests in the
field. I t is scarcely necessary to add t h a t such efforts
frequently have been unsuccessful. W h e n modern techniques are u.sed properly, staining tests can save valuable
time in identification work. No one method of claymineral analysis, however elaborate, is foolproof; b u t
color tests deserve a place beside the established tools of
clay scientists and technologists. The relatively great
simplicity and speed which characterize dye staining
tests, compared with X - r a y diflfraetion and differential
thermal analysis, for example, justify wider application
of the color methods. Tt is the purpose of this p a p e r to
review past work and current procedures and to discuss
the present state of theoretical knowledge concerning
mechanisms of color formation on clay-mineral surfaces.
Mielenz and K i n g (1951) have developed the following criteria as desirable a t t r i b u t e s of satisfactory
staining tests for clay m i n e r a l s :
1. ncvelopment in particles of flay of cidors which a r e :
(a) reproducible, (b) sufficiently characteristic to permit identification of one or more clay mineral species, (c) sufficiently
intense to permit ready perception in either reflected or transmitted light, (d) and preferably of hues not common in the
minerals themselves
2. Preliminary treatment of samples not sufficiently laborious to make application of the method impractical
.S. Interferences sufficiently uncommon as to jiermit application of the method to the great ma.iority of samples
• Research Associate, Production Research Division, Development
and Research Department, Continental Oil Company, Ponca City,
Oklahoma.
None of the staining procedures described in this
p a p e r meets all of the criteria suggested above; but
when several tests are combined in a systematic manner,
the complete procedure satisfies all requirements.
Staining tests are not capable of identifying unambiguously all of the clay minerals present in m a n y samples of n a t u r a l l y occurring complex mixtures in rocks
and soils. The occurrence of interlayered lattices of various clay minerals complicates the problem further. Color
tests, nevertheless, provide a wealth of information when
applied to a wide variety of samples.
Slandardiced
Color Descriptions
for Staining
Tests.
The problem of measurement and standardization of
color may be solved by comjmriiig colors observed in
staining tests with prepared charts of color chips available in the MvnseU Bool- of Color, published in 1942 by
Munsell Color Co., Inc., Baltimore, Md. The widely
accepted Munsell color system classifies colors according
to hue, value, and chroma. The pocket edition, volume
I, is adequate for purposes of identifying colors produced on clays by various reagents used in staining
tests. This loose leaf booklet contains 20 charts corresponding to each of the 10 hues of maximum chrcmia and
10 intermediate hues. E a c h c h a r t is a r r a n g e d according
to value a n d chroma. F o r exam])le a color designated as
5.0 P B 3 / 8 corresponds to the hue purple-blue, a value
of 3, and a chroma of 8. Increasing value corresponds to
lessening amounts of n e u t r a l gray. Increasing chroma
corresponds to greater intensity of the given hue. On
the charts, value ranges from 2 to 8, and chroma from
2 to 12. The use of a comparison eye piece with the
Munsell color charts has been described by Mielenz,
King, and Schieltz (1950).
Staining
Tests Dependent
on Artifjcialli/
Induced
PleocJiroism.
The earliest application of d3'e staining
methods to the identification of clay minerals a p p a r ently was made by Grand jean (1909), following the
work of Suida (1904) which was applied to n u m e r o u s
other silicate minerals. This early work was concerned
with the artificially induced pleochroism of kaolingroup minerals when aniline dyes were adsorbed (m
their surfaces. Later Ross and Kerr (1931) described
the n a t u r a l pleochroism of kaolinite and mentioned the
fact t h a t kaolinite becomes more markedly pleochroic
when stained by dyes. I n an excellent critical discussion
of staining techniques, F a u s t (1940) described the
results of his studies on the artificially induced jileochroism of kaolinite, dickite, and nacrite when t r e a t e d
with crystal violet, methylene blue, malachite green,
safranine " O " , and basic fuchsin. F a u s t found t h a t
these dyes produced strong artificial pleochroism when
applied to the larger crystal aggregates such as books
a n d vermicular forms of kaolinite, whereas dickite crystals stained less readily and nacrite crystals stained
verv weakly. Both dickite and nacrite exhibited much
( lOo )
106
CLAYS AXD CLAY TECHNOLOGY
less pleoehroism. Pleochroic tests are not eharaeteristie
of other clay minerals.
Observations of artifieially induced pleoehroism eon.stitute a necessary step in modern dye staining procedures, but the pleochroic effects are less valuable dia<>uostic criteria t h a n the characteristic colors which result
from the adsorption of aniline dyes a n d aromatic amines
and phenols on clay granules.
Staining
Tests Dependent
on Acid-Base
Reactions.
Dyestuffs which act as hydrogen-ion indicators in
aqueous solutions may be adsorbed on clay-mineral surfaces where the resulting color depends on the availability of hydrogen ions from the clay acid. Such
adsorbed dyes m a y be considered indicators of the hydrogen-ion content of the water film on the clay surface.
G. T. F a u s t ' s p a p e r (1940) clearly demonstrated the
acid-base mechanism of the color reactions of aniline
dyes on clay mineral surfaces. "VVeil-Malherbe and AVeiss
(1948) discu.ssed at length the dift'erences between color
reactions dependent on acid-base mechanisms and those
dependent on oxidation-reduction reactions. The recent
excellent work of Mielenz and K i n g (1951) indicates
t h a t the dyes malachite green, color index No. 657, and
safranine " Y , " color index No. 841, are the most satisfactory aniline dyes for clay-mineral identification.
Following the suggestion of Merwin (Merwin and
Posnjak 1938), F a u s t used aniline as a solvent for his
dye reagents. This was done because aniline penetrated
the clay graniiles better than other solvents and led to
more intense characteristic colors. F a u s t also used nitrobenzene as a dye solvent because it has a stable index
of refraction approximating t h a t of (luartz; and thus it
permits rapid recognition and quantitative determination of the q u a r t z content of a clay-mineral sample.
Nitrobenzene has been found to penetrate clay aggregates as well as aniline and does not participate in oxidation-reduction reactions with clays as does aniline. I t
is characteristic of the acid-base staining reactions t h a t
colors develop in the absence of any water other t h a n
t h a t present as an adsorbed film on clay samples in the
oven-dry condition. Samples should be dried immediately before making the test so t h a t the effective concentration of hydrogen ions in the adsorbed water film m a y
be as high as possible. If this is not done even the hydrogen montmoriUonoids m a y be stained the basic color of
the dye. On the other hand, colors are developed by oxidation-reduction reactions only in the presence of bulk
water. Another point of difference is the speed with
which characteristic colors are developed. I n acid-base
reactions the colors develop immediately whereas the full
development of color by oxidation-reduction mechanisms
may take from 5 minutes to a n u m b e r of hours.
P r i o r acid t r e a t m e n t of clay samples leads to uniformity in the characteristic colors produced by aniline
dyes on different clay minerals. The colors used for
identification thus are those developed by hydrogen
clays. Since the acidity of clay-mineral surfaces is
largely a function of the amount of exchangeable hydrogen, it follows t h a t characteristic colors produced by
various clay minerals in acid-base color tests are dependent on the respective base-exchange capacities. These
conditions obtain, and the test is characteristic, if the
p r o p e r concentration of dye solution is used together
[Bull. 169
with the proper relative amount of clay sample a n d
volume of dye solution. If an excess of dye is used for
a given clay sample, the clay granules will be s a t u r a t e d
with the basic color of the dye even in the presence of
hydrogen montmorillonoids which produce the acid color
under proper test conditions. This eff'ect is the result of
base exchange, the replacement of hydrogen ions in exchange positions by dye molecules.
Th(> necessity for adjusting the amount of d.vestnff
added to the sample at h a n d makes it impractical to
attempt to identify very small amounts of montmorillonoids admixed with clay minerals of markedly lower
base-exchange capacity, ilielenz and K i n g (1951), however, have been able to identifj' niontmorillonoids in concentrations of as little as 5 percent. Sensitivity of the
dye tests for montmorillonoids is also a function of the
size of the granules in the sample. The smaller particles
of montmorillonoids maj^ be saturated with the basic
color of the dyestnff' whereas the larger grains are
stained with the desired acidic color. Detailed jirocednres
for the use of malachite green and safranine " Y " have
been described by Mielenz, King, and Schieltz (1950)
and Mielenz a n d K i n g (1951).
An outstanding advantage of the acid-base tests is
t h a t they may be used for semiquantitative determinations of the various minerals present in a clay sample.
Using nitrobenzene as a dye solvent, it is possible to
identif.v many mineral constituents with the petrographic microscope. Samples are p r e p a r e d on microscope
slides with cover glasses. The numbers of grains of the
various minerals admixed with clays and the numbers
of clay granules or aggregates stained each color m a y
be counted to estimate the amounts of the various constituents present.
The acid-base tests employing malachite green and
safranine " Y " are the best color tests available for identification of minerals belonging to the kaolin group and
probably are applicable to a wider variety of samples
t h a n the oxidation-reduction color tests. Heetorite samples, however, are decomposed completely by the necessary acid treatment. The tests are useful for the identification of montmorillonoids if samples contain large
amounts of these minerals, but are unsuitable if only
small percentages are present. The acid-base dye tests
should be u-sed in eon.iunetion with one or more of the
oxidation-reduction color tests in order to permit more
complete and accurate identification of unknown samples.
Hendricks and. Alexander's
Benzidine Test.
I n 1940
Hendricks and Alexander (1940) described a qualitative
procedure for the identification of members of the niontmorillonoid group. The test depended on the adsorption
and oxidation of benzidine on clay mineral surfaces.
Bisenack (19,38) and H a u s e r and Leggett fl940) indep e n d e n t l y described cla.v color changes resulting from
oxidation-reduction reactions, b u t they did not propose
to use them as identification methods. Hendricks a n d
Alexander found t h a t any diamines capable of producing
colored semiqninone forms on oxidation exhibit the same
colors after adsorption a n d oxidation on montmorillonite
surfaces. Semiquinones are " o d d e l e c t r o n " compounds,
free radicals stable in aqueous solution, which are formed
by the loss or addition of one electron at one end of a
Part III]
107
M E T H O D S OF IDEXTIFYIXG CLAYS AND INTERPRETATION OF R E S U L T S
system of fonjuo-atpcl double bonds. Semiquinones are
stabilized by resonance energy and usually are colored.
Tliey are stable only in restricted rantres of p l l . Kenii(juinone cations result fi'om one-electron oxidation of
various aromatic diamines, phenols, and certain other
orji-anie compounds. The chemistry of scmicpunoncs has
been discussed in detail by Miehaelis (1935).
H e n d r i c k s aud Alexander believed their test was the
result of fortuitous oxidation of a very small amount of
diamine to the semi(iuiuone form on a montmorillonite
surface. They found that ferric iron salts and or<i'anie
m a t t e r were capable of producing the blue color in the
absence of montmorillonoids. They also found that manganese dioxide was a strong enough oxidizing agent to
cause color formation. The inorganic interferences coidd
be removed by prior t r e a t m e n t with hydrochloric acid
and the organic m a t t e r by peroxides. Keducing agents
such as ferrous iron salts and stannous chloride were
found to inhibit formation of the blue color. I n addition
to the presence of an oxidizing agent, it was necessary
t h a t the reagent be adsorbed, presumably on niontmorillonoid 001 lattice planes. H e n d r i c k s and Alexander
found prior adsorption of large molecules such as codeine
or brueine on montmorillonite samples prevented formation of the blue color. The benzidine test has been the
target of much criticism as a result of its indefinite nat u r e (Bndell et al., 1 9 4 1 ; Kriiger and Oberlies 1943;
P a g e 1 9 4 1 ; Siegl 1945). Experience with the test has
shown that positive tests for montmorillonoids can be
obtained with clay samples know'n to contain n o n e ; and,
conversely, negative tests have been obtained with
known montmorillonite samples.
Difficulties encountered in application of the benzidine
test to u n k n o w n samples probably result from the apparently low oxidation potential of benzidine in n e u t r a l
aqueous solution and from anomalous adsorx)tion effects.
]\Iany relatively weak oxidizing agents are capable of
removing one electron from benzidine in n e u t r a l aqueous
solutions. The presence of such substances in non-montmorillonoid clays would result in false positive tests.
Bndell, Zorn, and Hofmann (1941) thought t h a t members of the nioutmorillonoid g r o u p containing large
amounts of magnesium or aluminum do not produce blue
colors with benzidine, or produce only very weak colors.
I n agreement with their findings, the writer and others
(Mielenz a n d K i n g 1951) have found that p u r e hectorite
samples produce little or no blue color when treated with
benzidine. On the other hand, certain pure, typical montmorillonite samples such as A P I 11 and 30 from Santa
Rita, New Mexico, and A P I 24 from Otay, California
( K e r r et al. 1949), also give weak or negative benzidine
tests. Kerr, Kulp, a n d H a m i l t o n (1949) calculated, according to the method of Ross and Hendricks (1945),
the amount of aluminum in t e t r a h e d r a l co-ordination for
] 0 of the A P I collection of montmorillonoid samples.
The above-named montmorillonite samples which give
v e r y weak or negative benzidine tests, A P I 11, 24, a n d
30, and hectorite, A P I 34a, were found to contain essentially no a k u n i n u m substituted for silicon in the tetrahedral layers. This suggests t h a t oxidation of benzidine
to the semiquinone form in n e u t r a l solution can occur on
001 lattice planes of montmorillonoid minerals only at
those active spots where silicon has been replaced by
aluminum.
Mielenz and King's Modified Benzidine Tesi. Recently
Mielenz and King (1951) have siiggested an improved
modification of the benzidine test which they have effected
by ad,iusting the p l l of the reagent solution in contact
with a clay sample to approximately one. W i t h montmorillonoid clays, this results in a lemon-yellow particle
color r a t h e r t h a n the purple-blue obtained in n e u t r a l
solution. The yellow colors also are stable upon drying.
This permits mounting the dried granules on a microscope' slide in immersion oil for semi-(juantitative determinations in the same nmnner as the acid-base tests. The
newer benzidine test is much more reliable t h a n the
older one. It gives positive tests with known montmorillonoid samples and negative tests with samples known
not to contain mendiers of the montmorillonoid group.
A tentative reaction equation has been proposed by
AYeiss (1938) to describe the course of oxidation of benzidine to its semiquinone.
H=N
•
[0]
-le-
NH2
Blue cation?
I t would be of interest to determine if a different cationic, yellow semiquinone is stable in acid solution at
a p l l of about 1 such t h a t :
H=N-
-Nm + n
-xm
Yellow cation?
Identification of the blue- a n d yellow'-colored substances
necessarily is tentative. Clarification of the n a t u r e of
benzidine semiquinones stable in various p H ranges
awaits precise potentiometric studies.
I n addition to identification of montmorillonoid minerals, Mielenz and K i n g have found, empirically, t h a t
their modified benzidine procedure permits diff'erentiation of kaolinite from anauxite. Anauxite is stained
yellow whereas kaolinite is not aft'ected. This is useful
in conjunction with acid-base dye tests.
Ilamhleton
and Dodd's Para-Amino
Phenol Test.
In
1951 Hambleton a n d D o d d (1953) described the use of
2J-amino phenol as a reagent for the identification of
clay ndnerals by an oxidation-reduction mechanism. Test
procedures were developed using various concentrations
of alcoholic solutions of the p-amino phenol reagent, followed by d r y i n g of the alcohol solvent, t r e a t m e n t of the
sample with 1 : 1 hydrochloric acid, and observation of
the sample wet and after d r y i n g . Reflected light was
most satisfactory. W h i t e porcelain spot plates, a stereoscopic microscope, and a reproducible intense light
source were the (mly equipment items needed. Characteristic colors ^^ ere obtained only in the presence of a highly
acidic solution. Another test procedure using a s a t u r a t e d
aqueous solution of the jj-amino phenol reagent in a
m a n n e r similar to the newer benzidine test of Mielenz
and K i n g was tried, b u t it was le.ss successful.
The new ^^-amino phenol test is most sensitive and
accurate for the montmorillonoids and the h v d r o u s mica
CLAYS AND CLAY
108
minerals. It is less satisfaetory for members of the kaolin
group when these occur in mixtures with other clays.
The test is particularly sensitive to relatively small concentrations of montmorillonite or members of the montmorillonoid group in mixtures of clay minerals. Tentative equations for the oxidation of 2-'-amino phenol to
semiquinone forms are wi'itten below:
•XIL
:XIL
[<)1
^,
J
+ H
-to>
:OII
•XII
i
J
-XH.
•
:.OII
Blue
cation?
:XII:.
:OII
^ 11
Blue cation?
•on
•.OH
The p-amino phenol reagent is not oxidized to characteristic, blue colors by ferric iron or extraneous organic
matter, although there is some evidence that the presence
of manganese dioxide will give anomalous tests when
present in clay-mineral samples. The new reagent appears to have a higher oxidation potential in acid solution t h a n does benzidine in n e u t r a l solution.
The most sensitive montmorillonoid test is obtained
by use of a 0.1 percent alcoholic solution of p-amino
phenol as a test reagent. A d r o p of this solution is applied to about 3 to 5 cubic nullimeters bulk volume of
clay sample in a depression of a white porcelain spot
plate, and the sample is stirred with a toothpick. After
evaporation of the alcohol solvent, the dried grains are
mixed with a t h i n glass s t i r r i n g r o d ; and one drop of
1 : 1 hydrochloric acid is added. After 10 or 15 minutes
the color of the wet sample is observed. Members of the
montmorillonite family are identified by the presence
of a characteristic d a r k blue edging a r o u n d clay granules
or aggregates.
If 0.5 percent and 2 x:)ercent reagent solutions are
applied to separate portions and the same procedure
is followed, it is found t h a t hydrous micas are colored
characteristic m u r k y greens, olive greens, browns, yellow
greens, or tans on the surface of the main mass of the
clay sample. This test often is definitive for the almost
invariablj' i m p u r e hj'drous micas, but it should be confirmed by acid-base tests.
If 4 percent or 2 percent solutions of the p-amino
phenol reagent are used in these tests and the samples are
allowed to d r y following application of 1 : 1 hydrochloric
acid, it is found t h a t blue or purple-blue colors are characteristic of montmorillonoids and t h a t various shades of
pink are characteristic of members of the kaolin group
iniless the latter are present in relatively small amounts.
W h e n samples are stained with s a t u r a t e d aqueous solutions of the 2'-amino phenol reagent less satisfactory
differentiations are possible, although clay granules may
be colored more intensely.
One advantage of the j^-amino phenol color tests is
t h a t they generally m a y be applied without a n y previous
chemical p r e p a r a t i o n of the sample. The best procedure
is to p r e t r e a t u n k n o w n samples with dilute hydrochloric
acid to remove excess ferric salts a n d anomalous oxidizing
TECHNOLOGY
IBull. 169
agents such as manganese dioxide. "When the u n k n o w n
sample contains hectorite or one of the nontronites, the
usual p r e t r e a t m e n t with 1: 1 hydrochloric acid will decompose hectorite r a p i d l y a n d nontronites slowly. F o r
this reason it is advisable to test for the presence of
montmorillonoids before making any routine p r e t r e a t ment with acid. The time required to examine the sample
for the presence of montmorillonite minerals nsing a 0.1
pei'cent solution of ^j-amino phenol is so short after the
addition of the hydrochloric acid, only about 10 miniites,
t h a t neither hectorites nor nontronites decompose before
a characteristic montmorillonoid test has been observed.
I t seems reasonable to assume that color intensities
observed in oxidation-reduction staining tests are a function of the relative densities of adsorbed colored semiquinone cations. W i t h mixed-layer minerals in which
only a fraction of the total area of 001 lattice planes
resembles montmorillonoid (crystal surfaces, less intense
colors would be expected. The stain would not penetrate
those 001 surfaces bonded by potassium ions as in the
micas. I n the case of relatively p u r e illite samples, t h e r e
would be no intervening 001 planes Avhich the stain could
penetrate between exterior surfaces. Although these
exterior surfaces might resemble montmorillonoid planes,
they would constitute only a small percentage of the
total 001 planes. T h u s there would not be enough of the
colored semiquinone cations adsorbed to produce an
over-all color effect similar to t h a t observed in the
presence of montmorillonoid minerals. The result would
be an opaque color which would be a mixture of the semiquinone color and the body color of the sample. B y
noting these variaticms in color, it should be possible to
identify illite samples by empirically determining the
colors produced with known illites after the removal of
extraneous coloring material such as ferric salts. This
has been done in the development of the p-amino phenol
procedures. Although not detectable by X - r a y diffraction, some penetration of 001 montmorillonoid planes by
adsorbed reagents probably occurs.
Nature of the Oxidizing Agents on Clay Mineral Surfaces.
Hendricks a n d Alexander (1940) and AVeilMalherbe a n d Weiss (1948) have assumed that the
oxidizing agent responsible for the one-electron oxidation of benzidine to its blue semiquinone on montmorillonite surfaces is lattice-bound ferric iron. Other
oxidizing agents probably contribute to formation of the
blue benzidine color also. On the other hand, reagent
solutions of p-amino phenol are not oxidized by ferric
ions in acid solution, and montmorillonoid clay-mineral
samples which contain essentially no iron cause formation of the blue semiquinone color when treated with
p-aniino phenol and hydrochloric acid. One or more
oxidizing agents other t h a n ferric iron must be responsible for semicpiinone formation by p-amino phenol, a n d
these same agents probably play a role in the oxidation
of benzidine.
The empirical n a t u r e of oxidation-reduction color tests
is in p a r t the result of ignorance concerning oxidation
mechanisms which can account for the characteristic
formation of colored semicpiinone cations only on montmorillonoid minerals. E x p e r i m e n t a l results suggest there
must be a p a r t i c u l a r kind of oxidant associated with the
silicon t e t r a h e d r a l layers of montmorillonoids, b u t n o t
METHODS OF IDKXTIFYING CI-AYS AND INTERPRETATION OF R E S T L T S
Part III]
with those of the kaolin minerals, pj^rophyllite, or talc.
The most outstanding s t r u c t u r a l differences between
these minerals result from isomorphous substitutions in
the montmorillonoids of a l u m i n u m for silicon in the
t e t r a h e d r a l layers a n d of magnesium, and other divalent
cations for aluminum in the octahedral layer. These
substitutions are the cause of the inherent electrical
charge on the three-layer clay minerals which results in
their capacity to adsorb cations on 001 lattice planes.
W h e n silicon is replaced by a l u m i n u m in the t e t r a h e d r a l
layer, there is an excess of electrons associated with the
oxygen t e t r a h e d r o n s u r r o u n d i n g the aluminum ion.
^Yheu magnesium is substituted for aluminum in the
octahedral layer, the resulting electron excess is distributed more diffusely in oxygen t e t r a h e d r a on each side
of the substituted site. I n each ease active spots probably
are formed on 001 lattice planes. Oxide ions associated
with these spots are potential donors of electrons. E x changeable cations are adsorbed over these sites; nonexchangeable potassium ions are embedded within the
open oxygen hexagons adjacent to the activated tetrahedra. Large cations such as the semiquinones of benzidine or p-amino phenol must be adsorbed and held by
both ionic and van der Waals forces over one or more
active spots.
Recently W. A. AYeyl (1949) has postulated t h a t the
surfaces of clays and finely divided silica exert a
". . . strongly oxidizing effect wliicli is comparable with that of
ozone. This effect is very strong for the dust of freshly ground
(]uartz. silica gel, clay, and for some other silicates and persists
even in the presence of moisture. The o.xidizing effect of SiOa
is enhanced by light, elevated temperature and is proportional
to the surface. The quantity of oxygen available is small and
its detection requires .sensitive methods. The oxidation potential, however, is very high, as can be seen from the fact
that silver is oxidized to its peroxide."
"Weyl a t t r i b u t e d the specific toxicity of silica a n d clay
dust in the lungs, t h a t is, silicosis, to this powerful
oxidizing effect, and t h u s explained the eft'ectiveness of
reducing agents such as aluminum metal dust in therapeutic treatment. Weyl claimed t h a t a specific reagent,
4^ 4'^ 4"-hexamethyl triamino t r i p h e n y l methane, indicated the presence of " a t o m i c o x y g e n " on silica gel at
room t e m p e r a t u r e and on finely divided silica or clay at
150° C. Ilauser, Le Beau, and P e v e a r (1951) have proposed that the release of atomic oxygen on clay surfaces
is responsible for oxidation-reduction color reactions.
AVeyl ^ considers t h a t all oxide surfaces undergo O2—O2'
ion exchange, one step of which is a one-electron transfer
to a chemisorbed oxygen molecule.
I n the absence of any more plausible theory, the writer
proposes a tentative hypothesis based on the assumption
t h a t activated oxygen ions in montmorillonoid 001 lattice
planes are capable of t r a n s f e r r i n g an electron to adjacent
adsorbed oxygen atoms to form superoxide or perhydroxyl ions, O-.r. This reaction might be represented by
the following e q u a t i o n :
O2
Adsorbed
oxygen
molecule
+
0"
.Activated eleefrniidonor oxide ion
bound in tetrahedral
lattice layer
> 0-2
.Available
oxidizing
agent
(superoxide ion)
^ P e r s o n a l communication, A u g u s t 1-S, 10r>l.
+
0Bound in
tetrahedral
lattice
layer
109
The concentration of superoxide ion oxidizing agent on
001 montmorillonoid planes would be greater if the elect r o n excess resulted from substitution of a l u m i n u m for
silicon in t e t r a h e d r a l layers. Thus, in n e u t r a l solution it
ai^i^ears t h a t the effective oxidation potential would be
too low to oxidize benzidine to its blue semiquinone
cation unless there were such t e t r a h e d r a l layer substitution. If the montmorillonoid sample contained no alumin u m in the t e t r a h e d r a l layer, the oxidation potential in
n e u t r a l solution apparentlj- would be lower t h a n t h a t of
the ferric-ferrous ion couple.
I n acid solution, however, the oxidation potential of
superoxide ion would be distinctly h i g h e r ; a n d this
greater potential would be capable of oxidizing benzidine
to its yellow semiquinone and p-amino phenol to its blue
semiquinone. I t is necessary, also, to assume t h a t the
blue semiquinone form of benzidine is stable only in
n e u t r a l and basic solutions, whereas its yellow form and
the blue semiquinone of p-amino phenol are stable only
in acid solution. I n acidic solution the oxidation iiotetitial
a p p a r e n t l y is sufficient to cause semiquinone formation
even when the positive charge deficiency arises from octah e d r a l layer substitution. Thus hcctorite produces semiquinone colors in acid systems (before decomijosition)
b u t not in n e u t r a l solutions.
I t is possible t h a t other reagents besides benzidine and
p-amino phenol may be found suitable for oxidationreduction color tests. The selection of these two reagents
is based on the empirical observation t h a t their semiquinones are sufficiently stable in a p l l range which may
be utilized in test procedures. A p p a r e n t l y the blue semiquinone of p-amino phenol is stable onlj^ in a much more
acid medium t h a n the yellow benzidine semiquinone.
Color Test Procedures.
Mielenz a n d King (1951) have
developed a s t a n d a r d procedure for the identification of
clay minerals by staining tests. They emplo.v characteristic differences in birefringence to differentiate between
the two- and three-layer clay minerals, followed by observation of the colors produced by malachite green and
safranine " Y " dye solutions. I n this m a n n e r they detect
divisions in the kaolin group, t h a t is, kaolinite a n d anauxite, dickite and nacrite, and thehalloysites. Pleochroie
effects aid in distinguishing kaolinite and ananxite from
dickite and nacrite. The acid benzidine test m a y be used
(with a different sample) to differentiate kaolinite and
anauxite.
Montmorillonoids are recognized if present in amounts
greater t h a n 5 to 10 percent, and glauconite and eeladonite fail to stain sufficiently to mask their yellow or
greenish-yellow body colors. The acid benzidine test then
may be applied to distinguish small percentages of
montmorillonoids. Experience is necessary to distinguish
between illites, vermicnlites, a n d palygorskite or a t t a p u l gite. I n place of the acid benzidine test at this stage of
the analysis, 27-amino phenol can be used to confirm the
presence of montmorillonoids, including hectorite, a n d
to aid in determining the hydroiis micas. I n addition,
27-ainino phenol is helpful in identifying halloysites by
producing characteristic mottled brown granule edges,
and it may aid in confirming other members of the kaoliji
group.
Finally, a word of caution is in order. Before applying staining tests as a new procedure, it is advisable to
110
C L A Y S AND C L A Y
observe results obtained -with k n o w n m i n e r a l
Experience is indispensable.
samples.
Discussion.
S t a i n i n g test procedures described in this
Ijaper generally are too involved to p e r m i t r e a d y a d a p t a tion to field identification of u n k n o w n samples. Much of
the unwillingness to a p p l y staining tests stems from unf o r t u n a t e experience accjuired when the tests were used
in the absence of information concerning t h e i r limitations. Development of color tests has reached the point
today where reliable l a b o r a t o r y a n a l y t i c a l p r o c e d u r e s can
be utilized to save time a n d expense. N u m e r o u s laboratories, such as the B u r e a u of Reclamation P e t r o g r a p h i c
L a b o r a t o r y in Denver, have developed r o u t i n e procedures
which have proved to be nu)st dependable in t h e h a n d s of
p r o p e r l y t r a i n e d analysts. Difficulties in the use of staining tests have arisen when general " s h o t g u n " applications of one overly simplified p r o c e d u r e have been made
to complex u n k n o w n clay samples.
E n o u g h develo])nient work has been done a t this time
to p e r m i t a n y clay l a b o r a t o r y to set u p generally reliable
procedures for the identification of most of the known
clay minerals. M a n y cdiecks and counterchecks have been
developed to improve the accuracy of the tests. If the
color tests are p r o p e r l y used in conjunction w i t h petrog r a p h i c a n d other more complex analytical procedures,
they will be found to be a most helpful a d d i t i o n to the
tools of the clay technologist.
I n a d d i t i o n to wider application to analysis, there is a
possibility t h a t color tests, p a r t i c u l a r l y those based on
oxidation-reduction reaction mechanisms, will be useful
in elucidating differences in s t r u c t u r e among the threelayer clay minerals. Less is k n o w i \ c o n c e r n i n g the h y d r o u s
mica clays t h a n those of the kaolin or montmorillonoid
g r o u p s . G. B r o w n (1951) has p r e s e n t e d t h e h y d r o u s
micas as a contiiuions series of m i n e r a l s between the wellcrystallized micas a n d the e x p a n d i n g montmorillonoids
a n d vermiculites. Mixed-layer m i n e r a l s were included
w i t h other h y d r o u s micas. The use of r e a g e n t s such as
benzidine, p-amino phenol, and other similar compounds
which can be oxidized to colored, stable semiqiiinone
cations m a y be used to differentiate between h y d r o u s
micas of v a r y i n g c r y s t a l l i n i t y a n d to detect mixed-layer
m i n e r a l s c o n t a i n i n g various amounts of montmorillonoid
or vermiculite layers. Such differentiations m i g h t be
made b y s t u d y i n g t h e v a r i a t i o n s in color p r o d u c e d w i t h
a given reagent.
Acknowledgment.
This p a p e r is based on work done
while t h e w r i t e r was employed a t the P e t r o l e u m E x p e r i ment S t a t i o n of the IJ. S. B u r e a u of Mines, Bartlesville,
Oklahoma. T h a n k s are due D r . C. W . Seibel, Director
Region V I , IT. S. B u r e a u of Mines, a n d to t h e managem e n t of t h e C o n t i n e n t a l Oil Company, for permission
to publish this p a p e r .
DISCUSSION
J. L. Hall:
Could differentiation between kaolinite and anau-xite by the dye
adsorption technique be discussed in more detail?
M. E. King:
An aqueous solution of l)enzidine is added to
sample. After the sample is dried, it is acidified
and again dried. When such a treated sample is
mersion oils and is viewed under a petrographic
an acid-treated
with 1 : 1 HCl
mounted in immicroscope and
TECHXOIJOGY
[Bull. 169
transmitted light, anauxite is lemon-yellow whereas liaolinite is
colorless. This procedure is desoi-il)ed in detail in a recent paper
by Jlielenz and King (1051). The method has been eheelved on
anauxite from the type locality at liilin. Czechoslovakia, and
from tJie lone formation of California.
B. B. Osthaus:
^Vhat method is used to remove iron compounds and organic
matter?
M. E. K i n g :
A 1 : 1 solution of water and hydrochloric acid is used
move iron compounds and a solution (}f hydrogen peroxide
move organic nuitter. However, caution must he used in
ment of samples thought to contain minerals susceptible to
down in this concentration of acid.
to reto retreatbreak-
R. E. Grim:
How successful are you in using the dye tests for the identification of clay minerals in samples which are an intimate mixture
of various clay minerals? For example, in a mixture of kaolinite,
illite, and montmorillonite? What is the sensitivity of the dye
test?
M. E. K i n g :
A combination of the various dye tests generally enables a satisfactory separation of clay minerals in such a mixture. The main
difficulty arises in detection of clay minerals not reacting positively
in the tests where they are intimately admixed in a very finely
divided condition with clay minerals which react positively. No
difficulty is experienced with mixtures if the particles are large
enough for easy inspection microscopically. An illustration of the
application of staining tests is the differentiation of illite from
montmorillonite by the combination of Dodd's test using paraamino phenol with the benzidine test. If a material develops a
deep prussian-blue <'olor with para-amino plienol, and shows no
change in color.after acidification and the addition of a drop of
benzidine, then the tests indicate that the material consists of
montmorillonite. If the color disappears entirely or changes to a
very light pink, then the tests indicate that the material contains
illite or hydrous mica.
With the use of malachite-green and safranine "Y,'' identification of montmorillonite is reliable if montmorillonite constitutes
more than 5 percent of the sample, but with the modified benzidine
test even one particle of montmorillonite in a mixture can be
identified. In general, the dye tests are as sensitive as are any
of the other methods.
Isaac Barshad:
Did you ever use dye tests to differentiate the different mica
minerals or a K+-saturated montmorillonite and a K+-saturated
illite?
M. E. King:
Yes, we found that muscovite, biotite, phlogopite, and lepidolite, for example, do not react with any of the dyes regardless
of the p l l . All of our dye tests are carried out in an acid medium ;
therefore, even thoTigh montmorillonite might be K+-saturated to
begin with, it becomes H*-saturated and reacts with dyes. K+saturated illite, after acidification, causes only a slight change in
the color of safranine "Y" or malachite-green. Moreover, when
illite treated with benzidine is acidified to pH 1 and subsequently
dried, no coloration is evident with transmitted light. Details of
these reactions are described by Mielenz and King (1951).
R. C. Mielenz:
We have come to feel that the adsorption of dye molecules by
clay minerals offers a tool whereby a better understanding of the
phenomenon of adsorption may be gained, for upon adsorption
of dye molecules not only can one measure the amount adsorbed
and the changes in the crystal lattice dimensions, but also the
color transformation in the dye indicates some change in the
organic molecule itself which may lie related in some way to the
forces with which it is bonded to the surface.
Regarding the likeness of the montmorillonite and the illite surfaces, it may be true theoretically that these surfaces should be
alike ; l)ut, since the dye tests indicate that they are not alike, the
task before us is to find out what the difference is between them
rather than to ignore it.
Part III]
^IETHODS OF IDENTIFYING CLAYS AND INTERPRETATION
V. T. Allen:
H a s tiiiyoiie hail difficulties w i t h t h e b e n z i d i n e test on the mine r a l s of t h e kaolin Kroup? W e h a d s e v e r a l kaolins which by all
o t h e r t e s t s c o n t a i n e d no m o n t n i o r i l l o n i t e , b u t t h e s a m p l e still gave
a blue test w i t h benzidine.
M. E. King:
I observed t h e s a m e t h i n g w i t h a k a o l i n i t e from i l a c o n . ( l e o r g i a .
b u t X - r a y a n a l y s i s revealed t h a t the s a m p l e c o n t a i n e d a b o u t 5
p e r c e n t m o n t n i o r i l l o n i t e . t h u s e x p l a i n i n g t h e anomah)Us r e s u l t obt a i n e d w i t h the dye test.
I n kaolin s a m p l e s which do not c o n t a i n a n y m o n t m o r i l l o n i t e b u t
still give a n a b n o r m a l color t e s t w i t h benzidine, the a b n o r m a l i t y
m a y possibly be associated witli the degree of tineness of the m i n eral g r a i n s . A high degree of fineness ma.v i n d i c a t e the p r e s e n c e
of a considerable numl)er of b r o k e n b o n d s which nia.v c a u s e oxid a t i o n of t h e benzidine, b u t t h e blue color t h a t r e s u l t s is never as
i n t e n s e as t h a t iiroduced b.v m o n t n i o r i l l o n i t e . Such possible diffic u l t i e s m a y be overcome by acidifying t h e clay saniide d u r i n g t h e
t e s t to a i i H of a b o u t 1. T'pon subseiinent dr.ving of t h e s a m p l e ,
k a o l i n i t e will not show a n y blue or yellow coloration in t r a n s m i t t e d light u n d e r the p e t r o g r a p h i c m i c r o s c o p e .
Padraic Partridge:
W i l l an i n c r e a s e in surface a r e a of kacdinite. which m a y r e s u l t
u p o n g r i n d i n g , e x p l a i n some of t h e a b n o r m a l test colors as well a s
t h e large increase in t h e c a t i o n - e x c h a n g e caiiacit,\' wliich h a v e been
mentioned ?
W. F. Bradley:
B e c a u s e of t h e h e m i m o r p h i c n a t u r e of k a o l i n i t e , it is doubtful
w h e t h e r a n y s u r f a c e s should really be called aluKU'iiial. E a c h
c r y s t a l m u s t of necessity p r e s e n t both an oxygen and a h y d r o x y l
s u r f a c e in a d d i t i o n to less e x t e n s i v e edges. T h e large i n c r e a s e in
t h e b a s e - e x c h a n g e c a p a c i t y of k a o l i n i t e u p o n g r i n d i n g , a s in t h e
e x i i e r i m e n t reiiorted by Kelle.x', is not necessaril.v due to an increase
in surface a r e a . A l t h o u g h a n i n c r e a s e in s u r f a c e a r e a u n d o u b t e d l y
r e s u l t s from g r i n d i n g , the new s u r f a c e s w h i c h a r e exposed would
m o s t likely lie a d d i t i o n a l oxygen a n d h y d r o x y l surfaces p a r a l l e l to
t h e ()f)l p l a n e ; a n d since such s u r f a c e s a r e electrically n e u t r a l , t h e
l a r g e i n c r e a s e in t h e base-exchange c a p a c i t y is due most likely to
some o t h e r }>henonienon which s i m u l a t e s b a s e e x c h a n g e .
Isaac Barshad:
I s it possible t h a t a p e r m u t i t e - l i k c s u b s t a n c e is formed from t h e
d e c o m p o s i t i o n p r o d u c t s of k a o l i n i t e released d u r i n g g r i n d i n g ? T h a t
k a o l i n i t e decomposed d u r i n g g r i n d i n g w a s revealed by differential
t h e r m a l a n d X - r a y a n a l y s e s . T h i s decomposition is revealed in
D T A liy a r e d u c t i o n in t h e m a g n i t u d e of t h e e n d o t h e r m i c b r e a k
c h a r a c t e r i s t i c of k a o l i n i t e and in X - r a y a n a l y s i s b.v a w e a k e n i n g
in t h e i n t e n s i t i e s of diffrjiction lines of t h e k a o l i n i t e p a t t e r n . I see
no r e a s o n to q u e s t i o n the possibility of t h e f o r m a t i o n of a p e r m u tlte-like m a t e r i a l of high e x c h a n g e c a p a c i t y from the released silica
and alumina.
W. P. Kelley:
C a n p e r m u t i t e be produced from a m i x t u r e of silicon dioxide a n d
a l u m i n u m oxide w i t h no o t h e r e l e m e n t s p r e s e n t except w a t e r ?
G. A. Mickelson:
B y b r i n g i n g t o g e t h e r v a r i o u s p r o p o r t i o n s of t e t r a e t h y l o r t h o silicate a n d a l u m i n u m isopropoxide, C. L . T h o m a s ( 1 9 4 9 ) p r o d u c e d
a whole series of p e r m u t i t e - l i k e s u b s t a n c e s of v a r y i n g silicaa l u m i n a r a t i o s r a n g i n g in e x c h a n g e c a p a c i t y from 100 to 600 me.
p e r 100 g. A l t h o u g h t h e r e w e r e n o c a t i o n s s u c h a s X a * a n d K+
p r e s e n t , I do not see w h y a l u m i n u m ions, simple or complex,
c a n n o t a c t a s e x c h a n g e a b l e c a t i o n s to satisfy t h e exchange p o s i t i o n s
due to t e t r a h e d r a l a l u m i n u m ions c h a r a c t e r i s t i c of such p e r m u t i t e s .
OF R E S U L T S
SELECTED
in
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