R E F E R E N C E C H L O R I T E C H A R A C T E R I Z A T I O N FOR
C H L O R I T E I D E N T I F I C A T I O N I N SOIL CLAYS
R. TORRENCE MARTIN
Massachusetts Institute of Technology
ABSTRACT
Literature pertaining to differential thermal and X-ray diffraction of chlorite minerals
is reviewed. Optical, DTA, and X-ray data for eleven chlorite samples of clinochlore,
prochlorite, thuringite, corundophilite, and leuchtenbergite are given. The effect of
particle size (105 t o - - 1 ~) on DTA, X-ray diffraction, glycol retention, and cation
exchange capacity are given for two thuringites, one clinochlore, and one prochlorite.
Identification of chlorite by DTA in a soil clay containing a mixture of minerals is
improbable at the present time except under very favorable circumstances. However,
for relatively pure chlorite samples, variations in chemical composition are reflected in
the differential thermal curves. The largest change in the thermogram is produced by
ferric iron which lowers the peak temperature from 720~ C to 610~ C. Differences in
thermal behavior between low and high ferric iron chlorite species are maintained for
any given particle size; Chlorite thermograms obtained by different investigators show
much greater variation than the differences in thermograms for other clay minerals
determined on different equipment.
X-ray diffraction can be used to positively identify chlorite in a soil clay, (a) by
careful analysis of reflections at least as great as 14 A, and (b) by the influence heat
treatment (550 ~ C for 30 minutes) has on the X-ray pattern. Heat treatment produces
marked changes in the X-ray pattern of the finer particle size samples and the magnitude of the change effected is greater for high iron chlorites (thuringite) than for low
iron chlorites (clinochlore and prochlorite). Olivine is not the recrystallization product
for thuringite. The smallest size fractions show no tendency toward vermiculite or
montmorillonoid.
Cation exchange capacity for silt size chlorites varies from 4 to 32 m.e./100gm., and
for --2 ~ chlorite particles from 30 to 47 m.e./100gm. Cation exchange capacities
for --2 ~ and --1 ~ chlorites are essentially the same.
Ethylene glycol retention increases with decreasing particle size. Glycol retention
for --2 ~ chlorite samples varies from 25 to 40 mg. glycol/gm, clay. For --1 /z chlorite
material, glycol retention is 2 to 4 times greater than for --2 # material.
INTRODUCTION
S t r u c t u r a l l y chlorite is very closely related to the clay minerals. I n a
general m a n n e r if the K § of illite were replaced with a charged brucite sheet
the product would be a chlorite and, in fact, both illite a n d chlorite have
been produced synthetically from m o n t m o r i l l o n i t e by rather mild treatments
(Cailfire a n d H~nin, 1949a, 1949b). I n view of the relative ease with which
chlorite may be synthesized, it is not s u r p r i s i n g that as analytical methods
are refined, chlorite is f o u n d to be a fairly c o m m o n soil m i n e r a t (Jeffries,
1953; and M a r t i n , 1954).
T h e chlorite g r o u p is similar to the m o n t m 0 r i l l o n o i d clay mineral g r o u p
117
118
CHLORITE IDENTIFICATION IN S0IL CLAYS
in that the sixteen chlorite species are not radically different structurally
but are mostly variations due to isomorphous substitution. In the montmorillonoid group where the species differences are due to isomorphous
substitution, there are still rather marked variations in properties which may
seriously hinder adequate analysis. For example, the thermogram of nontronite is strikingly similar to that of illite but is entirely different from
that of montmorillonite proper. Likewise, it has been found that chlorites
high in ferric iron lose most of their hydroxyl water at a lower temperature
than do the magnesian chlorites.
Because analytical data on chlorite and particularly fine grained chlorites
are meager, they have been examined in some detail using the methods
normally employed in clay mineral investigations. To facilitate identification
of chlorites in soil clays differential thermal analysis ( D T A ) , X-ray, glycol
retention, and cation exchange capacity data for a group of chlorites embracing as wide a composition range as possible have been compiled.
Since no single survey of work already done in this field is readily available, it seems worthwhile to consider first the contributions of Orcel,
Sabatier, Brindley and All, and Hey.
LITERATURE REVIEW
The only detailed study of chlorite with the differential thermal analyzer
is the work of Orcel (1927, pp. 273-322) done before the structure of
chlorite was known. Although Sabatier (1950) has examined the influence
of particle size, the particular species he studied is not known so that the
usefulness of his data is very limited. Brindley and Ali (1950) have done
an excellent job of providing valuable X-ray criteria for the identification
of magnesian chlorites as well as explanation of their thermal behavior.
Orcel
Orcel (1927) in his comprehensive differential thermal study of 28 different chlorite samples showed that (a) the departure of H20 gave different
endothermic peaks for different chlorites, (b) the first endothermic reaction
was generally the largest, (c) the differences in the observed intensity of
thermal reaction were apparent only on the second inflection, (d) leuchtenbergite, aphrosiderite and thuringite appeared to give only one endothermic
peak. Two further observations from Orcel's chlorite thermograms might
well be noted: (1) a few of the samples exhibited exothermic peaks following the second endothermic inflection, and (2) thermograms for sheridanite, ripidolite and clinochlore were given where the second endothermic
reaction was barely visible while thermograms for other .samples of these
same species showed a second inflection nearly as large as the first.
In addition to DTA, Orcel made direct dehydration tests in a vacuum.
The most significant observation of these experiments was that even where
only one peak was observed by DTA, two definite stages were found by
R. TORRENCE MARTIN
119
direct dehydration. Orcel's conclusion that the stages were simply overlapped in the D T A process overlooks the fact that the temperature difference between the two dehydration stages for the sample showing only one
endotherm is equal to or greater than the temperature difference for samples
showing two endothermic peaks. For example, the temperature difference
between the two dehydration stages was 410 ~ C for clinochlore which has
two differential thermal endothermic peaks, and 445 ~ C for thuringite which
has one differential thermal endothermic peak. The peak temperature for
the first dehydration was 60 ~ C lower for thuringite than for clinochlore.
Orcel concluded that the degree of fineness of the powder did not influence the departure of water because aphrosiderite, ba~alite, and thuringite
crystals which had a maximum dimension less than 0.01 mm. showed the
same departure of water as other samples that had to be ground.
As a supplement to this paper, Orcel (1929) showed that the thermal
behavior of a high FeO content chlorite (ripidolite 18.7% FeO) was markedly influenced by the nature of the furnace atmosphere. The nature of this
influence, he believed, arose from the oxidation of Fe 2+ by the water; i.e.,
2FeO+H20~--Fe203+H21'. This exothermic reaction was observed at 775 ~ C
on thermograms run in vacuum or in a current of N2 but was not observed on thermograms run ,in air because air oxidizes all the FeO prior
to reaching a temperature where the above reaction can take place. Further
evidence in support of this hypothesis was later given by Orcel and Renaud
(1941) in which they made a spectral analysis of the gas emitted and
measured the pressure built up by the escaping gases. They found that the
same repidolite referred to above gave a very much greater concentration
of H2 than did a chlorite sample containing only 1.24% FeO. Although he
does not specifically state it, Orcel's work suggests that for the very high
FeO content chlorites such as thuringite (FeO content up to 40 percent),
the exothermic reaction accompanying the oxidation of Fe 2§ and the endothermic reaction associated with the second stage of dehydration cancelled
each other and thus only a single endothermic peak was observed.
Sabatier
Sabatier (1950) specifically studied the influence of crystal size on D T A
of 10 chlorites. The unground samples were approximately 0.1 mm. in
diameter and ground samples were too small to be observed with a microscope but were still definitely crystalline as evidenced by X-ray diffraction.
Sabatier reported that from his data thermograms for ground and unground chlorites were very different, and that his thermograms were different from Orcel's even for unground samples. He also reported that thermograms of ground chlorites all looked alike which was not apparent from
thermograms for unground samples. The peak temperatures for ground
samples were all the same _+50 ~ C with major endothermic peak temperature at 650 ~ C. It is Sabatier's hypothesis, which is supported by dehydration curves, that grinding assures loss of all brucite H 2 0 at a sufficiently
120
CHLORITE IDENTIFICATION IN SOIL CLAYS
low temperature so that when the recrystallization temperature is reached,
the dehydrated brucite sheet reacts with external SiO4 groups on the mica
sheet yielding an exothermic peak. Sabatier's experiments indicated that
particle size was the major factor controlling the thermal curves on the
different chlorite species.
Brindley and Ali
Brindley and All (1950) examined the effect of heat on the crystal structure of three magnesian chlorites. During the first dehydration stage, 50
percent of the removable water was lost and the intensities, especially of
basal reflections, showed marked changes; the first order reflection became
very intense while the second and third order reflections were weakened.
Fourier analysis showed that the loss of half the removable water was accompanied by a migration of the Mg ions of the brucite sheet towards the
hydroxyls of the brucite sheet.
The second dehydration stage, during which the chlorite structure disappears, is accompanied by the appearance of an olivine phase with a high
degree of preferential orientation. As the alumina content of the magnesian
chlorites increases, the temperature at which the above change takes place
also increases.
In contrast to the similarity of the X-ray results for the three minerals,
the thermograms show surprising differences. The first endothermic reaction corresponding to the first dehydration stage is the only constant feature
of the thermograms. Penninite and clinochlore have exothermic peaks about
8 3 0 ~ corresponding to olivine crystallization from dehydrated chlorite
but the second endothermic peak, attributed to second stage dehydration, is
absent from clinochtore. Sheridanite has a very large second endothermic
reaction about 850 ~ C which would correspond to the second stage dehydration. Although not mentioned by Brindley and Ali, the small exothermic
bump about 925 ~ C may be the olivine crystallization reaction in sheridanite
because they did not observe olivine by X-ray until 950 ~ C, which is 100 ~ C
higher than for clinochlore. Brindley and Ali (1950, p. 30) bring up the
point already mentioned that Orcel's explanation of overlapping thermal
peaks where only one endothermic reaction was observed is unlikely because
from X-ray data the two dehydration stages appear to be just as well
separated in these samples as in samples where two endothermic peaks
do occur.
The persistence of a modified chlorite structure after the first dehydration stage and the appearance of a new phase after the second dehydration
stage, confirm earlier data of Orcel and Caill~re (1938).
Hey
A recent review (Hey, 1954) provides, at least, a clue as to the relative
abundance of the different chlorite species. Although more than 100 specimens were classified, 75 percent of all occurrences reported were of four
R. TORRENCE MARTIN
121
species. The high iron chlorite thuringite (_>4.0% Fe2Oa) accounted for
one fourth of all occurrences and the tow iron chlorites clinochlore, ripidolite, and sheridanite ( < 4 . 0 % Fe~Os) accounted for one half of all occurrences.* These three low iron chlorites and prochlorite are very similar in
composition and the differences among them are smaller in magnitude than
the differences within the single species thuringite. Thus, differences between the various low iron chlorites are small compared to the differences
between the low and high iron chlorites, and the differences within the
thuringite species may exceed the differences within the low iron chlorite
group.
Summary
Chlorites have been found to dehydrate in two separate stages which
dehydration curves and X - r a y data indicate as being well separated, but by
D T A , the second dehydration stage is not always observed. This may be
because oxidation of Fe 2+ and/or recrystallization proceeds simultaneously
with the second dehydration stage.
OPTICAL, DTA, A N D X - R A Y D A T A F O R E L E V E N C H L O R I T E S
Optical
Eleven chlorite samples were obtained from the H a r v a r d Mineralogical
Laboratory and W a r d s Natural Science Establishment. Classification of
the chlorites, given in Table I, is based on optical data (Table I I ) and
Winchell's charts (1951, pp. 383 and 385). Where the optic sign and optic
angle are given, Ny is the true Ny, but for the thuringite samples N~ is
only approximate because the material was extremely fine grained, making
T~L~ I . - CLASSIFICATION,SOURCE,AND ORIGINFOR ELEVENCHLORITES
Lab. ~
460
459
487
465
462
492
466
488
497
464
463
Classification
based on optics
Clinochlore
Clinochlore
Clinochlore
Prochlorite
Prochlorite
Prochlorite
Thuringite
Thuringite
Thuringite
Corundophilite
Leuchtenbergite
Locality
Chester, Pa. (Harvard # 805421)
Birmingham, Pa. (Harvard # 870381)
Hiawassee, Ga. (Wards *)
Tyrol, Austria (Harvard # 871431)
Cross Island, Me.
Chester, Vt. (Wards ~)
? (Harvard # 871521)
Dona Ana Co., N. Mex. (Wards ~)
Gosheneratp, Switzerland (Wards a)
Kenya, Africa (Harvard # 977031)
Ural Mrs., Russia (Harvard # 807511)
1 Obtained from Prof. C. Frondel, Harvard University.
Obtained from Wards Natural Science Establishment, Rochester, N.Y.
* The low-high iron chlorite division is based on ferric iron so that a low iron chlorite
may contain considerable iron in the ferrous state.
122
CHLORITE
IDENTIFICATION
'4=
~:~
r~
IN
SOIL
~ "~
,,-~ ~
.~
~ r"
~
CLAYS
~
~ ~
r
-~
~
V
.~
~
A
~.~.~ ~ . m
~~~~
L~
+
+
+I
+I
++
~
+
~.~
"
+
~
.~
+
O
I
+I§
+I
§247
m m
m
m
+I
.
§
.
§
.
.
.~
~
+I
<
o
o
-
o.r~
....
o
o~
~
0
t.,
i,.-
~,.-~
~
~
0
"~
R. TORRENeE MARTIN
123
proper orientation on the universal stage impossible. Oxidation of iron in
ripidolite :g: 497 may increase N~ and thus change its classification to
thuringite; this cannot be the case for ripidolite :~ 495 because N~ is too
low. It is recognized that minor isomorphous substitution of such optically
active ions as Ti and Ni may result in anomalous optical behavior, and that
certainly with the extensive isomorphous substitution in chlorites, anomalous optical behavior might be expected. Nevertheless, some classification
is necessary, and that based on optical data generally has proved adequate.
Methods
Samples for D T A and X - r a y were prepared by dry grinding the material
to pass a sieve with 0.07 ram. openings. F o r the fine grained samples very
little grinding was required and for the samples that were large macroscopic flakes, the flakes were cut into thin ribbons prior to grinding so that
a minimum of grinding was required. Thermograms were obtained on the
differential thermal equipment at M.I.T. (Lambe, 1952).t Specimens for
X-ray were prepared by rolling a thin pencil of collodion containing the
test specimen. After the sample had hardened it was placed in a 114.6
ram. diameter evacuated powder camera and a powder photograph obtained by the use of unfiltered Cr K radiation.
Differential Thermal dnalysis
As shown by the thermograms in Figure 1, chlorites are thermally very
active, producing in general a large endothermic peak followed by minor
endothermic and exothermic reactions. With the exception of leuehtenbergite, the peak temperature for the major endot.hermic reaction which is
caused by loss of crystalline water ( O H groups), occurs at 7 2 0 + 2 0 ~ C or
610___ 10 ~ C. It should be pointed out that the size of this major peak is
more than twice the size of the major endothermic peak for the clay mineral
montmorillonite which occurs at 740 ~ C and is about one half the size of
the prominent endothermic reaction for the clay mineral kaolinite which
occurs about 600 ~ C. The general shape of the major high temperature
endotherms on kaolinite, montmorillonite and chlorite are very simliar which
could easily lead to considerable difficulty in analyzing an unknown thermogram (see Fig. 1).
The small differences observed on the minor peaks between the clinot In accordance with the recommendations of Mackenzie and Farquharson (1952),
the following information concerning the DTA equipment is given:
Heating rate - - 12.5~ C/rain., with maximum variation of less than 1~ C/rain.
Thermocouples - - Single Mock, Pt-Pt (10% RH) ; multiple block, Cr-alumel.
Sample s i z e - single block, 1.35 cc. ; multiple block, 0.40 cc.
Pretreatment--7 days over saturated Ca(NOs)~.4H,O solution.
Temperature control couple- in Ni steel block, temperatures are uncorrected.
Temperature calibration - - single and multiple ; quartz a "-->~ at 569 -----3 ~ C, Ba COs
to a at 819 + 3 ~ C, Ba CO, to ~ at 988 ---+-3 ~ C.
124
CHLORITE IDENTIFICATION
TEMPERATURE
0
I
t
3
4
5
I lo
CLINOCH
6
"r
8
9
I0
ii
IN
IN
0
SOIL ChIYS
I00 ~ C
I
2
5
4
5
6
7
8
9
I0
.
'
RE
i
I
i
I
I
I
_A,
~,
I
I
I
487
\
3
4
5
6
7
S
9
I0
II
TEMPERATURE
Fzc. la
0
I
IN
Z
I00 ~ C
3
4
5
6
7
8
9
~0
II
R. TORRENCE M A R T I N
TEMPERATURE
i
2
3
4.
5
6
7
8
9
fo
ii
IN
125
I00 ~ C
0
i
2
3
4
5
6
7
0
9
IO
II
I
'
i
THURINGITE
~-.-.--.--.....
w ~_
C OR U N D OPHI L I TE
464
j
LEU C H T EN B E RG I TE ")("
463
I
/ ~ ~
i
THURINGITE
488
t
11
--\
9
!
"\
I
~ ,
i
j
l
MONTMORILLONITE
,
/
!
J
I
v
I
I
THORINGITE
497
F~.._
KArOLINrTE
")'
i
;'--X
__
,..,_._
1
i
'
1
I
0
I
3
4
5
6
7
s
9
to
iI
TEMPERATURE
0
I
IN
2
3
4
5
6
V
7
8
9
to
I00 a C
F I G U R E 1 ( a and b ) . Thermograms of chlorites, kaolinite and montmorillonite.
Size fraction: --0.07 mm. ; single block. Scale: 1,000, except leuchtenbergite* 500, and
kaolinite * 2,000.
FIG. lb
ii
126
CHLORITE IDENTIFICATION IN SOIL CLAYS
chlore and prochlorite samples need further study before any significance
can be attached to them. In fact, the difference between corundophilite,
clinochlore and prochlorite may be merely one of particle size and/or the
extent of disorder in the crystal structure. A comparison of the prochlorite
thermograms of Figure 1 with those of Figure 3 shows that for the prochlorite samples examined, the frst endotherm completely dominates the
curve which was not the case for the prochlorite thermograms from the
literature. A detailed discussion of the differential thermal behavior of
prochlorite from Chester, Vt., is given in Section V. Likewise, the differences between the thuringite samples require further investigation before
definite conclusions can be drawn.
Based upon the major endothermic reaction of the thermogram, the
chlorites examined can be divided into three groups, namely, (a) a low iron
chlorite group containing clinochlore, prochlorite, and corundophilite with
the major endotherm at 720 ~ C; (b) a high iron chlorite group containing
thuringite with the major endotherm at 610 ~ C; and (c) leuchtenbergite
with the major endothermic reaction at 860 ~ C.:~
X-ray Diffraction
The X-ray data of low iron chlorites, clinochlore, prochlorite, corundophilite, and leuchtenbergite, are so similar that it would be impossible to tell
one from another on the basis of X-ray powder data alone. There are two
differentiating characteristics between the low iron chlorites (Table I I I )
and thuringites (Table IV). First, the basal spacing (00L) of thuringite
is less than that for the low iron chlorites and, second, the 4.58 A line which,
although weak, is consistent for the samples in Table I I I does not occur
on thuringite X-ray patterns.
The basal spacings may be more accurately compared by computing
(001) from higher order reflections. Accordingly, (001) was computed
from (003), (004), and (005) reflections. The (001) and (002) reflections were not used because very small misalignment of the sample can
produce sizeable errors for d values greater than 5 A; basal reflections
greater than (005) were omitted because of overlapping with other reflections. The computed values given in Table V clearly demonstrate that on
the basis of X-ray data these chlorites may be divided into two groups, low
and high iron chlorites. Sample :~497 is either an interstratified complex
or a very poorly crystallized chlorite because of the diffuse (001) reflection
and the non integral series formed by (00L) reflections. It should be noted
that the leuchtenbergite X-ray data fit very well with the low iron chlorite
group but that the thermogram of leuchtenbergite was very different from
the rest of the group. Basal spacings in Table V are consistently greater
than the basal spacing for the corresponding minerals given by Brindley
:~The thermogram of leuchtenbergite is interesting because leuchtenbergite has been
classified as a variety of clinochlore (Hey, 1954) ; unfortunately, there was insufficient
sample to perform extensive tests.
127
R. TORRENCE MARTIN"
I~0
r~
N
~
N
~
N
~
I
I
~
N
~
~
Ii1
~.j...
8
~t<
.
L"~
L)
~ J....
N
N
~l~J ~!~
128
CHLORITE IDENTIFICATION IN SOIL CLAYS
TARLS IV.--X-RAY DAtA FOR THURINGITE I
# 466
9
.~
d(A)
# 488
,,,
A
f
14.12 3
7.09 10
4.69 3
3.52 5
2.81 89
2.68 x
2.57 1
2.48 1
2.40 2
# 497
A
"d(A)
I"
"d(A)
f
14.1
2
5
2
4
2
3
2
2
3
15.2
7.14
4.65
3.55
2.83
2.68
2.59
2.46
2.40
2.28
2.21
2.01
2
4
1
3
1
x
2
1
1
1
x
2
7.07
4.67
3.52
2.82
2.68
2.61
2.46
2.39
2.28
2
2.27
2
2.21
2.01
1
3
2.21
2.01
x
3
1.89
2
1.89
1
1.89
1
1.83
1
1.83
1
1.83
x
Intensities estimated visually ; x indicates very weak line.
(1951, p. 197). The difference varies from 0.02 to 0.24 A, indicating that
there is considerable variability within a given chlorite species
As an aid in distinguishing between chlorite and kaolinite type structures,
Brindley (1951, p. 188) recommends heat treatment at 550 ~ C-600~ C for
30 minutes, which is sufficient to decompose kaolin minerals, but the chlorite
structure largely resists the treatment. The effect of heating the chlorites
at 550 ~ C for 30 minutes is summarized in Table VI. H e a t treatment produces a partial dehydration which enhances the 14 A reflection and weakens
the 7.0 A line. The extent of dehydration and corresponding change in the
X - r a y pattern varies somewhat for the different samples; nevertheless, two
groups again emerge: (a) thuringites (high Fe) where d values are s i g nificantly reduced as well as marked line intensity changes, and (b) clinoTABLE V.--CoNIPUTED
Sample
460
459
487
465
462
492
464
463
466
488
497
1 See text for explanation.
(00L) FOR E~EVR~r CHLORITES I
Name
Clinochlore
Clinochlore
Clinochlore
Prochlorite
Prochlorite
Prochlorite
Corundophilite
Leuchtenbergite
Thuringite
Thuringite
Thuringite
(00L) A
14.23)
14.18/
14.17/
1417/
§
14118~14"t9-0.03
14,18]
14.24[
14.19.]
14,07
14.06
R. TORRENCE MARTIN
TABLE VI. - - CHANGES IN
X-RAY
PATTERNSO F
CHLORITES
129
DUE TO HEAT TREATMENT
Sample #
Effect
460
Slight; intensity of 14 X line increased and most lines broadened
slightly; 4.7 A line broadened so that 4.58 3, line no longer
resolved.
Great; intensity of 14 ~_ line increased two to three times; 7.0 and
2.84 A_ line disappeared; 4.73 and 4.57 lines combined to form
weak band about 4.60A; intensity of 3.54 and 2.53 ~_ lines
decreased and intensity of 2.78 and 2.68 lines increased.
Great ; very similar to # 459 except 14 _h line not quite as strong
and 7.0 A line still faintly visible.
Slight ; intensity of 14 A line increased ; intensity of 7.0 and 4.7 3,
lines decreased; very slight line broadening; 4.58 A line still
resolved from 4.73 A line.
Moderate; all lines still strong; intensity of 14 A line increased;
intensity of 7.0 -h line decreased more than intensity of 4.7 and
3.5 -h lines; 4.58 line still resolved.
Great ; very similar to # 487,
Very great; d values decrease; intensity of 14 3, line increases
and intensity of 7.0 .A_ line decreases so that the intensities are
about equal.
Very great; d values' decrease; intensity of 14 A line increases
slightly and intensity of 7.0 A line decreases so that 14 _A_line
twice aS intense as 7.0 A line; other prominent lines weaken.
Very great; 14 A line sharpened and decreased d, intensity unchanged; 7.0 A line disappeared; all other lines very faint.
Slight; very similar to # 465.
Moderate; very similar to # 462, except that 4.58 A_ line no
longer resolved from 4.7 • line.
459
487
465
462
492
466
488
497
464
463
chlore, prochlorite, corundophilite and leuchtenbergite (low F e ) where the
changes a r e largely confined to line intensity variations. W h e t h e r the differences within g r o u p ( b ) a r e significant or a r e the result of m i n o r crystallographic imperfections still has to be investigated.
Summary of Data
Based upon the results of optical, X - r a y , and differential thermal d a t a
the chlorite samples studied m a y be divided into two general groups, ( a )
high iron chlorites, a n d ( b ) low iron chlorites. T h e characteristics o f
these groups are briefly s u m m a r i z e d in Table V I I .
EFFECT
OF PARTICLE
SIZE ON CHLORITE
PROPERTIES
Materials and Methods
T w o samples f r o m each of the above groups were chosen for detailed
study. T h e low iron chlorites (prochlorite ~ 492 and clinochlore :~ 487)
and high iron chlorites ( t h u r i n g i t e A ~ 488 and thuringite B =~ 497) w e r e
g r o u n d with a mechanical m o r t a r and pestle, a n d different size fractions
130
CHLORITE IDENTIFICATION IN SOIL CLAYS
TABLE VII. - - S ~ t A R ~ OF PROPERTIESFORELEVENCHLORITESAMPLES
Property
Low iron 1
Optical - - N~ -----0.01
X-ray - - basal spacing
--0.03A
heat treatment
on d
DTA - - peak temperature
"+'20~ C
High iron 8
1.59
1.67
14.19
14.07
none
decreased basal reflection
7208
610
1 Sample # 460, 459, 487, 465, 462, 492, 464, 463.
8 Sample # 466, 488, 497.
8 Sample # 463 excluded.
(105--44 ~, 4 4 - - 2 ~, --2/~, --1/~) were separated by sedimentation using
N a O H as a dispersing agent. The - - 2 /~ samples were composed almost
entirely of particles - 2 / ~ + 1 ~ in size because attempts to separate --1 /~
material from the --2 ~ chlorite gave very low yield without further grinding. The various size fractions were made homoionic to calcium and examined by D T A and X-ray methods already described. Additional tests
performed were glycol retention and exchange capacity.w
Differential Thermal Analysis
Thermograms for the two coarser fractions (105--44 /~ and 4 4 - 2 / , )
were so similar that thermograms for the 1 0 5 - 4 4 ~ fraction of clinochlore
and prochlorite were omitted from Figure 2. Thuringite A thermograms
for the 105--44/~ and 4 4 - 2 ~ fractions illustrate the similarity of these
fractions. The most striking features of the thermal curves are that: (a)
for a given particle size, the major endothermic peak continues to divide
the chlorites into low and high iron species, (b) the size of the adsorbed
water reaction and the exothermic peak increase with decreasing particle
size, and (c) a particle size is reached where the amplitude of the major
endothermic peak is markedly reduced; the decrease in amplitude may or
may not be accompanied by a lowering of the peak temperature.
F o r both the low and high iron chlorites, the reduction in peak amplitude
was accompanied by a reduction in peak temperature where the original
specimen contained euhedral crystals, while the low and high iron chlorites
of poor morphology showed no change in peak temperature when the
marked reduction in amplitude occurred. The particle size at which the
reduction in amplitude occurred was --2/~ for the samples of poor morphology and - 1 /~ for the specimens that were originally euhedral crystals.
F r o m dehydration curves, Spell (1945, pp. 20-24) concluded that the
lower temperature and smaller peak on kaolinite thermograms with decreaswGlycol retention is determined by a modified Dyal and Hendricks gravimetrie
method (Martin, 1954, in press), and exchange capacity by the ammonium acetate
method (Peech et al., 1947, pp. 9-11).
R. TORRENCE 1ViARTIN
TEMPERATURE
9
io
IN
ii
131
I00" C
i
6
2
7
8
9
io
,HL
,LEL,o,!4.
"x~_
I
if'
/ \
~/'i ~'~
/
/I
THURINGITE A < I./J
I
r
\
THURIN
/
~.
".~
i
9
I0
II
TEMPERATURE
i
IN
4
5
6
7
8
I00 ~ C
FIGURE 2 . - Thermograms of different size fractions of four chlorites.
single block--1,000 scale.
9
i0
tl
132
CHLORITE IDENTIFICATION IN SOIL CLAYS
ing particle size was due to the fact that it required less energy to expel the
water from the finer particles. In order to check that chlorites behave similarly, selected samples were heated at D T A rate to temperatures that would
bracket the major endothermic peak and the weight loss during this interval
ascribed to water loss. Table V I I I shows that the amount of water lost is
about the same or perhaps slightly more for the --1 t~ samples than for the
larger particles of the same chlorite. A decrease in the amount of material
reacting will also decrease the peak size. The weight of material reacting
was 30 percent less for clinochlore --2 t~ than for clinochlore 44--2 tz; however, the amplitude for the 700 ~ C endotherm for - - 2 / , clinochlore was less
than half the amplitude for the same peak on 44--2/~ clinochlore or a decrease of over 100 percent. These data suggest that the decrease in peak
size and temperature of the major endotherm is related to the energy required to expel the structure water. The low water loss for thuringite may
be ascribed to oxidation of Fe z+ by water as suggested by Orcel and Renaud
(1941).
Since the experimental conditions for all samples were identical, the
increase in the adsorbed water peak is indicative of the increase in specific
surface with decreasing particle size. Whether or not the double adsorbed
water peak on clinochlore and thuringite B has any relation to crystal
morphology is uncertain.
Thermograms for the small particle sizes of clinochlore and prochlorite
in Figure 2 show two dehydration stages, endothermic peaks at 660 ~ C7 2 0 ~ C and 780 o C-820 ~ C, followed by a sharp exothermic peak 830 ~ C870 ~ C. The behavior of these low iron chlorites is consistent with the
changes produced by heat in the crystal structure considered by Brindley
and Ali, and Orcel and Renaud. The absence of the second endothermic
peak for clinochlore noted by Brindley and Ali is probably related to the
particle size of the material used in DTA, because of the effect that particle
size has on the thermograms as shown in Figure 2.
The increase in the exothermic peak with decreasing particle size for the
low iron chlorites substantiates the observations of Sabafier; however, the
increased exothermic reaction for the high iron chlorites may be interpreted
as confirming the results of either Sabatier or as an expression of Fe 2§
oxidation suggested by Orcel. Sabatier's hypothesis for the marked exotherm on fine grained material appears unlikely because Brindley and Ali
TABLE V I I I . - - W A T E R Loss FROM CHLORITES DURING TEMPERATURE RANGE OF
~r
ENDOtrHRRI~IC PI~AK
Lab. ~:
493
530
489
340
Sample
Proehlorite 44--2 ~
Prochlorite < 1/z
Thuringite A<~2 t~
Thuringite <1 t~
THE
Temperature
range (0 C)
H~O
lost (%)
425 M 750
425 - - 750
3 2 5 - 675
325 - - 675
7.9
8.2
4.1
4.6
R. TORRENCE MARTIN
133
noted that the changes in X-ray diffraction were the same whether or not
the second endothermic peak appeared on the thermogram. In fact, comparing the data of Brindley and Ali and of Sabatier with the results obtained here, it seems more likely that no exothermic peak occurs on the
coarser fractions because of steric hindrance and simultaneous reactions.
The large particles prevent the rapid expulsion of O H water and the gradual elimination of water takes place at the same time that the recrystallization reaction occurs. In other words, crystallization of the new phase takes
place in the larger particle size material but its expression on the thermogram is obscured by the continuing O H water loss. It is felt that this explanation satisfactorily explains the observations of Brindley and Ali, those
of Sabatier, and the thermograms in Figure 2.
The exothermic peak of thuringite A is ascribed to recrystallization
rather than oxidation of Fe 2+ because (1) the apparent low H 2 0 loss for
the first dehydration stage suggests that the oxidation of Fe v occurred
during this interval, and (2) the exothermic peak is preceded by a small
endotherm. The lower temperature of the exothermic peak on thuringite
A is attributed to the iron content of the sample, which strains the original
structure, causing it to break down at a lower temperature; and the large
amount of iron probably means that the product of recrystallization is
different from that obtained with low iron chlorite.
The temperature difference between --1 t~ and - 2 ~ high and low iron
chlorites are 85 ~ C and 75 ~ C respectively; this difference is 8 to 10 times
the difference within either the low or high iron chlorite group. Sabatier's
observation that ground chlorites had a single major endotherm at 650__.
5 0 ~ would include the ground and ~nground chlorites examined here.
Since Sabatier does not identify the specimens he studied, further consideration of the difference between his data and those obtained here is
impossible.
X-ray Diffraction
The X-ray diffraction patterns of the 1 0 5 - 4 4 ~ and 4 4 - 2 / L fractions
were, like the thermograms, so similar that only data for the 4 4 - 2 ~ and
finer fractions are given in Tables IX-XI. In general, the X-ray data for
the various size fractions of a given sample are the same and agree fairly
well with results in Section III. On unheated samples there is the expected
broadening of reflections with decreasing particle size, ll and the minor variations in intensity of the reflection appear to be mostly accounted for by
this broadening. Basal spacings (00L) calculated for the unheated specimens as in Section III, from (003), (004), and (005) reflections, for the
different particle size samples are given in Table XII. Pressure oriented
(Mitchell, 1954) specimens were used to obtain data for (00L) calculations
II The major cause for broadened reflections on the samples examined is probably
structural disorders both natural occurring and those produced by grinding rather than
merely a reduction in particle size.
13~
CHLORITE IDENTIFICATION IN SOIL CLAYS
I
~,~ i ~
I~I
I
<i
~.i ~ ~"
I,~
~4
M
i
I~'-~
I
~.I ~-~,
-,~ I~I
I~ I<~
L:.-~ L."~
I
u-+
7~tTr~
I
~
"'+
~I
~.~
~'~3"ml"
,-'+
X
t'+~..-.+
x~
t
~4
I
I
I
III
I C',3'{'~,,.-~.,.-J
I (~,,-+
I
.P-i
O
0
r/'j
.~ L'~
.]:I
I
R. TORRS~CE MARTIN
135
TABLE X . - X - R A y D A T A FOR D I F F E R E N T SIZE FRA~rlONS
PROCHLORITE- HEATED AND U N H E A T E D SAMPI~S I
44--2#
2~
~,
1#
A
lJnheated
Heated g'
OF
A
Ilnheated
Heated d
Unheated
Heated s
'd(A) f
'd(A) f
~a(A) f
h(A) I"
)(A) f
)(A) f
14.0
7.05
4.71
4
5
3
14.1
7.11
14.3
7.11
4.72
2
3
3b
14.3
5
14.4
7.10
14.4
7.13
5
2
4.59
lb
4.61
4B
4.61
4B
4.58
3.54
2.83
2.68
2.59
2.54
2.44
2.39
2.26
2.00
1.88
1
4
2b
I
2
3
3
2
1
3
1
x
1
1
1
3.53
2.82
3
1
3.53
2.82
1
1
2.46
1
2.26
1
2.59
2.54
2.44
2.39
2.27
2.01
1
2
1
1
1
2
2.59
2.54
2.44
2.39
2.27
2.01
1
1
1
1
1
1
1
2b
2
lb
x
1
2
2
1
1
2
x
x
3.52
2.81
2.67
2.60
1.83
4.55
3.56
2.83
2.68
2.59
2.54
2.45
2.39
2.27
2.00
1.89
1.83
1.82
1
1.82
1
10
2
4.61
3.52
2.80
2.67
2.59
2.53
2.44
2.39
2.26
2.00
1.87
3b
2
2
2
1
1
3
x
1
x
1
4
5
Considerable low a n g l e
scattering observed on all
treatments.
Intensities estimated visually; X indicates very weak reflection.
Heated at 550 ~ C for 30 minutes.
TABLE
XI.-
X-RAY
THURINGITE
DATA
A -
FOR D I F F F . , R E N T S I Z E F R A C T I O N S OF
HF~TED
44--2~
"d(A) I
14.2
2
7.045
A
Heated ~
0nheated
A
9
A
,
I
2
2
x t
'd(A)
14.3
7.05
4.66
I
2
4
1
4.68
3b
1~
3.52
3
3.52
2.81
2.68
2.59
2.53
x
x
x
x
2A0
x
2.82
2.68
2.56
2.53
2.46
2.40
2.28
2.01
1
x
x
x
x
1
1
1
1.87
x
4.65
2
d(A)
13.9
6.98
{ 4.49
5.03
3.51
4
J3.49
2.81
2.66
2.60
2.53
2.45
2.39
2.27
2.01
2
2
2
x
2
3
2
3
2b
2b
1.88
1.82
1~
A
9
A
SAMPLES 1
2~
^
Unheated
AND UNHEATED
t2.90
2S
N
HHeated~"
A
d(A)
13.9
6.9
4.6
Ilnheated
9
A
I
3
lb
2b
"d(A)
14.1
7.07
Heated 2"
~
I"
3
5
9
d(A)
13.8
6.94
4.95
~4.48
I
5
3
1
3J
3
3.46
1
2.81
2.68
2.58
1
2
1
2.79
2.68
2.60
2.51
1
1
2
2
2.46
2.39
2.28
2.01
1
2
I
1
2.37
1
1.82
1
_
1 Intensities estimated visually; x indicates very weak reflection.
Heated at 550 ~ C for 30 minutes.
136
CHLORITE IDENTIFICATION IN SOIL CLAYS
TABLE X l I . - COMPUTED (00L) FOR VARIOUS SIZE FRACTIONS
Sample
Clinoehlore 104--44
Clinochlore 44--2
Clinoehlore
--2 z
Proehlorite 104--44
Prochlorite 44--2
Prochlorite
--2 ~
Prochlorite
--1/~
Thuringite A 104--44
Thuringite
44--2
Thuringite
--2 #
Thuringite
--1 i*
d (A)
14.17
14.17
14.20
14.17
14.15
14.19
14.12
14.07
14.02
14.08
14.07
Average
14.18
14.16
14.06
on the -- 1 ~ samples. The spacings show no change with particle size. For
-- 1/L prochlorite the line broadening is sufficient that even with an oriented
sample accurate measurement of the reflection is difficult; also --1 ~ prochlorite showed considerable low angle scattering (d value greater than
25 A ) .
The effect of mild heat treatment (550 ~ C for 30 minutes) on the --2
and --1/~ fractions shows some differences from the coarser fractions; the
effects are most pronounced on thuringite. This mild heat treatment is
insufficient to produce the change in basal spacing observed by Brindley and
All on magnesian chlorites, but the changes in line intensity become quite
pronounced for the finer fractions. H e a t treatment on thuringite produces
marked changes on all size fractions and the effect intensifies with decreasing particle size. Oddly, the X-ray pattern has almost disappeared on the
-- 2 ~ thuringite samples but the -- 1/~ fraction X-ray pattern contains nearly
as many lines as the unheated sample. Fresh samples of --1 /z and - - 2 / ~
thuringite A were heat treated and when the products were X-rayed the
patterns were identical to those first obtained. Even doubling the exposure
time for the --2 ~ sample failed to bring out any additional lines. No explanation has yet been found for this anomalous behavior.
T h e fact that the X-ray pattern of heated - 2 ~ thuringite samples almost
disappears is of particular interest in identifying chloritic material in soil
clays because, as discussed by Brindley (1951), the behavior on heat treatment is often a major point in deciding whether or not chlorite is present.
I f only two reflections remain as for - - 2 # thuringite B, the interpretation
hinges almost solely on the 14 A line because the 4.5 A line could easily
become part o f the strong broad hk band that is characteristic of many soil
clays. For this reason, check tests used to identify montmorillonoids and
vermiculite were performed on -- 1 ~ and - 2 ~ fractions. Neither glycerol
treatment,# which will detect expanding lattice minerals, nor potassium
acetate treatment, which will collapse vermiculite, produced any noticeable
# See Brindley (1951, p. 116) for details.
137
R. TORRENCE MARTIN
change in d values or in relative intensity of the reflections. Therefore, it
is concluded that grinding produced no marked changes in crystal structure
even for prochlorite --1 t~ where considerable low angle scattering was
observed.
The appearance of lines not attributable to chlorite on most of the heated
samples of thuringite A (see Table X I ) suggests that a new crystalline
phase may be forming. In order to check this possibility, samples of
thuringite ~ 490 were heated for 30 minutes at 550 ~ C, 650 ~ C, 750 ~ C,
and 1,000 ~ C and the products X-rayed. Reflections on the resultant patterns not attributable to chlorite, given in Table XlII, confirm that a new
crystalline phase does begin to appear after mild heat treatment of thuringite
and that the product is different from that observed by Brindley and All
(1950) after more severe heating of magnesian chlorites. The new phase
has not yet been identified. The change in the chlorite pattern between
550 ~ C and 750 ~ C is very slight but at 1,000 ~ C the chlorite pattern has
disappeared except for weak lines at 2.69 A and 1.88 A that could be assigned to chlorite but which are not necessarily chlorite reflections. It is
interesting that the new crystalline phase seems to be most well developed
at 650 ~ C which is at least 100 ~ C below the temperature where the modified chlorite structure disappears. After destruction of the modified chlorite
structure there is a broadening of reflections observed for the product of
recrystallization which is believed to be the result of disorder produced by
the removal of the stabilizing influence of the modified chlorite structure.
That a specimen contains chlorite can b e easily determined from X-ray
diffraction patterns, but the only subdivision of the chlorite group that
appears feasible is the low and high iron groups, and even this requires
further confirmation.
TABLE
Xlll.- X - R A Y D A T A FOR T H E RECRYSTALLIZATION PRODUCT OF THURINGITE
AT VARIOUS TEI~PERATURES I
550~ C
650~ C
750~ C
~d(A)
I"
d(A)
I"
'd(A)
5.03
4.49
4.01
2.90
2.16
2.06
1
89
1
4
89
89
4.99
4
4.00
2.90
2.16
2.06
3
6
2
3
4.98
4.51
3.99
2.90
2.16
2.06
1,000~ C
I
3B ~
3B 8
2b
6
1
1
~I(A)
I
5.00"]
3BB
4.00
2.90
2.16
2.05
3B
1B
1B
1Thuringite # 490 (44--2 g), only reflections not attributable to chlorite included.
Chlorite pattern persists at 750~ C but at 1,000~ C very weak lines at 2.69 and 1.88 A
are only possible chlorite lines.
Broad reflection.
8Somewhat broadened reflection.
138
CHLORITE IDENTIFICATION IN SOIL CLAYS
Exchange Capacity and Glycol Retention
It is generally assumed that the exchange capacity and specific surface of
chlorite minerals is very low; however, the data in Table XIV show that
for clay size ( - - 2 / ~ ) chlorite material both exchange capacity and specific
surface are appreciable. The increase in glycol retention and exchange
capacity as the particle size is reduced to 2/~ by grinding is what might be
expected.
Even silt size chlorite material has an exchange capacity as large as the
clay mineral kaolinite. Exchange capacity of the finer fractions are within
the illite range (25-40 m.e./100 g.), except for thuringite B --2/~, which is
somewhat greater. The constant exchange capacity ( _ 5%) for --1 ~ and
- - 2 / , samples indicates either that the major source of exchange sites is a
charge deficiency in the lattice or that the reduction in particle size from
--2 ~ to 1 ~ was accomplished predominantly by basal cleavage. In view of
the platey character of chlorite minerals, the latter explanation would appear
the more plausible.
The increase in glycol retention with decreasing particle size reflects an
increase in specific surface. It should be noted that the glycol retention
increased markedly between --2 t~ and 1 /~ material where the exchange
capacity remained constant. Maximum glycol retention is about half that
of illite except for - 1 ~ prochlorite. Glycol retention for - 1 ~ prochlorite
exceeds considerably that of illite; this may be associated with the change
in the mineral indicated by the observed low angle scattering. The high
glycol value for --1 ~ prochlorite is not due to any induced expansive
character to the mineral because all the various X-ray treatments showed
the same low angle scattering.
From the data in Table XIV, it is certainly evident that the contribution
TABLE X I V . - ETHYLENE GLYCOL RETENTION AND CATION EXCHANGE CAPACITY
FOR DIFFERENT SIZE FRACTIONS OF VARIOUS CHLORITES
Glycol
(mg./g.)
Exchange
capacity
(m.e./100 g.)
3.8
6.2
36.0
5.0
16.0
42
39
5.2
6.4
30
27
32
Lab. #
Sample
Particle
size (#)
486
485
484
494
493
495
530
491
490
489
340
498
Clinochlore
Clinochlore
Clinochlore
Prochlorite
Prochlorite
Prochlorite
Prochlorite
Thuringite (A)
Thuringite (A)
Thuringite (A)
Thuringite (A)
Thuringite (B)
105--44
44--2
<2
105--44
44--2
<2
<1
105--44
44--2
<2
<1
44~2
11
22
40
15
17
25
106
6
8
25
40
25
499
Thuringite (B)
<2
34
47
R. TORRENCE MARTIN
139
of chlorite minerals to glycol retention and exchange capacity of a soil clay
must be considered in making clay mineral analyses.
DISCUSSION
The fact that Brindley and Grim include chlorites in their books dealing
with clay minerals indicates that chlorite is more and more being considered
as a clay mineral. Certainly the DTA, X-ray, glycol retention, and exchange capacity data presented here show that chlorites have all the attributes of the clay minerals except perhaps plasticity, and unpublished
data by the author show that some silt size chlorites are plastic.
To many geologists, chlorite is a macroscopic or at least microscopic
metamorphic mineral unlikely to be formed by normal weathering in the
soil. Perhaps one of the reasons that chlorite has only recently been reported in soil clays, is that heretofore nobody was looking for chlorite. It
would be fairly easy to explain the mineralogical properties of clay size
chlorite, particularly when occurring in mixtures with other clay minerals,
in terms of one of the usual clay minerals. So, again, chlorite was not
observed simply because it was not considered as a possibility. Even when
chlorite is considered as a possibility it is not always an easy matter to
establish its presence or absence. Thermograms of chlorite and the other
clay minerals are sufficiently different that in most instances at least a
tentative identification could be made, but if chlorite is mixed with another
clay, illite, for example, the occurrence of chlorite might never be suspected.
The thermograms in Figure 1 showed that chlorite may be difficult to
identify when mixed with kaolinite or montmorillonoid.
The difficulty in differentiating chlorite from other clay minerals by DTA
is further complicated by the rather marked differences obtained by different laboratories on the same chlorite species, whereas the differences in the
thermograms for the other clay minerals are generally quite minor. This
variation is illustrated in Figure 3 where thermograms by three different
laboratories on prochlorite from Chester, Vt., are given. Where the data
were available, a thermogram of kaolinite was included for comparison.
According to Grim (1953, p. 199) Barshad's A T scale (Barshad, 1948,
p. 667) is only slightly less than Grim's A T scale. Since the kaolinites involved are very similar (API, 1951 ), it was possible to compare the amplitude of the first endothermic peak of the different prochlorite thermograms
by adjusting the different kaolinite amplitudes to a constant. The amplitudes of the first prochlorite inflection were 1.5, 2.6, 2.7, for curves A, C,
and D respectively. It was assumed that the scale differences between prochlorite and kaolinite are linear. In an attempt to resolve this difficulty
with chlorite, the author obtained the prochlorite from Chester, Vt., used
by Barshad (see Fig. 3) and compared the thermograms for two samples
of the same mineral in the same DTA equipment. The results in Figure 4
show that the thermograms for the two different prochlorite samples from
Chester, Vt., are very similar whether run in a multiple block or a single
140
CHLORITE IDENTIFICATION IN SOIL CLAYS
A
from
6
~
f
8
r
4
GRIM(1955)
I0
o
6
m
8
BARSHAD(1948)
I0
J
4
6
8
TEMPERATURE IN IO0~
I0
from
SPELL
(1945)
FIGURE 3.--Thermograms of procl~}6ri~e from the literature. (A) Prochlorite,
Chester, Vt. from Grim. (B) Kaolinite, Ga. from Grim same scale as curve A. By
permission from Clay Mineralogy by R. E. Grim, copyright 1953, McGraw-Hill Book
Co., Inc. (C) Prochlorite, Chester, Vt. from Barshad. (D) Prochlorite, Chester, Vt.
from Speil (200 fl). (E) Kaolinite, S.C. from Speil (600 ~2).
block, but that there is considerable difference between the thermal curves
run on the multiple block and those run on a single block. For the other
clay minerals, kaolinite, illite, and montmorillonoid, the multiple block runs
show a proportional decrease in peak size over the whole temperature range.
The fact that chlorites do not show a proportional change in the same
temperature range must mean that the difference arises from the chlorite
sample.
Thermograms in Figure 3 were all obtained on very similar equipment.
The broad endothermic reaction below 500 ~ C on curve A, Figure 3, could
arise from a loosely packed sample and a tightly packed inert reference material (Arens, 1951, p. 47), but the differences above 600 ~ C between curve
A and curves C and D in Figure 3 cannot be explained on this basis. The
only difference in experimental factors between curve C, Figure 3, and
curve C, Figure 4, both on the same sample of prochlorite, is the thermocouple material. Curve A, Figure 4, and curve C, Figure 3, give a comparison of the influence of sample size (1.5 gm. and 0.5 gin. respectively),
since both these tests were with P t - P t ( 1 0 ~ Rh) thermocouples. It has
long been recognized that the comparison of an unknown clay thermogram
with literature thermal curves does not constitute a very reliable identifica-
R. TORRENCE MARTIN
141
A
500
700
900
temperature ~
500
700
900
temperature ~
SINGLE BLOCK
MULTIPLE BLOCK
1.Sg., P t - P t (10% Rh) couple
0.5g., Cr-AI couple
FIGURI~4. u Thermograms of prochlorite. (A) Prochlorite, Chester, Vt. from Barshad. (B) Prochlorite, Chester, Vt. from Wards (Lab # 492). (C) Prochlorite,
Chester, Vt. from Barshad. (D) Prochlorite, Chester, Vt. from Wards (Lab # 492).
(E) Kaolinite, S.C.
tion; in the case of chlorites, identification from a comparison with thermal
curves in the literature would appear to be virtually impossible at the present
time.
Differences in the thermograms for the chlorite species studied can be
attributed to differences in isomorphous substitution in octahedral lattice
positions. I f substitution in tetrahedral positions determined differences in
thermal behavior, thermograms of leuchtenbergite and sheridanite are out
of place with their composition; however, for substitution differences in
octahedral positions the chlorite thermograms form a consistent picture.
142
CHLORITE IDENTIFICATION IN SOIL CLAYS
The high-low iron content division in chlorites is analogous to the nontronite-montmorillonlte division in the montmorillonoid group as shown by
the lower peak temperature for thuringite thermograms which corresponds
closely to the lower temperature observed for nontronite, a high iron content montmorillonoid. Iron atoms produce a marked change in the thermal
behavior of chlorite because (a) the F e - O H bond is weaker than the A1-OH
or M g - O H bond, (b) Fe atoms are larger than A1 or Mg atoms and therefore do not fit into the structure as well, and (c) ferrous ions are subject
to oxidation.
Thermograms of several minerals reveal that O H ions associated with
M g 2§ are more tightly bound than O H ions associated with AI~§ ** The
peak temperature on the thermogram of leuchtenbergite, 100~ higher
than for most other chlorites, could be explained by a high magnesium
content, a low alumina content, and a very low iron content. F o r sheridanite,
Brindley and All (1950) observed two large distinct endothermic peaks at
630 ~ C and 850 ~ C. The alumina content of sheridanite is fairly high, and
the gradual increase noted on the leuchtenbergite thermogram which was
attributed to AI has separated into a distinct peak On the sheridanite thermogram. As with leuchtenbergite, the iron content of sheridanite is very low
so that the second endothermic reaction may be attributed to O H ions associated with magnesium; however, it must be remembered that D T A measures only the net heat effect at any given time so that in attributing the
second endotherm to O H water only, one has to assume that no other
thermal reactions are taking place. Since all the chlorite dehydration data
show water loss in two stages, there is little doubt that at least a part of
the second endotherm is due to water loss. Nevertheless, in view of the
fact that Brindley and Ali have shown that the chlorite structure breaks
down about 850 ~ C, there must be an increase in entropy accompanying
this change and the heat adsorbed could account, at least in part, for the
second endothermic p e a k . t t It is also reasonable to assume that A1 rich
chlorites would require a more complete structure breakdown prior to
olivine formation than would Mg rich chlorites ; hence, the entropy increases
should be greater for an A1 rich chlorite and part of the large second endotherm on sheridanite may be due to this entropy change.
In the chlorite species prochlorite, clinochlore, and eorundophilite there
is sufficient iron, mostly ferrous, to so weaken the structure that nearly all
the O H water is lost at the same temperature. This composition is expressed by the large endotherm about 700 ~ C for these minerals. The pronounced changes in thermal behavior observed by Sabatier (1950) with
decreasing particle size were not substantiated by the present investigation.
** Compare thermograms of gibbsite and brucite, pyrophylite and talc, and muscovite
and biotite; in all three cases the mineral containing A1 ions has lower temperature
thermal peaks.
J't Earley et al. (1953, p. 777) found that for montmorillonite the second endotherm
w a s due entirely to this entropy increase.
R. TORRENCE MARTIN
143
Like the thermogram, the X-ray diffraction pattern of a chlorite is fairly
easily distinguished from the pattern of another clay mineral, but in a clay
mineral mixture, conclusive proof that chlorite is present generally requires
one or more supplementary tests used in conjunction with X-ray diffraction
(Brindley, 1951, pp. 187-189). Destruction of chlorite with warm acid has
not proved successful since the diminution of chlorite line intensity for a
50:50 mixture of chlorite and kaolinite after acid treatment was hardly
perceptible.:~:~ As Brindley concluded, heat treatment is preferred over acid
treatment because chlorites are not sufficiently acid soluble.
Brindley (1951, pp. 189-191) presents data to show that there is a fair
correlation between the basal spacing and the ratio of Si to A1 in tetrahedral
coordination. Data presented here show that for the species examined, the
ferric iron content of the sample was apparently the factor governing the
basal spacing. That both ferric iron content and A1 in tetrahedral coordination influence the basal spacing is evident from the regression equation given
by Hey (1954, p. 286), dool=13.925+O.23(Si-2)--O.O5FeS§
Although chemical analyses are not available for the samples studied, the
species designation based on optical properties permits an average composition for each species to be assumed; when this is done the (00L) calculated
from Hey's equation and the observed basal spacing for the different samples agree within the estimated standard deviation. The calculated refractive indices agree very well with the observed refractive indices, indicating
that the assumed compositions are about right, and that the difference between Winchell's and Hey's classifications is slight for the compositions
studied. The calculated (00L) for sheridanite, a chlorite containing very
little iron, is 14.08 A ; therefore, the low-high iron chlorite division made on
basal spacing does not apply generally. Since the change in basal spacing
from one extreme in composition to the other is only the order of 0,3 A,
very careful determination of (00L) must be obtained if a good correlation
is to be found between composition and basal spacing.
The influence of heat treatment at 550 ~ C for 30 minutes offers another
possibility for distinguishing between the chlorite species because this mild
treatment has a pronounced effect on high iron chlorites and very little
effect on the low iron chlorites examined here or those studied by Brindley
and All (1951). Further, the recrystallization product from thuringite
appears to be something other than olivine. At first, this seems rather surprising because the Forsterite-Fayalite series shows complete isomorphism;
however, the fact that the recrystallizati0n product of thuringite appears
at such a low temperature may mean that only ions from the brucite sheet
are involved, i.e., no silica is available for Fayalite formation so that recrystallization product may not be a silicate. Further work on this is underway.
Exchange capacity and glycol retention for the --2 ~ fraction chlorite
samples show no variations that could be used to differentiate the chlorite
species; however, as already mentioned, the glycol retention and exchange
~ I n 6 N HCI heated at 60 ~ C for 1 hour.
144
CHLORITE IDENTIFICATION
IN
SOIL CLAYS
capacity of clay size chlorite material cannot be neglected in clay mineral
studies where chlorite is found.
By careful analysis chlorite can be positively identified, but identification
of the different chlorite species in soil clays remains problematical. Quantitative estimation of chlorite is very uncertain and the presence of chlorite
increases the uncertainty in quantitative estimation of the other clay
minerals.
ACKNOWLEDGMENTS
This paper is a contribution of the Massachusetts Institute of Technology
Soil Stabilization Laboratory sponsored by industrial contributions. The
author gratefully acknowledges the financial assistance given by the sponsoring organizations and the advice and constructive criticism offered by the
Soil Stabilization Laboratory staff members.
Special thanks are due to Prof. Clifford Frondel of H a r v a r d for making
available the chlorite samples in the H a r v a r d collection and to Prof. Isaac
Barshad of the University of California f o r a sample of his prochlorite
from Chester, Vt., as well as to Mrs. Margaret M. Martin for translating
the French articles.
REFERENCES
Arens, P. L. (1951) A study of differential thermal analysis of clays and clay minerals:
Doctorate dissertation, Wageningen, Netherlands, 131 pp.
Barshad, I. (1948) Vermiculite and its relation to biotite as revealed by base exchange
reactions, X-ray analyses, differential thermal curves, and water content: Am.
Mineral., vol. 33, pp. 655-678.
Brindley, G. W. (1951) X-ray identification and crystal structures of clay minerals:
London, The Mineralogical Society, 345 pp.
Brindley, G. W., and All, S. Z. (1950) X-ray study o] thermal transformation~ in some
magnesian chlorite minerals: Acta Cryst., vol. 3, pp. 25-30.
Call,re, S., and Hgnin, S. (1949a) Transformation of minerals of montmorillorite
family into 10.4 mica,r: Min. Mag., vol. 28, pp. 606-611.
Call,re S., and Hgnin, S. (1949b) Experimental formation of chlorite from montmorillonite: Min. Mag., vol. 28, pp. 612-620.
Earley, J. W., et al. (1953) Thermal dehydration, and X-ray studies on montmorillonite: Am. Mineral., vol. 38, pp. 770-783.
Grim, R. E. (1953) Clay mineralogy: New York, McGraw-Hill, 384 pp.
Hey, M. H. (1954) .4 new reviezv of chlorites: Min. Mag., vol. 30, pp. 277-292.
Jeffries, C. D., et al. (1953) Mica z~eathering sequence in the Highfield and Chester
soil profiles: Soil Sci. Sac. Am. Proc., vol. 17, pp. 337-339.
Lambe, T. W. (1952) Differential thermal analysis: High. Res. Board Proc., vol. 31,
pp. 621-642.
Kerr, P. F., et al. (1949) Differential thermal analysis of reference clay mh*era~ spedmens: New York, Columbia University, 48 pp.
Mackenzie, R. C., and Farquharson, K. R. (1952) Standardisation of differential thermal analysis technique, Comit~ International pour L'~tude des Argiles.
Martin, R. T. (1954) Clay minerals of five New York Soil profiles: Soil Sci., vol. 77,
pp. 389-399.
Martin, R. T. (In press, 1954) Ethylene glycol retention by clays: Soil Sci. Sac. Am.
Proc.
R. TORRENCE MARTIN
145
Mitchell, W. A. (1953) Oriented--aggregate specimens of clay for X-ray analysis
made by pressure: Clay Minerals Bull. vol. 2, pp. 76-78.
Orcel, J. (1927) Recherches sur la composition chimique des chlorites; Chapter IV,
L'eau de chlorites: Soc. Fran. Min. Bull., vol. 50, pp. 273-322.
Orcel, J. (1929) Compl~ment a l'analyse thermique des chlorites: Soc. Fran. Min.
Bull., vol. 52, pp. 194-197.
Orcel, J., and Cail~re, S. (1938) Nouvelles observations sur les transformations des
prochlorites magnesiennes sous l'action de la chaleur: Compt. Rend., vol. 207,
pp. 788-790.
Orcel, J., and Renaud, P. (1941) Etude du d~gagement d'hydrog~ne associd au depart
de l'eau de constitution des chlorites ferromogndsiennes: Compt. Rend., vol. 212,
pp. 918-921.
Peeeh, M. (1947) Methods of soil analysis for soil-lertility investigations: U.S.D.A.,
Circular No. 757, pp. 9-11.
Sabatier, G. (1950) Sur l" influence de la dimension des cristaux de chlorites sur leurs
courbes d'analyse thermique diffdrentielle: Soc. Fran. Min. Bull., vol. 73, pp. 43-48.
Spell, S., et al. (1945) Differential thermal analysis: U.S. Bur. Mines Tech. Paper
664, 81 pp.
Winchell, A. N. (1951) Elements of optical mineralogy. Part II: DescriPtions of
minerals- New York, John Wiley & Sons, 551 pp.