THE AMERICAN MINERALOGIST, VOL. 52, MAY_JUNE, 1967

STRUCTURAL ASPECTS OF DIOCTAHEDRAL CHLORITE

R. A. Eccr.BsroNl aNl S. W. Barr,Bu, Deparlment of Geology,U niztersity oJ W isconsin, M ad,is on, W i.sconsin.

AsstnA,crDioctahedral chlorites described to date can be classified into three categories: 1) chlo-

rites with two dioctahedral sheets and an octahedral cation total slightly greater than 4.0per formula unit, 2) chlorites with one dioctahedral and one trioctahedral sheet and anoctahedral cation total near 5.0, and 3) imperfectly crystallized specimens with incompleteinterlayers.

The existence of di/trioctahedral sheets in category 2) has been confirmed by structuralanalysis for a particular Al-Mg specimen in powder form and for a similar chlorite inter-stratified with montmorillonite. The 2:llayer portion of the structure is dioctahedral andthe interlayer is trioctahedral. The layer type is 116. Chemical analysis, structure factors,average T-O bond length of 1.65 A, average basal O-O distance of 2.64 A,, and tetrahedraltwist angle of 7.60 indicate a composition for both specimens near [AL.o(Sir.*Alo.r)0.(OH;rl-o ?

l(Mgr.sAlg t (OH; ]u+0.2. The lateral misfit that might be expected betweenmixed di/trioctahedral sheetshas been accommodated structurally by thickening the triocta-hedral sheet and thinning the dioctahedral sheet so that the mean octahedral edges areidentical. Existing d(001) x-ray spacing graphs constructed to derive tetrahedral composi-tions for trioctahedral chlorites are not valid for dioctahedral species.

fNrnonucrtoN

Although most chlorites are trioctahedral, dioctahedral species arebeing reported in the literature with increasing frequency. Specimensthat have been called dioctahedral chlorite actually can be grouped intothree different categories: (1) Chlorites with two dioctahedral sheets,i.e. with a pyrophyllite-like 2:I layer and a gibbsite-like interlayer.The octahedral cation total is slightly greater than 4.0 per formulaunit, primarily Al, and d(060) measures between 1.49 and 1.50 A. (2)Chlorites with one dioctahedral sheet and one trioctahedral sheet. Theoctahedral cation total is near 5.0 per formula unit, and d(060) measuresbetween 1.49 and 1.51 A. (3) Less well-crystallized, layer silicates com-parable to (1) and (2) above, but with incompletely developed interlayermaterial. Such specimens, occurring mainly in soils and sediments, com-prise the majority of reported dioctahedral chlorites. The incompletenessof the interlayer material is indicated by partial collapse of the layerson heating, expansion on solvation for some specimens, and relativelyeasy dissolution in sodium citrate or other reagents.

Because most specimens are fine grained, no structural details havebeen reported for dioctahedral chlorites previously. The present paperwill consider only the best crystallized examples within the first twocategories above. Systematic refinements of cell parameters from the

r Present address: Dept. of Geology, Australian National University, Canberra, A.C.T.

673

674 R. A. EGGLESTON AND S. W. BAILEY

powder pattern of a chlorite with di/trioctahedral sheets and the Fourrer

transform analysis of the same chlorite in regular interstratifi,cation with

montmorillonite will be reported.

Cnronrrns wrru Two DlocrenBlner- SnBBrs

Several natural occurrences have been reported of chlorites believed

to have two dioctahedral sheets. In most cases the chlorite is so inter-

mixed with other materials that it cannot be characterized adequately.

In other cases the interlayer material proves to be incomplete or unstable.

Only the few examples that have been best chatacterized and that ap-

pear to have the most complete gibbsite-like interlayers are listed in

Table !a. In these specimens the substitution of Al for Si ranges from

0.7 to 1.3 per 4.0 positions. The octahedral cation total is always greater

than 4.00 atoms, ranging from 4.27 to 4.4. This suggests that the positive

charge on the gibbsite interlayer arises primarily from the presence of Al

cations in excess of 2.0 in this sheet, in accord with the theoretical for-

mula of Alr++"rar(Sil_"At)Oro(OH)s given by Brindley and Gillery

(1956). Some positive charge may arise also from anion vacancies or

from the presence of extra protons.The present writers have been able to study a dioctahedral chlorite

of this type, kindly supplied by Professor G. Miiller. The chlorite comes

from a Permian sandstone in NW Germany and is to be described by

Miiller elsewhere. Although mixed with quartz and mica, suffi.cient puri-

fication could be obtained to identify the chlorite as being dioctahedral

with d(060):1.49 A and as having layers classified as the Io type by

Bailey and Brown (1962). A regular interstratification of an aluminous

chlorite with swell ing chlorite (Heckroodt and Roering, 1965) can also

be identified as having Ia type Iayers from the published X-ray pattern.

Although the d(060) value of 1.49 A suggests two dioctahedral sheets,

the appreciable amount of Li present by analysis in this latter specimen

also ind.icates a possible analogy with the Io chlorite cookeite, which has

both dioctahedral and trioctahedral sheets.

Cnronrrns wrrn Dr/ruocrAHEDRAr SuBers

Cookeite, a mineral known since 1862, is the earliest and best known

example of a chlorite having one dioctahedral sheet and one trioctahedral

sheet. Brammall, Leech, and Bannister (1937) give an ideal composition

(LiAl4) (SiaAl)Oro(OH)s for cookeite. They allocate two AI to the 2:1

unit and 2Al+1Li to the interlayer "brucite" sheet. Dr. K. Norrish

(personal communication, 1961) has computed a one-dimensional elec-

tron density projection that confirms the di/trioctahedral nature of

cookeite, but suggests that the composition for his specimen should be

Alr.zslio.ao for the 2:1 unit and LiAlz for the brucite sheet. Bailey and

675DIOCTAHEDRAL CHLORITE

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676 R. A. EGGLESTON AND S. W, BAILET/

Brown (1962) examined six cookeites and found all to be based on theIalayer type. The d(060) value is l.4g A, which is typical of dioctahedralIayer silicates and considerably smaller than the 1.53-1.56 A range foundfor trioctahedral chlorites. Because dioctahedral sheets are characterizedby two small octahedral sites occupied by AI and a larger, vacant site,it is believed that Li fi.ts into this larger stie in the cookeite interlayerwithout increasing d(060). Cookeite is the only dioctahedral chloriteknown so far to occur in single crystals. At least two different 2-layer

polytypes occur and are in the process of structural refinement (Lister,

1966). The exact status of mananodite, l isted by Frank-Kamenetsky(1960) as a dioctahedral Li-Al chlorite with some substitution of boron

in the tetrahedral positions, is uncertain.In 1954 Sudo and his co-workers in Japan identified an AI-Mg dioc-

tahedral chlorite as one of the constituents of a regular interstratifica-

tion of chlorite with montmorillonite. This material has now been re-ported from the hydrothermal alteration zones around five different ore

bodies in western Japan (Sudo, 1959). The chloritic component was

first described as having pyrophyllite-like and gibbsite-like sheets, i.e.having two dioctahedral sheets. Ilowever, allocation of a chemical analy-

sis of pure material from the Kamikita mine (Sudo and Kodama, 1957)gives an octahedral cation total of 4.75 for the chlorite component. The

allocation shown by these authors for the "gibbsite" sheet contains

more Mg than Al, and is probably trioctahedral. A di/trioctahedral

structure is also suggested by the d(060) value of 1.506 A, which is

rather large for a combination of two dioctahedral sheets composed only

of Al.Bailey and Tyler (1960) reported two similar occurrences in Michigan

of an apparent regular interstratification of dioctahedral chlorite andmontmorillonite, one from an alteration zone around an ore body andthe other in fractures in an altered igneous rock. A partial chemical

analysis of the former material shows slightly more MgO and SiOz thanin the Kamikita material and slightly less AlzOa. The d(060) value is

1.507 A for the Michigan material.Bailey and Tyler also report several occurrences in Michigan of a

similar chlorite in random interstratification with montmorillonite and

of a non-interstratified dioctahedral chlorite. The d(060) value of the

latter ranges from 1.505 to 1.511 A in diffe.e.tt specimens. The material

does not expand on solvation with glycerol. The 001 reflection at 14.23

A itrc.eases in intensity by a factor between four and five on heating at

500oC and decreases in spacing only to 14.0 A. Static heating at 1000"Cproduces spinel and at 1200oC produces cordierite plus spinel. The ideal

formulas of both products contain twice as much Al as Mg, confirming

DIOCTAHEDRAL CHLORITE 677

the dioctahedral nature. It seems likely that this is the same chlorite asthat in the regular and random interstratif ications and that it is a secondexample of a chlorite with di/trioctahedral sheets. The indexed powderpattern (Table 6) indicates the layer type is 116.

Schultz (1963) has described the occurrence of 20 samples of aluminouschlorite from sediments in Triassic rocks of the Colorado Plateau. Thed(060) values are between 1.50 and 1.51 A, and the 003 reflection is con-siderably more intense than should be the case for a trioctahedral chlo-rite. A chemical analysis of impure material shows slightly more AIzO:than MgO and, according to allocation by the present writers, yields anoctahedral total near five atoms. These data suggest the presence ofmixed di/trioctahedral sheets, but further details are needed for verif ica-tion.

Hayashi and Oinuma (1964) report dioctahedral chlorite in the altera-tion zone of the Kamikita mine, Japan. The d(060) value is 1.509 A, thed(001) value of 14.13 A is s table on heat ing and solvat ion, and the in-tensity of 001 increases by a factor of f ive to six on heating at 600oC. Be-cause the powder pattern is similar to that given by Bailey and Tyler, theIayer type is probably IID. Allocation of a chemical analysis yields theformula l isted in Table 16. Minor impurit ies of mixedJayer clay andpyrite are included in this formula. Both Schultz (1963) and Hayashi andOinuma (1964) note that dioctahedral chlorite is more resistant to HCIthan are the trioctahedral varieties.

Frenzel and Schembra (1965) have described dioctahedral chloritefrom hydrothermally altered arkose in the Kaiserbach Valley. The d(001)value of 14.11 A does not change aftersolvation. The X-ray powder pat-tern is similar to those of Bailey and Tyler (1960) and of Hayashi andOinuma (1964), but contains impurities oI qttartz, dolomite, and feldspar.The d(060) value is 1.507 A. Allocation of a chemical analysis yields 4.80octahedral cations plus 0.13 exchangeable cations (Table 16).

The evidence cited above for the existence of Al-Mg chlorites withdi/trioctahedral sheets is indirect. It depends on the intermediate valueof d(060), on chemical analysis of impure material, and on the ideal com-positions of the recrystall ization products of static heating. The presentwriters have undertaken the structural refinement of the Bailey andTyler (1960) specimens, which occur pure, in order (1) to verify the pres-ence of di/trioctahedral sheets, (2) to determine whether the dioctahe-dral sheet is in the 2:lI 'a.1rer or in the interlayer, and (3) to determine anystructural adjustments within the layers resulting from the presence ofboth dioctahedral and trioctahedrerl sheets in the same layer. Both thedioctahedral chlorite and the interstratif ication of a similar chlorite withmontmorillonite have been examined.

678 R. A. EC,GLESTON AND S, W. BAILEV

Iwtpnsrnarrlrnn DTocTAHEDRAL Cnr.onrrB-uoNTMoRrLLoNrrE

An apparent regular interstratification of dioctahedral chlorite withmontmorillonite occurs as a fine-grained greenish coating on shear sur-faces in an altered dike in the Geneva-Davis iron mine, Gogebic district,Michigan. A partial chemical analysis is listed in Table 2. Assuming thatthe tetrahedral sheets of the chlorite and montmorillonite have the samecomposition, the analysis can be allocated as follows:

Chlorite: (Sir.nAlo.o)Alr(Mgz.rAlo.o)Oto(OH)s

Montmoril lonite : (Si3.4410.6) Al2(Mgo.zCao. t) Oro(OH) z . 4 .4 II2O

'fhe chloritic component has one dioctahedral sheet and one trioctahedralsheet.

Tarr,n 2. Cururclr AN,rr,vsrs or INrsrsrnarmrno Cnlomrn-Molrruonrr-ror.rrtn

w t ' 70

SiOzTiOzAlzOsCrzOgFezOtFeOMgoCaOKzONazOHrO

4 2 . 1 5nil28.95tracetracenil1 1 . 2 00. 65

niln.d.n.d.

Difiractometer patterns were obtained from oriented aggregates of theinterstratified clay in a water-saturated state, after solvation with glyc-erol, and after heating at 500'C. The average basal spacing d(001) is28.9 L for the natural material, 32.1 A after solvation, and 24.1 A. afterdehydration. Because the 001 spacings deviate slightly from integral sub-multiples, some deviation from exact regularity of interstratification issuggestid. The observed patterns are l isted in Table 3.

Fourier transforms for the solvated and heated clays were calculatedaccording to the methods of MacEwan (1956) and MacEwan, RuizAmil, and Brown (1961). An average layer structure factor was used totake into account the different interlayer scattering powers in the twocomponents. The observed peak heights obtained from the two trans-forms were averaged. To test the consistency of the results, theoreticalpeak heights for threeJayer packets were calculated from the probabili-ties derived from the one- and two-layer packet sequences. Some dis-

DIOCTAEEDRAL CELORITE

agreement was noted between observed and calculated peak heights sothe transform was reevaluated as an integral, the function being summedat intervals of 0.1" 0. Agreement between the observed and calculatedpeak heights was much closer with the integral method than with theseries summation (Table 4).

The failure of the series transform to give internally consistent resultscan be attributed partly to the breadth of the diffraction maxima. Mac-Ewan's derivation of the series transform assumes a rapid change in in-tensity for a small change in theta. The peaks from the material used inthis study had appreciable intensity over a range of 3" 2d, hence the use ofa Fourier series is not completely justified.

Tesr-n 3. Dn'lnecrounrnn Perrpnrs or OnrrNrnn Cnlonrr:n-MoNruonu.roNrtn Cr-ny

Water slurry Solvated Ileated 500"C

679

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0100220

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0308000

100950

230

250

200

1007rI60

220

100

0

The Fourier transforms show that the interstratified material is 60percent chlorite and 40 percent montmorillonite. Reallocating the chemi-cal analysis in these proportions gives an approximate composition:

Chlorite: (Sis.6A10.6)Al2(Mgz.aAlo.o)Oro(OH)s

Montmoril lonite: (Si3.6A10.6)Alz(Cao.6)Oro(OH)r. 5.5 HrO

The sample was not analyzed for sodium, and this may explain the dis-crepancy between the tetrahedral sheet charge and the interlayer chargein the montmorillonite component. Also, there is no requirement that thetetrahedral sheets have the same composition in both components. Theyare listed in this manner because of lack of evidence to the contrary.

An attempt was made to distinguish between two possible structuralmodels for the chloritic component of the interstratified clay by means ofthe transform integral. Model t has a dioctahedral 2:l layer and a tri-

680 R. A. EGGLESTON AND S. W. BAILEV

octahedral interlayer. Model 2 has a trioctahedral 2:l layer and a dioc-tahedral interlayer. Although Model 1 gives slightly more selfconsistentresults than Model 2, especially for four-layer packets, the transformintegral is only slightly affected by the change in distribution of cationsbetween the octahedral sheets (Table 4).

More certain evidence for the dioctahedral nature of the 2:l layer of

Taern 4. l'ounrrn Tnaxsronrt Prax Hrrcnrs ron lNrunstrerrlrno

Series summation Integral method

Packet* Model 1 Model 2

Obs.Calc,Obs.

L

M

CCCMMM

CCCCMCMCMMMM

CCCCCMCMCCCMMMMCMMMM

*J

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6040

30664

15 1559 6026 250 0

29 22J 5 J /

18 200 1

t2 1335 3553 520 00 0

r 2 366 6422 330 0

I J I

54 4527 446 40 0

the chlorite is found by comparison of observed and calculated structureamplitudes for the basal reflections of water saturated material (Table 5).The agreement is markedly better for Model 1 than it is for Model 2. In-tuitively one would expect this result because it gives similar composi-tions to the 2:l unit in both the chlorite and montmorillonite compo-nents. The primary difference is in the interlayers.

It is not possible to have exact alternation of different layer types if thecomponents are present in unequal amounts. However, the probabilitiesof layer sequences expected for any ratio of layers in regular or randominterstratification can be calculated and converted into expected peak

Cnr,onrru-MoNTMoRrLLoNrrE

* C : Chlorite, M : Montmorillonite.

DIOCTA H EDRA L CH I.ORI T E

Taerr 5. CorrapaarsoN or'001 Srnucrunr Auplrruons lon HynnareoCnr,onrrn-MowrMoRrlr,oNrrE

681

Model 1 Model 2

Fo* F" F.-F"

t 21 a

- 1 0I J

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3- l

x Adjusted to )F":2P".

heights. Comparison of these calculated values, Iisted below, with theobserved peak heights in Table 4 gives the unexpected result that theseries approximation suggests a very regular system whereas the morerigorous integral method suggests one intermediate between regular andrandom. The latter result is in agreement with calculations by MacEwanand others (1961), who found that quite a modest degree of regularity canimpart diffraction effects that resemble regular interstratification.

DFo*

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3:2 regular

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Fine grained, whitish dioctahedral chlorite occurs disseminated inhematite ore at the Tracy mine, Michigan. ft can be obtained pure, al-though not in amount suftcient for chemical analysis. The indexedpowder pattern is given in Table 6. Cell dimensions, refined from the pat-tern by a least-squares procedure, are a:5.237 - l 3 , b:9.070 * 5, c:14 .285+ 13 A , and B :97o92 '+05 ' .

The powder pattern corresponds to that of a IID layer type accordingto the data of Baileyand Brown (1962). Regularlayer stacking is indicatedby the presence of a series of weak kl3n reflections. Two regular one-layerpolytypes are possible for this layer type, namely IIb-2 and 116-4 with

682 R. A. EGGLESTON AND S. W. BAILEY

Tesr-B 6. Pownnn PerrntN Droctannon.tr- Cur,onrre, Tn.lcv MrNn

) - f - dcuto I ak Iob"hkl hhl, d" b Icalc

14.23 807 .13 704.7s5 804.517 854.314 54.180 54.Or7 53.840 23.552 653 .272 23.084 42.923 22.839 202.783 1

2.ff i6 10

2 .5M 252.501 1002.409 502.348 252.230 252.160 22.024 5

001002003020, 11002t111tt202200,1023113r14005024

1zoo, rs1 \120T,130 J202, r3r2Ol,132203,132202,133204,133042007

14 .187 . W4.7264. 5354.2894 .1714.0023 .8203.5443 .2723.1022.9432.8362.798

2.599

2.5422 .5032.4052.3492.2302.1592.025

1.9841 .8681.8121.710\r.6$l1 .5561 .511r .4781.4391 . 4 1 8

1.389

1 .331

t .297

1 .274

1.983 531.866 171.809 271 . 7 0 21 .6511 . 5 5 5 4 I1 . 5 r 2 5 11 . 4 7 8 4t .440 31 . 4 1 8 3

1 .389

1 3 3 4 |

1 .297 16

t . 2 7 5

J '

5283

508 204,13515 206,13525B 205,136t'B 207,t3658 206,137

408 208,13760 060,33110 062,331,3335 063,332,3342 0.0. 10

on o1zos, rso, _\^" - [064, 333,335J

2 065,334,336,,. 1+oo, zo2,\

\403,261 J17 l40l'263'\

\404,262 J

60

t9

r.256 101 .2 r7 108 2 .0 . I I ; 1 .3 . l 0 r . z t sr . 182 2 0 .o . l 2 1 . 1811.163 2 407,265 r.169I .129 2 2 .O .T2 ; r . 3 . l l l . I l 91.082 2 n6.268 1.084

2

Pattern taken in 114.6 mm camera, CuKa radiation, corrected for film shrinkage, in-

tensities measured photometrically.

ideal space groups C2f m and C1 respectively. Attempts to identify which

polytype is present were inconclusive; best agreement between observed

and calculated intensities was obtained for a mixture of both polytypes.

Because Bailey (1966) has pointed out that almost all dioctahedral layer

silicates are two-layer polytypes, consideration was also given to the

possibility of a two-layer polytype in which the kl3n, l:2u reflections

are much stronger than the k*3n, l:2n*l reflections. If one assumes a

c glide plane to be present, there are only three two-layer polytypes

having IID layers. AII three theoretical structures' however, predict

hl3n, l:2nll reflections that are not observed.lBy use of the more intense h:3n reflections the average structure

within the layer can be obtained, regardless of the sequence of layer

I After the present paper was in press, an abstract by V. A. Drits appeared in Abstracts,7th congr. Int. union crystallogr., Moscow, describing a2-layer chlorite with di/triocta-hedral sheets. The structure proposed is the same as one o{ the three 2-layer models consid-

ered in the present paper, except for the choice of an alternate Z direction.

t 2

29110522620

DIOCTAH EDRA L CE I.ORIT E 683

stacking. For these reflections atoms repeating at intervals of bf 3 arein-distinguishable. Atoms of this type were grouped together and their yparameters fixed at the ideal values. The other 11 atomic parametersplus a scale factor were refi.ned by a least-squares procedure, using a totalof 47 reflections. The multiplicity of the powder lines was allowed for bydistributing the observed line intensity among the contributing spectraaccording to the ratio calculated from the model in use. The distributionof intensity for each line was changed where necessary as refinement pro-gressed. Reflections that make unequal contributions to a given powderline were assigned lesser weights in the least squares procedure, as werethe weakest reflections. Because of the lack of evidence to the contrary,the symmetry of the average layer was assumed tobe C2/m. Isotropictemperature factors were arbitrarily fixed al B:1.0 for cations and B:2.0 for anions.

The agreement between observed and calculated intensities is onlymoderately good for a structure with two dioctahedral sheets, but is verygood for a structure with one dioctahedral and one trioctahedral sheet.This latter structure was tested further by running four least-squares re-finement cycles for each of the two possible models of octahedral cationdistribution. Model 1, which has a dioctahedral 2:l layer and a triocta-hedral interlayer, proved superior to Model 2 because of the lower reli-ability factor attained by refinement (weighted R:6.07o relative to9.t/p) and the more reasonable values of the bond lengths.

The X-ray spacing graphs in the literature that show d(001) as a func-tion of tetrahedral composition for trioctahedral chlorites are not validfor dioctahedral chlorites. With the exception of the Caillbre et al. speci-men, analyzed dioctahedral chlorites show 0.3 to 0.6 fewer atoms oftetrahedral Al than would be expected from the d(001) values. For thisreason at the start of the investigation the same tetrahedral compositionwas assumed for the pure chlorite specimen as found for the chloriticcomponent in the interstratified material, namely SL.6A10.b. The averageT-O bond length for this composition should be 1.641 A according to thestandard values of Smith and Bailey (1963). The refined values found inthe present study are 1.655 A for Model I and 1.685 A for Model 2. Fromother published data the average tetrahedral basal oxygen-oxygen dis-tance can be predicted as2.64 A for this composition. The refined valuefor Model t is 2.64 A, but for Model 2 it is 2.70 A. tfre angle of tetra-hedral twist to be expected for this composition is 7.5" (Brown and Bai-ley, 1963). The observed values are 7 .6" for Model I and 14.7" for Model2 .

The three criteria above indicate a mean tetrahedral composition ofSia.arAlo.oo for Model 1 and Siz.aaAlr.sz for Model 2. The mean value for

ffiA R. A. EGGLESTON AND S. W. BAILEY

Model 2 is unreasonable, but the mean value for Model 1 is within experi-mental error of the composition of Sia.26A10 zr found by chemical analysisof the Hayashi and Oinuma (1964) specimen, which has nearly identicald-values and intensities to the Tracy mine specimen. An over-all com-position for the latter specimen of

[Alz o(Sia 3A10.?) Or0(OH) r]-o.t [(Msr.rAlo.z) (oH; u]+o.z

is therefore consistent with the structural results, with the d-values bycomparison with a similar analyzed specimen, and with the relative ratiosof Si:Al:Mg found by analysis of the interstratified material. An attemptto obtain a quantitative analysis of the Tracy mine specimen on the Uni-versity of Chicago electron probe was unsuccessful.

The final atomic coordinates for Model 1 are listed in Table 7. Bond

T.cnrn 7. FrNu, Arourc Peaaunrnns Drocr.qgnnnar Cnromrn

8 T2 M(1)4 M(2)4 M(2), 2 M(4)8 O (1) ,4 (OH)(1)4 O ( 2 )8 O ( 3 )4 (OH)(2),8 (oH)(3)

(sio,$Alo,17)(vacant)(Al)(Mgc,zzAlo,z:)

.2312000.19001 or'-

.50661 4J,

r/60r/sr/6, r/2t/6, t/20.233to, r/3

.190s00r/2i 7 )

.2368

.2332

.4243

lengths and angles computed form these coordinates are listed in Table 8.Standard deviations are not listed because of uncertainty as to the correctweighting scheme to be used for the case of structural analysis of a pow-der. If the crystal has a two-layer structure, the refined r and y coordi-nates will be average values, and the standard deviations will incorporatethe real differences from these average positions. The accuracy is prob-ably greater that might first be assumed because (1) the refinement is ofa layer type whose parameters have already been studied previously bysingle crystal techniques, and (2) the reflections available are primarily oftype 201', which are the most useful for refining details within a layer. Theinternal consistency of the structure and the agreement of bond lengthsand angles both within the structure and with those of similar structuressuggest standard deviations not greater than 0.03 A for bond lengths.

Use of only h:3n reflections during refinement necessarily introducessome indeterminancy into the structure because most of the l parameterscannot be varied from their ideal positions. Tetrahedral and octahedral

DIOCTAHEDRAL CHLORITE

'I'egr,n 8. Inrnnlrourc DrsreNcns ANo ANcr.ss

(l) Tetrahedral distances in A (* :apical oxygen)T-O (1) * :1 .68 O (1) * O (2 ) :2 .79

T-O (2) -1.67 O (1)--O (3) :2.73

T-o (3) :1.61 o (1)---o (3)'-2.7sT-O (3)', :1.66 O (2) -O (3) :2 68

o (2 ) -o (3 ) ' :2 .6 rMean =1.650 O (3) -O (3)':2.6+

Mean :2 .7O

(2) Octahedral distances in A (t:edge shared by two octahedra)M (1)-O (1) :2.02X4 M (2)-O (1) :2.02X2,2.o3X2

M (1) - (Or r ) (1 ) :2 .o3X2 M (2) - (OH) (1 ) :2 .o2x2M (4) - (OH) (2 ) :20sX2 M (3) - (OH) (2 ) :20sX2M (4)-(OH) (3):2.0sXa M (3)-(OH) (3):2.0sX2,2.0sX2

around M (1) around M (2)

O (1) -O (1) :302X2,2 .6eX2 O (1) -O (1) :3 .o2X2 '2 .69X21,2 .6e

o (l)-(oH) (r):3.o2x+, 2.6eX4 O (l)-(OH) (1) :3.02X2, 3.o2X2, 2.6eX2(oH) (1)-(oH) (1) :2.6e1

around M (3) around M (4)(oH) (2 ) (OH) (2 ) :2 .771 (OH) (z ) - (OH) (3 ) :3 .o2X4,2 .77X41(OH) (2)-(OH) (3) :3.o2x.2, 3.02x2, 2.77 Xzt (OH) (3)-(OH) (3):3.02X2, 2.17 ;z.21(oH) (3)-(OH) (s) : 3.02X2, 2.7 7 X.zl, 2.7 7 1

(3) O-(OH) distances in A between sheetso (2 ) - (oH) (2 ) :2 .72X2

o (3 ) - (oH) (3 ) :280X2o (3)'-(oH) (s):2.8oX2

Mean :2.77

(4) Tetrahedral bond angles in degrees (*:apigal ettt.r;o {t;*-t o (2):1r2.eo ttl--t-o (3) :ttz to (1)*-T-o (3)',:111.0 T-O (2)-T:r30.2o (2) -T--{ (3) :10e.s T-O (3)-T:135.1X2o (2) -T--O (3)':103 3O (3) -T--{ (3)':107.5 Mean :133.5

Mean = 109.4

ordering, if present, cannot be detected. Although the space group re-quires that the vacant octahedral site be at Mr on the mirror plane, thereflections used require that the Mr and Mz octahedra be identical (Table8). In reality, it may be expected by analogy with other dioctahedralstructures that the M1 vacant site is larger than the occupied Ms sites,that the apical oxygens tilt away from the vacant site, and that the basaloxygen surface is corrugated. Although the basal oxygens are not pre-cisely coplanar in the present structure, the deviation is small and is notconsidered to be real.

Miiller (1963) states that it is questionable whether a trioctahedral

685

686 R. A. EGGLESTON AND S. W, BAILEV

brucite interlayer can exist in combination with a pyrophyllitelike 2: 1layer because of the different lateral dimensions involved. fn this speci-men a structural accomrnodation has been reached by thickening the tri-octahedral sheet considerably and by thinning the dioctahedral sheetslightly so that the lateral dimensions of the two sheets are identical. Themean lateral octahedral edge is 3.02 A in both sheets. The blucite inter-layer is thicker (2.15 A) than in any other chlorite so far investigated(1.90 to 2.03 A range). The dioctahedral sheet is probably as thin (2.05 A)as such sheets can be without greatly increasing anion repulsion. Thethicknesses recorded in other dioctahedral 2:1 species studied to daterange- from 2.04 to 2.t0 h, exclusive of celadonite, for which a value of2.46 A is reported.

The larger tetrahedral sheets have adjusted to the size of the smalleroctahedral sheets by tetrahedral rotation and by a slight thickening.Bailey (1966) in a survey of layer silicate structures cites tetrahedral rota-tion angles of 7.3" to 11.3o for dioctahedral species having no substitutionof tetrahedral Al for Si and values of 12.8o to 2l.Oo for tetrahedral com-positions from SigAl to SizAlz. The observed angle of twist (7.6') for di-octahedral chlorite is smaller than normally expected for its tetrahedralcomposition, but is appropriate when the adjusted thicknesses of theoctahedral sheets are considered. The direction of twist is such as to moveeach basal oxygen closer to the nearest brucite OH group, as found for allother chlorites.

The thinned dioctahedral sheet is compensated by a slightly thickenedtetrahedral sheet so that the total thickness of the 2:1 pyrophyllite layer(6.65 A) is similar to those in muscovite and parag_onite (6.58 A) and tothe 2:l talc layers in other chlorites (6.64 to 6.69 A ran-ge). Despite thethickened brucite interlayer, the d(001) value of 14.177 A is smaller thanwould be expected for a trioctahedral chlorite of similar net layer charge.This is the case for most other dioctahedral chlorites as well. For theTracy mine specimen with di/trioctahedral sheets, comparison of sheetthicknesses with other chlorites indicates that this smaller d(001) value isdue primarily to a closer approach of the basal oxygens to the brucitehydroxyls. The mean perpendicular O-OH distance is 2.69 A comparedto 2.75 to 2.85 A for trioctahedral chlorites. For chlorites with two di-octahedral sheets the additional effect of a thinner interlayer sheet maybe expected.

GBNnsrs

According to Shirozu and Bailey (1965) the most stable type of chloritelayer is IIb. This layer type is to be expected, therefore, when sufficientthermal energy is available in the environment of crystallization. Bailey

DIOCTAHEDRAL CEINRITE 687

and Tyler (1960) suggest a hydrothermal origin for the Tracy mine di-octahedral chlorite because of its proximity to known dikes and closeassociation in the ore with dickite and nacrite. The other chlorites withdi/trioctahedral sheets that can be identified as IID types, those of Hay-ashi and Oinuma (1964) and of Frenzel and Schembra (1965), are alsodescribed as hydrothermal in origin.

The sedimentary dioctahedral chlorite supplied to the writers by Pro-fessor Miiller is the Ialayer type. Data from additional specimens will beneeded to determine whether the difierence noted in layer type is due tostructural causes, i.e. because of having two dioctahedral sheets, or to itssedimentary rather than hydrothermal origin. Bailey and Brown (1962)and Shirozu and Bailey (1965) suggest thatla and 16 layer type chloritesare high structural energy forms that may crystallize metastably andpersist indefinitely within a sedimentary environment. Thus, they maybe indicative of a low temperature, authigenic origin.

NouBNcr,nrunn

Chlorites with di/trioctahedral sheets pose a problem in nomenclaturebecause they are intermediate structurally between wholly dioctahedraland wholly trioctahedral species. The Clay Minerals Group of the Miner-alogical Society (London) originally proposed the term leptochlorile Iorspecies of this type (Brown, 1955). Cookeite was listed as the only certainrepresentative of the di/trioctahedral group at that time. The most re-cent recommendation is that of the nomenclature subcommittee of theinternational clay organization, Comite International pour I'Etude desArgiles (C.I.P.E.A.). Their 1966 report (G.W. Brindley, pers. commun.)suggests that for the present the term d.i.octahedral chlorite should includeall species with four to five octahedral cations per formula unit and thattri.octahed.ral chlorite should include species with five to six octahedral cat-ions per formula unit. Eventually, terms such as d,i- tri octohedral chloriteand, tri,- d.i octahed,ral chlori,te may be used when these structural schemescan be identified clearly, the first prefix referring to the silicate sheet. Theusage in the present paper is in accord with these recommendations, ex-cept that diftrioctahedrol is preferred to di- tri octahed,ral.

Miiller (1963) has proposed the name sudoite as a group or subgroupname for dioctahedral chlorite, assumed at that time always to have twodioctahedral sheets. Frank-Kamenetsky, Logvinenko, and Drits (1965)have proposed the name tosud.ite for a regular interstratification of di-octahedral chlorite with montmorillonite, similar to the interstratifica-tion described in the present paper. Both names are in honor of ProfessorT. Sudo of Tokyo University. The nomenclalure committee of The CIayMinerals Society does not favor usage of specific group or subgroup

688 R. .4. EGGLESTON AND S, W, BAILEY

names within the chlorite series, but considers that the name sud,oitewould be suitable as a species designation (G. W. Brindley, pers. com-mun., 1966). The present writers propose that sudoite would be especiallyappropriate as a species name for chlorites of the type studied by Pro-fessor Sudo, i.e. Al-Mg specimens having 116 layers consisting of a di-octahedral 2: 1 sil icate unit and a trioctahedral interlayer. No names areconsidered necessary for interstratified materials.

AcruowIBoGMENTS

This study has been supported in part by Grant l l76-L2 from thePetroleum Research Fund administered by the American ChemicalSociety, by Grants GP-3748 and GP-4843 from the National ScienceFoundation, and by support by the Research Committee of the GraduateSchool from funds supplied by the Wisconsin Alumni Research Founda-tion. Computations were carried out in the University of Wisconsin Com-puting Center, which is also supported in part by National Science Foun-dation and Research Committee funds.

Rrrnnnmcns

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Kaiserbachtal (Siidpfalz). Neuu Jahrb. Min., Monatsh., 108-114.Ilnvesur, H. .q,Nn K. Onruu.q (1964) Aluminian chlorite from Kamikita mine, Japan. Clay

Sci.,2, 22-30.

DIOCTAHEDRAL CHINRITE 689

Hr,cr<roont, R. C. aNo C. RonnrNc (1965) A high-aluminous chlorite-swelling chloriteregular mixedJayer clay mineral. CI'a5' Minu., Bull. 6' 83-89.

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Sudoit-Chlorit- Gruppe). P r o c. I nt. Cl ay C onJ., S to ckhol'm, I, l2l-130.Scrrur-tz, L. G. (1963) Clay Minerals in Triassic rocks of the Colorado Plateau. U . S. Geol '

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Monuscriptreceived,June 16, 1966; acceptetllor pwblicalion,August 19,1966.