REFERENCE CHLORITE CHARACTERIZATION FOR CHLORITE IDENTIFICATION IN 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 to--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 magni- tude 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 Structurally chlorite is very closely related to the clay minerals. In a general manner if the K § of illite were replaced with a charged brucite sheet the product would be a chlorite and, in fact, both illite and chlorite have been produced synthetically from montmorillonite by rather mild treatments (Cailfire and H~nin, 1949a, 1949b). In view of the relative ease with which chlorite may be synthesized, it is not surprising that as analytical methods are refined, chlorite is found to be a fairly common soil minerat (Jeffries, 1953; and Martin, 1954). The chlorite group is similar to the montm0rillonoid clay mineral group 117
Transcript
R E F E R E N C E CHLORITE C H A R A C T E R I Z A T I O N FOR CHLORITE I D E N T I F I C A T I O N IN SOIL CLAYS
R. TORRENCE MARTIN Massachusetts Institute of Technology
A B S T R A C T
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 to - -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 magni- tude 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.
I N T R O D U C T I O N
Structural ly chlorite is very closely related to the clay minerals. I n a general manner if the K § of illite were replaced with a charged brucite sheet the product would be a chlorite and, in fact, both illite and chlorite have been produced synthetically from montmori l loni te by rather mild t reatments (Cailfire and H~nin, 1949a, 1949b). I n view of the relative ease with which chlorite may be synthesized, it is not surpr is ing that as analytical methods are refined, chlorite is found to be a fairly common soil minerat (Jeffries, 1953; and Mart in , 1954).
The chlorite group is similar to the montm0ri l lonoid clay mineral group
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 mont- morillonoid 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 non- tronite 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 (DTA) , X-ray, glycol retention, and cation exchange capacity data for a group of chlorites em- bracing as wide a composition range as possible have been compiled.
Since no single survey of work already done in this field is readily avail- able, it seems worthwhile to consider first the contributions of Orcel, Sabatier, Brindley and All, and Hey.
L I T E R A T U R E R E V I E W
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 differ- ent 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) leuchten- bergite, 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 fol- lowing the second endothermic inflection, and (2) thermograms for sherida- nite, 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 over- lapped in the DTA process overlooks the fact that the temperature differ- ence 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 influ- ence 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 mark- edly 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 ob- served 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 endo- thermic reaction associated with the second stage of dehydration cancelled each other and thus only a single endothermic peak was observed.
S a b a t i e r
Sabatier (1950) specifically studied the influence of crystal size on D TA of 10 chlorites. The unground samples were approximately 0.1 mm. in diameter and ground samples were too small to be observed with a micro- scope but were still definitely crystalline as evidenced by X-ray diffraction.
Sabatier reported that from his data thermograms for ground and un- ground chlorites were very different, and that his thermograms were differ- ent from Orcel's even for unground samples. He also reported that thermo- grams 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 tempera- ture at 650 ~ C. It is Sabatier's hypothesis, which is supported by dehydra- tion 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 struc- ture 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 ac- companied 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 dis- appears, 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 reac- tion corresponding to the first dehydration stage is the only constant feature of the thermograms. Penninite and clinochlore have exothermic peaks about 830~ 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 dehydra- tion. 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 dehydra- tion 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 speci- mens 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, ripido- lite, and sheridanite ( < 4 . 0 % Fe~Os) accounted for one half of all occur- rences.* 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 be- tween 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-ray data indicate as being well separated, but by DTA, 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, AND X-RAY DATA FOR ELEVEN CHLORITES
Optical
Eleven chlorite samples were obtained from the Harvard Mineralogical Laboratory and Wards 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 ORIGIN FOR ELEVEN CHLORITES
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
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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. I t 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, anoma- lous 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-ray were prepared by dry grinding the material to pass a sieve with 0.07 ram. openings. For the fine grained samples very little grinding was required and for the samples that were large macro- scopic 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 ob- tained 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 leuehten- bergite, the peak temperature for the major endot.hermic reaction which is caused by loss of crystalline water ( O H groups), occurs at 720+20 ~ C or 610___ 10 ~ C. I t 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 thermo- gram (see Fig. 1).
The small differences observed on the minor peaks between the clino-
t 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 IN SOIL ChIYS
T E M P E R A T U R E IN I 0 0 ~ C
0 I t 3 4 5 6 "r 8 9 I0 i i 0 I 2
I lo ' C L I N O C H RE
i I
i
I I
I
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I I
I
487
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5 4 5 6 7 8 9 I0 .
3 4 5 6 7 S 9 I0 II 0 I Z 3 4 5 6 7 8 9 ~0 II
T E M P E R A T U R E IN I 0 0 ~ C
Fzc. la
R. TORRENCE M A R T I N 125
THURINGITE 4 8 8
j , l ! J I I I
THORINGITE 4 9 7
F~.._
T E M P E R A T U R E IN I 0 0 ~ C
i 2 3 4. 5 6 7 8 9 fo i i 0 i 2 3 4 5 6 7 0
' I i
THURINGITE
w ~-.-.--.--..... ~ _ CORUNDOPHIL ITE 4 6 4
LEUCHTENBERGITE ")(" 11 i
j 4 6 3 v
I / ~ ~ �9 --\
I ~ , i
MONTMORILLONITE
/ "\ i KArOLINrTE ")'
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9 IO I I
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!
0 I 3 4 5 6 7 s 9 to iI 0 I 2 3 4 5 6 7 8 9 to ii
T E M P E R A T U R E IN I 0 0 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
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 pro- chlorite samples examined, the f rs t 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 differ- ences 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, corundoph-
ilite, 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) reflec- tions 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 reflec- tions. 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.
Intensities estimated visually ; x indicates very weak line.
(1951, p. 197). The difference varies f rom 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. Hea t treatment pro- duces 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-ray 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) clino-
TABLE VI. - - CHANGES IN X-RAY PATTERNS OF C H L O R I T E S DUE TO HEAT TREATMENT
Sample # Effect
460
459
487
465
462
492 466
488
497
464 463
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 un- changed; 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.
chlore, prochlor i te , corundophi l i te and leuchtenbergi te ( low F e ) where the changes are largely confined to line intensi ty var iat ions. W h e t h e r the differ- ences within g roup (b ) a re significant or a re the resul t of minor crys ta l lo- graphic imperfec t ions still has to be invest igated.
Summary of Data
Based upon the resul ts of optical, X- ray , and differential thermal da ta the chlorite samples s tudied may be divided into two general groups, ( a ) high i ron chlorites, and (b ) low iron chlorites. The character is t ics of these groups are briefly summar ized in Table V I I .
E F F E C T O F P A R T I C L E S I Z E O N C H L O R I T E P R O P E R T I E S
Materials and Methods
Two samples f rom each of the above groups were chosen for detai led study. The low iron chlori tes (prochlor i te ~ 492 and clinochlore :~ 487) and high i ron chlorites ( thur ingi te A ~ 488 and thur ingi te B =~ 497) were ground with a mechanical mor ta r and pestle, and different size f ract ions
130 CHLORITE IDENTIFICATION IN SOIL CLAYS
TABLE VII. - - S~ tAR~ OF PROPERTIES FOR ELEVEN CHLORITE SAMPLES
(105--44 ~, 44- -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 f rom the - -2 ~ chlorite gave very low yield without further grind- ing. The various size fractions were made homoionic to calcium and ex- amined 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 prochlori te 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.
For 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 morphol- ogy and - 1 /~ for the specimens that were originally euhedral crystals.
From dehydration curves, Spell (1945, pp. 20-24) concluded that the lower temperature and smaller peak on kaolinite thermograms with decreas-
w Glycol 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 IN I00" C
9 io i i i 2
"x~_ I
/
\
131
i 9 I 0 II
TEMPERATURE IN I00 ~ C
,HL ,LEL,o,!4. 6 7 8 9 io t l
if'
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/I THURINGITE A < I./J
I r
THURIN / ~ . " . ~
i 4 5 6 7 8 9 i0
FIGURE 2 . - Thermograms of different size fractions of four chlorites. single block--1,000 scale.
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 simi- larly, 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 VI 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; how- ever, 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 de- crease of over 100 percent. These data suggest that the decrease in peak size and temperature of the major endotherm is related to the energy re- quired 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 ~ C- 7 2 0 ~ C and 780 o C-820 ~ C, followed by a sharp exothermic peak 830 ~ C- 870 ~ 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 exo- therm on fine grained material appears unlikely because Brindley and Ali
TABLE VIII . - -WATER Loss FROM CHLORITES DURING TEMPERATURE RANGE OF T H E ~r ENDOtrHRRI~IC PI~AK
Temperature H~O Lab. ~: Sample range (0 C) lost (%)
noted that the changes in X-ray diffraction were the same whether or not the second endothermic peak appeared on the thermogram. In fact, com- paring the data of Brindley and Ali and of Sabatier with the results ob- tained 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 grad- ual elimination of water takes place at the same time that the recrystalliza- tion reaction occurs. In other words, crystallization of the new phase takes place in the larger particle size material but its expression on the thermo- gram is obscured by the continuing O H water loss. It is felt that this ex- planation 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 H20 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__. 50~ would include the ground and ~nground chlorites examined here. Since Sabatier does not identify the specimens he studied, further con- sideration 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 varia- tions in intensity of the reflection appear to be mostly accounted for by this broadening. Basal spacings (00L) calculated for the unheated speci- mens 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
~4
I
0
r/'j
I
~ , ~ i ~
~.i ~ ~"
I~ I<~ L:.-~ L."~
7 ~tTr~
.~ L'~
.] :I
I I~I I
I , ~ M i I~'-~ I
-,~ I~I ~.I ~-~,
u-+ I ~ " '+ X
~I ~.~
x ~ t ~4 I I I
III
~'~3"ml" ,-'+ t'+~..-.+ I C ' , 3 ' { ' ~ , , . - ~ . , . - J I ( ~ , , - + I
.P-i
O
R. TORRS~CE MARTIN
TABLE X.- X-RAy DATA FOR D I F F E R E N T SIZE FRA~rlONS OF
PROCHLORITE- HEATED AND UNHEATED SAMPI~S I
135
4 4 - - 2 # 2 ~ 1 # ~, A A
lJnheated Heated g' I lnheated Heated d Unheated Heated s
'd(A) f 'd(A) f ~a(A) f h(A) I" )(A) f )(A) f 14.0 4 14.1 10 7.05 5 7.11 2 4.71 3
3.51 4 J3.49 1 ~ 3.52 3 t2.90 2 S 2.81 2 2.81 x 2.82 1 2.66 2 2.68 x 2.68 x 2.60 2 2.59 x 2.56 x 2.53 x 2.53 x 2.53 x 2.45 2 2.46 x 2.39 3 2A0 x 2.40 1 2.27 2 2.28 1 2.01 3 2.01 1 1 . 8 8 2b 1.87 x N _
1 . 8 2 2b
14.1 3 13.8 5 7.07 5 6.94 3
4.95 1 4.68 3b ~4.48 3 J
3.52 3 3.46 1
2.81 1 2.79 1 2.68 2 2.68 1 2.58 1 2.60 2
2.51 2 2.46 1 2.39 2 2.37 1 2.28 I 2.01 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
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 ~ pro- chlorite 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. Hea t treatment on thuringite produces marked changes on all size fractions and the effect intensifies with decreas- ing 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 ex- planation has yet been found for this anomalous behavior.
The 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 t reat- ment 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.
R. TORRENCE MARTIN 137
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 XI ) 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 pat- terns 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 as- signed 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 modi- fied 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 be 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-RAY DATA FOR THE RECRYSTALLIZATION PRODUCT OF THURINGITE AT VARIOUS TEI~PERATURES I
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.
I t 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
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 ex- change capacity data presented here show that chlorites have all the at- tributes 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 re- ported in soil clays, is that heretofore nobody was looking for chlorite. I t 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 differ- ent 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 in- volved are very similar (API, 1951 ), it was possible to compare the ampli- tude of the first endothermic peak of the different prochlorite thermograms by adjusting the different kaolinite amplitudes to a constant. The ampli- tudes 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 pro- chlorite 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 GRIM (1955) 6 8 I0
~ f r o m BARSHAD (1948) 4 6 8 I0
J
from SPELL 4 6 8 I0 (1945)
TEMPERATURE IN IO0~
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 ma- terial (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 thermo- couple material. Curve A, Figure 4, and curve C, Figure 3, give a com- parison 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. I t 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
500 700 900 500 700 900
temperature ~ temperature ~ SINGLE BLOCK MULTIPLE BLOCK 1.Sg., Pt -Pt (10% Rh) couple 0.5g., Cr-AI couple
FIGURI~ 4. u Thermograms of prochlorite. (A) Prochlorite, Chester, Vt. from Bar- shad. (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 non- tronite-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 con- tent 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 there- fore 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 Mg 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. For 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 thermo- gram. As with leuchtenbergite, the iron content of sheridanite is very low so that the second endothermic reaction may be attributed to O H ions asso- ciated with magnesium; however, it must be remembered that D T A meas- ures 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 peak . t t I t 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 endo- therm 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 ex- pressed by the large endotherm about 700 ~ C for these minerals. The pro- nounced 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 coordina- tion 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 composi- tion 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 sam- ples agree within the estimated standard deviation. The calculated refrac- tive indices agree very well with the observed refractive indices, indicating that the assumed compositions are about right, and that the difference be- tween 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 sur- prising 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 recrys- tallization 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 I N 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. Quanti- tative estimation of chlorite is very uncertain and the presence of chlorite increases the uncertainty in quantitative estimation of the other clay minerals.
A C K N O W L E D G M E N T S
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 spon- soring organizations and the advice and constructive criticism offered by the Soil Stabilization Laboratory staff members.
Special thanks are due to Prof. Clifford Frondel of Harvard for making available the chlorite samples in the Harvard collection and to Prof . Isaac Barshad of the University of California f o r a sample of his prochlorite f rom Chester, Vt., as well as to Mrs. Margaret M. Martin for translating the French articles.
R E F E R E N C E S
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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 montmo- rillonite: Min. Mag., vol. 28, pp. 612-620.
Earley, J. W., et al. (1953) Thermal dehydration, and X-ray studies on montmoril- lonite: Am. Mineral., vol. 38, pp. 770-783.
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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.
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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.
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